fix: add support for go 1.26

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

.github/workflows/build.yml

@@ -24,7 +24,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: ['1.20.x', '1.21.x', '1.22.x', '1.23.x', '1.24.x', '1.25.x']
+        go-version: ['1.20.x', '1.21.x', '1.22.x', '1.23.x', '1.24.x', '1.25.x', '1.26.x']
       fail-fast: false  # Continue with other versions if one fails
     steps:
       # full checkout for semantic-release
@@ -64,7 +64,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: ['1.24.x', '1.25.x']
+        go-version: ['1.24.x', '1.25.x', '1.26.x']
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:

v2/array/array.go

@@ -21,7 +21,7 @@ import (
 	"github.com/IBM/fp-go/v2/internal/array"
 	M "github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/option"
-	"github.com/IBM/fp-go/v2/tuple"
+	"github.com/IBM/fp-go/v2/pair"
 )
 
 // From constructs an array from a set of variadic arguments
@@ -163,11 +163,11 @@ func FilterMapWithIndex[A, B any](f func(int, A) Option[B]) Operator[A, B] {
 	return G.FilterMapWithIndex[[]A, []B](f)
 }
 
-// FilterChain maps an array with an iterating function that returns an [Option] of an array. It keeps only the Some values discarding the Nones and then flattens the result.
+// ChainOptionK maps an array with an iterating function that returns an [Option] of an array. It keeps only the Some values discarding the Nones and then flattens the result.
 //
 //go:inline
-func FilterChain[A, B any](f option.Kleisli[A, []B]) Operator[A, B] {
-	return G.FilterChain[[]A](f)
+func ChainOptionK[A, B any](f option.Kleisli[A, []B]) Operator[A, B] {
+	return G.ChainOptionK[[]A](f)
 }
 
 // FilterMapRef filters an array using a predicate on pointers and maps the matching elements using a function on pointers.
@@ -453,7 +453,7 @@ func Size[A any](as []A) int {
 // the second contains elements for which it returns true.
 //
 //go:inline
-func MonadPartition[A any](as []A, pred func(A) bool) tuple.Tuple2[[]A, []A] {
+func MonadPartition[A any](as []A, pred func(A) bool) pair.Pair[[]A, []A] {
 	return G.MonadPartition(as, pred)
 }
 
@@ -461,7 +461,7 @@ func MonadPartition[A any](as []A, pred func(A) bool) tuple.Tuple2[[]A, []A] {
 // for which the predicate returns false, the right one those for which the predicate returns true
 //
 //go:inline
-func Partition[A any](pred func(A) bool) func([]A) tuple.Tuple2[[]A, []A] {
+func Partition[A any](pred func(A) bool) func([]A) pair.Pair[[]A, []A] {
 	return G.Partition[[]A](pred)
 }
 

v2/array/array_test.go

@@ -24,8 +24,8 @@ import (
 	"github.com/IBM/fp-go/v2/internal/utils"
 	N "github.com/IBM/fp-go/v2/number"
 	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
 	S "github.com/IBM/fp-go/v2/string"
-	T "github.com/IBM/fp-go/v2/tuple"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -163,11 +163,11 @@ func TestPartition(t *testing.T) {
 		return n > 2
 	}
 
-	assert.Equal(t, T.MakeTuple2(Empty[int](), Empty[int]()), Partition(pred)(Empty[int]()))
-	assert.Equal(t, T.MakeTuple2(From(1), From(3)), Partition(pred)(From(1, 3)))
+	assert.Equal(t, pair.MakePair(Empty[int](), Empty[int]()), Partition(pred)(Empty[int]()))
+	assert.Equal(t, pair.MakePair(From(1), From(3)), Partition(pred)(From(1, 3)))
 }
 
-func TestFilterChain(t *testing.T) {
+func TestChainOptionK(t *testing.T) {
 	src := From(1, 2, 3)
 
 	f := func(i int) O.Option[[]string] {
@@ -177,7 +177,7 @@ func TestFilterChain(t *testing.T) {
 		return O.None[[]string]()
 	}
 
-	res := FilterChain(f)(src)
+	res := ChainOptionK(f)(src)
 
 	assert.Equal(t, From("a1", "b1", "a3", "b3"), res)
 }

v2/array/generic/array.go

@@ -21,7 +21,7 @@ import (
 	FC "github.com/IBM/fp-go/v2/internal/functor"
 	M "github.com/IBM/fp-go/v2/monoid"
 	O "github.com/IBM/fp-go/v2/option"
-	"github.com/IBM/fp-go/v2/tuple"
+	"github.com/IBM/fp-go/v2/pair"
 )
 
 // Of constructs a single element array
@@ -215,7 +215,7 @@ func Filter[AS ~[]A, PRED ~func(A) bool, A any](pred PRED) func(AS) AS {
 	return FilterWithIndex[AS](F.Ignore1of2[int](pred))
 }
 
-func FilterChain[GA ~[]A, GB ~[]B, A, B any](f func(a A) O.Option[GB]) func(GA) GB {
+func ChainOptionK[GA ~[]A, GB ~[]B, A, B any](f func(a A) O.Option[GB]) func(GA) GB {
 	return F.Flow2(
 		FilterMap[GA, []GB](f),
 		Flatten[[]GB],
@@ -234,7 +234,7 @@ func FilterMapWithIndex[GA ~[]A, GB ~[]B, A, B any](f func(int, A) O.Option[B])
 	return F.Bind2nd(MonadFilterMapWithIndex[GA, GB, A, B], f)
 }
 
-func MonadPartition[GA ~[]A, A any](as GA, pred func(A) bool) tuple.Tuple2[GA, GA] {
+func MonadPartition[GA ~[]A, A any](as GA, pred func(A) bool) pair.Pair[GA, GA] {
 	left := Empty[GA]()
 	right := Empty[GA]()
 	array.Reduce(as, func(c bool, a A) bool {
@@ -246,10 +246,10 @@ func MonadPartition[GA ~[]A, A any](as GA, pred func(A) bool) tuple.Tuple2[GA, G
 		return c
 	}, true)
 	// returns the partition
-	return tuple.MakeTuple2(left, right)
+	return pair.MakePair(left, right)
 }
 
-func Partition[GA ~[]A, A any](pred func(A) bool) func(GA) tuple.Tuple2[GA, GA] {
+func Partition[GA ~[]A, A any](pred func(A) bool) func(GA) pair.Pair[GA, GA] {
 	return F.Bind2nd(MonadPartition[GA, A], pred)
 }
 

v2/array/generic/zip.go

@@ -18,7 +18,7 @@ package generic
 import (
 	F "github.com/IBM/fp-go/v2/function"
 	N "github.com/IBM/fp-go/v2/number"
-	T "github.com/IBM/fp-go/v2/tuple"
+	"github.com/IBM/fp-go/v2/pair"
 )
 
 // ZipWith applies a function to pairs of elements at the same index in two arrays, collecting the results in a new array. If one
@@ -34,19 +34,19 @@ func ZipWith[AS ~[]A, BS ~[]B, CS ~[]C, FCT ~func(A, B) C, A, B, C any](fa AS, f
 
 // Zip takes two arrays and returns an array of corresponding pairs. If one input array is short, excess elements of the
 // longer array are discarded
-func Zip[AS ~[]A, BS ~[]B, CS ~[]T.Tuple2[A, B], A, B any](fb BS) func(AS) CS {
-	return F.Bind23of3(ZipWith[AS, BS, CS, func(A, B) T.Tuple2[A, B]])(fb, T.MakeTuple2[A, B])
+func Zip[AS ~[]A, BS ~[]B, CS ~[]pair.Pair[A, B], A, B any](fb BS) func(AS) CS {
+	return F.Bind23of3(ZipWith[AS, BS, CS, func(A, B) pair.Pair[A, B]])(fb, pair.MakePair[A, B])
 }
 
 // Unzip is the function is reverse of [Zip]. Takes an array of pairs and return two corresponding arrays
-func Unzip[AS ~[]A, BS ~[]B, CS ~[]T.Tuple2[A, B], A, B any](cs CS) T.Tuple2[AS, BS] {
+func Unzip[AS ~[]A, BS ~[]B, CS ~[]pair.Pair[A, B], A, B any](cs CS) pair.Pair[AS, BS] {
 	l := len(cs)
 	as := make(AS, l)
 	bs := make(BS, l)
 	for i := range l {
 		t := cs[i]
-		as[i] = t.F1
-		bs[i] = t.F2
+		as[i] = pair.Head(t)
+		bs[i] = pair.Tail(t)
 	}
-	return T.MakeTuple2(as, bs)
+	return pair.MakePair(as, bs)
 }

v2/array/zip.go

@@ -17,7 +17,7 @@ package array
 
 import (
 	G "github.com/IBM/fp-go/v2/array/generic"
-	T "github.com/IBM/fp-go/v2/tuple"
+	"github.com/IBM/fp-go/v2/pair"
 )
 
 // ZipWith applies a function to pairs of elements at the same index in two arrays,
@@ -55,8 +55,8 @@ func ZipWith[FCT ~func(A, B) C, A, B, C any](fa []A, fb []B, f FCT) []C {
 //	// Result: [(a, 1), (b, 2)]
 //
 //go:inline
-func Zip[A, B any](fb []B) func([]A) []T.Tuple2[A, B] {
-	return G.Zip[[]A, []B, []T.Tuple2[A, B]](fb)
+func Zip[A, B any](fb []B) func([]A) []pair.Pair[A, B] {
+	return G.Zip[[]A, []B, []pair.Pair[A, B]](fb)
 }
 
 // Unzip is the reverse of Zip. It takes an array of pairs (tuples) and returns
@@ -78,6 +78,6 @@ func Zip[A, B any](fb []B) func([]A) []T.Tuple2[A, B] {
 //	ages := result.Tail   // [30, 25, 35]
 //
 //go:inline
-func Unzip[A, B any](cs []T.Tuple2[A, B]) T.Tuple2[[]A, []B] {
+func Unzip[A, B any](cs []pair.Pair[A, B]) pair.Pair[[]A, []B] {
 	return G.Unzip[[]A, []B](cs)
 }

v2/array/zip_test.go

@@ -19,7 +19,7 @@ import (
 	"fmt"
 	"testing"
 
-	T "github.com/IBM/fp-go/v2/tuple"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -40,7 +40,7 @@ func TestZip(t *testing.T) {
 
 	res := Zip[string](left)(right)
 
-	assert.Equal(t, From(T.MakeTuple2("a", 1), T.MakeTuple2("b", 2), T.MakeTuple2("c", 3)), res)
+	assert.Equal(t, From(pair.MakePair("a", 1), pair.MakePair("b", 2), pair.MakePair("c", 3)), res)
 }
 
 func TestUnzip(t *testing.T) {
@@ -51,6 +51,6 @@ func TestUnzip(t *testing.T) {
 
 	unzipped := Unzip(zipped)
 
-	assert.Equal(t, right, unzipped.F1)
-	assert.Equal(t, left, unzipped.F2)
+	assert.Equal(t, right, pair.Head(unzipped))
+	assert.Equal(t, left, pair.Tail(unzipped))
 }

v2/di/erasure/injector.go

@@ -87,8 +87,8 @@ var (
 	// assembleProviders constructs the provider map for item and non-item providers
 	assembleProviders = F.Flow3(
 		A.Partition(isItemProvider),
-		T.Map2(collectProviders, collectItemProviders),
-		T.Tupled2(mergeProviders.Concat),
+		pair.BiMap(collectProviders, collectItemProviders),
+		pair.Paired(mergeProviders.Concat),
 	)
 )
 

v2/effect/bind.go

@@ -68,7 +68,7 @@ func ApS[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
 	fa Effect[C, T],
 ) Operator[C, S1, S2] {
-	return readerreaderioresult.ApS[C](setter, fa)
+	return readerreaderioresult.ApS(setter, fa)
 }
 
 //go:inline
@@ -76,7 +76,7 @@ func ApSL[C, S, T any](
 	lens Lens[S, T],
 	fa Effect[C, T],
 ) Operator[C, S, S] {
-	return readerreaderioresult.ApSL[C](lens, fa)
+	return readerreaderioresult.ApSL(lens, fa)
 }
 
 //go:inline
@@ -84,7 +84,7 @@ func BindL[C, S, T any](
 	lens Lens[S, T],
 	f func(T) Effect[C, T],
 ) Operator[C, S, S] {
-	return readerreaderioresult.BindL[C](lens, f)
+	return readerreaderioresult.BindL(lens, f)
 }
 
 //go:inline
@@ -132,7 +132,7 @@ func BindReaderK[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
 	f reader.Kleisli[C, S1, T],
 ) Operator[C, S1, S2] {
-	return readerreaderioresult.BindReaderK[C](setter, f)
+	return readerreaderioresult.BindReaderK(setter, f)
 }
 
 //go:inline
@@ -140,7 +140,7 @@ func BindReaderIOK[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
 	f readerio.Kleisli[C, S1, T],
 ) Operator[C, S1, S2] {
-	return readerreaderioresult.BindReaderIOK[C](setter, f)
+	return readerreaderioresult.BindReaderIOK(setter, f)
 }
 
 //go:inline
@@ -172,7 +172,7 @@ func BindReaderKL[C, S, T any](
 	lens Lens[S, T],
 	f reader.Kleisli[C, T, T],
 ) Operator[C, S, S] {
-	return readerreaderioresult.BindReaderKL[C](lens, f)
+	return readerreaderioresult.BindReaderKL(lens, f)
 }
 
 //go:inline
@@ -180,7 +180,7 @@ func BindReaderIOKL[C, S, T any](
 	lens Lens[S, T],
 	f readerio.Kleisli[C, T, T],
 ) Operator[C, S, S] {
-	return readerreaderioresult.BindReaderIOKL[C](lens, f)
+	return readerreaderioresult.BindReaderIOKL(lens, f)
 }
 
 //go:inline
@@ -204,7 +204,7 @@ func ApReaderS[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
 	fa Reader[C, T],
 ) Operator[C, S1, S2] {
-	return readerreaderioresult.ApReaderS[C](setter, fa)
+	return readerreaderioresult.ApReaderS(setter, fa)
 }
 
 //go:inline
@@ -212,7 +212,7 @@ func ApReaderIOS[C, S1, S2, T any](
 	setter func(T) func(S1) S2,
 	fa ReaderIO[C, T],
 ) Operator[C, S1, S2] {
-	return readerreaderioresult.ApReaderIOS[C](setter, fa)
+	return readerreaderioresult.ApReaderIOS(setter, fa)
 }
 
 //go:inline
@@ -244,7 +244,7 @@ func ApReaderSL[C, S, T any](
 	lens Lens[S, T],
 	fa Reader[C, T],
 ) Operator[C, S, S] {
-	return readerreaderioresult.ApReaderSL[C](lens, fa)
+	return readerreaderioresult.ApReaderSL(lens, fa)
 }
 
 //go:inline
@@ -252,7 +252,7 @@ func ApReaderIOSL[C, S, T any](
 	lens Lens[S, T],
 	fa ReaderIO[C, T],
 ) Operator[C, S, S] {
-	return readerreaderioresult.ApReaderIOSL[C](lens, fa)
+	return readerreaderioresult.ApReaderIOSL(lens, fa)
 }
 
 //go:inline

v2/effect/bind_test.go

@@ -61,7 +61,7 @@ func TestBind(t *testing.T) {
 	t.Run("binds effect result to state", func(t *testing.T) {
 		initial := BindState{Name: "Alice"}
 
-		eff := Bind[TestContext](
+		eff := Bind(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -69,7 +69,7 @@ func TestBind(t *testing.T) {
 				}
 			},
 			func(s BindState) Effect[TestContext, int] {
-				return Of[TestContext, int](30)
+				return Of[TestContext](30)
 			},
 		)(Do[TestContext](initial))
 
@@ -83,7 +83,7 @@ func TestBind(t *testing.T) {
 	t.Run("chains multiple binds", func(t *testing.T) {
 		initial := BindState{}
 
-		eff := Bind[TestContext](
+		eff := Bind(
 			func(email string) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Email = email
@@ -91,9 +91,9 @@ func TestBind(t *testing.T) {
 				}
 			},
 			func(s BindState) Effect[TestContext, string] {
-				return Of[TestContext, string]("alice@example.com")
+				return Of[TestContext]("alice@example.com")
 			},
-		)(Bind[TestContext](
+		)(Bind(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -101,9 +101,9 @@ func TestBind(t *testing.T) {
 				}
 			},
 			func(s BindState) Effect[TestContext, int] {
-				return Of[TestContext, int](30)
+				return Of[TestContext](30)
 			},
-		)(Bind[TestContext](
+		)(Bind(
 			func(name string) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Name = name
@@ -111,7 +111,7 @@ func TestBind(t *testing.T) {
 				}
 			},
 			func(s BindState) Effect[TestContext, string] {
-				return Of[TestContext, string]("Alice")
+				return Of[TestContext]("Alice")
 			},
 		)(Do[TestContext](initial))))
 
@@ -127,7 +127,7 @@ func TestBind(t *testing.T) {
 		expectedErr := errors.New("bind error")
 		initial := BindState{Name: "Alice"}
 
-		eff := Bind[TestContext](
+		eff := Bind(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -182,7 +182,7 @@ func TestLet(t *testing.T) {
 			func(s BindState) string {
 				return s.Name + "@example.com"
 			},
-		)(Bind[TestContext](
+		)(Bind(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -190,7 +190,7 @@ func TestLet(t *testing.T) {
 				}
 			},
 			func(s BindState) Effect[TestContext, int] {
-				return Of[TestContext, int](25)
+				return Of[TestContext](25)
 			},
 		)(Do[TestContext](initial)))
 
@@ -270,7 +270,7 @@ func TestBindTo(t *testing.T) {
 
 		eff := BindTo[TestContext](func(v int) SimpleState {
 			return SimpleState{Value: v}
-		})(Of[TestContext, int](42))
+		})(Of[TestContext](42))
 
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
@@ -296,7 +296,7 @@ func TestBindTo(t *testing.T) {
 			},
 		)(BindTo[TestContext](func(x int) State {
 			return State{X: x}
-		})(Of[TestContext, int](10)))
+		})(Of[TestContext](10)))
 
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
@@ -309,9 +309,9 @@ func TestBindTo(t *testing.T) {
 func TestApS(t *testing.T) {
 	t.Run("applies effect and binds result to state", func(t *testing.T) {
 		initial := BindState{Name: "Alice"}
-		ageEffect := Of[TestContext, int](30)
+		ageEffect := Of[TestContext](30)
 
-		eff := ApS[TestContext](
+		eff := ApS(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -333,7 +333,7 @@ func TestApS(t *testing.T) {
 		initial := BindState{Name: "Alice"}
 		ageEffect := Fail[TestContext, int](expectedErr)
 
-		eff := ApS[TestContext](
+		eff := ApS(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -388,7 +388,7 @@ func TestBindIOEitherK(t *testing.T) {
 				}
 			},
 			func(s BindState) ioeither.IOEither[error, int] {
-				return ioeither.Of[error, int](30)
+				return ioeither.Of[error](30)
 			},
 		)(Do[TestContext](initial))
 
@@ -411,7 +411,7 @@ func TestBindIOEitherK(t *testing.T) {
 				}
 			},
 			func(s BindState) ioeither.IOEither[error, int] {
-				return ioeither.Left[int, error](expectedErr)
+				return ioeither.Left[int](expectedErr)
 			},
 		)(Do[TestContext](initial))
 
@@ -434,7 +434,7 @@ func TestBindIOResultK(t *testing.T) {
 				}
 			},
 			func(s BindState) ioresult.IOResult[int] {
-				return ioresult.Of[int](30)
+				return ioresult.Of(30)
 			},
 		)(Do[TestContext](initial))
 
@@ -450,7 +450,7 @@ func TestBindReaderK(t *testing.T) {
 	t.Run("binds Reader operation to state", func(t *testing.T) {
 		initial := BindState{Name: "Alice"}
 
-		eff := BindReaderK[TestContext](
+		eff := BindReaderK(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -476,7 +476,7 @@ func TestBindReaderIOK(t *testing.T) {
 	t.Run("binds ReaderIO operation to state", func(t *testing.T) {
 		initial := BindState{Name: "Alice"}
 
-		eff := BindReaderIOK[TestContext](
+		eff := BindReaderIOK(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -512,7 +512,7 @@ func TestBindEitherK(t *testing.T) {
 				}
 			},
 			func(s BindState) either.Either[error, int] {
-				return either.Of[error, int](30)
+				return either.Of[error](30)
 			},
 		)(Do[TestContext](initial))
 
@@ -535,7 +535,7 @@ func TestBindEitherK(t *testing.T) {
 				}
 			},
 			func(s BindState) either.Either[error, int] {
-				return either.Left[int, error](expectedErr)
+				return either.Left[int](expectedErr)
 			},
 		)(Do[TestContext](initial))
 
@@ -566,9 +566,9 @@ func TestLensOperations(t *testing.T) {
 
 	t.Run("ApSL applies effect using lens", func(t *testing.T) {
 		initial := BindState{Name: "Alice", Age: 25}
-		ageEffect := Of[TestContext, int](30)
+		ageEffect := Of[TestContext](30)
 
-		eff := ApSL[TestContext](ageLens, ageEffect)(Do[TestContext](initial))
+		eff := ApSL(ageLens, ageEffect)(Do[TestContext](initial))
 
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
@@ -580,10 +580,10 @@ func TestLensOperations(t *testing.T) {
 	t.Run("BindL binds effect using lens", func(t *testing.T) {
 		initial := BindState{Name: "Alice", Age: 25}
 
-		eff := BindL[TestContext](
+		eff := BindL(
 			ageLens,
 			func(age int) Effect[TestContext, int] {
-				return Of[TestContext, int](age + 5)
+				return Of[TestContext](age + 5)
 			},
 		)(Do[TestContext](initial))
 
@@ -667,7 +667,7 @@ func TestApOperations(t *testing.T) {
 		initial := BindState{Name: "Alice"}
 		readerEffect := func(ctx TestContext) int { return 30 }
 
-		eff := ApReaderS[TestContext](
+		eff := ApReaderS(
 			func(age int) func(BindState) BindState {
 				return func(s BindState) BindState {
 					s.Age = age
@@ -685,7 +685,7 @@ func TestApOperations(t *testing.T) {
 
 	t.Run("ApEitherS applies Either effect", func(t *testing.T) {
 		initial := BindState{Name: "Alice"}
-		eitherEffect := either.Of[error, int](30)
+		eitherEffect := either.Of[error](30)
 
 		eff := ApEitherS[TestContext](
 			func(age int) func(BindState) BindState {
@@ -742,7 +742,7 @@ func TestComplexBindChain(t *testing.T) {
 			func(s ComplexState) string {
 				return s.Name + "@example.com"
 			},
-		)(Bind[TestContext](
+		)(Bind(
 			func(age int) func(ComplexState) ComplexState {
 				return func(s ComplexState) ComplexState {
 					s.Age = age
@@ -750,11 +750,11 @@ func TestComplexBindChain(t *testing.T) {
 				}
 			},
 			func(s ComplexState) Effect[TestContext, int] {
-				return Of[TestContext, int](25)
+				return Of[TestContext](25)
 			},
 		)(BindTo[TestContext](func(name string) ComplexState {
 			return ComplexState{Name: name}
-		})(Of[TestContext, string]("Alice"))))))
+		})(Of[TestContext]("Alice"))))))
 
 		result, err := runEffect(eff, TestContext{Value: "test"})
 

v2/effect/dependencies_test.go

@@ -36,7 +36,7 @@ type InnerContext struct {
 func TestLocal(t *testing.T) {
 	t.Run("transforms context for inner effect", func(t *testing.T) {
 		// Create an effect that uses InnerContext
-		innerEffect := Of[InnerContext, string]("result")
+		innerEffect := Of[InnerContext]("result")
 
 		// Transform OuterContext to InnerContext
 		accessor := func(outer OuterContext) InnerContext {
@@ -52,7 +52,7 @@ func TestLocal(t *testing.T) {
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -61,9 +61,9 @@ func TestLocal(t *testing.T) {
 
 	t.Run("allows accessing outer context fields", func(t *testing.T) {
 		// Create an effect that reads from InnerContext
-		innerEffect := Chain[InnerContext](func(_ string) Effect[InnerContext, string] {
-			return Of[InnerContext, string]("inner value")
-		})(Of[InnerContext, string]("start"))
+		innerEffect := Chain(func(_ string) Effect[InnerContext, string] {
+			return Of[InnerContext]("inner value")
+		})(Of[InnerContext]("start"))
 
 		// Transform context
 		accessor := func(outer OuterContext) InnerContext {
@@ -78,7 +78,7 @@ func TestLocal(t *testing.T) {
 			Value:  "original",
 			Number: 100,
 		})(outerEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -100,7 +100,7 @@ func TestLocal(t *testing.T) {
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
@@ -119,7 +119,7 @@ func TestLocal(t *testing.T) {
 		}
 
 		// Effect at deepest level
-		level3Effect := Of[Level3, string]("deep result")
+		level3Effect := Of[Level3]("deep result")
 
 		// Transform Level2 -> Level3
 		local23 := Local[Level2, Level3, string](func(l2 Level2) Level3 {
@@ -137,7 +137,7 @@ func TestLocal(t *testing.T) {
 
 		// Run with Level1 context
 		ioResult := Provide[Level1, string](Level1{A: "a"})(level1Effect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -158,7 +158,7 @@ func TestLocal(t *testing.T) {
 		}
 
 		// Effect that needs only DatabaseConfig
-		dbEffect := Of[DatabaseConfig, string]("connected")
+		dbEffect := Of[DatabaseConfig]("connected")
 
 		// Extract DB config from AppConfig
 		accessor := func(app AppConfig) DatabaseConfig {
@@ -178,7 +178,7 @@ func TestLocal(t *testing.T) {
 			APIKey:  "secret",
 			Timeout: 30,
 		})(appEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -188,7 +188,7 @@ func TestLocal(t *testing.T) {
 
 func TestContramap(t *testing.T) {
 	t.Run("is equivalent to Local", func(t *testing.T) {
-		innerEffect := Of[InnerContext, int](42)
+		innerEffect := Of[InnerContext](42)
 
 		accessor := func(outer OuterContext) InnerContext {
 			return InnerContext{Value: outer.Value}
@@ -206,11 +206,11 @@ func TestContramap(t *testing.T) {
 
 		// Run both
 		localIO := Provide[OuterContext, int](outerCtx)(localEffect)
-		localReader := RunSync[int](localIO)
+		localReader := RunSync(localIO)
 		localResult, localErr := localReader(context.Background())
 
 		contramapIO := Provide[OuterContext, int](outerCtx)(contramapEffect)
-		contramapReader := RunSync[int](contramapIO)
+		contramapReader := RunSync(contramapIO)
 		contramapResult, contramapErr := contramapReader(context.Background())
 
 		assert.NoError(t, localErr)
@@ -219,7 +219,7 @@ func TestContramap(t *testing.T) {
 	})
 
 	t.Run("transforms context correctly", func(t *testing.T) {
-		innerEffect := Of[InnerContext, string]("success")
+		innerEffect := Of[InnerContext]("success")
 
 		accessor := func(outer OuterContext) InnerContext {
 			return InnerContext{Value: outer.Value + " modified"}
@@ -232,7 +232,7 @@ func TestContramap(t *testing.T) {
 			Value:  "original",
 			Number: 50,
 		})(outerEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -254,7 +254,7 @@ func TestContramap(t *testing.T) {
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
-		readerResult := RunSync[int](ioResult)
+		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
@@ -275,7 +275,7 @@ func TestLocalAndContramapInteroperability(t *testing.T) {
 		}
 
 		// Effect at deepest level
-		effect3 := Of[Config3, string]("result")
+		effect3 := Of[Config3]("result")
 
 		// Use Local for first transformation
 		local23 := Local[Config2, Config3, string](func(c2 Config2) Config3 {
@@ -293,7 +293,7 @@ func TestLocalAndContramapInteroperability(t *testing.T) {
 
 		// Run
 		ioResult := Provide[Config1, string](Config1{Value: "test"})(effect1)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -312,24 +312,24 @@ func TestLocalEffectK(t *testing.T) {
 		}
 
 		// Effect that needs DatabaseConfig
-		dbEffect := Of[DatabaseConfig, string]("query result")
+		dbEffect := Of[DatabaseConfig]("query result")
 
 		// Transform AppConfig to DatabaseConfig effectfully
 		loadConfig := func(app AppConfig) Effect[AppConfig, DatabaseConfig] {
-			return Of[AppConfig, DatabaseConfig](DatabaseConfig{
+			return Of[AppConfig](DatabaseConfig{
 				ConnectionString: "loaded from " + app.ConfigPath,
 			})
 		}
 
 		// Apply the transformation
-		transform := LocalEffectK[string, DatabaseConfig, AppConfig](loadConfig)
+		transform := LocalEffectK[string](loadConfig)
 		appEffect := transform(dbEffect)
 
 		// Run with AppConfig
 		ioResult := Provide[AppConfig, string](AppConfig{
 			ConfigPath: "/etc/app.conf",
 		})(appEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -345,7 +345,7 @@ func TestLocalEffectK(t *testing.T) {
 			Path string
 		}
 
-		innerEffect := Of[InnerCtx, string]("success")
+		innerEffect := Of[InnerCtx]("success")
 
 		expectedErr := assert.AnError
 		// Context transformation that fails
@@ -353,11 +353,11 @@ func TestLocalEffectK(t *testing.T) {
 			return Fail[OuterCtx, InnerCtx](expectedErr)
 		}
 
-		transform := LocalEffectK[string, InnerCtx, OuterCtx](failingTransform)
+		transform := LocalEffectK[string](failingTransform)
 		outerEffect := transform(innerEffect)
 
 		ioResult := Provide[OuterCtx, string](OuterCtx{Path: "test"})(outerEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
@@ -378,14 +378,14 @@ func TestLocalEffectK(t *testing.T) {
 
 		// Successful context transformation
 		transform := func(outer OuterCtx) Effect[OuterCtx, InnerCtx] {
-			return Of[OuterCtx, InnerCtx](InnerCtx{Value: outer.Path})
+			return Of[OuterCtx](InnerCtx{Value: outer.Path})
 		}
 
-		transformK := LocalEffectK[string, InnerCtx, OuterCtx](transform)
+		transformK := LocalEffectK[string](transform)
 		outerEffect := transformK(innerEffect)
 
 		ioResult := Provide[OuterCtx, string](OuterCtx{Path: "test"})(outerEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
@@ -402,25 +402,25 @@ func TestLocalEffectK(t *testing.T) {
 		}
 
 		// Effect that uses Config
-		configEffect := Chain[Config](func(cfg Config) Effect[Config, string] {
-			return Of[Config, string]("processed: " + cfg.Data)
+		configEffect := Chain(func(cfg Config) Effect[Config, string] {
+			return Of[Config]("processed: " + cfg.Data)
 		})(readerreaderioresult.Ask[Config]())
 
 		// Effectful transformation that simulates loading config
 		loadConfigEffect := func(app AppContext) Effect[AppContext, Config] {
 			// Simulate IO operation (e.g., reading file)
-			return Of[AppContext, Config](Config{
+			return Of[AppContext](Config{
 				Data: "loaded from " + app.ConfigFile,
 			})
 		}
 
-		transform := LocalEffectK[string, Config, AppContext](loadConfigEffect)
+		transform := LocalEffectK[string](loadConfigEffect)
 		appEffect := transform(configEffect)
 
 		ioResult := Provide[AppContext, string](AppContext{
 			ConfigFile: "config.json",
 		})(appEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -439,16 +439,16 @@ func TestLocalEffectK(t *testing.T) {
 		}
 
 		// Effect at deepest level
-		level3Effect := Of[Level3, string]("deep result")
+		level3Effect := Of[Level3]("deep result")
 
 		// Transform Level2 -> Level3 effectfully
-		transform23 := LocalEffectK[string, Level3, Level2](func(l2 Level2) Effect[Level2, Level3] {
-			return Of[Level2, Level3](Level3{C: l2.B + "-c"})
+		transform23 := LocalEffectK[string](func(l2 Level2) Effect[Level2, Level3] {
+			return Of[Level2](Level3{C: l2.B + "-c"})
 		})
 
 		// Transform Level1 -> Level2 effectfully
-		transform12 := LocalEffectK[string, Level2, Level1](func(l1 Level1) Effect[Level1, Level2] {
-			return Of[Level1, Level2](Level2{B: l1.A + "-b"})
+		transform12 := LocalEffectK[string](func(l1 Level1) Effect[Level1, Level2] {
+			return Of[Level1](Level2{B: l1.A + "-b"})
 		})
 
 		// Compose transformations
@@ -457,7 +457,7 @@ func TestLocalEffectK(t *testing.T) {
 
 		// Run with Level1 context
 		ioResult := Provide[Level1, string](Level1{A: "a"})(level1Effect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -477,8 +477,8 @@ func TestLocalEffectK(t *testing.T) {
 		}
 
 		// Effect that needs DatabaseConfig
-		dbEffect := Chain[DatabaseConfig](func(cfg DatabaseConfig) Effect[DatabaseConfig, string] {
-			return Of[DatabaseConfig, string](fmt.Sprintf("%s:%d", cfg.Host, cfg.Port))
+		dbEffect := Chain(func(cfg DatabaseConfig) Effect[DatabaseConfig, string] {
+			return Of[DatabaseConfig](fmt.Sprintf("%s:%d", cfg.Host, cfg.Port))
 		})(readerreaderioresult.Ask[DatabaseConfig]())
 
 		// Transform using outer context
@@ -488,13 +488,13 @@ func TestLocalEffectK(t *testing.T) {
 			if app.Environment == "prod" {
 				prefix = "prod-"
 			}
-			return Of[AppConfig, DatabaseConfig](DatabaseConfig{
+			return Of[AppConfig](DatabaseConfig{
 				Host: prefix + app.DBHost,
 				Port: app.DBPort,
 			})
 		}
 
-		transform := LocalEffectK[string, DatabaseConfig, AppConfig](transformWithContext)
+		transform := LocalEffectK[string](transformWithContext)
 		appEffect := transform(dbEffect)
 
 		ioResult := Provide[AppConfig, string](AppConfig{
@@ -502,7 +502,7 @@ func TestLocalEffectK(t *testing.T) {
 			DBHost:      "localhost",
 			DBPort:      5432,
 		})(appEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -518,31 +518,31 @@ func TestLocalEffectK(t *testing.T) {
 			APIKey string
 		}
 
-		innerEffect := Of[ValidatedConfig, string]("success")
+		innerEffect := Of[ValidatedConfig]("success")
 
 		// Validation that can fail
 		validateConfig := func(raw RawConfig) Effect[RawConfig, ValidatedConfig] {
 			if raw.APIKey == "" {
 				return Fail[RawConfig, ValidatedConfig](assert.AnError)
 			}
-			return Of[RawConfig, ValidatedConfig](ValidatedConfig{
+			return Of[RawConfig](ValidatedConfig{
 				APIKey: raw.APIKey,
 			})
 		}
 
-		transform := LocalEffectK[string, ValidatedConfig, RawConfig](validateConfig)
+		transform := LocalEffectK[string](validateConfig)
 		outerEffect := transform(innerEffect)
 
 		// Test with invalid config
 		ioResult := Provide[RawConfig, string](RawConfig{APIKey: ""})(outerEffect)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
 
 		// Test with valid config
 		ioResult2 := Provide[RawConfig, string](RawConfig{APIKey: "valid-key"})(outerEffect)
-		readerResult2 := RunSync[string](ioResult2)
+		readerResult2 := RunSync(ioResult2)
 		result, err2 := readerResult2(context.Background())
 
 		assert.NoError(t, err2)
@@ -561,11 +561,11 @@ func TestLocalEffectK(t *testing.T) {
 		}
 
 		// Effect at deepest level
-		effect3 := Of[Level3, string]("result")
+		effect3 := Of[Level3]("result")
 
 		// Use LocalEffectK for first transformation (effectful)
-		localEffectK23 := LocalEffectK[string, Level3, Level2](func(l2 Level2) Effect[Level2, Level3] {
-			return Of[Level2, Level3](Level3{Info: l2.Data})
+		localEffectK23 := LocalEffectK[string](func(l2 Level2) Effect[Level2, Level3] {
+			return Of[Level2](Level3{Info: l2.Data})
 		})
 
 		// Use Local for second transformation (pure)
@@ -579,7 +579,7 @@ func TestLocalEffectK(t *testing.T) {
 
 		// Run
 		ioResult := Provide[Level1, string](Level1{Value: "test"})(effect1)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -596,22 +596,22 @@ func TestLocalEffectK(t *testing.T) {
 		}
 
 		// Effect that uses InnerCtx
-		innerEffect := Chain[InnerCtx](func(ctx InnerCtx) Effect[InnerCtx, int] {
-			return Of[InnerCtx, int](ctx.Value * 2)
+		innerEffect := Chain(func(ctx InnerCtx) Effect[InnerCtx, int] {
+			return Of[InnerCtx](ctx.Value * 2)
 		})(readerreaderioresult.Ask[InnerCtx]())
 
 		// Complex transformation with nested effects
 		complexTransform := func(outer OuterCtx) Effect[OuterCtx, InnerCtx] {
-			return Of[OuterCtx, InnerCtx](InnerCtx{
+			return Of[OuterCtx](InnerCtx{
 				Value: outer.Multiplier * 10,
 			})
 		}
 
-		transform := LocalEffectK[int, InnerCtx, OuterCtx](complexTransform)
+		transform := LocalEffectK[int](complexTransform)
 		outerEffect := transform(innerEffect)
 
 		ioResult := Provide[OuterCtx, int](OuterCtx{Multiplier: 3})(outerEffect)
-		readerResult := RunSync[int](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)

v2/effect/effect_test.go

@@ -31,13 +31,13 @@ type TestContext struct {
 
 func runEffect[A any](eff Effect[TestContext, A], ctx TestContext) (A, error) {
 	ioResult := Provide[TestContext, A](ctx)(eff)
-	readerResult := RunSync[A](ioResult)
+	readerResult := RunSync(ioResult)
 	return readerResult(context.Background())
 }
 
 func TestSucceed(t *testing.T) {
 	t.Run("creates successful effect with value", func(t *testing.T) {
-		eff := Succeed[TestContext, int](42)
+		eff := Succeed[TestContext](42)
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
 		assert.NoError(t, err)
@@ -45,7 +45,7 @@ func TestSucceed(t *testing.T) {
 	})
 
 	t.Run("creates successful effect with string", func(t *testing.T) {
-		eff := Succeed[TestContext, string]("hello")
+		eff := Succeed[TestContext]("hello")
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
 		assert.NoError(t, err)
@@ -58,7 +58,7 @@ func TestSucceed(t *testing.T) {
 			Age  int
 		}
 		user := User{Name: "Alice", Age: 30}
-		eff := Succeed[TestContext, User](user)
+		eff := Succeed[TestContext](user)
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
 		assert.NoError(t, err)
@@ -88,7 +88,7 @@ func TestFail(t *testing.T) {
 
 func TestOf(t *testing.T) {
 	t.Run("lifts value into effect", func(t *testing.T) {
-		eff := Of[TestContext, int](100)
+		eff := Of[TestContext](100)
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
 		assert.NoError(t, err)
@@ -97,8 +97,8 @@ func TestOf(t *testing.T) {
 
 	t.Run("is equivalent to Succeed", func(t *testing.T) {
 		value := "test value"
-		eff1 := Of[TestContext, string](value)
-		eff2 := Succeed[TestContext, string](value)
+		eff1 := Of[TestContext](value)
+		eff2 := Succeed[TestContext](value)
 
 		result1, err1 := runEffect(eff1, TestContext{Value: "test"})
 		result2, err2 := runEffect(eff2, TestContext{Value: "test"})
@@ -111,7 +111,7 @@ func TestOf(t *testing.T) {
 
 func TestMap(t *testing.T) {
 	t.Run("maps over successful effect", func(t *testing.T) {
-		eff := Of[TestContext, int](10)
+		eff := Of[TestContext](10)
 		mapped := Map[TestContext](func(x int) int {
 			return x * 2
 		})(eff)
@@ -123,7 +123,7 @@ func TestMap(t *testing.T) {
 	})
 
 	t.Run("maps to different type", func(t *testing.T) {
-		eff := Of[TestContext, int](42)
+		eff := Of[TestContext](42)
 		mapped := Map[TestContext](func(x int) string {
 			return fmt.Sprintf("value: %d", x)
 		})(eff)
@@ -148,7 +148,7 @@ func TestMap(t *testing.T) {
 	})
 
 	t.Run("chains multiple maps", func(t *testing.T) {
-		eff := Of[TestContext, int](5)
+		eff := Of[TestContext](5)
 		result := Map[TestContext](func(x int) int {
 			return x + 1
 		})(Map[TestContext](func(x int) int {
@@ -164,9 +164,9 @@ func TestMap(t *testing.T) {
 
 func TestChain(t *testing.T) {
 	t.Run("chains successful effects", func(t *testing.T) {
-		eff := Of[TestContext, int](10)
-		chained := Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
+		eff := Of[TestContext](10)
+		chained := Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
 		})(eff)
 
 		result, err := runEffect(chained, TestContext{Value: "test"})
@@ -176,9 +176,9 @@ func TestChain(t *testing.T) {
 	})
 
 	t.Run("chains to different type", func(t *testing.T) {
-		eff := Of[TestContext, int](42)
-		chained := Chain[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](fmt.Sprintf("number: %d", x))
+		eff := Of[TestContext](42)
+		chained := Chain(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](fmt.Sprintf("number: %d", x))
 		})(eff)
 
 		result, err := runEffect(chained, TestContext{Value: "test"})
@@ -190,8 +190,8 @@ func TestChain(t *testing.T) {
 	t.Run("propagates first error", func(t *testing.T) {
 		expectedErr := errors.New("first error")
 		eff := Fail[TestContext, int](expectedErr)
-		chained := Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
+		chained := Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
 		})(eff)
 
 		_, err := runEffect(chained, TestContext{Value: "test"})
@@ -202,8 +202,8 @@ func TestChain(t *testing.T) {
 
 	t.Run("propagates second error", func(t *testing.T) {
 		expectedErr := errors.New("second error")
-		eff := Of[TestContext, int](10)
-		chained := Chain[TestContext](func(x int) Effect[TestContext, int] {
+		eff := Of[TestContext](10)
+		chained := Chain(func(x int) Effect[TestContext, int] {
 			return Fail[TestContext, int](expectedErr)
 		})(eff)
 
@@ -214,11 +214,11 @@ func TestChain(t *testing.T) {
 	})
 
 	t.Run("chains multiple operations", func(t *testing.T) {
-		eff := Of[TestContext, int](5)
-		result := Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x + 10)
-		})(Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
+		eff := Of[TestContext](5)
+		result := Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x + 10)
+		})(Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
 		})(eff))
 
 		value, err := runEffect(result, TestContext{Value: "test"})
@@ -230,10 +230,10 @@ func TestChain(t *testing.T) {
 
 func TestAp(t *testing.T) {
 	t.Run("applies function effect to value effect", func(t *testing.T) {
-		fn := Of[TestContext, func(int) int](func(x int) int {
+		fn := Of[TestContext](func(x int) int {
 			return x * 2
 		})
-		value := Of[TestContext, int](21)
+		value := Of[TestContext](21)
 
 		result := Ap[int](value)(fn)
 		val, err := runEffect(result, TestContext{Value: "test"})
@@ -243,10 +243,10 @@ func TestAp(t *testing.T) {
 	})
 
 	t.Run("applies function to different type", func(t *testing.T) {
-		fn := Of[TestContext, func(int) string](func(x int) string {
+		fn := Of[TestContext](func(x int) string {
 			return fmt.Sprintf("value: %d", x)
 		})
-		value := Of[TestContext, int](42)
+		value := Of[TestContext](42)
 
 		result := Ap[string](value)(fn)
 		val, err := runEffect(result, TestContext{Value: "test"})
@@ -258,7 +258,7 @@ func TestAp(t *testing.T) {
 	t.Run("propagates error from function effect", func(t *testing.T) {
 		expectedErr := errors.New("function error")
 		fn := Fail[TestContext, func(int) int](expectedErr)
-		value := Of[TestContext, int](42)
+		value := Of[TestContext](42)
 
 		result := Ap[int](value)(fn)
 		_, err := runEffect(result, TestContext{Value: "test"})
@@ -269,7 +269,7 @@ func TestAp(t *testing.T) {
 
 	t.Run("propagates error from value effect", func(t *testing.T) {
 		expectedErr := errors.New("value error")
-		fn := Of[TestContext, func(int) int](func(x int) int {
+		fn := Of[TestContext](func(x int) int {
 			return x * 2
 		})
 		value := Fail[TestContext, int](expectedErr)
@@ -285,9 +285,9 @@ func TestAp(t *testing.T) {
 func TestSuspend(t *testing.T) {
 	t.Run("suspends effect computation", func(t *testing.T) {
 		callCount := 0
-		eff := Suspend[TestContext, int](func() Effect[TestContext, int] {
+		eff := Suspend(func() Effect[TestContext, int] {
 			callCount++
-			return Of[TestContext, int](42)
+			return Of[TestContext](42)
 		})
 
 		// Effect not executed yet
@@ -302,7 +302,7 @@ func TestSuspend(t *testing.T) {
 
 	t.Run("suspends failing effect", func(t *testing.T) {
 		expectedErr := errors.New("suspended error")
-		eff := Suspend[TestContext, int](func() Effect[TestContext, int] {
+		eff := Suspend(func() Effect[TestContext, int] {
 			return Fail[TestContext, int](expectedErr)
 		})
 
@@ -314,8 +314,8 @@ func TestSuspend(t *testing.T) {
 
 	t.Run("allows lazy evaluation", func(t *testing.T) {
 		var value int
-		eff := Suspend[TestContext, int](func() Effect[TestContext, int] {
-			return Of[TestContext, int](value)
+		eff := Suspend(func() Effect[TestContext, int] {
+			return Of[TestContext](value)
 		})
 
 		value = 10
@@ -334,8 +334,8 @@ func TestSuspend(t *testing.T) {
 func TestTap(t *testing.T) {
 	t.Run("executes side effect without changing value", func(t *testing.T) {
 		sideEffectValue := 0
-		eff := Of[TestContext, int](42)
-		tapped := Tap[TestContext](func(x int) Effect[TestContext, any] {
+		eff := Of[TestContext](42)
+		tapped := Tap(func(x int) Effect[TestContext, any] {
 			sideEffectValue = x * 2
 			return Of[TestContext, any](nil)
 		})(eff)
@@ -350,7 +350,7 @@ func TestTap(t *testing.T) {
 	t.Run("propagates original error", func(t *testing.T) {
 		expectedErr := errors.New("original error")
 		eff := Fail[TestContext, int](expectedErr)
-		tapped := Tap[TestContext](func(x int) Effect[TestContext, any] {
+		tapped := Tap(func(x int) Effect[TestContext, any] {
 			return Of[TestContext, any](nil)
 		})(eff)
 
@@ -362,8 +362,8 @@ func TestTap(t *testing.T) {
 
 	t.Run("propagates tap error", func(t *testing.T) {
 		expectedErr := errors.New("tap error")
-		eff := Of[TestContext, int](42)
-		tapped := Tap[TestContext](func(x int) Effect[TestContext, any] {
+		eff := Of[TestContext](42)
+		tapped := Tap(func(x int) Effect[TestContext, any] {
 			return Fail[TestContext, any](expectedErr)
 		})(eff)
 
@@ -375,11 +375,11 @@ func TestTap(t *testing.T) {
 
 	t.Run("chains multiple taps", func(t *testing.T) {
 		values := []int{}
-		eff := Of[TestContext, int](10)
-		result := Tap[TestContext](func(x int) Effect[TestContext, any] {
+		eff := Of[TestContext](10)
+		result := Tap(func(x int) Effect[TestContext, any] {
 			values = append(values, x+2)
 			return Of[TestContext, any](nil)
-		})(Tap[TestContext](func(x int) Effect[TestContext, any] {
+		})(Tap(func(x int) Effect[TestContext, any] {
 			values = append(values, x+1)
 			return Of[TestContext, any](nil)
 		})(eff))
@@ -394,13 +394,13 @@ func TestTap(t *testing.T) {
 
 func TestTernary(t *testing.T) {
 	t.Run("executes onTrue when predicate is true", func(t *testing.T) {
-		kleisli := Ternary[TestContext, int, string](
+		kleisli := Ternary(
 			func(x int) bool { return x > 10 },
 			func(x int) Effect[TestContext, string] {
-				return Of[TestContext, string]("greater")
+				return Of[TestContext]("greater")
 			},
 			func(x int) Effect[TestContext, string] {
-				return Of[TestContext, string]("less or equal")
+				return Of[TestContext]("less or equal")
 			},
 		)
 
@@ -411,13 +411,13 @@ func TestTernary(t *testing.T) {
 	})
 
 	t.Run("executes onFalse when predicate is false", func(t *testing.T) {
-		kleisli := Ternary[TestContext, int, string](
+		kleisli := Ternary(
 			func(x int) bool { return x > 10 },
 			func(x int) Effect[TestContext, string] {
-				return Of[TestContext, string]("greater")
+				return Of[TestContext]("greater")
 			},
 			func(x int) Effect[TestContext, string] {
-				return Of[TestContext, string]("less or equal")
+				return Of[TestContext]("less or equal")
 			},
 		)
 
@@ -429,13 +429,13 @@ func TestTernary(t *testing.T) {
 
 	t.Run("handles errors in onTrue branch", func(t *testing.T) {
 		expectedErr := errors.New("true branch error")
-		kleisli := Ternary[TestContext, int, string](
+		kleisli := Ternary(
 			func(x int) bool { return x > 10 },
 			func(x int) Effect[TestContext, string] {
 				return Fail[TestContext, string](expectedErr)
 			},
 			func(x int) Effect[TestContext, string] {
-				return Of[TestContext, string]("less or equal")
+				return Of[TestContext]("less or equal")
 			},
 		)
 
@@ -447,10 +447,10 @@ func TestTernary(t *testing.T) {
 
 	t.Run("handles errors in onFalse branch", func(t *testing.T) {
 		expectedErr := errors.New("false branch error")
-		kleisli := Ternary[TestContext, int, string](
+		kleisli := Ternary(
 			func(x int) bool { return x > 10 },
 			func(x int) Effect[TestContext, string] {
-				return Of[TestContext, string]("greater")
+				return Of[TestContext]("greater")
 			},
 			func(x int) Effect[TestContext, string] {
 				return Fail[TestContext, string](expectedErr)
@@ -466,9 +466,9 @@ func TestTernary(t *testing.T) {
 
 func TestEffectComposition(t *testing.T) {
 	t.Run("composes Map and Chain", func(t *testing.T) {
-		eff := Of[TestContext, int](5)
-		result := Chain[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](fmt.Sprintf("result: %d", x))
+		eff := Of[TestContext](5)
+		result := Chain(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](fmt.Sprintf("result: %d", x))
 		})(Map[TestContext](func(x int) int {
 			return x * 2
 		})(eff))
@@ -481,12 +481,12 @@ func TestEffectComposition(t *testing.T) {
 
 	t.Run("composes Chain and Tap", func(t *testing.T) {
 		sideEffect := 0
-		eff := Of[TestContext, int](10)
-		result := Tap[TestContext](func(x int) Effect[TestContext, any] {
+		eff := Of[TestContext](10)
+		result := Tap(func(x int) Effect[TestContext, any] {
 			sideEffect = x
 			return Of[TestContext, any](nil)
-		})(Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
+		})(Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
 		})(eff))
 
 		value, err := runEffect(result, TestContext{Value: "test"})
@@ -499,7 +499,7 @@ func TestEffectComposition(t *testing.T) {
 
 func TestEffectWithResult(t *testing.T) {
 	t.Run("converts result to effect", func(t *testing.T) {
-		res := result.Of[int](42)
+		res := result.Of(42)
 		// This demonstrates integration with result package
 		assert.True(t, result.IsRight(res))
 	})

v2/effect/monoid_test.go

@@ -30,11 +30,11 @@ func TestApplicativeMonoid(t *testing.T) {
 			"",
 		)
 
-		effectMonoid := ApplicativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := ApplicativeMonoid[TestContext](stringMonoid)
 
-		eff1 := Of[TestContext, string]("Hello")
-		eff2 := Of[TestContext, string](" ")
-		eff3 := Of[TestContext, string]("World")
+		eff1 := Of[TestContext]("Hello")
+		eff2 := Of[TestContext](" ")
+		eff3 := Of[TestContext]("World")
 
 		combined := effectMonoid.Concat(eff1, effectMonoid.Concat(eff2, eff3))
 		result, err := runEffect(combined, TestContext{Value: "test"})
@@ -49,11 +49,11 @@ func TestApplicativeMonoid(t *testing.T) {
 			0,
 		)
 
-		effectMonoid := ApplicativeMonoid[TestContext, int](intMonoid)
+		effectMonoid := ApplicativeMonoid[TestContext](intMonoid)
 
-		eff1 := Of[TestContext, int](10)
-		eff2 := Of[TestContext, int](20)
-		eff3 := Of[TestContext, int](30)
+		eff1 := Of[TestContext](10)
+		eff2 := Of[TestContext](20)
+		eff3 := Of[TestContext](30)
 
 		combined := effectMonoid.Concat(eff1, effectMonoid.Concat(eff2, eff3))
 		result, err := runEffect(combined, TestContext{Value: "test"})
@@ -68,7 +68,7 @@ func TestApplicativeMonoid(t *testing.T) {
 			"empty",
 		)
 
-		effectMonoid := ApplicativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := ApplicativeMonoid[TestContext](stringMonoid)
 
 		result, err := runEffect(effectMonoid.Empty(), TestContext{Value: "test"})
 
@@ -83,10 +83,10 @@ func TestApplicativeMonoid(t *testing.T) {
 			"",
 		)
 
-		effectMonoid := ApplicativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := ApplicativeMonoid[TestContext](stringMonoid)
 
 		eff1 := Fail[TestContext, string](expectedErr)
-		eff2 := Of[TestContext, string]("World")
+		eff2 := Of[TestContext]("World")
 
 		combined := effectMonoid.Concat(eff1, eff2)
 		_, err := runEffect(combined, TestContext{Value: "test"})
@@ -102,9 +102,9 @@ func TestApplicativeMonoid(t *testing.T) {
 			"",
 		)
 
-		effectMonoid := ApplicativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := ApplicativeMonoid[TestContext](stringMonoid)
 
-		eff1 := Of[TestContext, string]("Hello")
+		eff1 := Of[TestContext]("Hello")
 		eff2 := Fail[TestContext, string](expectedErr)
 
 		combined := effectMonoid.Concat(eff1, eff2)
@@ -120,13 +120,13 @@ func TestApplicativeMonoid(t *testing.T) {
 			1,
 		)
 
-		effectMonoid := ApplicativeMonoid[TestContext, int](intMonoid)
+		effectMonoid := ApplicativeMonoid[TestContext](intMonoid)
 
 		effects := []Effect[TestContext, int]{
-			Of[TestContext, int](2),
-			Of[TestContext, int](3),
-			Of[TestContext, int](4),
-			Of[TestContext, int](5),
+			Of[TestContext](2),
+			Of[TestContext](3),
+			Of[TestContext](4),
+			Of[TestContext](5),
 		}
 
 		combined := effectMonoid.Empty()
@@ -152,11 +152,11 @@ func TestApplicativeMonoid(t *testing.T) {
 			Counter{Count: 0},
 		)
 
-		effectMonoid := ApplicativeMonoid[TestContext, Counter](counterMonoid)
+		effectMonoid := ApplicativeMonoid[TestContext](counterMonoid)
 
-		eff1 := Of[TestContext, Counter](Counter{Count: 5})
-		eff2 := Of[TestContext, Counter](Counter{Count: 10})
-		eff3 := Of[TestContext, Counter](Counter{Count: 15})
+		eff1 := Of[TestContext](Counter{Count: 5})
+		eff2 := Of[TestContext](Counter{Count: 10})
+		eff3 := Of[TestContext](Counter{Count: 15})
 
 		combined := effectMonoid.Concat(eff1, effectMonoid.Concat(eff2, eff3))
 		result, err := runEffect(combined, TestContext{Value: "test"})
@@ -173,10 +173,10 @@ func TestAlternativeMonoid(t *testing.T) {
 			"",
 		)
 
-		effectMonoid := AlternativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := AlternativeMonoid[TestContext](stringMonoid)
 
-		eff1 := Of[TestContext, string]("First")
-		eff2 := Of[TestContext, string]("Second")
+		eff1 := Of[TestContext]("First")
+		eff2 := Of[TestContext]("Second")
 
 		combined := effectMonoid.Concat(eff1, eff2)
 		result, err := runEffect(combined, TestContext{Value: "test"})
@@ -191,10 +191,10 @@ func TestAlternativeMonoid(t *testing.T) {
 			"",
 		)
 
-		effectMonoid := AlternativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := AlternativeMonoid[TestContext](stringMonoid)
 
 		eff1 := Fail[TestContext, string](errors.New("first failed"))
-		eff2 := Of[TestContext, string]("Second")
+		eff2 := Of[TestContext]("Second")
 
 		combined := effectMonoid.Concat(eff1, eff2)
 		result, err := runEffect(combined, TestContext{Value: "test"})
@@ -210,7 +210,7 @@ func TestAlternativeMonoid(t *testing.T) {
 			"",
 		)
 
-		effectMonoid := AlternativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := AlternativeMonoid[TestContext](stringMonoid)
 
 		eff1 := Fail[TestContext, string](errors.New("first error"))
 		eff2 := Fail[TestContext, string](expectedErr)
@@ -228,7 +228,7 @@ func TestAlternativeMonoid(t *testing.T) {
 			"default",
 		)
 
-		effectMonoid := AlternativeMonoid[TestContext, string](stringMonoid)
+		effectMonoid := AlternativeMonoid[TestContext](stringMonoid)
 
 		result, err := runEffect(effectMonoid.Empty(), TestContext{Value: "test"})
 
@@ -242,12 +242,12 @@ func TestAlternativeMonoid(t *testing.T) {
 			0,
 		)
 
-		effectMonoid := AlternativeMonoid[TestContext, int](intMonoid)
+		effectMonoid := AlternativeMonoid[TestContext](intMonoid)
 
 		eff1 := Fail[TestContext, int](errors.New("error 1"))
 		eff2 := Fail[TestContext, int](errors.New("error 2"))
-		eff3 := Of[TestContext, int](42)
-		eff4 := Of[TestContext, int](100)
+		eff3 := Of[TestContext](42)
+		eff4 := Of[TestContext](100)
 
 		combined := effectMonoid.Concat(
 			effectMonoid.Concat(eff1, eff2),
@@ -273,11 +273,11 @@ func TestAlternativeMonoid(t *testing.T) {
 			Result{Value: "", Code: 0},
 		)
 
-		effectMonoid := AlternativeMonoid[TestContext, Result](resultMonoid)
+		effectMonoid := AlternativeMonoid[TestContext](resultMonoid)
 
 		eff1 := Fail[TestContext, Result](errors.New("failed"))
-		eff2 := Of[TestContext, Result](Result{Value: "success", Code: 200})
-		eff3 := Of[TestContext, Result](Result{Value: "backup", Code: 201})
+		eff2 := Of[TestContext](Result{Value: "success", Code: 200})
+		eff3 := Of[TestContext](Result{Value: "backup", Code: 201})
 
 		combined := effectMonoid.Concat(effectMonoid.Concat(eff1, eff2), eff3)
 		result, err := runEffect(combined, TestContext{Value: "test"})
@@ -295,11 +295,11 @@ func TestMonoidComparison(t *testing.T) {
 			"",
 		)
 
-		applicativeMonoid := ApplicativeMonoid[TestContext, string](stringMonoid)
-		alternativeMonoid := AlternativeMonoid[TestContext, string](stringMonoid)
+		applicativeMonoid := ApplicativeMonoid[TestContext](stringMonoid)
+		alternativeMonoid := AlternativeMonoid[TestContext](stringMonoid)
 
-		eff1 := Of[TestContext, string]("A")
-		eff2 := Of[TestContext, string]("B")
+		eff1 := Of[TestContext]("A")
+		eff2 := Of[TestContext]("B")
 
 		// Applicative combines values
 		applicativeResult, err1 := runEffect(
@@ -325,11 +325,11 @@ func TestMonoidComparison(t *testing.T) {
 			0,
 		)
 
-		applicativeMonoid := ApplicativeMonoid[TestContext, int](intMonoid)
-		alternativeMonoid := AlternativeMonoid[TestContext, int](intMonoid)
+		applicativeMonoid := ApplicativeMonoid[TestContext](intMonoid)
+		alternativeMonoid := AlternativeMonoid[TestContext](intMonoid)
 
 		eff1 := Fail[TestContext, int](errors.New("error 1"))
-		eff2 := Of[TestContext, int](42)
+		eff2 := Of[TestContext](42)
 
 		// Applicative fails on first error
 		_, err1 := runEffect(

v2/effect/retry_test.go

@@ -34,7 +34,7 @@ func TestRetrying(t *testing.T) {
 			policy,
 			func(status retry.RetryStatus) Effect[TestContext, string] {
 				attemptCount++
-				return Of[TestContext, string]("success")
+				return Of[TestContext]("success")
 			},
 			func(res Result[string]) bool {
 				return result.IsLeft(res) // retry on error
@@ -59,7 +59,7 @@ func TestRetrying(t *testing.T) {
 				if attemptCount < 3 {
 					return Fail[TestContext, string](errors.New("temporary error"))
 				}
-				return Of[TestContext, string]("success after retries")
+				return Of[TestContext]("success after retries")
 			},
 			func(res Result[string]) bool {
 				return result.IsLeft(res) // retry on error
@@ -78,7 +78,7 @@ func TestRetrying(t *testing.T) {
 		maxRetries := uint(3)
 		policy := retry.LimitRetries(maxRetries)
 
-		eff := Retrying[TestContext, string](
+		eff := Retrying(
 			policy,
 			func(status retry.RetryStatus) Effect[TestContext, string] {
 				attemptCount++
@@ -103,7 +103,7 @@ func TestRetrying(t *testing.T) {
 			policy,
 			func(status retry.RetryStatus) Effect[TestContext, int] {
 				attemptCount++
-				return Of[TestContext, int](42)
+				return Of[TestContext](42)
 			},
 			func(res Result[int]) bool {
 				return result.IsLeft(res) // retry on error
@@ -125,7 +125,7 @@ func TestRetrying(t *testing.T) {
 			policy,
 			func(status retry.RetryStatus) Effect[TestContext, int] {
 				attemptCount++
-				return Of[TestContext, int](attemptCount * 10)
+				return Of[TestContext](attemptCount * 10)
 			},
 			func(res Result[int]) bool {
 				// Retry if value is less than 30
@@ -155,7 +155,7 @@ func TestRetrying(t *testing.T) {
 				if len(statuses) < 3 {
 					return Fail[TestContext, string](errors.New("retry"))
 				}
-				return Of[TestContext, string]("done")
+				return Of[TestContext]("done")
 			},
 			func(res Result[string]) bool {
 				return result.IsLeft(res)
@@ -188,7 +188,7 @@ func TestRetrying(t *testing.T) {
 				if attemptCount < 3 {
 					return Fail[TestContext, string](errors.New("retry"))
 				}
-				return Of[TestContext, string]("success")
+				return Of[TestContext]("success")
 			},
 			func(res Result[string]) bool {
 				return result.IsLeft(res)
@@ -218,7 +218,7 @@ func TestRetrying(t *testing.T) {
 				if attemptCount < 2 {
 					return Fail[TestContext, string](errors.New("retry"))
 				}
-				return Of[TestContext, string]("success")
+				return Of[TestContext]("success")
 			},
 			func(res Result[string]) bool {
 				return result.IsLeft(res)
@@ -250,7 +250,7 @@ func TestRetrying(t *testing.T) {
 					return Fail[TestContext, string](err)
 				}
 				attemptCount++
-				return Of[TestContext, string]("finally succeeded")
+				return Of[TestContext]("finally succeeded")
 			},
 			func(res Result[string]) bool {
 				return result.IsLeft(res)
@@ -268,7 +268,7 @@ func TestRetrying(t *testing.T) {
 		attemptCount := 0
 		policy := retry.LimitRetries(5)
 
-		eff := Retrying[TestContext, string](
+		eff := Retrying(
 			policy,
 			func(status retry.RetryStatus) Effect[TestContext, string] {
 				attemptCount++
@@ -297,7 +297,7 @@ func TestRetrying(t *testing.T) {
 				if attemptCount < 2 {
 					return Fail[TestContext, string](errors.New("retry"))
 				}
-				return Of[TestContext, string]("success with context")
+				return Of[TestContext]("success with context")
 			},
 			func(res Result[string]) bool {
 				return result.IsLeft(res)
@@ -335,7 +335,7 @@ func TestRetryingWithComplexScenarios(t *testing.T) {
 				if status.IterNumber < 2 {
 					return Fail[TestContext, State](errors.New("retry"))
 				}
-				return Of[TestContext, State](state)
+				return Of[TestContext](state)
 			},
 			func(res Result[State]) bool {
 				return result.IsLeft(res)
@@ -353,8 +353,8 @@ func TestRetryingWithComplexScenarios(t *testing.T) {
 		attemptCount := 0
 		policy := retry.LimitRetries(3)
 
-		eff := Chain[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string]("final: " + string(rune('0'+x)))
+		eff := Chain(func(x int) Effect[TestContext, string] {
+			return Of[TestContext]("final: " + string(rune('0'+x)))
 		})(Retrying[TestContext, int](
 			policy,
 			func(status retry.RetryStatus) Effect[TestContext, int] {
@@ -362,7 +362,7 @@ func TestRetryingWithComplexScenarios(t *testing.T) {
 				if attemptCount < 2 {
 					return Fail[TestContext, int](errors.New("retry"))
 				}
-				return Of[TestContext, int](attemptCount)
+				return Of[TestContext](attemptCount)
 			},
 			func(res Result[int]) bool {
 				return result.IsLeft(res)

v2/effect/run_test.go

@@ -26,10 +26,10 @@ import (
 func TestProvide(t *testing.T) {
 	t.Run("provides context to effect", func(t *testing.T) {
 		ctx := TestContext{Value: "test-value"}
-		eff := Of[TestContext, string]("result")
+		eff := Of[TestContext]("result")
 
 		ioResult := Provide[TestContext, string](ctx)(eff)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -43,10 +43,10 @@ func TestProvide(t *testing.T) {
 		}
 
 		cfg := Config{Host: "localhost", Port: 8080}
-		eff := Of[Config, string]("connected")
+		eff := Of[Config]("connected")
 
 		ioResult := Provide[Config, string](cfg)(eff)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -59,7 +59,7 @@ func TestProvide(t *testing.T) {
 		eff := Fail[TestContext, string](expectedErr)
 
 		ioResult := Provide[TestContext, string](ctx)(eff)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
@@ -72,10 +72,10 @@ func TestProvide(t *testing.T) {
 		}
 
 		ctx := SimpleContext{ID: 42}
-		eff := Of[SimpleContext, int](100)
+		eff := Of[SimpleContext](100)
 
 		ioResult := Provide[SimpleContext, int](ctx)(eff)
-		readerResult := RunSync[int](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -85,12 +85,12 @@ func TestProvide(t *testing.T) {
 	t.Run("provides context to chained effects", func(t *testing.T) {
 		ctx := TestContext{Value: "base"}
 
-		eff := Chain[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string]("result")
-		})(Of[TestContext, int](42))
+		eff := Chain(func(x int) Effect[TestContext, string] {
+			return Of[TestContext]("result")
+		})(Of[TestContext](42))
 
 		ioResult := Provide[TestContext, string](ctx)(eff)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -102,10 +102,10 @@ func TestProvide(t *testing.T) {
 
 		eff := Map[TestContext](func(x int) string {
 			return "mapped"
-		})(Of[TestContext, int](42))
+		})(Of[TestContext](42))
 
 		ioResult := Provide[TestContext, string](ctx)(eff)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -116,10 +116,10 @@ func TestProvide(t *testing.T) {
 func TestRunSync(t *testing.T) {
 	t.Run("runs effect synchronously", func(t *testing.T) {
 		ctx := TestContext{Value: "test"}
-		eff := Of[TestContext, int](42)
+		eff := Of[TestContext](42)
 
 		ioResult := Provide[TestContext, int](ctx)(eff)
-		readerResult := RunSync[int](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -128,10 +128,10 @@ func TestRunSync(t *testing.T) {
 
 	t.Run("runs effect with context.Context", func(t *testing.T) {
 		ctx := TestContext{Value: "test"}
-		eff := Of[TestContext, string]("hello")
+		eff := Of[TestContext]("hello")
 
 		ioResult := Provide[TestContext, string](ctx)(eff)
-		readerResult := RunSync[string](ioResult)
+		readerResult := RunSync(ioResult)
 
 		bgCtx := context.Background()
 		result, err := readerResult(bgCtx)
@@ -146,7 +146,7 @@ func TestRunSync(t *testing.T) {
 		eff := Fail[TestContext, int](expectedErr)
 
 		ioResult := Provide[TestContext, int](ctx)(eff)
-		readerResult := RunSync[int](ioResult)
+		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
@@ -156,14 +156,14 @@ func TestRunSync(t *testing.T) {
 	t.Run("runs complex effect chains", func(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 
-		eff := Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
-		})(Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x + 10)
-		})(Of[TestContext, int](5)))
+		eff := Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
+		})(Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x + 10)
+		})(Of[TestContext](5)))
 
 		ioResult := Provide[TestContext, int](ctx)(eff)
-		readerResult := RunSync[int](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -172,10 +172,10 @@ func TestRunSync(t *testing.T) {
 
 	t.Run("handles multiple sequential runs", func(t *testing.T) {
 		ctx := TestContext{Value: "test"}
-		eff := Of[TestContext, int](42)
+		eff := Of[TestContext](42)
 
 		ioResult := Provide[TestContext, int](ctx)(eff)
-		readerResult := RunSync[int](ioResult)
+		readerResult := RunSync(ioResult)
 
 		// Run multiple times
 		result1, err1 := readerResult(context.Background())
@@ -198,10 +198,10 @@ func TestRunSync(t *testing.T) {
 
 		ctx := TestContext{Value: "test"}
 		user := User{Name: "Alice", Age: 30}
-		eff := Of[TestContext, User](user)
+		eff := Of[TestContext](user)
 
 		ioResult := Provide[TestContext, User](ctx)(eff)
-		readerResult := RunSync[User](ioResult)
+		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
 		assert.NoError(t, err)
@@ -219,10 +219,10 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 		cfg := AppConfig{APIKey: "secret", Timeout: 30}
 
 		// Create an effect that uses the config
-		eff := Of[AppConfig, string]("API call successful")
+		eff := Of[AppConfig]("API call successful")
 
 		// Provide config and run
-		result, err := RunSync[string](Provide[AppConfig, string](cfg)(eff))(context.Background())
+		result, err := RunSync(Provide[AppConfig, string](cfg)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "API call successful", result)
@@ -238,7 +238,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 
 		eff := Fail[AppConfig, string](expectedErr)
 
-		_, err := RunSync[string](Provide[AppConfig, string](cfg)(eff))(context.Background())
+		_, err := RunSync(Provide[AppConfig, string](cfg)(eff))(context.Background())
 
 		assert.Error(t, err)
 		assert.Equal(t, expectedErr, err)
@@ -249,11 +249,11 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 
 		eff := Map[TestContext](func(x int) string {
 			return "final"
-		})(Chain[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
-		})(Of[TestContext, int](21)))
+		})(Chain(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
+		})(Of[TestContext](21)))
 
-		result, err := RunSync[string](Provide[TestContext, string](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[TestContext, string](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "final", result)
@@ -267,7 +267,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 
 		ctx := TestContext{Value: "test"}
 
-		eff := Bind[TestContext](
+		eff := Bind(
 			func(y int) func(State) State {
 				return func(s State) State {
 					s.Y = y
@@ -275,13 +275,13 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 				}
 			},
 			func(s State) Effect[TestContext, int] {
-				return Of[TestContext, int](s.X * 2)
+				return Of[TestContext](s.X * 2)
 			},
 		)(BindTo[TestContext](func(x int) State {
 			return State{X: x}
-		})(Of[TestContext, int](10)))
+		})(Of[TestContext](10)))
 
-		result, err := RunSync[State](Provide[TestContext, State](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[TestContext, State](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, 10, result.X)
@@ -297,14 +297,14 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 		}
 
 		outerCtx := OuterCtx{Value: "outer"}
-		innerEff := Of[InnerCtx, string]("inner result")
+		innerEff := Of[InnerCtx]("inner result")
 
 		// Transform context
 		transformedEff := Local[OuterCtx, InnerCtx, string](func(outer OuterCtx) InnerCtx {
 			return InnerCtx{Data: outer.Value + "-transformed"}
 		})(innerEff)
 
-		result, err := RunSync[string](Provide[OuterCtx, string](outerCtx)(transformedEff))(context.Background())
+		result, err := RunSync(Provide[OuterCtx, string](outerCtx)(transformedEff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "inner result", result)
@@ -314,11 +314,11 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		input := []int{1, 2, 3, 4, 5}
 
-		eff := TraverseArray[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
+		eff := TraverseArray(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
 		})(input)
 
-		result, err := RunSync[[]int](Provide[TestContext, []int](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[TestContext, []int](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)

v2/effect/traverse_test.go

@@ -27,8 +27,8 @@ import (
 func TestTraverseArray(t *testing.T) {
 	t.Run("traverses empty array", func(t *testing.T) {
 		input := []int{}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](strconv.Itoa(x))
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -39,8 +39,8 @@ func TestTraverseArray(t *testing.T) {
 
 	t.Run("traverses array with single element", func(t *testing.T) {
 		input := []int{42}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](strconv.Itoa(x))
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -51,8 +51,8 @@ func TestTraverseArray(t *testing.T) {
 
 	t.Run("traverses array with multiple elements", func(t *testing.T) {
 		input := []int{1, 2, 3, 4, 5}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](strconv.Itoa(x))
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -63,8 +63,8 @@ func TestTraverseArray(t *testing.T) {
 
 	t.Run("transforms to different type", func(t *testing.T) {
 		input := []string{"hello", "world", "test"}
-		kleisli := TraverseArray[TestContext](func(s string) Effect[TestContext, int] {
-			return Of[TestContext, int](len(s))
+		kleisli := TraverseArray(func(s string) Effect[TestContext, int] {
+			return Of[TestContext](len(s))
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -76,11 +76,11 @@ func TestTraverseArray(t *testing.T) {
 	t.Run("stops on first error", func(t *testing.T) {
 		expectedErr := errors.New("traverse error")
 		input := []int{1, 2, 3, 4, 5}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
 			if x == 3 {
 				return Fail[TestContext, string](expectedErr)
 			}
-			return Of[TestContext, string](strconv.Itoa(x))
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		_, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -96,8 +96,8 @@ func TestTraverseArray(t *testing.T) {
 		}
 
 		input := []int{1, 2, 3}
-		kleisli := TraverseArray[TestContext](func(id int) Effect[TestContext, User] {
-			return Of[TestContext, User](User{
+		kleisli := TraverseArray(func(id int) Effect[TestContext, User] {
+			return Of[TestContext](User{
 				ID:   id,
 				Name: fmt.Sprintf("User%d", id),
 			})
@@ -118,15 +118,15 @@ func TestTraverseArray(t *testing.T) {
 	t.Run("chains with other operations", func(t *testing.T) {
 		input := []int{1, 2, 3}
 
-		eff := Chain[TestContext](func(strings []string) Effect[TestContext, int] {
+		eff := Chain(func(strings []string) Effect[TestContext, int] {
 			total := 0
 			for _, s := range strings {
 				val, _ := strconv.Atoi(s)
 				total += val
 			}
-			return Of[TestContext, int](total)
-		})(TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](strconv.Itoa(x * 2))
+			return Of[TestContext](total)
+		})(TraverseArray(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x * 2))
 		})(input))
 
 		result, err := runEffect(eff, TestContext{Value: "test"})
@@ -137,10 +137,10 @@ func TestTraverseArray(t *testing.T) {
 
 	t.Run("uses context in transformation", func(t *testing.T) {
 		input := []int{1, 2, 3}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Chain[TestContext](func(ctx TestContext) Effect[TestContext, string] {
-				return Of[TestContext, string](fmt.Sprintf("%s-%d", ctx.Value, x))
-			})(Of[TestContext, TestContext](TestContext{Value: "prefix"}))
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
+			return Chain(func(ctx TestContext) Effect[TestContext, string] {
+				return Of[TestContext](fmt.Sprintf("%s-%d", ctx.Value, x))
+			})(Of[TestContext](TestContext{Value: "prefix"}))
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -151,8 +151,8 @@ func TestTraverseArray(t *testing.T) {
 
 	t.Run("preserves order", func(t *testing.T) {
 		input := []int{5, 3, 8, 1, 9, 2}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 10)
+		kleisli := TraverseArray(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 10)
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -168,8 +168,8 @@ func TestTraverseArray(t *testing.T) {
 			input[i] = i
 		}
 
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, int] {
-			return Of[TestContext, int](x * 2)
+		kleisli := TraverseArray(func(x int) Effect[TestContext, int] {
+			return Of[TestContext](x * 2)
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -184,16 +184,16 @@ func TestTraverseArray(t *testing.T) {
 		input := []int{1, 2, 3}
 
 		// First traversal: int -> string
-		kleisli1 := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](strconv.Itoa(x))
+		kleisli1 := TraverseArray(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		// Second traversal: string -> int (length)
-		kleisli2 := TraverseArray[TestContext](func(s string) Effect[TestContext, int] {
-			return Of[TestContext, int](len(s))
+		kleisli2 := TraverseArray(func(s string) Effect[TestContext, int] {
+			return Of[TestContext](len(s))
 		})
 
-		eff := Chain[TestContext](kleisli2)(kleisli1(input))
+		eff := Chain(kleisli2)(kleisli1(input))
 
 		result, err := runEffect(eff, TestContext{Value: "test"})
 
@@ -203,8 +203,8 @@ func TestTraverseArray(t *testing.T) {
 
 	t.Run("handles nil array", func(t *testing.T) {
 		var input []int
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](strconv.Itoa(x))
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		result, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -222,8 +222,8 @@ func TestTraverseArray(t *testing.T) {
 				result += s + ","
 			}
 			return result
-		})(TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
-			return Of[TestContext, string](strconv.Itoa(x))
+		})(TraverseArray(func(x int) Effect[TestContext, string] {
+			return Of[TestContext](strconv.Itoa(x))
 		})(input))
 
 		result, err := runEffect(eff, TestContext{Value: "test"})
@@ -235,11 +235,11 @@ func TestTraverseArray(t *testing.T) {
 	t.Run("error in middle of array", func(t *testing.T) {
 		expectedErr := errors.New("middle error")
 		input := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
 			if x == 5 {
 				return Fail[TestContext, string](expectedErr)
 			}
-			return Of[TestContext, string](strconv.Itoa(x))
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		_, err := runEffect(kleisli(input), TestContext{Value: "test"})
@@ -251,11 +251,11 @@ func TestTraverseArray(t *testing.T) {
 	t.Run("error at end of array", func(t *testing.T) {
 		expectedErr := errors.New("end error")
 		input := []int{1, 2, 3, 4, 5}
-		kleisli := TraverseArray[TestContext](func(x int) Effect[TestContext, string] {
+		kleisli := TraverseArray(func(x int) Effect[TestContext, string] {
 			if x == 5 {
 				return Fail[TestContext, string](expectedErr)
 			}
-			return Of[TestContext, string](strconv.Itoa(x))
+			return Of[TestContext](strconv.Itoa(x))
 		})
 
 		_, err := runEffect(kleisli(input), TestContext{Value: "test"})

v2/either/either.go

@@ -188,6 +188,81 @@ func MonadChain[E, A, B any](fa Either[E, A], f Kleisli[E, A, B]) Either[E, B] {
 	return f(fa.r)
 }
 
+// MonadChainLeft sequences a computation on the Left (error) value, allowing error recovery or transformation.
+// If the Either is Left, applies the provided function to the error value, which returns a new Either.
+// If the Either is Right, returns the Right value unchanged with the new error type.
+//
+// This is the dual of [MonadChain] - while MonadChain operates on Right values (success),
+// MonadChainLeft operates on Left values (errors). It's useful for error recovery, error transformation,
+// or chaining alternative computations when an error occurs.
+//
+// Note: MonadChainLeft is identical to [OrElse] - both provide the same functionality for error recovery.
+//
+// The error type can be transformed from EA to EB, allowing flexible error type conversions.
+//
+// Example:
+//
+//	// Error recovery: convert specific errors to success
+//	result := either.MonadChainLeft(
+//	    either.Left[int](errors.New("not found")),
+//	    func(err error) either.Either[string, int] {
+//	        if err.Error() == "not found" {
+//	            return either.Right[string](0) // default value
+//	        }
+//	        return either.Left[int](err.Error()) // transform error
+//	    },
+//	) // Right(0)
+//
+//	// Error transformation: change error type
+//	result := either.MonadChainLeft(
+//	    either.Left[int](404),
+//	    func(code int) either.Either[string, int] {
+//	        return either.Left[int](fmt.Sprintf("Error code: %d", code))
+//	    },
+//	) // Left("Error code: 404")
+//
+//	// Right values pass through unchanged
+//	result := either.MonadChainLeft(
+//	    either.Right[error](42),
+//	    func(err error) either.Either[string, int] {
+//	        return either.Left[int]("error")
+//	    },
+//	) // Right(42)
+//
+//go:inline
+func MonadChainLeft[EA, EB, A any](fa Either[EA, A], f Kleisli[EB, EA, A]) Either[EB, A] {
+	return MonadFold(fa, f, Of[EB])
+}
+
+// ChainLeft is the curried version of [MonadChainLeft].
+// Returns a function that sequences a computation on the Left (error) value.
+//
+// Note: ChainLeft is identical to [OrElse] - both provide the same functionality for error recovery.
+//
+// This is useful for creating reusable error handlers or transformers that can be
+// composed with other Either operations using pipes or function composition.
+//
+// Example:
+//
+//	// Create a reusable error handler
+//	handleNotFound := either.ChainLeft[error, string](func(err error) either.Either[string, int] {
+//	    if err.Error() == "not found" {
+//	        return either.Right[string](0)
+//	    }
+//	    return either.Left[int](err.Error())
+//	})
+//
+//	// Use in a pipeline
+//	result := F.Pipe1(
+//	    either.Left[int](errors.New("not found")),
+//	    handleNotFound,
+//	) // Right(0)
+//
+//go:inline
+func ChainLeft[EA, EB, A any](f Kleisli[EB, EA, A]) Kleisli[EB, Either[EA, A], A] {
+	return Fold(f, Of[EB])
+}
+
 // MonadChainFirst executes a side-effect computation but returns the original value.
 // Useful for performing actions (like logging) without changing the value.
 //
@@ -471,6 +546,8 @@ func Alt[E, A any](that Lazy[Either[E, A]]) Operator[E, A, A] {
 // If the Either is Left, it applies the provided function to the error value,
 // which returns a new Either that replaces the original.
 //
+// Note: OrElse is identical to [ChainLeft] - both provide the same functionality for error recovery.
+//
 // This is useful for error recovery, fallback logic, or chaining alternative computations.
 // The error type can be widened from E1 to E2, allowing transformation of error types.
 //

v2/either/either_test.go

@@ -124,79 +124,52 @@ func TestStringer(t *testing.T) {
 func TestZeroWithIntegers(t *testing.T) {
 	e := Zero[error, int]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-	assert.False(t, IsLeft(e), "Zero should not create a Left value")
-
-	value, err := Unwrap(e)
-	assert.Equal(t, 0, value, "Right value should be zero for int")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	assert.Equal(t, Of[error](0), e, "Zero should create a Right value with zero for int")
 }
 
 // TestZeroWithStrings tests Zero function with string types
 func TestZeroWithStrings(t *testing.T) {
 	e := Zero[error, string]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-	assert.False(t, IsLeft(e), "Zero should not create a Left value")
-
-	value, err := Unwrap(e)
-	assert.Equal(t, "", value, "Right value should be empty string")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	assert.Equal(t, Of[error](""), e, "Zero should create a Right value with empty string")
 }
 
 // TestZeroWithBooleans tests Zero function with boolean types
 func TestZeroWithBooleans(t *testing.T) {
 	e := Zero[error, bool]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
-	assert.Equal(t, false, value, "Right value should be false for bool")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	assert.Equal(t, Of[error](false), e, "Zero should create a Right value with false for bool")
 }
 
 // TestZeroWithFloats tests Zero function with float types
 func TestZeroWithFloats(t *testing.T) {
 	e := Zero[error, float64]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
-	assert.Equal(t, 0.0, value, "Right value should be 0.0 for float64")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	assert.Equal(t, Of[error](0.0), e, "Zero should create a Right value with 0.0 for float64")
 }
 
 // TestZeroWithPointers tests Zero function with pointer types
 func TestZeroWithPointers(t *testing.T) {
 	e := Zero[error, *int]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
-	assert.Nil(t, value, "Right value should be nil for pointer type")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	var nilPtr *int
+	assert.Equal(t, Of[error](nilPtr), e, "Zero should create a Right value with nil pointer")
 }
 
 // TestZeroWithSlices tests Zero function with slice types
 func TestZeroWithSlices(t *testing.T) {
 	e := Zero[error, []int]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
-	assert.Nil(t, value, "Right value should be nil for slice type")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	var nilSlice []int
+	assert.Equal(t, Of[error](nilSlice), e, "Zero should create a Right value with nil slice")
 }
 
 // TestZeroWithMaps tests Zero function with map types
 func TestZeroWithMaps(t *testing.T) {
 	e := Zero[error, map[string]int]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
-	assert.Nil(t, value, "Right value should be nil for map type")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	var nilMap map[string]int
+	assert.Equal(t, Of[error](nilMap), e, "Zero should create a Right value with nil map")
 }
 
 // TestZeroWithStructs tests Zero function with struct types
@@ -208,23 +181,16 @@ func TestZeroWithStructs(t *testing.T) {
 
 	e := Zero[error, TestStruct]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
 	expected := TestStruct{Field1: 0, Field2: ""}
-	assert.Equal(t, expected, value, "Right value should be zero value for struct")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	assert.Equal(t, Of[error](expected), e, "Zero should create a Right value with zero value for struct")
 }
 
 // TestZeroWithInterfaces tests Zero function with interface types
 func TestZeroWithInterfaces(t *testing.T) {
 	e := Zero[error, interface{}]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
-	assert.Nil(t, value, "Right value should be nil for interface type")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	var nilInterface interface{}
+	assert.Equal(t, Of[error](nilInterface), e, "Zero should create a Right value with nil interface")
 }
 
 // TestZeroWithCustomErrorType tests Zero function with custom error types
@@ -236,12 +202,7 @@ func TestZeroWithCustomErrorType(t *testing.T) {
 
 	e := Zero[CustomError, string]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-	assert.False(t, IsLeft(e), "Zero should not create a Left value")
-
-	value, err := Unwrap(e)
-	assert.Equal(t, "", value, "Right value should be empty string")
-	assert.Equal(t, CustomError{Code: 0, Message: ""}, err, "Error should be zero value for CustomError")
+	assert.Equal(t, Of[CustomError](""), e, "Zero should create a Right value with empty string")
 }
 
 // TestZeroCanBeUsedWithOtherFunctions tests that Zero Eithers work with other either functions
@@ -252,17 +213,13 @@ func TestZeroCanBeUsedWithOtherFunctions(t *testing.T) {
 	mapped := MonadMap(e, func(n int) string {
 		return fmt.Sprintf("%d", n)
 	})
-	assert.True(t, IsRight(mapped), "Mapped Zero should still be Right")
-	value, _ := Unwrap(mapped)
-	assert.Equal(t, "0", value, "Mapped value should be '0'")
+	assert.Equal(t, Of[error]("0"), mapped, "Mapped Zero should be Right with '0'")
 
 	// Test with Chain
 	chained := MonadChain(e, func(n int) Either[error, string] {
 		return Right[error](fmt.Sprintf("value: %d", n))
 	})
-	assert.True(t, IsRight(chained), "Chained Zero should still be Right")
-	chainedValue, _ := Unwrap(chained)
-	assert.Equal(t, "value: 0", chainedValue, "Chained value should be 'value: 0'")
+	assert.Equal(t, Of[error]("value: 0"), chained, "Chained Zero should be Right with 'value: 0'")
 
 	// Test with Fold
 	folded := MonadFold(e,
@@ -295,23 +252,15 @@ func TestZeroWithComplexTypes(t *testing.T) {
 
 	e := Zero[error, ComplexType]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
 	expected := ComplexType{Nested: nil, Ptr: nil}
-	assert.Equal(t, expected, value, "Right value should be zero value for complex struct")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	assert.Equal(t, Of[error](expected), e, "Zero should create a Right value with zero value for complex struct")
 }
 
 // TestZeroWithOption tests Zero with Option type
 func TestZeroWithOption(t *testing.T) {
 	e := Zero[error, O.Option[int]]()
 
-	assert.True(t, IsRight(e), "Zero should create a Right value")
-
-	value, err := Unwrap(e)
-	assert.True(t, O.IsNone(value), "Right value should be None for Option type")
-	assert.Nil(t, err, "Error should be nil for Right value")
+	assert.Equal(t, Of[error](O.None[int]()), e, "Zero should create a Right value with None option")
 }
 
 // TestZeroIsNotLeft tests that Zero never creates a Left value
@@ -343,3 +292,211 @@ func TestZeroEqualsDefaultInitialization(t *testing.T) {
 	assert.Equal(t, IsRight(defaultInit), IsRight(zero), "Both should be Right")
 	assert.Equal(t, IsLeft(defaultInit), IsLeft(zero), "Both should not be Left")
 }
+
+// TestMonadChainLeft tests the MonadChainLeft function with various scenarios
+func TestMonadChainLeft(t *testing.T) {
+	t.Run("Left value is transformed by function", func(t *testing.T) {
+		// Transform error to success
+		result := MonadChainLeft(
+			Left[int](errors.New("not found")),
+			func(err error) Either[string, int] {
+				if err.Error() == "not found" {
+					return Right[string](0) // default value
+				}
+				return Left[int](err.Error())
+			},
+		)
+		assert.Equal(t, Of[string](0), result)
+	})
+
+	t.Run("Left value error type is transformed", func(t *testing.T) {
+		// Transform error type from int to string
+		result := MonadChainLeft(
+			Left[int](404),
+			func(code int) Either[string, int] {
+				return Left[int](fmt.Sprintf("Error code: %d", code))
+			},
+		)
+		assert.Equal(t, Left[int]("Error code: 404"), result)
+	})
+
+	t.Run("Right value passes through unchanged", func(t *testing.T) {
+		// Right value should not be affected
+		result := MonadChainLeft(
+			Right[error](42),
+			func(err error) Either[string, int] {
+				return Left[int]("should not be called")
+			},
+		)
+		assert.Equal(t, Of[string](42), result)
+	})
+
+	t.Run("Chain multiple error transformations", func(t *testing.T) {
+		// First transformation
+		step1 := MonadChainLeft(
+			Left[int](errors.New("error1")),
+			func(err error) Either[error, int] {
+				return Left[int](errors.New("error2"))
+			},
+		)
+		// Second transformation
+		step2 := MonadChainLeft(
+			step1,
+			func(err error) Either[string, int] {
+				return Left[int](err.Error())
+			},
+		)
+		assert.Equal(t, Left[int]("error2"), step2)
+	})
+
+	t.Run("Error recovery with fallback", func(t *testing.T) {
+		// Recover from specific errors
+		result := MonadChainLeft(
+			Left[int](errors.New("timeout")),
+			func(err error) Either[error, int] {
+				if err.Error() == "timeout" {
+					return Right[error](999) // fallback value
+				}
+				return Left[int](err)
+			},
+		)
+		assert.Equal(t, Of[error](999), result)
+	})
+
+	t.Run("Transform error to different Left", func(t *testing.T) {
+		// Transform one error to another
+		result := MonadChainLeft(
+			Left[string]("original error"),
+			func(s string) Either[int, string] {
+				return Left[string](len(s))
+			},
+		)
+		assert.Equal(t, Left[string](14), result) // length of "original error"
+	})
+}
+
+// TestChainLeft tests the curried ChainLeft function
+func TestChainLeft(t *testing.T) {
+	t.Run("Curried function transforms Left value", func(t *testing.T) {
+		// Create a reusable error handler
+		handleNotFound := ChainLeft[error, string](func(err error) Either[string, int] {
+			if err.Error() == "not found" {
+				return Right[string](0)
+			}
+			return Left[int](err.Error())
+		})
+
+		result := handleNotFound(Left[int](errors.New("not found")))
+		assert.Equal(t, Of[string](0), result)
+	})
+
+	t.Run("Curried function with Right value", func(t *testing.T) {
+		handler := ChainLeft[error, string](func(err error) Either[string, int] {
+			return Left[int]("should not be called")
+		})
+
+		result := handler(Right[error](42))
+		assert.Equal(t, Of[string](42), result)
+	})
+
+	t.Run("Use in pipeline with Pipe", func(t *testing.T) {
+		// Create error transformer
+		toStringError := ChainLeft[int, string](func(code int) Either[string, string] {
+			return Left[string](fmt.Sprintf("Error: %d", code))
+		})
+
+		result := F.Pipe1(
+			Left[string](404),
+			toStringError,
+		)
+		assert.Equal(t, Left[string]("Error: 404"), result)
+	})
+
+	t.Run("Compose multiple ChainLeft operations", func(t *testing.T) {
+		// First handler: convert error to string
+		handler1 := ChainLeft[error, string](func(err error) Either[string, int] {
+			return Left[int](err.Error())
+		})
+
+		// Second handler: add prefix to string error
+		handler2 := ChainLeft[string, string](func(s string) Either[string, int] {
+			return Left[int]("Handled: " + s)
+		})
+
+		result := F.Pipe2(
+			Left[int](errors.New("original")),
+			handler1,
+			handler2,
+		)
+		assert.Equal(t, Left[int]("Handled: original"), result)
+	})
+
+	t.Run("Error recovery in pipeline", func(t *testing.T) {
+		// Handler that recovers from specific errors
+		recoverFromTimeout := ChainLeft(func(err error) Either[error, int] {
+			if err.Error() == "timeout" {
+				return Right[error](0) // recovered value
+			}
+			return Left[int](err) // propagate other errors
+		})
+
+		// Test with timeout error
+		result1 := F.Pipe1(
+			Left[int](errors.New("timeout")),
+			recoverFromTimeout,
+		)
+		assert.Equal(t, Of[error](0), result1)
+
+		// Test with other error
+		result2 := F.Pipe1(
+			Left[int](errors.New("other error")),
+			recoverFromTimeout,
+		)
+		assert.True(t, IsLeft(result2))
+	})
+
+	t.Run("Transform error type in pipeline", func(t *testing.T) {
+		// Convert numeric error codes to descriptive strings
+		codeToMessage := ChainLeft(func(code int) Either[string, string] {
+			messages := map[int]string{
+				404: "Not Found",
+				500: "Internal Server Error",
+			}
+			if msg, ok := messages[code]; ok {
+				return Left[string](msg)
+			}
+			return Left[string](fmt.Sprintf("Unknown error: %d", code))
+		})
+
+		result := F.Pipe1(
+			Left[string](404),
+			codeToMessage,
+		)
+		assert.Equal(t, Left[string]("Not Found"), result)
+	})
+
+	t.Run("ChainLeft with Map combination", func(t *testing.T) {
+		// Combine ChainLeft with Map to handle both channels
+		errorHandler := ChainLeft(func(err error) Either[string, int] {
+			return Left[int]("Error: " + err.Error())
+		})
+
+		valueMapper := Map[string](S.Format[int]("Value: %d"))
+
+		// Test with Left
+		result1 := F.Pipe2(
+			Left[int](errors.New("fail")),
+			errorHandler,
+			valueMapper,
+		)
+		assert.Equal(t, Left[string]("Error: fail"), result1)
+
+		// Test with Right
+		result2 := F.Pipe2(
+			Right[error](42),
+			errorHandler,
+			valueMapper,
+		)
+		assert.Equal(t, Of[string]("Value: 42"), result2)
+	})
+}

v2/either/filterable.go

@@ -0,0 +1,351 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package either provides implementations of the Either type and related operations.
+//
+// This package implements several Fantasy Land algebraic structures:
+//   - Filterable: https://github.com/fantasyland/fantasy-land#filterable
+//
+// The Filterable specification defines operations for filtering and partitioning
+// data structures based on predicates and mapping functions.
+package either
+
+import (
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
+)
+
+// Partition separates an [Either] value into a [Pair] based on a predicate function.
+// It returns a function that takes an Either and produces a Pair of Either values,
+// where the first element contains values that fail the predicate and the second
+// contains values that pass the predicate.
+//
+// This function implements the Filterable specification's partition operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is Left, both elements of the resulting Pair will be the same Left value
+//   - If the input is Right and the predicate returns true, the result is (Left(empty), Right(value))
+//   - If the input is Right and the predicate returns false, the result is (Right(value), Left(empty))
+//
+// This function is useful for separating Either values into two categories based on
+// a condition, commonly used in filtering operations where you want to keep track of
+// both the values that pass and fail a test.
+//
+// Parameters:
+//   - p: A predicate function that tests values of type A
+//   - empty: The default Left value to use when creating Left instances for partitioning
+//
+// Returns:
+//
+//	A function that takes an Either[E, A] and returns a Pair where:
+//	  - First element: Either values that fail the predicate (or original Left)
+//	  - Second element: Either values that pass the predicate (or original Left)
+//
+// Example:
+//
+//	import (
+//	    E "github.com/IBM/fp-go/v2/either"
+//	    N "github.com/IBM/fp-go/v2/number"
+//	    P "github.com/IBM/fp-go/v2/pair"
+//	)
+//
+//	// Partition positive and non-positive numbers
+//	isPositive := N.MoreThan(0)
+//	partition := E.Partition(isPositive, "not positive")
+//
+//	// Right value that passes predicate
+//	result1 := partition(E.Right[string](5))
+//	// result1 = Pair(Left("not positive"), Right(5))
+//	left1, right1 := P.Unpack(result1)
+//	// left1 = Left("not positive"), right1 = Right(5)
+//
+//	// Right value that fails predicate
+//	result2 := partition(E.Right[string](-3))
+//	// result2 = Pair(Right(-3), Left("not positive"))
+//	left2, right2 := P.Unpack(result2)
+//	// left2 = Right(-3), right2 = Left("not positive")
+//
+//	// Left value passes through unchanged in both positions
+//	result3 := partition(E.Left[int]("error"))
+//	// result3 = Pair(Left("error"), Left("error"))
+//	left3, right3 := P.Unpack(result3)
+//	// left3 = Left("error"), right3 = Left("error")
+func Partition[E, A any](p Predicate[A], empty E) func(Either[E, A]) Pair[Either[E, A], Either[E, A]] {
+	l := Left[A](empty)
+	return func(e Either[E, A]) Pair[Either[E, A], Either[E, A]] {
+		if e.isLeft {
+			return pair.Of(e)
+		}
+		if p(e.r) {
+			return pair.MakePair(l, e)
+		}
+		return pair.MakePair(e, l)
+	}
+}
+
+// Filter creates a filtering operation for [Either] values based on a predicate function.
+// It returns a function that takes an Either and produces an Either, where Right values
+// that fail the predicate are converted to Left values with the provided empty value.
+//
+// This function implements the Filterable specification's filter operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is Left, it passes through unchanged
+//   - If the input is Right and the predicate returns true, the Right value passes through unchanged
+//   - If the input is Right and the predicate returns false, it's converted to Left(empty)
+//
+// This function is useful for conditional validation or filtering of Either values,
+// where you want to reject Right values that don't meet certain criteria by converting
+// them to Left values with a default error.
+//
+// Parameters:
+//   - p: A predicate function that tests values of type A
+//   - empty: The default Left value to use when filtering out Right values that fail the predicate
+//
+// Returns:
+//
+//	An Operator function that takes an Either[E, A] and returns an Either[E, A] where:
+//	  - Left values pass through unchanged
+//	  - Right values that pass the predicate remain as Right
+//	  - Right values that fail the predicate become Left(empty)
+//
+// Example:
+//
+//	import (
+//	    E "github.com/IBM/fp-go/v2/either"
+//	    N "github.com/IBM/fp-go/v2/number"
+//	)
+//
+//	// Filter to keep only positive numbers
+//	isPositive := N.MoreThan(0)
+//	filterPositive := E.Filter(isPositive, "not positive")
+//
+//	// Right value that passes predicate - remains Right
+//	result1 := filterPositive(E.Right[string](5))
+//	// result1 = Right(5)
+//
+//	// Right value that fails predicate - becomes Left
+//	result2 := filterPositive(E.Right[string](-3))
+//	// result2 = Left("not positive")
+//
+//	// Left value passes through unchanged
+//	result3 := filterPositive(E.Left[int]("original error"))
+//	// result3 = Left("original error")
+//
+//	// Chaining filters
+//	isEven := func(n int) bool { return n%2 == 0 }
+//	filterEven := E.Filter(isEven, "not even")
+//
+//	// Apply multiple filters in sequence
+//	result4 := filterEven(filterPositive(E.Right[string](4)))
+//	// result4 = Right(4) - passes both filters
+//
+//	result5 := filterEven(filterPositive(E.Right[string](3)))
+//	// result5 = Left("not even") - passes first, fails second
+func Filter[E, A any](p Predicate[A], empty E) Operator[E, A, A] {
+	l := Left[A](empty)
+	return func(e Either[E, A]) Either[E, A] {
+		if e.isLeft || p(e.r) {
+			return e
+		}
+		return l
+	}
+}
+
+// FilterMap combines filtering and mapping operations for [Either] values using an [Option]-returning function.
+// It returns a function that takes an Either[E, A] and produces an Either[E, B], where Right values
+// are transformed by applying the function f. If f returns Some(B), the result is Right(B). If f returns
+// None, the result is Left(empty).
+//
+// This function implements the Filterable specification's filterMap operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is Left, it passes through with its error value preserved as Left[B]
+//   - If the input is Right and f returns Some(B), the result is Right(B)
+//   - If the input is Right and f returns None, the result is Left(empty)
+//
+// This function is useful for operations that combine validation/filtering with transformation,
+// such as parsing strings to numbers (where invalid strings result in None), or extracting
+// optional fields from structures.
+//
+// Parameters:
+//   - f: An Option Kleisli function that transforms values of type A to Option[B]
+//   - empty: The default Left value to use when f returns None
+//
+// Returns:
+//
+//	An Operator function that takes an Either[E, A] and returns an Either[E, B] where:
+//	  - Left values pass through with error preserved
+//	  - Right values are transformed by f: Some(B) becomes Right(B), None becomes Left(empty)
+//
+// Example:
+//
+//	import (
+//	    E "github.com/IBM/fp-go/v2/either"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	    "strconv"
+//	)
+//
+//	// Parse string to int, filtering out invalid values
+//	parseInt := func(s string) O.Option[int] {
+//	    if n, err := strconv.Atoi(s); err == nil {
+//	        return O.Some(n)
+//	    }
+//	    return O.None[int]()
+//	}
+//	filterMapInt := E.FilterMap(parseInt, "invalid number")
+//
+//	// Valid number string - transforms to Right(int)
+//	result1 := filterMapInt(E.Right[string]("42"))
+//	// result1 = Right(42)
+//
+//	// Invalid number string - becomes Left
+//	result2 := filterMapInt(E.Right[string]("abc"))
+//	// result2 = Left("invalid number")
+//
+//	// Left value passes through with error preserved
+//	result3 := filterMapInt(E.Left[string]("original error"))
+//	// result3 = Left("original error")
+//
+//	// Extract optional field from struct
+//	type Person struct {
+//	    Name  string
+//	    Email O.Option[string]
+//	}
+//	extractEmail := func(p Person) O.Option[string] { return p.Email }
+//	filterMapEmail := E.FilterMap(extractEmail, "no email")
+//
+//	result4 := filterMapEmail(E.Right[string](Person{Name: "Alice", Email: O.Some("alice@example.com")}))
+//	// result4 = Right("alice@example.com")
+//
+//	result5 := filterMapEmail(E.Right[string](Person{Name: "Bob", Email: O.None[string]()}))
+//	// result5 = Left("no email")
+func FilterMap[E, A, B any](f option.Kleisli[A, B], empty E) Operator[E, A, B] {
+	l := Left[B](empty)
+	return func(e Either[E, A]) Either[E, B] {
+		if e.isLeft {
+			return Left[B](e.l)
+		}
+		if b, ok := option.Unwrap(f(e.r)); ok {
+			return Right[E](b)
+		}
+		return l
+	}
+}
+
+// PartitionMap separates and transforms an [Either] value into a [Pair] of Either values using a mapping function.
+// It returns a function that takes an Either[E, A] and produces a Pair of Either values, where the mapping
+// function f transforms the Right value into Either[B, C]. The result is partitioned based on whether f
+// produces a Left or Right value.
+//
+// This function implements the Filterable specification's partitionMap operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is Left, both elements of the resulting Pair will be Left with the original error
+//   - If the input is Right and f returns Left(B), the result is (Right(B), Left(empty))
+//   - If the input is Right and f returns Right(C), the result is (Left(empty), Right(C))
+//
+// This function is useful for operations that need to categorize and transform values simultaneously,
+// such as separating valid and invalid data while applying different transformations to each category.
+//
+// Parameters:
+//   - f: A Kleisli function that transforms values of type A to Either[B, C]
+//   - empty: The default error value to use when creating Left instances for partitioning
+//
+// Returns:
+//
+//	A function that takes an Either[E, A] and returns a Pair[Either[E, B], Either[E, C]] where:
+//	  - If input is Left: (Left(original_error), Left(original_error))
+//	  - If f returns Left(B): (Right(B), Left(empty))
+//	  - If f returns Right(C): (Left(empty), Right(C))
+//
+// Example:
+//
+//	import (
+//	    E "github.com/IBM/fp-go/v2/either"
+//	    P "github.com/IBM/fp-go/v2/pair"
+//	)
+//
+//	// Classify and transform numbers: negative -> error message, positive -> squared value
+//	classifyNumber := func(n int) E.Either[string, int] {
+//	    if n < 0 {
+//	        return E.Left[int]("negative: " + strconv.Itoa(n))
+//	    }
+//	    return E.Right[string](n * n)
+//	}
+//	partitionMap := E.PartitionMap(classifyNumber, "not classified")
+//
+//	// Positive number - goes to right side as squared value
+//	result1 := partitionMap(E.Right[string](5))
+//	// result1 = Pair(Left("not classified"), Right(25))
+//	left1, right1 := P.Unpack(result1)
+//	// left1 = Left("not classified"), right1 = Right(25)
+//
+//	// Negative number - goes to left side with error message
+//	result2 := partitionMap(E.Right[string](-3))
+//	// result2 = Pair(Right("negative: -3"), Left("not classified"))
+//	left2, right2 := P.Unpack(result2)
+//	// left2 = Right("negative: -3"), right2 = Left("not classified")
+//
+//	// Original Left value - appears in both positions
+//	result3 := partitionMap(E.Left[int]("original error"))
+//	// result3 = Pair(Left("original error"), Left("original error"))
+//	left3, right3 := P.Unpack(result3)
+//	// left3 = Left("original error"), right3 = Left("original error")
+//
+//	// Validate and transform user input
+//	type ValidationError struct{ Field, Message string }
+//	type User struct{ Name string; Age int }
+//
+//	validateUser := func(input map[string]string) E.Either[ValidationError, User] {
+//	    name, hasName := input["name"]
+//	    ageStr, hasAge := input["age"]
+//	    if !hasName {
+//	        return E.Left[User](ValidationError{"name", "missing"})
+//	    }
+//	    if !hasAge {
+//	        return E.Left[User](ValidationError{"age", "missing"})
+//	    }
+//	    age, err := strconv.Atoi(ageStr)
+//	    if err != nil {
+//	        return E.Left[User](ValidationError{"age", "invalid"})
+//	    }
+//	    return E.Right[ValidationError](User{name, age})
+//	}
+//	partitionUsers := E.PartitionMap(validateUser, ValidationError{"", "not processed"})
+//
+//	validInput := map[string]string{"name": "Alice", "age": "30"}
+//	result4 := partitionUsers(E.Right[string](validInput))
+//	// result4 = Pair(Left(ValidationError{"", "not processed"}), Right(User{"Alice", 30}))
+//
+//	invalidInput := map[string]string{"name": "Bob"}
+//	result5 := partitionUsers(E.Right[string](invalidInput))
+//	// result5 = Pair(Right(ValidationError{"age", "missing"}), Left(ValidationError{"", "not processed"}))
+func PartitionMap[E, A, B, C any](f Kleisli[B, A, C], empty E) func(Either[E, A]) Pair[Either[E, B], Either[E, C]] {
+	return func(e Either[E, A]) Pair[Either[E, B], Either[E, C]] {
+		if e.isLeft {
+			return pair.MakePair(Left[B](e.l), Left[C](e.l))
+		}
+		res := f(e.r)
+		if res.isLeft {
+			return pair.MakePair(Right[E](res.l), Left[C](empty))
+		}
+		return pair.MakePair(Left[B](empty), Right[E](res.r))
+	}
+}

v2/either/types.go

@@ -21,6 +21,7 @@ import (
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/lens"
 	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/predicate"
 	"github.com/IBM/fp-go/v2/reader"
 )
@@ -53,4 +54,6 @@ type (
 	// Predicate represents a function that tests a value of type A and returns a boolean.
 	// It's commonly used for filtering and conditional operations.
 	Predicate[A any] = predicate.Predicate[A]
+
+	Pair[L, R any] = pair.Pair[L, R]
 )

v2/ioeither/ioeither.go

@@ -489,6 +489,8 @@ func After[E, A any](timestamp time.Time) Operator[E, A, A] {
 // If the input is a Left value, it applies the function f to transform the error and potentially
 // change the error type from EA to EB. If the input is a Right value, it passes through unchanged.
 //
+// Note: MonadChainLeft is identical to [OrElse] - both provide the same functionality for error recovery.
+//
 // This is useful for error recovery or error transformation scenarios where you want to handle
 // errors by performing another computation that may also fail.
 //
@@ -523,6 +525,8 @@ func MonadChainLeft[EA, EB, A any](fa IOEither[EA, A], f Kleisli[EB, EA, A]) IOE
 // ChainLeft is the curried version of [MonadChainLeft].
 // It returns a function that chains a computation on the left (error) side of an [IOEither].
 //
+// Note: ChainLeft is identical to [OrElse] - both provide the same functionality for error recovery.
+//
 // This is particularly useful in functional composition pipelines where you want to handle
 // errors by performing another computation that may also fail.
 //
@@ -644,6 +648,8 @@ func TapLeft[A, EA, EB, B any](f Kleisli[EB, EA, B]) Operator[EA, A, A] {
 // If the IOEither is Left, it applies the provided function to the error value,
 // which returns a new IOEither that replaces the original.
 //
+// Note: OrElse is identical to [ChainLeft] - both provide the same functionality for error recovery.
+//
 // This is useful for error recovery, fallback logic, or chaining alternative IO computations.
 // The error type can be widened from E1 to E2, allowing transformation of error types.
 //

v2/ioeither/ioeither_test.go

@@ -490,3 +490,148 @@ func TestOrElseW(t *testing.T) {
 	preserved := preserveRecover(preservedRight)()
 	assert.Equal(t, E.Right[AppError](42), preserved)
 }
+
+// TestChainLeftIdenticalToOrElse proves that ChainLeft and OrElse are identical functions.
+// This test verifies that both functions produce the same results for all scenarios:
+// - Left values with error recovery
+// - Left values with error transformation
+// - Right values passing through unchanged
+func TestChainLeftIdenticalToOrElse(t *testing.T) {
+	// Test 1: Left value with error recovery - both should recover to Right
+	t.Run("Left value recovery - ChainLeft equals OrElse", func(t *testing.T) {
+		recoveryFn := func(e string) IOEither[string, int] {
+			if e == "recoverable" {
+				return Right[string](42)
+			}
+			return Left[int](e)
+		}
+
+		input := Left[int]("recoverable")
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(recoveryFn)(input)()
+
+		// Using OrElse
+		resultOrElse := OrElse(recoveryFn)(input)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](42), resultChainLeft)
+	})
+
+	// Test 2: Left value with error transformation - both should transform error
+	t.Run("Left value transformation - ChainLeft equals OrElse", func(t *testing.T) {
+		transformFn := func(e string) IOEither[string, int] {
+			return Left[int]("transformed: " + e)
+		}
+
+		input := Left[int]("original error")
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(transformFn)(input)()
+
+		// Using OrElse
+		resultOrElse := OrElse(transformFn)(input)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Left[int]("transformed: original error"), resultChainLeft)
+	})
+
+	// Test 3: Right value - both should pass through unchanged
+	t.Run("Right value passthrough - ChainLeft equals OrElse", func(t *testing.T) {
+		handlerFn := func(e string) IOEither[string, int] {
+			return Left[int]("should not be called")
+		}
+
+		input := Right[string](100)
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(handlerFn)(input)()
+
+		// Using OrElse
+		resultOrElse := OrElse(handlerFn)(input)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](100), resultChainLeft)
+	})
+
+	// Test 4: Error type widening - both should handle type transformation
+	t.Run("Error type widening - ChainLeft equals OrElse", func(t *testing.T) {
+		widenFn := func(e string) IOEither[int, int] {
+			return Left[int](404)
+		}
+
+		input := Left[int]("not found")
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(widenFn)(input)()
+
+		// Using OrElse
+		resultOrElse := OrElse(widenFn)(input)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Left[int](404), resultChainLeft)
+	})
+
+	// Test 5: Composition in pipeline - both should work identically in F.Pipe
+	t.Run("Pipeline composition - ChainLeft equals OrElse", func(t *testing.T) {
+		recoveryFn := func(e string) IOEither[string, int] {
+			if e == "network error" {
+				return Right[string](0)
+			}
+			return Left[int](e)
+		}
+
+		input := Left[int]("network error")
+
+		// Using ChainLeft in pipeline
+		resultChainLeft := F.Pipe1(input, ChainLeft(recoveryFn))()
+
+		// Using OrElse in pipeline
+		resultOrElse := F.Pipe1(input, OrElse(recoveryFn))()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](0), resultChainLeft)
+	})
+
+	// Test 6: Multiple chained operations - both should behave identically
+	t.Run("Multiple operations - ChainLeft equals OrElse", func(t *testing.T) {
+		handler1 := func(e string) IOEither[string, int] {
+			if e == "error1" {
+				return Right[string](1)
+			}
+			return Left[int](e)
+		}
+
+		handler2 := func(e string) IOEither[string, int] {
+			if e == "error2" {
+				return Right[string](2)
+			}
+			return Left[int](e)
+		}
+
+		input := Left[int]("error2")
+
+		// Using ChainLeft
+		resultChainLeft := F.Pipe2(
+			input,
+			ChainLeft(handler1),
+			ChainLeft(handler2),
+		)()
+
+		// Using OrElse
+		resultOrElse := F.Pipe2(
+			input,
+			OrElse(handler1),
+			OrElse(handler2),
+		)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](2), resultChainLeft)
+	})
+}

v2/ioref/ioref.go

@@ -19,6 +19,7 @@ import (
 	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/pair"
+	"github.com/IBM/fp-go/v2/readerio"
 )
 
 // MakeIORef creates a new IORef containing the given initial value.
@@ -50,6 +51,32 @@ func MakeIORef[A any](a A) IO[IORef[A]] {
 	}
 }
 
+// Write atomically writes a new value to an IORef and returns the written value.
+//
+// This function returns a Kleisli arrow that takes an IORef and produces an IO
+// computation that writes the given value to the reference. The write operation
+// is atomic and thread-safe, using a write lock to ensure exclusive access.
+//
+// Parameters:
+//   - a: The new value to write to the IORef
+//
+// Returns:
+//   - A Kleisli arrow from IORef[A] to IO[A] that writes the value and returns it
+//
+// Example:
+//
+//	ref := ioref.MakeIORef(42)()
+//
+//	// Write a new value
+//	newValue := ioref.Write(100)(ref)()  // Returns 100, ref now contains 100
+//
+//	// Chain writes
+//	pipe.Pipe2(
+//	    ref,
+//	    ioref.Write(50),
+//	    io.Chain(ioref.Write(75)),
+//	)()  // ref now contains 75
+//
 //go:inline
 func Write[A any](a A) io.Kleisli[IORef[A], A] {
 	return func(ref IORef[A]) IO[A] {
@@ -180,6 +207,57 @@ func ModifyIOK[A any](f io.Kleisli[A, A]) io.Kleisli[IORef[A], A] {
 	}
 }
 
+// ModifyReaderIOK atomically modifies the value in an IORef using a ReaderIO-based transformation.
+//
+// This is a variant of ModifyIOK that works with ReaderIO computations, allowing the
+// transformation function to access an environment of type R while performing IO effects.
+// This is useful when the modification logic needs access to configuration, context,
+// or other shared resources.
+//
+// The modification is atomic and thread-safe, using a write lock to ensure exclusive
+// access during the read-modify-write cycle. The ReaderIO effect in the transformation
+// function is executed while holding the lock.
+//
+// Parameters:
+//   - f: A ReaderIO Kleisli arrow (readerio.Kleisli[R, A, A]) that takes the current value
+//     and an environment R, and returns an IO computation producing the new value
+//
+// Returns:
+//   - A ReaderIO Kleisli arrow from IORef[A] to ReaderIO[R, A] that returns the new value
+//
+// Example:
+//
+//	type Config struct {
+//	    multiplier int
+//	}
+//
+//	ref := ioref.MakeIORef(10)()
+//
+//	// Modify using environment
+//	modifyWithConfig := ioref.ModifyReaderIOK(func(x int) readerio.ReaderIO[Config, int] {
+//	    return func(cfg Config) io.IO[int] {
+//	        return func() int {
+//	            return x * cfg.multiplier
+//	        }
+//	    }
+//	})
+//
+//	config := Config{multiplier: 5}
+//	newValue := modifyWithConfig(ref)(config)()  // Returns 50, ref now contains 50
+func ModifyReaderIOK[R, A any](f readerio.Kleisli[R, A, A]) readerio.Kleisli[R, IORef[A], A] {
+	return func(ref IORef[A]) ReaderIO[R, A] {
+		return func(r R) readerio.IO[A] {
+			return func() A {
+				ref.mu.Lock()
+				defer ref.mu.Unlock()
+
+				ref.a = f(ref.a)(r)()
+				return ref.a
+			}
+		}
+	}
+}
+
 // ModifyWithResult atomically modifies the value in an IORef and returns both
 // the new value and an additional result computed from the old value.
 //
@@ -269,3 +347,62 @@ func ModifyIOKWithResult[A, B any](f io.Kleisli[A, Pair[A, B]]) io.Kleisli[IORef
 		}
 	}
 }
+
+// ModifyReaderIOKWithResult atomically modifies the value in an IORef and returns a result,
+// using a ReaderIO-based transformation function.
+//
+// This combines the capabilities of ModifyIOKWithResult and ModifyReaderIOK, allowing the
+// transformation function to:
+//   - Access an environment of type R (like configuration or context)
+//   - Perform IO effects during the transformation
+//   - Both update the stored value and compute a result based on the old value
+//   - Ensure atomicity of the entire read-transform-write-compute cycle
+//
+// The modification is atomic and thread-safe, using a write lock to ensure exclusive
+// access. The ReaderIO effect in the transformation function is executed while holding the lock.
+//
+// Parameters:
+//   - f: A ReaderIO Kleisli arrow (readerio.Kleisli[R, A, Pair[A, B]]) that takes the old value
+//     and an environment R, and returns an IO computation producing a Pair of (new value, result)
+//
+// Returns:
+//   - A ReaderIO Kleisli arrow from IORef[A] to ReaderIO[R, B] that produces the result
+//
+// Example:
+//
+//	type Config struct {
+//	    logEnabled bool
+//	}
+//
+//	ref := ioref.MakeIORef(42)()
+//
+//	// Increment with conditional logging, return old value
+//	incrementWithLog := ioref.ModifyReaderIOKWithResult(
+//	    func(x int) readerio.ReaderIO[Config, pair.Pair[int, int]] {
+//	        return func(cfg Config) io.IO[pair.Pair[int, int]] {
+//	            return func() pair.Pair[int, int] {
+//	                if cfg.logEnabled {
+//	                    fmt.Printf("Incrementing from %d\n", x)
+//	                }
+//	                return pair.MakePair(x+1, x)
+//	            }
+//	        }
+//	    },
+//	)
+//
+//	config := Config{logEnabled: true}
+//	oldValue := incrementWithLog(ref)(config)()  // Logs and returns 42, ref now contains 43
+func ModifyReaderIOKWithResult[R, A, B any](f readerio.Kleisli[R, A, Pair[A, B]]) readerio.Kleisli[R, IORef[A], B] {
+	return func(ref IORef[A]) ReaderIO[R, B] {
+		return func(r R) readerio.IO[B] {
+			return func() B {
+				ref.mu.Lock()
+				defer ref.mu.Unlock()
+
+				result := f(ref.a)(r)()
+				ref.a = pair.Head(result)
+				return pair.Tail(result)
+			}
+		}
+	}
+}

v2/ioref/ioref_test.go

@@ -24,9 +24,588 @@ import (
 	"github.com/IBM/fp-go/v2/io"
 	N "github.com/IBM/fp-go/v2/number"
 	"github.com/IBM/fp-go/v2/pair"
+	"github.com/IBM/fp-go/v2/readerio"
 	"github.com/stretchr/testify/assert"
 )
 
+func TestMakeIORef(t *testing.T) {
+	t.Run("creates IORef with integer value", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		assert.NotNil(t, ref)
+		assert.Equal(t, 42, Read(ref)())
+	})
+
+	t.Run("creates IORef with string value", func(t *testing.T) {
+		ref := MakeIORef("hello")()
+		assert.NotNil(t, ref)
+		assert.Equal(t, "hello", Read(ref)())
+	})
+
+	t.Run("creates IORef with slice value", func(t *testing.T) {
+		slice := []int{1, 2, 3}
+		ref := MakeIORef(slice)()
+		assert.NotNil(t, ref)
+		assert.Equal(t, slice, Read(ref)())
+	})
+
+	t.Run("creates IORef with struct value", func(t *testing.T) {
+		type Person struct {
+			Name string
+			Age  int
+		}
+		person := Person{Name: "Alice", Age: 30}
+		ref := MakeIORef(person)()
+		assert.NotNil(t, ref)
+		assert.Equal(t, person, Read(ref)())
+	})
+
+	t.Run("creates IORef with zero value", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		assert.NotNil(t, ref)
+		assert.Equal(t, 0, Read(ref)())
+	})
+
+	t.Run("creates IORef with nil pointer", func(t *testing.T) {
+		var ptr *int
+		ref := MakeIORef(ptr)()
+		assert.NotNil(t, ref)
+		assert.Nil(t, Read(ref)())
+	})
+
+	t.Run("multiple IORefs are independent", func(t *testing.T) {
+		ref1 := MakeIORef(10)()
+		ref2 := MakeIORef(20)()
+
+		assert.Equal(t, 10, Read(ref1)())
+		assert.Equal(t, 20, Read(ref2)())
+
+		Write(30)(ref1)()
+		assert.Equal(t, 30, Read(ref1)())
+		assert.Equal(t, 20, Read(ref2)()) // ref2 unchanged
+	})
+}
+
+func TestRead(t *testing.T) {
+	t.Run("reads initial value", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		value := Read(ref)()
+		assert.Equal(t, 42, value)
+	})
+
+	t.Run("reads updated value", func(t *testing.T) {
+		ref := MakeIORef(10)()
+		Write(20)(ref)()
+		value := Read(ref)()
+		assert.Equal(t, 20, value)
+	})
+
+	t.Run("multiple reads return same value", func(t *testing.T) {
+		ref := MakeIORef(100)()
+		value1 := Read(ref)()
+		value2 := Read(ref)()
+		value3 := Read(ref)()
+		assert.Equal(t, 100, value1)
+		assert.Equal(t, 100, value2)
+		assert.Equal(t, 100, value3)
+	})
+
+	t.Run("concurrent reads are thread-safe", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		var wg sync.WaitGroup
+		iterations := 100
+		results := make([]int, iterations)
+
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func(idx int) {
+				defer wg.Done()
+				results[idx] = Read(ref)()
+			}(i)
+		}
+
+		wg.Wait()
+
+		// All reads should return the same value
+		for _, v := range results {
+			assert.Equal(t, 42, v)
+		}
+	})
+
+	t.Run("reads during concurrent writes", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		var wg sync.WaitGroup
+		iterations := 50
+
+		// Start concurrent writes
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func(val int) {
+				defer wg.Done()
+				Write(val)(ref)()
+			}(i)
+		}
+
+		// Start concurrent reads
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				value := Read(ref)()
+				// Value should be valid (between 0 and iterations-1)
+				assert.GreaterOrEqual(t, value, 0)
+				assert.Less(t, value, iterations)
+			}()
+		}
+
+		wg.Wait()
+	})
+}
+
+func TestWrite(t *testing.T) {
+	t.Run("writes new value", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		result := Write(100)(ref)()
+		assert.Equal(t, 100, result)
+		assert.Equal(t, 100, Read(ref)())
+	})
+
+	t.Run("overwrites existing value", func(t *testing.T) {
+		ref := MakeIORef(10)()
+		Write(20)(ref)()
+		Write(30)(ref)()
+		assert.Equal(t, 30, Read(ref)())
+	})
+
+	t.Run("returns written value", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		result := Write(42)(ref)()
+		assert.Equal(t, 42, result)
+	})
+
+	t.Run("writes string value", func(t *testing.T) {
+		ref := MakeIORef("hello")()
+		result := Write("world")(ref)()
+		assert.Equal(t, "world", result)
+		assert.Equal(t, "world", Read(ref)())
+	})
+
+	t.Run("chained writes", func(t *testing.T) {
+		ref := MakeIORef(1)()
+		Write(2)(ref)()
+		Write(3)(ref)()
+		result := Write(4)(ref)()
+		assert.Equal(t, 4, result)
+		assert.Equal(t, 4, Read(ref)())
+	})
+
+	t.Run("concurrent writes are thread-safe", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		var wg sync.WaitGroup
+		iterations := 100
+
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func(val int) {
+				defer wg.Done()
+				Write(val)(ref)()
+			}(i)
+		}
+
+		wg.Wait()
+
+		// Final value should be one of the written values
+		finalValue := Read(ref)()
+		assert.GreaterOrEqual(t, finalValue, 0)
+		assert.Less(t, finalValue, iterations)
+	})
+
+	t.Run("write with zero value", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		Write(0)(ref)()
+		assert.Equal(t, 0, Read(ref)())
+	})
+}
+
+func TestModify(t *testing.T) {
+	t.Run("modifies value with simple function", func(t *testing.T) {
+		ref := MakeIORef(10)()
+		result := Modify(func(x int) int { return x * 2 })(ref)()
+		assert.Equal(t, 20, result)
+		assert.Equal(t, 20, Read(ref)())
+	})
+
+	t.Run("modifies with addition", func(t *testing.T) {
+		ref := MakeIORef(5)()
+		Modify(func(x int) int { return x + 10 })(ref)()
+		assert.Equal(t, 15, Read(ref)())
+	})
+
+	t.Run("modifies string value", func(t *testing.T) {
+		ref := MakeIORef("hello")()
+		result := Modify(func(s string) string { return s + " world" })(ref)()
+		assert.Equal(t, "hello world", result)
+		assert.Equal(t, "hello world", Read(ref)())
+	})
+
+	t.Run("chained modifications", func(t *testing.T) {
+		ref := MakeIORef(2)()
+		Modify(func(x int) int { return x * 3 })(ref)() // 6
+		Modify(func(x int) int { return x + 4 })(ref)() // 10
+		result := Modify(func(x int) int { return x / 2 })(ref)()
+		assert.Equal(t, 5, result)
+		assert.Equal(t, 5, Read(ref)())
+	})
+
+	t.Run("concurrent modifications are thread-safe", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		var wg sync.WaitGroup
+		iterations := 100
+
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				Modify(func(x int) int { return x + 1 })(ref)()
+			}()
+		}
+
+		wg.Wait()
+		assert.Equal(t, iterations, Read(ref)())
+	})
+
+	t.Run("modify with identity function", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		result := Modify(func(x int) int { return x })(ref)()
+		assert.Equal(t, 42, result)
+		assert.Equal(t, 42, Read(ref)())
+	})
+
+	t.Run("modify returns new value", func(t *testing.T) {
+		ref := MakeIORef(100)()
+		result := Modify(func(x int) int { return x - 50 })(ref)()
+		assert.Equal(t, 50, result)
+	})
+}
+
+func TestModifyWithResult(t *testing.T) {
+	t.Run("modifies and returns old value", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		oldValue := ModifyWithResult(func(x int) pair.Pair[int, int] {
+			return pair.MakePair(x+1, x)
+		})(ref)()
+		assert.Equal(t, 42, oldValue)
+		assert.Equal(t, 43, Read(ref)())
+	})
+
+	t.Run("swaps value and returns old", func(t *testing.T) {
+		ref := MakeIORef(100)()
+		oldValue := ModifyWithResult(func(x int) pair.Pair[int, int] {
+			return pair.MakePair(200, x)
+		})(ref)()
+		assert.Equal(t, 100, oldValue)
+		assert.Equal(t, 200, Read(ref)())
+	})
+
+	t.Run("returns different type", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		message := ModifyWithResult(func(x int) pair.Pair[int, string] {
+			return pair.MakePair(x*2, fmt.Sprintf("doubled from %d", x))
+		})(ref)()
+		assert.Equal(t, "doubled from 42", message)
+		assert.Equal(t, 84, Read(ref)())
+	})
+
+	t.Run("computes result based on old value", func(t *testing.T) {
+		ref := MakeIORef(10)()
+		wasPositive := ModifyWithResult(func(x int) pair.Pair[int, bool] {
+			return pair.MakePair(x+5, x > 0)
+		})(ref)()
+		assert.True(t, wasPositive)
+		assert.Equal(t, 15, Read(ref)())
+	})
+
+	t.Run("chained modifications with results", func(t *testing.T) {
+		ref := MakeIORef(5)()
+		result1 := ModifyWithResult(func(x int) pair.Pair[int, int] {
+			return pair.MakePair(x*2, x)
+		})(ref)()
+		result2 := ModifyWithResult(func(x int) pair.Pair[int, int] {
+			return pair.MakePair(x+10, x)
+		})(ref)()
+		assert.Equal(t, 5, result1)
+		assert.Equal(t, 10, result2)
+		assert.Equal(t, 20, Read(ref)())
+	})
+
+	t.Run("concurrent modifications with results are thread-safe", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		var wg sync.WaitGroup
+		iterations := 100
+		results := make([]int, iterations)
+
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func(idx int) {
+				defer wg.Done()
+				oldValue := ModifyWithResult(func(x int) pair.Pair[int, int] {
+					return pair.MakePair(x+1, x)
+				})(ref)()
+				results[idx] = oldValue
+			}(i)
+		}
+
+		wg.Wait()
+		assert.Equal(t, iterations, Read(ref)())
+
+		// All old values should be unique
+		seen := make(map[int]bool)
+		for _, v := range results {
+			assert.False(t, seen[v])
+			seen[v] = true
+		}
+	})
+
+	t.Run("extract and replace pattern", func(t *testing.T) {
+		ref := MakeIORef([]int{1, 2, 3})()
+		first := ModifyWithResult(func(xs []int) pair.Pair[[]int, int] {
+			if len(xs) == 0 {
+				return pair.MakePair(xs, 0)
+			}
+			return pair.MakePair(xs[1:], xs[0])
+		})(ref)()
+		assert.Equal(t, 1, first)
+		assert.Equal(t, []int{2, 3}, Read(ref)())
+	})
+}
+
+func TestModifyReaderIOK(t *testing.T) {
+	type Config struct {
+		multiplier int
+	}
+
+	t.Run("modifies with environment", func(t *testing.T) {
+		ref := MakeIORef(10)()
+		config := Config{multiplier: 5}
+
+		result := ModifyReaderIOK(func(x int) readerio.ReaderIO[Config, int] {
+			return func(cfg Config) io.IO[int] {
+				return io.Of(x * cfg.multiplier)
+			}
+		})(ref)(config)()
+
+		assert.Equal(t, 50, result)
+		assert.Equal(t, 50, Read(ref)())
+	})
+
+	t.Run("uses environment for computation", func(t *testing.T) {
+		ref := MakeIORef(100)()
+		config := Config{multiplier: 2}
+
+		result := ModifyReaderIOK(func(x int) readerio.ReaderIO[Config, int] {
+			return func(cfg Config) io.IO[int] {
+				return func() int {
+					return x / cfg.multiplier
+				}
+			}
+		})(ref)(config)()
+
+		assert.Equal(t, 50, result)
+		assert.Equal(t, 50, Read(ref)())
+	})
+
+	t.Run("chained modifications with different configs", func(t *testing.T) {
+		ref := MakeIORef(10)()
+		config1 := Config{multiplier: 2}
+		config2 := Config{multiplier: 3}
+
+		ModifyReaderIOK(func(x int) readerio.ReaderIO[Config, int] {
+			return func(cfg Config) io.IO[int] {
+				return io.Of(x * cfg.multiplier)
+			}
+		})(ref)(config1)()
+
+		result := ModifyReaderIOK(func(x int) readerio.ReaderIO[Config, int] {
+			return func(cfg Config) io.IO[int] {
+				return io.Of(x + cfg.multiplier)
+			}
+		})(ref)(config2)()
+
+		assert.Equal(t, 23, result) // (10 * 2) + 3
+		assert.Equal(t, 23, Read(ref)())
+	})
+
+	t.Run("concurrent modifications with environment are thread-safe", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		config := Config{multiplier: 1}
+		var wg sync.WaitGroup
+		iterations := 100
+
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				ModifyReaderIOK(func(x int) readerio.ReaderIO[Config, int] {
+					return func(cfg Config) io.IO[int] {
+						return io.Of(x + cfg.multiplier)
+					}
+				})(ref)(config)()
+			}()
+		}
+
+		wg.Wait()
+		assert.Equal(t, iterations, Read(ref)())
+	})
+
+	t.Run("environment provides configuration", func(t *testing.T) {
+		type Settings struct {
+			prefix string
+		}
+		ref := MakeIORef("world")()
+		settings := Settings{prefix: "hello "}
+
+		result := ModifyReaderIOK(func(s string) readerio.ReaderIO[Settings, string] {
+			return func(cfg Settings) io.IO[string] {
+				return io.Of(cfg.prefix + s)
+			}
+		})(ref)(settings)()
+
+		assert.Equal(t, "hello world", result)
+		assert.Equal(t, "hello world", Read(ref)())
+	})
+}
+
+func TestModifyReaderIOKWithResult(t *testing.T) {
+	type Config struct {
+		logEnabled bool
+		multiplier int
+	}
+
+	t.Run("modifies with environment and returns result", func(t *testing.T) {
+		ref := MakeIORef(42)()
+		config := Config{logEnabled: false, multiplier: 2}
+
+		oldValue := ModifyReaderIOKWithResult(func(x int) readerio.ReaderIO[Config, pair.Pair[int, int]] {
+			return func(cfg Config) io.IO[pair.Pair[int, int]] {
+				return io.Of(pair.MakePair(x*cfg.multiplier, x))
+			}
+		})(ref)(config)()
+
+		assert.Equal(t, 42, oldValue)
+		assert.Equal(t, 84, Read(ref)())
+	})
+
+	t.Run("returns different type based on environment", func(t *testing.T) {
+		ref := MakeIORef(10)()
+		config := Config{logEnabled: true, multiplier: 3}
+
+		message := ModifyReaderIOKWithResult(func(x int) readerio.ReaderIO[Config, pair.Pair[int, string]] {
+			return func(cfg Config) io.IO[pair.Pair[int, string]] {
+				return func() pair.Pair[int, string] {
+					newVal := x * cfg.multiplier
+					msg := fmt.Sprintf("multiplied %d by %d", x, cfg.multiplier)
+					return pair.MakePair(newVal, msg)
+				}
+			}
+		})(ref)(config)()
+
+		assert.Equal(t, "multiplied 10 by 3", message)
+		assert.Equal(t, 30, Read(ref)())
+	})
+
+	t.Run("conditional logic based on environment", func(t *testing.T) {
+		ref := MakeIORef(-10)()
+		config := Config{logEnabled: true, multiplier: 2}
+
+		message := ModifyReaderIOKWithResult(func(x int) readerio.ReaderIO[Config, pair.Pair[int, string]] {
+			return func(cfg Config) io.IO[pair.Pair[int, string]] {
+				return func() pair.Pair[int, string] {
+					if x < 0 {
+						return pair.MakePair(0, "reset negative value")
+					}
+					return pair.MakePair(x*cfg.multiplier, "multiplied positive value")
+				}
+			}
+		})(ref)(config)()
+
+		assert.Equal(t, "reset negative value", message)
+		assert.Equal(t, 0, Read(ref)())
+	})
+
+	t.Run("chained modifications with results", func(t *testing.T) {
+		ref := MakeIORef(5)()
+		config := Config{logEnabled: false, multiplier: 2}
+
+		result1 := ModifyReaderIOKWithResult(func(x int) readerio.ReaderIO[Config, pair.Pair[int, int]] {
+			return func(cfg Config) io.IO[pair.Pair[int, int]] {
+				return io.Of(pair.MakePair(x*cfg.multiplier, x))
+			}
+		})(ref)(config)()
+
+		result2 := ModifyReaderIOKWithResult(func(x int) readerio.ReaderIO[Config, pair.Pair[int, int]] {
+			return func(cfg Config) io.IO[pair.Pair[int, int]] {
+				return io.Of(pair.MakePair(x+cfg.multiplier, x))
+			}
+		})(ref)(config)()
+
+		assert.Equal(t, 5, result1)
+		assert.Equal(t, 10, result2)
+		assert.Equal(t, 12, Read(ref)())
+	})
+
+	t.Run("concurrent modifications with environment are thread-safe", func(t *testing.T) {
+		ref := MakeIORef(0)()
+		config := Config{logEnabled: false, multiplier: 1}
+		var wg sync.WaitGroup
+		iterations := 100
+		results := make([]int, iterations)
+
+		for i := 0; i < iterations; i++ {
+			wg.Add(1)
+			go func(idx int) {
+				defer wg.Done()
+				oldValue := ModifyReaderIOKWithResult(func(x int) readerio.ReaderIO[Config, pair.Pair[int, int]] {
+					return func(cfg Config) io.IO[pair.Pair[int, int]] {
+						return io.Of(pair.MakePair(x+cfg.multiplier, x))
+					}
+				})(ref)(config)()
+				results[idx] = oldValue
+			}(i)
+		}
+
+		wg.Wait()
+		assert.Equal(t, iterations, Read(ref)())
+
+		// All old values should be unique
+		seen := make(map[int]bool)
+		for _, v := range results {
+			assert.False(t, seen[v])
+			seen[v] = true
+		}
+	})
+
+	t.Run("environment provides validation rules", func(t *testing.T) {
+		type ValidationConfig struct {
+			maxValue int
+		}
+		ref := MakeIORef(100)()
+		config := ValidationConfig{maxValue: 50}
+
+		message := ModifyReaderIOKWithResult(func(x int) readerio.ReaderIO[ValidationConfig, pair.Pair[int, string]] {
+			return func(cfg ValidationConfig) io.IO[pair.Pair[int, string]] {
+				return func() pair.Pair[int, string] {
+					if x > cfg.maxValue {
+						return pair.MakePair(cfg.maxValue, fmt.Sprintf("capped at %d", cfg.maxValue))
+					}
+					return pair.MakePair(x, "value within limits")
+				}
+			}
+		})(ref)(config)()
+
+		assert.Equal(t, "capped at 50", message)
+		assert.Equal(t, 50, Read(ref)())
+	})
+}
+
 func TestModifyIOK(t *testing.T) {
 	t.Run("basic modification with IO effect", func(t *testing.T) {
 		ref := MakeIORef(42)()

v2/ioref/types.go

@@ -45,29 +45,110 @@ import (
 	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/pair"
+	"github.com/IBM/fp-go/v2/readerio"
 )
 
 type (
 	// ioRef is the internal implementation of a mutable reference.
-	// It uses a read-write mutex to ensure thread-safe access.
+	// It uses a read-write mutex to ensure thread-safe access to the stored value.
+	//
+	// The mutex allows multiple concurrent readers (using RLock) but ensures
+	// exclusive access for writers (using Lock), preventing race conditions
+	// when reading or modifying the stored value.
+	//
+	// This type is not exported; users interact with it through the IORef type alias.
 	ioRef[A any] struct {
-		mu sync.RWMutex
-		a  A
+		mu sync.RWMutex // Protects concurrent access to the stored value
+		a  A            // The stored value
 	}
 
 	// IO represents a synchronous computation that may have side effects.
 	// It's a function that takes no arguments and returns a value of type A.
+	//
+	// IO computations are lazy - they don't execute until explicitly invoked
+	// by calling the function. This allows for composing and chaining effects
+	// before execution.
+	//
+	// Example:
+	//
+	//	// Define an IO computation
+	//	computation := func() int {
+	//	    fmt.Println("Computing...")
+	//	    return 42
+	//	}
+	//
+	//	// Nothing happens yet - the computation is lazy
+	//	result := computation()  // Now it executes and prints "Computing..."
 	IO[A any] = io.IO[A]
 
+	// ReaderIO represents a computation that requires an environment of type R
+	// and produces an IO effect that yields a value of type A.
+	//
+	// This combines the Reader pattern (dependency injection) with IO effects,
+	// allowing computations to access shared configuration or context while
+	// performing side effects.
+	//
+	// Example:
+	//
+	//	type Config struct {
+	//	    multiplier int
+	//	}
+	//
+	//	// A ReaderIO that uses config to compute a value
+	//	computation := func(cfg Config) io.IO[int] {
+	//	    return func() int {
+	//	        return 42 * cfg.multiplier
+	//	    }
+	//	}
+	//
+	//	// Execute with specific config
+	//	result := computation(Config{multiplier: 2})()  // Returns 84
+	ReaderIO[R, A any] = readerio.ReaderIO[R, A]
+
 	// IORef represents a mutable reference to a value of type A.
 	// Operations on IORef are thread-safe and performed within the IO monad.
 	//
 	// IORef provides a way to work with mutable state in a functional style,
 	// where mutations are explicit and contained within IO computations.
+	// This makes side effects visible in the type system and allows for
+	// better reasoning about code that uses mutable state.
+	//
+	// All operations on IORef (Read, Write, Modify, etc.) are atomic and
+	// thread-safe, making it safe to share IORefs across goroutines.
+	//
+	// Example:
+	//
+	//	// Create a new IORef
+	//	ref := ioref.MakeIORef(42)()
+	//
+	//	// Read the current value
+	//	value := ioref.Read(ref)()  // 42
+	//
+	//	// Write a new value
+	//	ioref.Write(100)(ref)()
+	//
+	//	// Modify the value atomically
+	//	ioref.Modify(func(x int) int { return x * 2 })(ref)()
 	IORef[A any] = *ioRef[A]
 
 	// Endomorphism represents a function from A to A.
 	// It's commonly used with Modify to transform the value in an IORef.
+	//
+	// An endomorphism is a morphism (structure-preserving map) from a
+	// mathematical object to itself. In programming terms, it's simply
+	// a function that takes a value and returns a value of the same type.
+	//
+	// Example:
+	//
+	//	// An endomorphism that doubles an integer
+	//	double := func(x int) int { return x * 2 }
+	//
+	//	// An endomorphism that uppercases a string
+	//	upper := func(s string) string { return strings.ToUpper(s) }
+	//
+	//	// Use with IORef
+	//	ref := ioref.MakeIORef(21)()
+	//	ioref.Modify(double)(ref)()  // ref now contains 42
 	Endomorphism[A any] = endomorphism.Endomorphism[A]
 
 	// Pair represents a tuple of two values of types A and B.
@@ -76,6 +157,8 @@ type (
 	//
 	// The head of the pair contains the new value to store in the IORef,
 	// while the tail contains the result to return from the operation.
+	// This allows atomic operations that both update the reference and
+	// compute a result based on the old value.
 	//
 	// Example:
 	//
@@ -85,5 +168,11 @@ type (
 	//	// Extract values
 	//	newVal := pair.Head(p)  // Gets the head (new value)
 	//	oldVal := pair.Tail(p)  // Gets the tail (old value)
+	//
+	//	// Use with ModifyWithResult to swap and return old value
+	//	ref := ioref.MakeIORef(42)()
+	//	oldValue := ioref.ModifyWithResult(func(x int) pair.Pair[int, int] {
+	//	    return pair.MakePair(100, x)  // Store 100, return old value
+	//	})(ref)()  // oldValue is 42, ref now contains 100
 	Pair[A, B any] = pair.Pair[A, B]
 )

v2/iterator/iter/iter.go

@@ -466,6 +466,11 @@ func Chain[A, B any](f func(A) Seq[B]) Operator[A, B] {
 	return F.Bind2nd(MonadChain[A, B], f)
 }
 
+//go:inline
+func FlatMap[A, B any](f func(A) Seq[B]) Operator[A, B] {
+	return Chain(f)
+}
+
 // Flatten flattens a sequence of sequences into a single sequence.
 //
 // RxJS Equivalent: [mergeAll] - https://rxjs.dev/api/operators/mergeAll

v2/iterator/iter/option.go

@@ -0,0 +1,158 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iter
+
+import (
+	"github.com/IBM/fp-go/v2/option"
+)
+
+// MonadChainOptionK chains a function that returns an Option into a sequence,
+// filtering out None values and unwrapping Some values.
+//
+// This is useful for operations that may or may not produce a value for each element
+// in the sequence. Only the successful (Some) results are included in the output sequence,
+// while None values are filtered out.
+//
+// This is the monadic form that takes the sequence as the first parameter.
+//
+// RxJS Equivalent: [concatMap] combined with [filter] - https://rxjs.dev/api/operators/concatMap
+//
+// Type parameters:
+//   - A: The element type of the input sequence
+//   - B: The element type of the output sequence (wrapped in Option by the function)
+//
+// Parameters:
+//   - as: The input sequence to transform
+//   - f: A function that takes an element and returns an Option[B]
+//
+// Returns:
+//
+//	A new sequence containing only the unwrapped Some values
+//
+// Example:
+//
+//	import (
+//	    "strconv"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	    I "github.com/IBM/fp-go/v2/iterator/iter"
+//	)
+//
+//	// Parse strings to integers, filtering out invalid ones
+//	parseNum := func(s string) O.Option[int] {
+//	    if n, err := strconv.Atoi(s); err == nil {
+//	        return O.Some(n)
+//	    }
+//	    return O.None[int]()
+//	}
+//
+//	seq := I.From("1", "invalid", "2", "3", "bad")
+//	result := I.MonadChainOptionK(seq, parseNum)
+//	// yields: 1, 2, 3 (invalid strings are filtered out)
+func MonadChainOptionK[A, B any](as Seq[A], f option.Kleisli[A, B]) Seq[B] {
+	return MonadFilterMap(as, f)
+}
+
+// ChainOptionK returns an operator that chains a function returning an Option into a sequence,
+// filtering out None values and unwrapping Some values.
+//
+// This is the curried version of [MonadChainOptionK], useful for function composition
+// and creating reusable transformations.
+//
+// RxJS Equivalent: [concatMap] combined with [filter] - https://rxjs.dev/api/operators/concatMap
+//
+// Type parameters:
+//   - A: The element type of the input sequence
+//   - B: The element type of the output sequence (wrapped in Option by the function)
+//
+// Parameters:
+//   - f: A function that takes an element and returns an Option[B]
+//
+// Returns:
+//
+//	An Operator that transforms Seq[A] to Seq[B], filtering out None values
+//
+// Example:
+//
+//	import (
+//	    "strconv"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	    I "github.com/IBM/fp-go/v2/iterator/iter"
+//	)
+//
+//	// Create a reusable parser operator
+//	parsePositive := I.ChainOptionK(func(x int) O.Option[int] {
+//	    if x > 0 {
+//	        return O.Some(x)
+//	    }
+//	    return O.None[int]()
+//	})
+//
+//	result := F.Pipe1(
+//	    I.From(-1, 2, -3, 4, 5),
+//	    parsePositive,
+//	)
+//	// yields: 2, 4, 5 (negative numbers are filtered out)
+//
+//go:inline
+func ChainOptionK[A, B any](f option.Kleisli[A, B]) Operator[A, B] {
+	return FilterMap(f)
+}
+
+// FlatMapOptionK is an alias for [ChainOptionK].
+//
+// This provides a more familiar name for developers coming from other functional
+// programming languages or libraries where "flatMap" is the standard terminology
+// for the monadic bind operation.
+//
+// Type parameters:
+//   - A: The element type of the input sequence
+//   - B: The element type of the output sequence (wrapped in Option by the function)
+//
+// Parameters:
+//   - f: A function that takes an element and returns an Option[B]
+//
+// Returns:
+//
+//	An Operator that transforms Seq[A] to Seq[B], filtering out None values
+//
+// Example:
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	    I "github.com/IBM/fp-go/v2/iterator/iter"
+//	)
+//
+//	// Validate and transform data
+//	validateAge := I.FlatMapOptionK(func(age int) O.Option[string] {
+//	    if age >= 18 && age <= 120 {
+//	        return O.Some(fmt.Sprintf("Valid age: %d", age))
+//	    }
+//	    return O.None[string]()
+//	})
+//
+//	result := F.Pipe1(
+//	    I.From(15, 25, 150, 30),
+//	    validateAge,
+//	)
+//	// yields: "Valid age: 25", "Valid age: 30"
+//
+//go:inline
+func FlatMapOptionK[A, B any](f option.Kleisli[A, B]) Operator[A, B] {
+	return ChainOptionK(f)
+}

v2/iterator/iter/option_test.go

@@ -0,0 +1,387 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iter
+
+import (
+	"fmt"
+	"slices"
+	"strconv"
+	"testing"
+
+	A "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestMonadChainOptionK_AllSome tests MonadChainOptionK when all values produce Some
+func TestMonadChainOptionK_AllSome(t *testing.T) {
+	// Function that always returns Some
+	double := func(x int) O.Option[int] {
+		return O.Some(x * 2)
+	}
+
+	seq := From(1, 2, 3, 4, 5)
+	result := MonadChainOptionK(seq, double)
+	values := slices.Collect(result)
+
+	expected := A.From(2, 4, 6, 8, 10)
+	assert.Equal(t, expected, values)
+}
+
+// TestMonadChainOptionK_AllNone tests MonadChainOptionK when all values produce None
+func TestMonadChainOptionK_AllNone(t *testing.T) {
+	// Function that always returns None
+	alwaysNone := func(x int) O.Option[int] {
+		return O.None[int]()
+	}
+
+	seq := From(1, 2, 3, 4, 5)
+	result := MonadChainOptionK(seq, alwaysNone)
+	values := slices.Collect(result)
+
+	assert.Empty(t, values)
+}
+
+// TestMonadChainOptionK_MixedSomeNone tests MonadChainOptionK with mixed Some and None
+func TestMonadChainOptionK_MixedSomeNone(t *testing.T) {
+	// Function that returns Some for even numbers, None for odd
+	evenOnly := func(x int) O.Option[int] {
+		if x%2 == 0 {
+			return O.Some(x)
+		}
+		return O.None[int]()
+	}
+
+	seq := From(1, 2, 3, 4, 5, 6)
+	result := MonadChainOptionK(seq, evenOnly)
+	values := slices.Collect(result)
+
+	expected := A.From(2, 4, 6)
+	assert.Equal(t, expected, values)
+}
+
+// TestMonadChainOptionK_ParseStrings tests parsing strings to integers
+func TestMonadChainOptionK_ParseStrings(t *testing.T) {
+	// Parse strings to integers, returning None for invalid strings
+	parseNum := func(s string) O.Option[int] {
+		if n, err := strconv.Atoi(s); err == nil {
+			return O.Some(n)
+		}
+		return O.None[int]()
+	}
+
+	seq := From("1", "invalid", "2", "3", "bad", "4")
+	result := MonadChainOptionK(seq, parseNum)
+	values := slices.Collect(result)
+
+	expected := A.From(1, 2, 3, 4)
+	assert.Equal(t, expected, values)
+}
+
+// TestMonadChainOptionK_EmptySequence tests MonadChainOptionK with empty sequence
+func TestMonadChainOptionK_EmptySequence(t *testing.T) {
+	double := func(x int) O.Option[int] {
+		return O.Some(x * 2)
+	}
+
+	seq := From[int]()
+	result := MonadChainOptionK(seq, double)
+	values := slices.Collect(result)
+
+	assert.Empty(t, values)
+}
+
+// TestMonadChainOptionK_TypeTransformation tests transforming types
+func TestMonadChainOptionK_TypeTransformation(t *testing.T) {
+	// Convert integers to strings, only for positive numbers
+	positiveToString := func(x int) O.Option[string] {
+		if x > 0 {
+			return O.Some(fmt.Sprintf("num_%d", x))
+		}
+		return O.None[string]()
+	}
+
+	seq := From(-2, -1, 0, 1, 2, 3)
+	result := MonadChainOptionK(seq, positiveToString)
+	values := slices.Collect(result)
+
+	expected := A.From("num_1", "num_2", "num_3")
+	assert.Equal(t, expected, values)
+}
+
+// TestMonadChainOptionK_ComplexType tests with complex types
+func TestMonadChainOptionK_ComplexType(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	// Extract age only for adults
+	getAdultAge := func(p Person) O.Option[int] {
+		if p.Age >= 18 {
+			return O.Some(p.Age)
+		}
+		return O.None[int]()
+	}
+
+	seq := From(
+		Person{"Alice", 25},
+		Person{"Bob", 15},
+		Person{"Charlie", 30},
+		Person{"David", 12},
+	)
+	result := MonadChainOptionK(seq, getAdultAge)
+	values := slices.Collect(result)
+
+	expected := A.From(25, 30)
+	assert.Equal(t, expected, values)
+}
+
+// TestChainOptionK_BasicUsage tests ChainOptionK basic functionality
+func TestChainOptionK_BasicUsage(t *testing.T) {
+	// Create a reusable operator
+	parsePositive := ChainOptionK(func(x int) O.Option[int] {
+		if x > 0 {
+			return O.Some(x)
+		}
+		return O.None[int]()
+	})
+
+	seq := From(-1, 2, -3, 4, 5, -6)
+	result := parsePositive(seq)
+	values := slices.Collect(result)
+
+	expected := A.From(2, 4, 5)
+	assert.Equal(t, expected, values)
+}
+
+// TestChainOptionK_WithPipe tests ChainOptionK in a pipeline
+func TestChainOptionK_WithPipe(t *testing.T) {
+	// Validate and transform in a pipeline
+	validateRange := ChainOptionK(func(x int) O.Option[int] {
+		if x >= 0 && x <= 100 {
+			return O.Some(x)
+		}
+		return O.None[int]()
+	})
+
+	result := F.Pipe2(
+		From(-10, 20, 150, 50, 200, 75),
+		validateRange,
+		Map(func(x int) int { return x * 2 }),
+	)
+	values := slices.Collect(result)
+
+	expected := A.From(40, 100, 150)
+	assert.Equal(t, expected, values)
+}
+
+// TestChainOptionK_Composition tests composing multiple ChainOptionK operations
+func TestChainOptionK_Composition(t *testing.T) {
+	// First filter: only positive
+	onlyPositive := ChainOptionK(func(x int) O.Option[int] {
+		if x > 0 {
+			return O.Some(x)
+		}
+		return O.None[int]()
+	})
+
+	// Second filter: only even
+	onlyEven := ChainOptionK(func(x int) O.Option[int] {
+		if x%2 == 0 {
+			return O.Some(x)
+		}
+		return O.None[int]()
+	})
+
+	result := F.Pipe2(
+		From(-2, -1, 0, 1, 2, 3, 4, 5, 6),
+		onlyPositive,
+		onlyEven,
+	)
+	values := slices.Collect(result)
+
+	expected := A.From(2, 4, 6)
+	assert.Equal(t, expected, values)
+}
+
+// TestChainOptionK_StringParsing tests parsing with ChainOptionK
+func TestChainOptionK_StringParsing(t *testing.T) {
+	// Create a reusable string parser
+	parseInt := ChainOptionK(func(s string) O.Option[int] {
+		if n, err := strconv.Atoi(s); err == nil {
+			return O.Some(n)
+		}
+		return O.None[int]()
+	})
+
+	result := F.Pipe1(
+		From("10", "abc", "20", "xyz", "30"),
+		parseInt,
+	)
+	values := slices.Collect(result)
+
+	expected := A.From(10, 20, 30)
+	assert.Equal(t, expected, values)
+}
+
+// TestFlatMapOptionK_Equivalence tests that FlatMapOptionK is equivalent to ChainOptionK
+func TestFlatMapOptionK_Equivalence(t *testing.T) {
+	validate := func(x int) O.Option[int] {
+		if x >= 0 && x <= 10 {
+			return O.Some(x)
+		}
+		return O.None[int]()
+	}
+
+	seq := From(-5, 0, 5, 10, 15)
+
+	// Using ChainOptionK
+	result1 := ChainOptionK(validate)(seq)
+	values1 := slices.Collect(result1)
+
+	// Using FlatMapOptionK
+	result2 := FlatMapOptionK(validate)(seq)
+	values2 := slices.Collect(result2)
+
+	// Both should produce the same result
+	assert.Equal(t, values1, values2)
+	assert.Equal(t, A.From(0, 5, 10), values1)
+}
+
+// TestFlatMapOptionK_WithMap tests FlatMapOptionK combined with Map
+func TestFlatMapOptionK_WithMap(t *testing.T) {
+	// Validate age and convert to category
+	validateAge := FlatMapOptionK(func(age int) O.Option[string] {
+		if age >= 18 && age <= 120 {
+			return O.Some(fmt.Sprintf("Valid age: %d", age))
+		}
+		return O.None[string]()
+	})
+
+	result := F.Pipe1(
+		From(15, 25, 150, 30, 200),
+		validateAge,
+	)
+	values := slices.Collect(result)
+
+	expected := A.From("Valid age: 25", "Valid age: 30")
+	assert.Equal(t, expected, values)
+}
+
+// TestChainOptionK_LookupOperation tests using ChainOptionK for lookup operations
+func TestChainOptionK_LookupOperation(t *testing.T) {
+	// Simulate a lookup table
+	lookup := map[string]int{
+		"one":   1,
+		"two":   2,
+		"three": 3,
+	}
+
+	lookupValue := ChainOptionK(func(key string) O.Option[int] {
+		if val, ok := lookup[key]; ok {
+			return O.Some(val)
+		}
+		return O.None[int]()
+	})
+
+	result := F.Pipe1(
+		From("one", "invalid", "two", "missing", "three"),
+		lookupValue,
+	)
+	values := slices.Collect(result)
+
+	expected := A.From(1, 2, 3)
+	assert.Equal(t, expected, values)
+}
+
+// TestMonadChainOptionK_EarlyTermination tests that iteration stops when yield returns false
+func TestMonadChainOptionK_EarlyTermination(t *testing.T) {
+	callCount := 0
+	countCalls := func(x int) O.Option[int] {
+		callCount++
+		return O.Some(x)
+	}
+
+	seq := From(1, 2, 3, 4, 5)
+	result := MonadChainOptionK(seq, countCalls)
+
+	// Collect only first 3 elements
+	collected := make([]int, 0)
+	for v := range result {
+		collected = append(collected, v)
+		if len(collected) >= 3 {
+			break
+		}
+	}
+
+	// Should have called the function only 3 times due to early termination
+	assert.Equal(t, 3, callCount)
+	assert.Equal(t, A.From(1, 2, 3), collected)
+}
+
+// TestChainOptionK_WithReduce tests ChainOptionK with reduction
+func TestChainOptionK_WithReduce(t *testing.T) {
+	// Parse and sum valid numbers
+	parseInt := ChainOptionK(func(s string) O.Option[int] {
+		if n, err := strconv.Atoi(s); err == nil {
+			return O.Some(n)
+		}
+		return O.None[int]()
+	})
+
+	result := F.Pipe1(
+		From("10", "invalid", "20", "bad", "30"),
+		parseInt,
+	)
+
+	sum := MonadReduce(result, func(acc, x int) int {
+		return acc + x
+	}, 0)
+
+	assert.Equal(t, 60, sum)
+}
+
+// TestFlatMapOptionK_NestedOptions tests FlatMapOptionK with nested option handling
+func TestFlatMapOptionK_NestedOptions(t *testing.T) {
+	type Result struct {
+		Value int
+		Valid bool
+	}
+
+	// Extract value only if valid
+	extractValid := FlatMapOptionK(func(r Result) O.Option[int] {
+		if r.Valid {
+			return O.Some(r.Value)
+		}
+		return O.None[int]()
+	})
+
+	seq := From(
+		Result{10, true},
+		Result{20, false},
+		Result{30, true},
+		Result{40, false},
+		Result{50, true},
+	)
+
+	result := F.Pipe1(seq, extractValid)
+	values := slices.Collect(result)
+
+	expected := A.From(10, 30, 50)
+	assert.Equal(t, expected, values)
+}

v2/llms.txt

@@ -0,0 +1,99 @@
+# fp-go
+
+> A comprehensive functional programming library for Go, bringing type-safe monads, functors, applicatives, optics, and composable abstractions inspired by fp-ts and Haskell to the Go ecosystem. Created by IBM, licensed under Apache-2.0.
+
+fp-go v2 requires Go 1.24+ and leverages generic type aliases for a cleaner API. 
+
+Key concepts: `Option` for nullable values, `Either`/`Result` for error handling, `IO` for lazy side effects, `Reader` for dependency injection, `IOResult` for effectful error handling, `ReaderIOResult` for the full monad stack, and `Optics` (lens, prism, traversal, iso) for immutable data manipulation.
+
+## Core Documentation
+
+- [API Reference (pkg.go.dev)](https://pkg.go.dev/github.com/IBM/fp-go/v2): Complete API documentation for all packages
+- [README](https://github.com/IBM/fp-go/blob/main/v2/README.md): Overview, quick start, installation, and migration guide from v1 to v2
+- [Design Decisions](https://github.com/IBM/fp-go/blob/main/v2/DESIGN.md): Key design principles and patterns
+- [Functional I/O Guide](https://github.com/IBM/fp-go/blob/main/v2/FUNCTIONAL_IO.md): Understanding Context, errors, and the Reader pattern for I/O operations
+- [Idiomatic vs Standard Comparison](https://github.com/IBM/fp-go/blob/main/v2/IDIOMATIC_COMPARISON.md): Performance comparison and when to use each approach
+- [Optics README](https://github.com/IBM/fp-go/blob/main/v2/optics/README.md): Guide to lens, prism, optional, and traversal optics
+
+## Standard Packages (struct-based)
+
+- [option](https://pkg.go.dev/github.com/IBM/fp-go/v2/option): Option monad — represent optional values without nil
+- [either](https://pkg.go.dev/github.com/IBM/fp-go/v2/either): Either monad — type-safe error handling with Left/Right values
+- [result](https://pkg.go.dev/github.com/IBM/fp-go/v2/result): Result monad — simplified Either with `error` as Left type (recommended for error handling)
+- [io](https://pkg.go.dev/github.com/IBM/fp-go/v2/io): IO monad — lazy evaluation and side effect management
+- [iooption](https://pkg.go.dev/github.com/IBM/fp-go/v2/iooption): IOOption — IO combined with Option
+- [ioeither](https://pkg.go.dev/github.com/IBM/fp-go/v2/ioeither): IOEither — IO combined with Either for effectful error handling
+- [ioresult](https://pkg.go.dev/github.com/IBM/fp-go/v2/ioresult): IOResult — IO combined with Result (recommended over IOEither)
+- [reader](https://pkg.go.dev/github.com/IBM/fp-go/v2/reader): Reader monad — dependency injection pattern
+- [readeroption](https://pkg.go.dev/github.com/IBM/fp-go/v2/readeroption): ReaderOption — Reader combined with Option
+- [readeriooption](https://pkg.go.dev/github.com/IBM/fp-go/v2/readeriooption): ReaderIOOption — Reader + IO + Option
+- [readerioresult](https://pkg.go.dev/github.com/IBM/fp-go/v2/readerioresult): ReaderIOResult — Reader + IO + Result for complex workflows
+- [readerioeither](https://pkg.go.dev/github.com/IBM/fp-go/v2/readerioeither): ReaderIOEither — Reader + IO + Either
+- [statereaderioeither](https://pkg.go.dev/github.com/IBM/fp-go/v2/statereaderioeither): StateReaderIOEither — State + Reader + IO + Either
+
+## Idiomatic Packages (tuple-based, high performance)
+
+- [idiomatic/option](https://pkg.go.dev/github.com/IBM/fp-go/v2/idiomatic/option): Option using native Go `(value, bool)` tuples
+- [idiomatic/result](https://pkg.go.dev/github.com/IBM/fp-go/v2/idiomatic/result): Result using native Go `(value, error)` tuples
+- [idiomatic/ioresult](https://pkg.go.dev/github.com/IBM/fp-go/v2/idiomatic/ioresult): IOResult using `func() (value, error)`
+- [idiomatic/readerresult](https://pkg.go.dev/github.com/IBM/fp-go/v2/idiomatic/readerresult): ReaderResult with tuple-based results
+- [idiomatic/readerioresult](https://pkg.go.dev/github.com/IBM/fp-go/v2/idiomatic/readerioresult): ReaderIOResult with tuple-based results
+
+## Context Packages (context.Context specializations)
+
+- [context/readerioresult](https://pkg.go.dev/github.com/IBM/fp-go/v2/context/readerioresult): ReaderIOResult specialized for context.Context
+- [context/readerioresult/http](https://pkg.go.dev/github.com/IBM/fp-go/v2/context/readerioresult/http): Functional HTTP client utilities
+- [context/readerioresult/http/builder](https://pkg.go.dev/github.com/IBM/fp-go/v2/context/readerioresult/http/builder): Functional HTTP request builder
+- [context/statereaderioresult](https://pkg.go.dev/github.com/IBM/fp-go/v2/context/statereaderioresult): State + Reader + IO + Result for context.Context
+
+## Optics
+
+- [optics](https://pkg.go.dev/github.com/IBM/fp-go/v2/optics): Core optics package
+- [optics/lens](https://pkg.go.dev/github.com/IBM/fp-go/v2/optics/lens): Lenses for focusing on fields in product types
+- [optics/prism](https://pkg.go.dev/github.com/IBM/fp-go/v2/optics/prism): Prisms for focusing on variants in sum types
+- [optics/iso](https://pkg.go.dev/github.com/IBM/fp-go/v2/optics/iso): Isomorphisms for bidirectional transformations
+- [optics/optional](https://pkg.go.dev/github.com/IBM/fp-go/v2/optics/optional): Optionals for values that may not exist
+- [optics/traversal](https://pkg.go.dev/github.com/IBM/fp-go/v2/optics/traversal): Traversals for focusing on multiple values
+- [optics/codec](https://pkg.go.dev/github.com/IBM/fp-go/v2/optics/codec): Codecs for encoding/decoding with validation
+
+## Utility Packages
+
+- [array](https://pkg.go.dev/github.com/IBM/fp-go/v2/array): Functional array/slice operations (map, filter, fold, etc.)
+- [record](https://pkg.go.dev/github.com/IBM/fp-go/v2/record): Functional operations for maps
+- [function](https://pkg.go.dev/github.com/IBM/fp-go/v2/function): Function composition, pipe, flow, curry, identity
+- [pair](https://pkg.go.dev/github.com/IBM/fp-go/v2/pair): Strongly-typed pair/tuple data structure
+- [tuple](https://pkg.go.dev/github.com/IBM/fp-go/v2/tuple): Type-safe heterogeneous tuples
+- [predicate](https://pkg.go.dev/github.com/IBM/fp-go/v2/predicate): Predicate combinators (and, or, not, etc.)
+- [endomorphism](https://pkg.go.dev/github.com/IBM/fp-go/v2/endomorphism): Endomorphism operations (compose, chain)
+- [eq](https://pkg.go.dev/github.com/IBM/fp-go/v2/eq): Type-safe equality comparisons
+- [ord](https://pkg.go.dev/github.com/IBM/fp-go/v2/ord): Total ordering type class
+- [semigroup](https://pkg.go.dev/github.com/IBM/fp-go/v2/semigroup): Semigroup algebraic structure
+- [monoid](https://pkg.go.dev/github.com/IBM/fp-go/v2/monoid): Monoid algebraic structure
+- [number](https://pkg.go.dev/github.com/IBM/fp-go/v2/number): Algebraic structures for numeric types
+- [string](https://pkg.go.dev/github.com/IBM/fp-go/v2/string): Functional string utilities
+- [boolean](https://pkg.go.dev/github.com/IBM/fp-go/v2/boolean): Functional boolean utilities
+- [bytes](https://pkg.go.dev/github.com/IBM/fp-go/v2/bytes): Functional byte slice utilities
+- [json](https://pkg.go.dev/github.com/IBM/fp-go/v2/json): Functional JSON encoding/decoding
+- [lazy](https://pkg.go.dev/github.com/IBM/fp-go/v2/lazy): Lazy evaluation without side effects
+- [identity](https://pkg.go.dev/github.com/IBM/fp-go/v2/identity): Identity monad
+- [retry](https://pkg.go.dev/github.com/IBM/fp-go/v2/retry): Retry policies with configurable backoff
+- [tailrec](https://pkg.go.dev/github.com/IBM/fp-go/v2/tailrec): Trampoline for tail-call optimization
+- [di](https://pkg.go.dev/github.com/IBM/fp-go/v2/di): Dependency injection utilities
+- [effect](https://pkg.go.dev/github.com/IBM/fp-go/v2/effect): Functional effect system
+- [circuitbreaker](https://pkg.go.dev/github.com/IBM/fp-go/v2/circuitbreaker): Circuit breaker error types
+- [builder](https://pkg.go.dev/github.com/IBM/fp-go/v2/builder): Generic builder pattern with validation
+
+## Code Samples
+
+- [samples/builder](https://github.com/IBM/fp-go/tree/main/v2/samples/builder): Functional builder pattern example
+- [samples/http](https://github.com/IBM/fp-go/tree/main/v2/samples/http): HTTP client examples
+- [samples/lens](https://github.com/IBM/fp-go/tree/main/v2/samples/lens): Optics/lens examples
+- [samples/mostly-adequate](https://github.com/IBM/fp-go/tree/main/v2/samples/mostly-adequate): Examples adapted from "Mostly Adequate Guide to Functional Programming"
+- [samples/tuples](https://github.com/IBM/fp-go/tree/main/v2/samples/tuples): Tuple usage examples
+
+## Optional
+
+- [Source Code](https://github.com/IBM/fp-go): GitHub repository
+- [Issues](https://github.com/IBM/fp-go/issues): Bug reports and feature requests
+- [Go Report Card](https://goreportcard.com/report/github.com/IBM/fp-go/v2): Code quality report
+- [Coverage](https://coveralls.io/github/IBM/fp-go?branch=main): Test coverage report

v2/monoid/monoid.go

@@ -13,6 +13,50 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package monoid provides an implementation of the Monoid algebraic structure.
+//
+// # Monoid
+//
+// A Monoid is an algebraic structure that extends [Semigroup] by adding an identity element.
+// It consists of:
+//   - A type A
+//   - An associative binary operation Concat: (A, A) → A
+//   - An identity element Empty: () → A
+//
+// # Laws
+//
+// A Monoid must satisfy the following laws:
+//
+//  1. Associativity (from Semigroup):
+//     Concat(Concat(x, y), z) = Concat(x, Concat(y, z))
+//
+//  2. Left Identity:
+//     Concat(Empty(), x) = x
+//
+//  3. Right Identity:
+//     Concat(x, Empty()) = x
+//
+// # Common Examples
+//
+//   - Integer addition: Concat = (+), Empty = 0
+//   - Integer multiplication: Concat = (*), Empty = 1
+//   - String concatenation: Concat = (++), Empty = ""
+//   - List concatenation: Concat = (++), Empty = []
+//   - Boolean AND: Concat = (&&), Empty = true
+//   - Boolean OR: Concat = (||), Empty = false
+//   - Function composition: Concat = (∘), Empty = id
+//
+// # References
+//
+//   - Haskell Data.Monoid: https://hackage.haskell.org/package/base/docs/Data-Monoid.html
+//   - Fantasy Land Monoid: https://github.com/fantasyland/fantasy-land#monoid
+//   - Semigroup: https://github.com/IBM/fp-go/v2/semigroup
+//
+// # Related Concepts
+//
+//   - [Semigroup]: A Monoid without the identity element requirement
+//   - Magma: A set with a binary operation (no laws required)
+//   - Group: A Monoid where every element has an inverse
 package monoid
 
 import (
@@ -21,20 +65,31 @@ import (
 
 // Monoid represents an algebraic structure with an associative binary operation and an identity element.
 //
-// A Monoid extends Semigroup by adding an identity element (Empty) that satisfies:
+// A Monoid extends [Semigroup] by adding an identity element (Empty) that satisfies:
 //   - Left identity: Concat(Empty(), x) = x
 //   - Right identity: Concat(x, Empty()) = x
 //
 // The Monoid must also satisfy the associativity law from Semigroup:
 //   - Associativity: Concat(Concat(x, y), z) = Concat(x, Concat(y, z))
 //
-// Common examples:
+// # Methods
+//
+//   - Concat(x, y A) A: Inherited from Semigroup, combines two values associatively
+//   - Empty() A: Returns the identity element for the monoid
+//
+// # Common Examples
+//
 //   - Integer addition with 0 as identity
 //   - Integer multiplication with 1 as identity
 //   - String concatenation with "" as identity
 //   - List concatenation with [] as identity
 //   - Boolean AND with true as identity
 //   - Boolean OR with false as identity
+//
+// # References
+//
+//   - Haskell Monoid typeclass: https://hackage.haskell.org/package/base/docs/Data-Monoid.html#t:Monoid
+//   - Fantasy Land Monoid specification: https://github.com/fantasyland/fantasy-land#monoid
 type Monoid[A any] interface {
 	S.Semigroup[A]
 	Empty() A
@@ -58,16 +113,22 @@ func (m monoid[A]) Empty() A {
 // The provided concat function must be associative, and the empty element must
 // satisfy the identity laws (left and right identity).
 //
-// Parameters:
-//   - c: An associative binary operation func(A, A) A
-//   - e: The identity element of type A
+// This is the primary constructor for creating custom monoid instances. It's the
+// equivalent of defining a Monoid instance in Haskell or implementing the Fantasy Land
+// Monoid specification.
 //
-// Returns:
-//   - A Monoid[A] instance
+// # Parameters
 //
-// Example:
+//   - c: An associative binary operation func(A, A) A (equivalent to Haskell's mappend or <>)
+//   - e: The identity element of type A (equivalent to Haskell's mempty)
 //
-//	// Integer addition monoid
+// # Returns
+//
+//   - A [Monoid][A] instance
+//
+// # Example
+//
+//	// Integer addition monoid (Sum in Haskell)
 //	addMonoid := MakeMonoid(
 //	    func(a, b int) int { return a + b },
 //	    0,  // identity element
@@ -81,6 +142,11 @@ func (m monoid[A]) Empty() A {
 //	    "",  // identity element
 //	)
 //	result := stringMonoid.Concat("Hello", " World")  // "Hello World"
+//
+// # References
+//
+//   - Haskell Monoid instance: https://hackage.haskell.org/package/base/docs/Data-Monoid.html#t:Monoid
+//   - Fantasy Land Monoid.empty: https://github.com/fantasyland/fantasy-land#monoid
 func MakeMonoid[A any](c func(A, A) A, e A) Monoid[A] {
 	return monoid[A]{c: c, e: e}
 }
@@ -91,13 +157,18 @@ func MakeMonoid[A any](c func(A, A) A, e A) Monoid[A] {
 // operation in the opposite order. This is useful for operations that are
 // not commutative.
 //
-// Parameters:
+// This corresponds to the Dual newtype wrapper in Haskell's Data.Monoid, which
+// provides a Monoid instance with reversed operation order.
+//
+// # Parameters
+//
 //   - m: The monoid to reverse
 //
-// Returns:
-//   - A new Monoid[A] with reversed operation order
+// # Returns
 //
-// Example:
+//   - A new [Monoid][A] with reversed operation order
+//
+// # Example
 //
 //	// Subtraction monoid (not commutative)
 //	subMonoid := MakeMonoid(
@@ -116,6 +187,10 @@ func MakeMonoid[A any](c func(A, A) A, e A) Monoid[A] {
 //	)
 //	reversed := Reverse(stringMonoid)
 //	result := reversed.Concat("Hello", "World")  // "WorldHello"
+//
+// # References
+//
+//   - Haskell Data.Monoid.Dual: https://hackage.haskell.org/package/base/docs/Data-Monoid.html#t:Dual
 func Reverse[A any](m Monoid[A]) Monoid[A] {
 	return MakeMonoid(S.Reverse(m).Concat, m.Empty())
 }
@@ -125,13 +200,19 @@ func Reverse[A any](m Monoid[A]) Monoid[A] {
 // This is useful when you need to use a monoid in a context that only requires
 // a semigroup (associative binary operation without identity).
 //
-// Parameters:
+// Since every Monoid is also a Semigroup (Monoid extends Semigroup), this conversion
+// is always safe. This reflects the mathematical relationship where monoids form a
+// subset of semigroups.
+//
+// # Parameters
+//
 //   - m: The monoid to convert
 //
-// Returns:
-//   - A Semigroup[A] that uses the same Concat operation
+// # Returns
 //
-// Example:
+//   - A [Semigroup][A] that uses the same Concat operation
+//
+// # Example
 //
 //	addMonoid := MakeMonoid(
 //	    func(a, b int) int { return a + b },
@@ -139,6 +220,11 @@ func Reverse[A any](m Monoid[A]) Monoid[A] {
 //	)
 //	sg := ToSemigroup(addMonoid)
 //	result := sg.Concat(5, 3)  // 8 (identity not available)
+//
+// # References
+//
+//   - Haskell Semigroup: https://hackage.haskell.org/package/base/docs/Data-Semigroup.html
+//   - Fantasy Land Semigroup: https://github.com/fantasyland/fantasy-land#semigroup
 func ToSemigroup[A any](m Monoid[A]) S.Semigroup[A] {
 	return S.Semigroup[A](m)
 }

v2/optics/codec/alt.go

@@ -0,0 +1,480 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/monoid"
+	"github.com/IBM/fp-go/v2/optics/codec/validate"
+	"github.com/IBM/fp-go/v2/reader"
+)
+
+// validateAlt creates a validation function that tries the first codec's validation,
+// and if it fails, tries the second codec's validation as a fallback.
+//
+// This is an internal helper function that implements the Alternative pattern for
+// codec validation. It combines two codec validators using the validate.Alt operation,
+// which provides error recovery and fallback logic.
+//
+// # Type Parameters
+//
+//   - A: The target type that both codecs decode to
+//   - O: The output type that both codecs encode to
+//   - I: The input type that both codecs decode from
+//
+// # Parameters
+//
+//   - first: The primary codec whose validation is tried first
+//   - second: A lazy codec that serves as the fallback. It's only evaluated if the
+//     first validation fails.
+//
+// # Returns
+//
+// A Validate[I, A] function that tries the first codec's validation, falling back
+// to the second if needed. If both fail, errors from both are aggregated.
+//
+// # Behavior
+//
+//   - **First succeeds**: Returns the first result, second is never evaluated
+//   - **First fails, second succeeds**: Returns the second result
+//   - **Both fail**: Aggregates errors from both validators
+//
+// # Notes
+//
+//   - The second codec is lazily evaluated for efficiency
+//   - This function is used internally by MonadAlt and Alt
+//   - The validation context is threaded through both validators
+//   - Errors are accumulated using the validation error monoid
+func validateAlt[A, O, I any](
+	first Type[A, O, I],
+	second Lazy[Type[A, O, I]],
+) Validate[I, A] {
+
+	return F.Pipe1(
+		first.Validate,
+		validate.Alt(F.Pipe1(
+			second,
+			lazy.Map(F.Flip(reader.Curry(Type[A, O, I].Validate))),
+		)),
+	)
+}
+
+// MonadAlt creates a new codec that tries the first codec, and if it fails during
+// validation, tries the second codec as a fallback.
+//
+// This function implements the Alternative typeclass pattern for codecs, enabling
+// "try this codec, or else try that codec" logic. It's particularly useful for:
+//   - Handling multiple valid input formats
+//   - Providing backward compatibility with legacy formats
+//   - Implementing graceful degradation in parsing
+//   - Supporting union types or polymorphic data
+//
+// The resulting codec uses the first codec's encoder and combines both validators
+// using the Alternative pattern. If both validations fail, errors from both are
+// aggregated for comprehensive error reporting.
+//
+// # Type Parameters
+//
+//   - A: The target type that both codecs decode to
+//   - O: The output type that both codecs encode to
+//   - I: The input type that both codecs decode from
+//
+// # Parameters
+//
+//   - first: The primary codec to try first. Its encoder is used for the result.
+//   - second: A lazy codec that serves as the fallback. It's only evaluated if the
+//     first validation fails.
+//
+// # Returns
+//
+// A new Type[A, O, I] that combines both codecs with Alternative semantics.
+//
+// # Behavior
+//
+// **Validation**:
+//   - **First succeeds**: Returns the first result, second is never evaluated
+//   - **First fails, second succeeds**: Returns the second result
+//   - **Both fail**: Aggregates errors from both validators
+//
+// **Encoding**:
+//   - Always uses the first codec's encoder
+//   - This assumes both codecs encode to the same output format
+//
+// **Type Checking**:
+//   - Uses the generic Is[A]() type checker
+//   - Validates that values are of type A
+//
+// # Example: Multiple Input Formats
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec"
+//	)
+//
+//	// Accept integers as either strings or numbers
+//	intFromString := codec.IntFromString()
+//	intFromNumber := codec.Int()
+//
+//	// Try parsing as string first, fall back to number
+//	flexibleInt := codec.MonadAlt(
+//	    intFromString,
+//	    func() codec.Type[int, any, any] { return intFromNumber },
+//	)
+//
+//	// Can now decode both "42" and 42
+//	result1 := flexibleInt.Decode("42")   // Success(42)
+//	result2 := flexibleInt.Decode(42)     // Success(42)
+//
+// # Example: Backward Compatibility
+//
+//	// Support both old and new configuration formats
+//	newConfigCodec := codec.Struct(/* new format */)
+//	oldConfigCodec := codec.Struct(/* old format */)
+//
+//	// Try new format first, fall back to old format
+//	configCodec := codec.MonadAlt(
+//	    newConfigCodec,
+//	    func() codec.Type[Config, any, any] { return oldConfigCodec },
+//	)
+//
+//	// Automatically handles both formats
+//	config := configCodec.Decode(input)
+//
+// # Example: Error Aggregation
+//
+//	// Both validations will fail for invalid input
+//	result := flexibleInt.Decode("not a number")
+//	// Result contains errors from both string and number parsing attempts
+//
+// # Notes
+//
+//   - The second codec is lazily evaluated for efficiency
+//   - First success short-circuits evaluation (second not called)
+//   - Errors are aggregated when both fail
+//   - The resulting codec's name is "Alt[<first codec name>]"
+//   - Both codecs must have compatible input and output types
+//   - The first codec's encoder is always used
+//
+// # See Also
+//
+//   - Alt: The curried, point-free version
+//   - validate.MonadAlt: The underlying validation operation
+//   - Either: For codecs that decode to Either[L, R] types
+func MonadAlt[A, O, I any](first Type[A, O, I], second Lazy[Type[A, O, I]]) Type[A, O, I] {
+	return MakeType(
+		fmt.Sprintf("Alt[%s]", first.Name()),
+		Is[A](),
+		validateAlt(first, second),
+		first.Encode,
+	)
+}
+
+// Alt creates an operator that adds alternative fallback logic to a codec.
+//
+// This is the curried, point-free version of MonadAlt. It returns a function that
+// can be applied to codecs to add fallback behavior. This style is particularly
+// useful for building codec transformation pipelines using function composition.
+//
+// Alt implements the Alternative typeclass pattern, enabling "try this codec, or
+// else try that codec" logic in a composable way.
+//
+// # Type Parameters
+//
+//   - A: The target type that both codecs decode to
+//   - O: The output type that both codecs encode to
+//   - I: The input type that both codecs decode from
+//
+// # Parameters
+//
+//   - second: A lazy codec that serves as the fallback. It's only evaluated if the
+//     first codec's validation fails.
+//
+// # Returns
+//
+// An Operator[A, A, O, I] that transforms codecs by adding alternative fallback logic.
+// This operator can be applied to any Type[A, O, I] to create a new codec with
+// fallback behavior.
+//
+// # Behavior
+//
+// When the returned operator is applied to a codec:
+//   - **First succeeds**: Returns the first result, second is never evaluated
+//   - **First fails, second succeeds**: Returns the second result
+//   - **Both fail**: Aggregates errors from both validators
+//
+// # Example: Point-Free Style
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    "github.com/IBM/fp-go/v2/optics/codec"
+//	)
+//
+//	// Create a reusable fallback operator
+//	withNumberFallback := codec.Alt(func() codec.Type[int, any, any] {
+//	    return codec.Int()
+//	})
+//
+//	// Apply it to different codecs
+//	flexibleInt1 := withNumberFallback(codec.IntFromString())
+//	flexibleInt2 := withNumberFallback(codec.IntFromHex())
+//
+// # Example: Pipeline Composition
+//
+//	// Build a codec pipeline with multiple fallbacks
+//	flexibleCodec := F.Pipe2(
+//	    primaryCodec,
+//	    codec.Alt(func() codec.Type[T, O, I] { return fallback1 }),
+//	    codec.Alt(func() codec.Type[T, O, I] { return fallback2 }),
+//	)
+//	// Tries primary, then fallback1, then fallback2
+//
+// # Example: Reusable Transformations
+//
+//	// Create a transformation that adds JSON fallback
+//	withJSONFallback := codec.Alt(func() codec.Type[Config, string, string] {
+//	    return codec.JSONCodec[Config]()
+//	})
+//
+//	// Apply to multiple codecs
+//	yamlWithFallback := withJSONFallback(yamlCodec)
+//	tomlWithFallback := withJSONFallback(tomlCodec)
+//
+// # Notes
+//
+//   - This is the point-free style version of MonadAlt
+//   - Useful for building transformation pipelines with F.Pipe
+//   - The second codec is lazily evaluated for efficiency
+//   - First success short-circuits evaluation
+//   - Errors are aggregated when both fail
+//   - Can be composed with other codec operators
+//
+// # See Also
+//
+//   - MonadAlt: The direct application version
+//   - validate.Alt: The underlying validation operation
+//   - F.Pipe: For composing multiple operators
+func Alt[A, O, I any](second Lazy[Type[A, O, I]]) Operator[A, A, O, I] {
+	return F.Bind2nd(MonadAlt, second)
+}
+
+// AltMonoid creates a Monoid instance for Type[A, O, I] using alternative semantics
+// with a provided zero/default codec.
+//
+// This function creates a monoid where:
+//  1. The first successful codec wins (no result combination)
+//  2. If the first fails during validation, the second is tried as a fallback
+//  3. If both fail, errors are aggregated
+//  4. The provided zero codec serves as the identity element
+//
+// Unlike other monoid patterns, AltMonoid does NOT combine successful results - it always
+// returns the first success. This makes it ideal for building fallback chains with default
+// codecs, configuration loading from multiple sources, and parser combinators with alternatives.
+//
+// # Type Parameters
+//
+//   - A: The target type that all codecs decode to
+//   - O: The output type that all codecs encode to
+//   - I: The input type that all codecs decode from
+//
+// # Parameters
+//
+//   - zero: A lazy Type[A, O, I] that serves as the identity element. This is typically
+//     a codec that always succeeds with a default value, but can also be a failing
+//     codec if no default is appropriate.
+//
+// # Returns
+//
+// A Monoid[Type[A, O, I]] that combines codecs using alternative semantics where
+// the first success wins.
+//
+// # Behavior Details
+//
+// The AltMonoid implements a "first success wins" strategy:
+//
+//   - **First succeeds**: Returns the first result, second is never evaluated
+//   - **First fails, second succeeds**: Returns the second result
+//   - **Both fail**: Aggregates errors from both validators
+//   - **Concat with Empty**: The zero codec is used as fallback
+//   - **Encoding**: Always uses the first codec's encoder
+//
+// # Example: Configuration Loading with Fallbacks
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec"
+//	    "github.com/IBM/fp-go/v2/array"
+//	)
+//
+//	// Create a monoid with a default configuration
+//	m := codec.AltMonoid(func() codec.Type[Config, string, string] {
+//	    return codec.MakeType(
+//	        "DefaultConfig",
+//	        codec.Is[Config](),
+//	        func(s string) codec.Decode[codec.Context, Config] {
+//	            return func(c codec.Context) codec.Validation[Config] {
+//	                return validation.Success(defaultConfig)
+//	            }
+//	        },
+//	        encodeConfig,
+//	    )
+//	})
+//
+//	// Define codecs for different sources
+//	fileCodec := loadFromFile("config.json")
+//	envCodec := loadFromEnv()
+//	defaultCodec := m.Empty()
+//
+//	// Try file, then env, then default
+//	configCodec := array.MonadFold(
+//	    []codec.Type[Config, string, string]{fileCodec, envCodec, defaultCodec},
+//	    m.Empty(),
+//	    m.Concat,
+//	)
+//
+//	// Load configuration - tries each source in order
+//	result := configCodec.Decode(input)
+//
+// # Example: Parser with Multiple Formats
+//
+//	// Create a monoid for parsing dates in multiple formats
+//	m := codec.AltMonoid(func() codec.Type[time.Time, string, string] {
+//	    return codec.Date(time.RFC3339) // default format
+//	})
+//
+//	// Define parsers for different date formats
+//	iso8601 := codec.Date("2006-01-02")
+//	usFormat := codec.Date("01/02/2006")
+//	euroFormat := codec.Date("02/01/2006")
+//
+//	// Combine: try ISO 8601, then US, then European, then RFC3339
+//	flexibleDate := m.Concat(
+//	    m.Concat(
+//	        m.Concat(iso8601, usFormat),
+//	        euroFormat,
+//	    ),
+//	    m.Empty(),
+//	)
+//
+//	// Can parse any of these formats
+//	result1 := flexibleDate.Decode("2024-03-15")      // ISO 8601
+//	result2 := flexibleDate.Decode("03/15/2024")      // US format
+//	result3 := flexibleDate.Decode("15/03/2024")      // European format
+//
+// # Example: Integer Parsing with Default
+//
+//	// Create a monoid with default value of 0
+//	m := codec.AltMonoid(func() codec.Type[int, string, string] {
+//	    return codec.MakeType(
+//	        "DefaultZero",
+//	        codec.Is[int](),
+//	        func(s string) codec.Decode[codec.Context, int] {
+//	            return func(c codec.Context) codec.Validation[int] {
+//	                return validation.Success(0)
+//	            }
+//	        },
+//	        strconv.Itoa,
+//	    )
+//	})
+//
+//	// Try parsing as int, fall back to 0
+//	intOrZero := m.Concat(codec.IntFromString(), m.Empty())
+//
+//	result1 := intOrZero.Decode("42")        // Success(42)
+//	result2 := intOrZero.Decode("invalid")   // Success(0) - uses default
+//
+// # Example: Error Aggregation
+//
+//	// Both codecs fail - errors are aggregated
+//	m := codec.AltMonoid(func() codec.Type[int, string, string] {
+//	    return codec.MakeType(
+//	        "NoDefault",
+//	        codec.Is[int](),
+//	        func(s string) codec.Decode[codec.Context, int] {
+//	            return func(c codec.Context) codec.Validation[int] {
+//	                return validation.FailureWithMessage[int](s, "no default available")(c)
+//	            }
+//	        },
+//	        strconv.Itoa,
+//	    )
+//	})
+//
+//	failing1 := codec.MakeType(
+//	    "Failing1",
+//	    codec.Is[int](),
+//	    func(s string) codec.Decode[codec.Context, int] {
+//	        return func(c codec.Context) codec.Validation[int] {
+//	            return validation.FailureWithMessage[int](s, "error 1")(c)
+//	        }
+//	    },
+//	    strconv.Itoa,
+//	)
+//
+//	failing2 := codec.MakeType(
+//	    "Failing2",
+//	    codec.Is[int](),
+//	    func(s string) codec.Decode[codec.Context, int] {
+//	        return func(c codec.Context) codec.Validation[int] {
+//	            return validation.FailureWithMessage[int](s, "error 2")(c)
+//	        }
+//	    },
+//	    strconv.Itoa,
+//	)
+//
+//	combined := m.Concat(failing1, failing2)
+//	result := combined.Decode("input")
+//	// result contains errors: "error 1", "error 2", and "no default available"
+//
+// # Monoid Laws
+//
+// AltMonoid satisfies the monoid laws:
+//
+//  1. **Left Identity**: m.Concat(m.Empty(), codec) ≡ codec
+//  2. **Right Identity**: m.Concat(codec, m.Empty()) ≡ codec (tries codec first, falls back to zero)
+//  3. **Associativity**: m.Concat(m.Concat(a, b), c) ≡ m.Concat(a, m.Concat(b, c))
+//
+// Note: Due to the "first success wins" behavior, right identity means the zero is only
+// used if the codec fails.
+//
+// # Use Cases
+//
+//   - Configuration loading with multiple sources (file, env, default)
+//   - Parsing data in multiple formats with fallbacks
+//   - API versioning (try v2, fall back to v1, then default)
+//   - Content negotiation (try JSON, then XML, then plain text)
+//   - Validation with default values
+//   - Parser combinators with alternative branches
+//
+// # Notes
+//
+//   - The zero codec is lazily evaluated, only when needed
+//   - First success short-circuits evaluation (subsequent codecs not tried)
+//   - Error aggregation ensures all validation failures are reported
+//   - Encoding always uses the first codec's encoder
+//   - This follows the alternative functor laws
+//
+// # See Also
+//
+//   - MonadAlt: The underlying alternative operation for two codecs
+//   - Alt: The curried version for pipeline composition
+//   - validate.AltMonoid: The validation-level alternative monoid
+//   - decode.AltMonoid: The decode-level alternative monoid
+func AltMonoid[A, O, I any](zero Lazy[Type[A, O, I]]) Monoid[Type[A, O, I]] {
+	return monoid.AltMonoid(
+		zero,
+		MonadAlt[A, O, I],
+	)
+}

v2/optics/codec/alt_test.go

@@ -0,0 +1,921 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+	"strconv"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestMonadAltBasicFunctionality tests the basic behavior of MonadAlt
+func TestMonadAltBasicFunctionality(t *testing.T) {
+	t.Run("uses first codec when it succeeds", func(t *testing.T) {
+		// Create two codecs that both work with strings
+		stringCodec := Id[string]()
+
+		// Create another string codec that only accepts uppercase
+		uppercaseOnly := MakeType(
+			"UppercaseOnly",
+			Is[string](),
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					for _, r := range s {
+						if r >= 'a' && r <= 'z' {
+							return validation.FailureWithMessage[string](s, "must be uppercase")(c)
+						}
+					}
+					return validation.Success(s)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Create alt codec that tries uppercase first, then any string
+		altCodec := MonadAlt(
+			uppercaseOnly,
+			func() Type[string, string, string] { return stringCodec },
+		)
+
+		// Test with uppercase string - should succeed with first codec
+		result := altCodec.Decode("HELLO")
+
+		assert.True(t, either.IsRight(result), "should successfully decode with first codec")
+
+		value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+		assert.Equal(t, "HELLO", value)
+	})
+
+	t.Run("falls back to second codec when first fails", func(t *testing.T) {
+		// Create a codec that only accepts positive integers
+		positiveInt := MakeType(
+			"PositiveInt",
+			Is[int](),
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					if i <= 0 {
+						return validation.FailureWithMessage[int](i, "must be positive")(c)
+					}
+					return validation.Success(i)
+				}
+			},
+			F.Identity[int],
+		)
+
+		// Create a codec that accepts any integer (with same input type)
+		anyInt := MakeType(
+			"AnyInt",
+			Is[int](),
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.Success(i)
+				}
+			},
+			F.Identity[int],
+		)
+
+		// Create alt codec
+		altCodec := MonadAlt(
+			positiveInt,
+			func() Type[int, int, int] { return anyInt },
+		)
+
+		// Test with negative number - first fails, second succeeds
+		result := altCodec.Decode(-5)
+
+		assert.True(t, either.IsRight(result), "should successfully decode with second codec")
+
+		value := either.GetOrElse(reader.Of[validation.Errors, int](0))(result)
+		assert.Equal(t, -5, value)
+	})
+
+	t.Run("aggregates errors when both codecs fail", func(t *testing.T) {
+		// Create two codecs that will both fail
+		positiveInt := MakeType(
+			"PositiveInt",
+			Is[int](),
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					if i <= 0 {
+						return validation.FailureWithMessage[int](i, "must be positive")(c)
+					}
+					return validation.Success(i)
+				}
+			},
+			F.Identity[int],
+		)
+
+		evenInt := MakeType(
+			"EvenInt",
+			Is[int](),
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					if i%2 != 0 {
+						return validation.FailureWithMessage[int](i, "must be even")(c)
+					}
+					return validation.Success(i)
+				}
+			},
+			F.Identity[int],
+		)
+
+		// Create alt codec
+		altCodec := MonadAlt(
+			positiveInt,
+			func() Type[int, int, int] { return evenInt },
+		)
+
+		// Test with -3 (negative and odd) - both should fail
+		result := altCodec.Decode(-3)
+
+		assert.True(t, either.IsLeft(result), "should fail when both codecs fail")
+
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(int) validation.Errors { return nil },
+		)
+
+		require.NotNil(t, errors)
+		// Should have errors from both validation attempts
+		assert.GreaterOrEqual(t, len(errors), 2, "should have errors from both codecs")
+	})
+}
+
+// TestMonadAltNaming tests that the codec name is correctly generated
+func TestMonadAltNaming(t *testing.T) {
+	t.Run("generates correct name", func(t *testing.T) {
+		stringCodec := Id[string]()
+		anotherStringCodec := Id[string]()
+
+		altCodec := MonadAlt(
+			stringCodec,
+			func() Type[string, string, string] { return anotherStringCodec },
+		)
+
+		assert.Equal(t, "Alt[string]", altCodec.Name())
+	})
+}
+
+// TestMonadAltEncoding tests that encoding uses the first codec's encoder
+func TestMonadAltEncoding(t *testing.T) {
+	t.Run("uses first codec's encoder", func(t *testing.T) {
+		// Create a codec that encodes ints as strings with prefix
+		prefixedInt := MakeType(
+			"PrefixedInt",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					var n int
+					_, err := fmt.Sscanf(s, "NUM:%d", &n)
+					if err != nil {
+						return validation.FailureWithError[int](s, "expected NUM:n format")(err)(c)
+					}
+					return validation.Success(n)
+				}
+			},
+			func(n int) string {
+				return fmt.Sprintf("NUM:%d", n)
+			},
+		)
+
+		// Create a standard int from string codec
+		standardInt := IntFromString()
+
+		// Create alt codec
+		altCodec := MonadAlt(
+			prefixedInt,
+			func() Type[int, string, string] { return standardInt },
+		)
+
+		// Encode should use first codec's encoder
+		encoded := altCodec.Encode(42)
+		assert.Equal(t, "NUM:42", encoded)
+	})
+}
+
+// TestAltOperator tests the curried Alt function
+func TestAltOperator(t *testing.T) {
+	t.Run("creates reusable operator", func(t *testing.T) {
+		// Create a fallback operator that accepts any string
+		withStringFallback := Alt(func() Type[string, string, string] {
+			return Id[string]()
+		})
+
+		// Create a codec that only accepts "hello"
+		helloOnly := MakeType(
+			"HelloOnly",
+			Is[string](),
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if s != "hello" {
+						return validation.FailureWithMessage[string](s, "must be 'hello'")(c)
+					}
+					return validation.Success(s)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply fallback to the codec
+		altCodec := withStringFallback(helloOnly)
+
+		// Test that it works
+		result1 := altCodec.Decode("hello")
+		assert.True(t, either.IsRight(result1))
+
+		result2 := altCodec.Decode("world")
+		assert.True(t, either.IsRight(result2))
+	})
+
+	t.Run("works in pipeline with F.Pipe", func(t *testing.T) {
+		// Create a codec pipeline with multiple fallbacks
+		baseCodec := MakeType(
+			"StrictInt",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					if s != "42" {
+						return validation.FailureWithMessage[int](s, "must be exactly '42'")(c)
+					}
+					return validation.Success(42)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		fallback1 := MakeType(
+			"Fallback1",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					if s != "100" {
+						return validation.FailureWithMessage[int](s, "must be exactly '100'")(c)
+					}
+					return validation.Success(100)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		fallback2 := MakeType(
+			"AnyInt",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					n, err := strconv.Atoi(s)
+					if err != nil {
+						return validation.FailureWithError[int](s, "not an integer")(err)(c)
+					}
+					return validation.Success(n)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		// Build pipeline with multiple alternatives
+		pipeline := F.Pipe2(
+			baseCodec,
+			Alt(func() Type[int, string, string] { return fallback1 }),
+			Alt(func() Type[int, string, string] { return fallback2 }),
+		)
+
+		// Test with "42" - should use base codec
+		result1 := pipeline.Decode("42")
+		assert.True(t, either.IsRight(result1))
+		value1 := either.GetOrElse(reader.Of[validation.Errors, int](0))(result1)
+		assert.Equal(t, 42, value1)
+
+		// Test with "100" - should use fallback1
+		result2 := pipeline.Decode("100")
+		assert.True(t, either.IsRight(result2))
+		value2 := either.GetOrElse(reader.Of[validation.Errors, int](0))(result2)
+		assert.Equal(t, 100, value2)
+
+		// Test with "999" - should use fallback2
+		result3 := pipeline.Decode("999")
+		assert.True(t, either.IsRight(result3))
+		value3 := either.GetOrElse(reader.Of[validation.Errors, int](0))(result3)
+		assert.Equal(t, 999, value3)
+	})
+}
+
+// TestAltLazyEvaluation tests that the second codec is only evaluated when needed
+func TestAltLazyEvaluation(t *testing.T) {
+	t.Run("does not evaluate second codec when first succeeds", func(t *testing.T) {
+		evaluated := false
+
+		stringCodec := Id[string]()
+
+		altCodec := MonadAlt(
+			stringCodec,
+			func() Type[string, string, string] {
+				evaluated = true
+				return Id[string]()
+			},
+		)
+
+		// Decode with first codec succeeding
+		result := altCodec.Decode("hello")
+		assert.True(t, either.IsRight(result))
+
+		// Second codec should not have been evaluated
+		assert.False(t, evaluated, "second codec should not be evaluated when first succeeds")
+	})
+
+	t.Run("evaluates second codec when first fails", func(t *testing.T) {
+		evaluated := false
+
+		// Create a codec that always fails
+		failingCodec := MakeType(
+			"Failing",
+			Is[string](),
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					return validation.FailureWithMessage[string](s, "always fails")(c)
+				}
+			},
+			F.Identity[string],
+		)
+
+		altCodec := MonadAlt(
+			failingCodec,
+			func() Type[string, string, string] {
+				evaluated = true
+				return Id[string]()
+			},
+		)
+
+		// Decode with first codec failing
+		result := altCodec.Decode("hello")
+		assert.True(t, either.IsRight(result))
+
+		// Second codec should have been evaluated
+		assert.True(t, evaluated, "second codec should be evaluated when first fails")
+	})
+}
+
+// TestAltWithComplexTypes tests Alt with more complex codec scenarios
+func TestAltWithComplexTypes(t *testing.T) {
+	t.Run("works with string length validation", func(t *testing.T) {
+		// Create codec that accepts strings of length 5
+		length5 := MakeType(
+			"Length5",
+			Is[string](),
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if len(s) != 5 {
+						return validation.FailureWithMessage[string](s, "must be length 5")(c)
+					}
+					return validation.Success(s)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Create codec that accepts any string
+		anyString := Id[string]()
+
+		// Create alt codec
+		altCodec := MonadAlt(
+			length5,
+			func() Type[string, string, string] { return anyString },
+		)
+
+		// Test with length 5 - should use first codec
+		result1 := altCodec.Decode("hello")
+		assert.True(t, either.IsRight(result1))
+
+		// Test with different length - should fall back to second codec
+		result2 := altCodec.Decode("hi")
+		assert.True(t, either.IsRight(result2))
+	})
+}
+
+// TestAltTypeChecking tests that type checking works correctly
+func TestAltTypeChecking(t *testing.T) {
+	t.Run("type checking uses generic Is", func(t *testing.T) {
+		stringCodec := Id[string]()
+		anotherStringCodec := Id[string]()
+
+		altCodec := MonadAlt(
+			stringCodec,
+			func() Type[string, string, string] { return anotherStringCodec },
+		)
+
+		// Test Is with valid type
+		result1 := altCodec.Is("hello")
+		assert.True(t, either.IsRight(result1))
+
+		// Test Is with invalid type
+		result2 := altCodec.Is(42)
+		assert.True(t, either.IsLeft(result2))
+	})
+}
+
+// TestAltRoundTrip tests encoding and decoding round trips
+func TestAltRoundTrip(t *testing.T) {
+	t.Run("round-trip with first codec", func(t *testing.T) {
+		stringCodec := Id[string]()
+		anotherStringCodec := Id[string]()
+
+		altCodec := MonadAlt(
+			stringCodec,
+			func() Type[string, string, string] { return anotherStringCodec },
+		)
+
+		original := "hello"
+
+		// Decode
+		decodeResult := altCodec.Decode(original)
+		require.True(t, either.IsRight(decodeResult))
+
+		decoded := either.GetOrElse(reader.Of[validation.Errors, string](""))(decodeResult)
+
+		// Encode
+		encoded := altCodec.Encode(decoded)
+
+		// Verify
+		assert.Equal(t, original, encoded)
+	})
+
+	t.Run("round-trip with second codec", func(t *testing.T) {
+		// Create a codec that only accepts "hello"
+		helloOnly := MakeType(
+			"HelloOnly",
+			Is[string](),
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if s != "hello" {
+						return validation.FailureWithMessage[string](s, "must be 'hello'")(c)
+					}
+					return validation.Success(s)
+				}
+			},
+			F.Identity[string],
+		)
+
+		anyString := Id[string]()
+
+		altCodec := MonadAlt(
+			helloOnly,
+			func() Type[string, string, string] { return anyString },
+		)
+
+		original := "world"
+
+		// Decode (will use second codec)
+		decodeResult := altCodec.Decode(original)
+		require.True(t, either.IsRight(decodeResult))
+
+		decoded := either.GetOrElse(reader.Of[validation.Errors, string](""))(decodeResult)
+
+		// Encode (uses first codec's encoder, which is identity)
+		encoded := altCodec.Encode(decoded)
+
+		// Verify
+		assert.Equal(t, original, encoded)
+	})
+}
+
+// TestAltErrorMessages tests that error messages are informative
+func TestAltErrorMessages(t *testing.T) {
+	t.Run("provides detailed error context", func(t *testing.T) {
+		// Create two codecs with specific error messages
+		codec1 := MakeType(
+			"Codec1",
+			Is[int](),
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.FailureWithMessage[int](i, "codec1 error")(c)
+				}
+			},
+			F.Identity[int],
+		)
+
+		codec2 := MakeType(
+			"Codec2",
+			Is[int](),
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.FailureWithMessage[int](i, "codec2 error")(c)
+				}
+			},
+			F.Identity[int],
+		)
+
+		altCodec := MonadAlt(
+			codec1,
+			func() Type[int, int, int] { return codec2 },
+		)
+
+		result := altCodec.Decode(42)
+
+		assert.True(t, either.IsLeft(result))
+
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(int) validation.Errors { return nil },
+		)
+
+		require.NotNil(t, errors)
+		require.GreaterOrEqual(t, len(errors), 2)
+
+		// Check that both error messages are present
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+
+		hasCodec1Error := false
+		hasCodec2Error := false
+		for _, msg := range messages {
+			if msg == "codec1 error" {
+				hasCodec1Error = true
+			}
+			if msg == "codec2 error" {
+				hasCodec2Error = true
+			}
+		}
+
+		assert.True(t, hasCodec1Error, "should have error from first codec")
+		assert.True(t, hasCodec2Error, "should have error from second codec")
+	})
+}
+
+// TestAltMonoid tests the AltMonoid function
+func TestAltMonoid(t *testing.T) {
+	t.Run("with default value as zero", func(t *testing.T) {
+		// Create a monoid with a default value of 0
+		m := AltMonoid(func() Type[int, string, string] {
+			return MakeType(
+				"DefaultZero",
+				Is[int](),
+				func(s string) Decode[Context, int] {
+					return func(c Context) Validation[int] {
+						return validation.Success(0)
+					}
+				},
+				strconv.Itoa,
+			)
+		})
+
+		// Create codecs
+		intFromString := IntFromString()
+		failing := MakeType(
+			"Failing",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.FailureWithMessage[int](s, "always fails")(c)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		t.Run("first success wins", func(t *testing.T) {
+			// Combine two successful codecs - first should win
+			codec1 := MakeType(
+				"Returns10",
+				Is[int](),
+				func(s string) Decode[Context, int] {
+					return func(c Context) Validation[int] {
+						return validation.Success(10)
+					}
+				},
+				strconv.Itoa,
+			)
+			codec2 := MakeType(
+				"Returns20",
+				Is[int](),
+				func(s string) Decode[Context, int] {
+					return func(c Context) Validation[int] {
+						return validation.Success(20)
+					}
+				},
+				strconv.Itoa,
+			)
+
+			combined := m.Concat(codec1, codec2)
+			result := combined.Decode("input")
+
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, int](0))(result)
+			assert.Equal(t, 10, value, "first success should win")
+		})
+
+		t.Run("falls back to second when first fails", func(t *testing.T) {
+			combined := m.Concat(failing, intFromString)
+			result := combined.Decode("42")
+
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, int](0))(result)
+			assert.Equal(t, 42, value)
+		})
+
+		t.Run("uses zero when both fail", func(t *testing.T) {
+			combined := m.Concat(failing, m.Empty())
+			result := combined.Decode("invalid")
+
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, int](-1))(result)
+			assert.Equal(t, 0, value, "should use default zero value")
+		})
+	})
+
+	t.Run("with failing zero", func(t *testing.T) {
+		// Create a monoid with a failing zero
+		m := AltMonoid(func() Type[int, string, string] {
+			return MakeType(
+				"NoDefault",
+				Is[int](),
+				func(s string) Decode[Context, int] {
+					return func(c Context) Validation[int] {
+						return validation.FailureWithMessage[int](s, "no default available")(c)
+					}
+				},
+				strconv.Itoa,
+			)
+		})
+
+		failing1 := MakeType(
+			"Failing1",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.FailureWithMessage[int](s, "error 1")(c)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		failing2 := MakeType(
+			"Failing2",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.FailureWithMessage[int](s, "error 2")(c)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		t.Run("aggregates all errors when all fail", func(t *testing.T) {
+			combined := m.Concat(m.Concat(failing1, failing2), m.Empty())
+			result := combined.Decode("input")
+
+			assert.True(t, either.IsLeft(result))
+
+			errors := either.MonadFold(result,
+				F.Identity[validation.Errors],
+				func(int) validation.Errors { return nil },
+			)
+
+			require.NotNil(t, errors)
+			// Should have errors from all three: failing1, failing2, and zero
+			assert.GreaterOrEqual(t, len(errors), 3)
+
+			messages := make([]string, len(errors))
+			for i, err := range errors {
+				messages[i] = err.Messsage
+			}
+
+			hasError1 := false
+			hasError2 := false
+			hasNoDefault := false
+			for _, msg := range messages {
+				if msg == "error 1" {
+					hasError1 = true
+				}
+				if msg == "error 2" {
+					hasError2 = true
+				}
+				if msg == "no default available" {
+					hasNoDefault = true
+				}
+			}
+
+			assert.True(t, hasError1, "should have error from failing1")
+			assert.True(t, hasError2, "should have error from failing2")
+			assert.True(t, hasNoDefault, "should have error from zero")
+		})
+	})
+
+	t.Run("chaining multiple fallbacks", func(t *testing.T) {
+		m := AltMonoid(func() Type[string, string, string] {
+			return MakeType(
+				"Default",
+				Is[string](),
+				func(s string) Decode[Context, string] {
+					return func(c Context) Validation[string] {
+						return validation.Success("default")
+					}
+				},
+				F.Identity[string],
+			)
+		})
+
+		primary := MakeType(
+			"Primary",
+			Is[string](),
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if s == "primary" {
+						return validation.Success("from primary")
+					}
+					return validation.FailureWithMessage[string](s, "not primary")(c)
+				}
+			},
+			F.Identity[string],
+		)
+
+		secondary := MakeType(
+			"Secondary",
+			Is[string](),
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if s == "secondary" {
+						return validation.Success("from secondary")
+					}
+					return validation.FailureWithMessage[string](s, "not secondary")(c)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Chain: try primary, then secondary, then default
+		combined := m.Concat(m.Concat(primary, secondary), m.Empty())
+
+		t.Run("uses primary when it succeeds", func(t *testing.T) {
+			result := combined.Decode("primary")
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+			assert.Equal(t, "from primary", value)
+		})
+
+		t.Run("uses secondary when primary fails", func(t *testing.T) {
+			result := combined.Decode("secondary")
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+			assert.Equal(t, "from secondary", value)
+		})
+
+		t.Run("uses default when both fail", func(t *testing.T) {
+			result := combined.Decode("other")
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+			assert.Equal(t, "default", value)
+		})
+	})
+
+	t.Run("satisfies monoid laws", func(t *testing.T) {
+		m := AltMonoid(func() Type[int, string, string] {
+			return MakeType(
+				"DefaultZero",
+				Is[int](),
+				func(s string) Decode[Context, int] {
+					return func(c Context) Validation[int] {
+						return validation.Success(0)
+					}
+				},
+				strconv.Itoa,
+			)
+		})
+
+		codec1 := MakeType(
+			"Codec1",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.Success(10)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		codec2 := MakeType(
+			"Codec2",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.Success(20)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		codec3 := MakeType(
+			"Codec3",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.Success(30)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		t.Run("left identity", func(t *testing.T) {
+			// m.Concat(m.Empty(), codec) should behave like codec
+			// But with AltMonoid, if codec fails, it falls back to empty
+			combined := m.Concat(m.Empty(), codec1)
+			result := combined.Decode("input")
+
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, int](-1))(result)
+			// Empty (0) comes first, so it wins
+			assert.Equal(t, 0, value)
+		})
+
+		t.Run("right identity", func(t *testing.T) {
+			// m.Concat(codec, m.Empty()) tries codec first, falls back to empty
+			combined := m.Concat(codec1, m.Empty())
+			result := combined.Decode("input")
+
+			assert.True(t, either.IsRight(result))
+			value := either.GetOrElse(reader.Of[validation.Errors, int](-1))(result)
+			assert.Equal(t, 10, value, "codec1 should win")
+		})
+
+		t.Run("associativity", func(t *testing.T) {
+			// For AltMonoid, first success wins
+			left := m.Concat(m.Concat(codec1, codec2), codec3)
+			right := m.Concat(codec1, m.Concat(codec2, codec3))
+
+			resultLeft := left.Decode("input")
+			resultRight := right.Decode("input")
+
+			assert.True(t, either.IsRight(resultLeft))
+			assert.True(t, either.IsRight(resultRight))
+
+			valueLeft := either.GetOrElse(reader.Of[validation.Errors, int](-1))(resultLeft)
+			valueRight := either.GetOrElse(reader.Of[validation.Errors, int](-1))(resultRight)
+
+			// Both should return 10 (first success)
+			assert.Equal(t, valueLeft, valueRight)
+			assert.Equal(t, 10, valueLeft)
+		})
+	})
+
+	t.Run("encoding uses first codec", func(t *testing.T) {
+		m := AltMonoid(func() Type[int, string, string] {
+			return MakeType(
+				"Default",
+				Is[int](),
+				func(s string) Decode[Context, int] {
+					return func(c Context) Validation[int] {
+						return validation.Success(0)
+					}
+				},
+				func(n int) string { return "DEFAULT" },
+			)
+		})
+
+		codec1 := MakeType(
+			"Codec1",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.Success(42)
+				}
+			},
+			func(n int) string { return fmt.Sprintf("FIRST:%d", n) },
+		)
+
+		codec2 := MakeType(
+			"Codec2",
+			Is[int](),
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					return validation.Success(100)
+				}
+			},
+			func(n int) string { return fmt.Sprintf("SECOND:%d", n) },
+		)
+
+		combined := m.Concat(codec1, codec2)
+
+		// Encoding should use first codec's encoder
+		encoded := combined.Encode(42)
+		assert.Equal(t, "FIRST:42", encoded)
+	})
+}

v2/optics/codec/codec_test.go

@@ -710,6 +710,146 @@ func TestTranscodeEither(t *testing.T) {
 	})
 }
 
+func TestTranscodeEitherValidation(t *testing.T) {
+	t.Run("validates Left value with context", func(t *testing.T) {
+		eitherCodec := TranscodeEither(String(), Int())
+		result := eitherCodec.Decode(either.Left[any, any](123)) // Invalid: should be string
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(either.Either[string, int]) validation.Errors { return nil },
+		)
+		assert.NotEmpty(t, errors)
+		// Verify error contains type information
+		assert.Contains(t, fmt.Sprintf("%v", errors[0]), "string")
+	})
+
+	t.Run("validates Right value with context", func(t *testing.T) {
+		eitherCodec := TranscodeEither(String(), Int())
+		result := eitherCodec.Decode(either.Right[any, any]("not a number"))
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(either.Either[string, int]) validation.Errors { return nil },
+		)
+		assert.NotEmpty(t, errors)
+		// Verify error contains type information
+		assert.Contains(t, fmt.Sprintf("%v", errors[0]), "int")
+	})
+
+	t.Run("preserves Either structure on validation failure", func(t *testing.T) {
+		eitherCodec := TranscodeEither(String(), Int())
+
+		// Left with wrong type
+		leftResult := eitherCodec.Decode(either.Left[any, any]([]int{1, 2, 3}))
+		assert.True(t, either.IsLeft(leftResult))
+
+		// Right with wrong type
+		rightResult := eitherCodec.Decode(either.Right[any, any](true))
+		assert.True(t, either.IsLeft(rightResult))
+	})
+
+	t.Run("validates with custom codec that can fail", func(t *testing.T) {
+		// Create a codec that only accepts positive integers
+		positiveInt := MakeType(
+			"PositiveInt",
+			func(u any) either.Either[error, int] {
+				i, ok := u.(int)
+				if !ok || i <= 0 {
+					return either.Left[int](fmt.Errorf("not a positive integer"))
+				}
+				return either.Of[error](i)
+			},
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					if i <= 0 {
+						return validation.FailureWithMessage[int](i, "must be positive")(c)
+					}
+					return validation.Success(i)
+				}
+			},
+			F.Identity[int],
+		)
+
+		eitherCodec := TranscodeEither(String(), positiveInt)
+
+		// Valid positive integer
+		validResult := eitherCodec.Decode(either.Right[any](42))
+		assert.True(t, either.IsRight(validResult))
+
+		// Invalid: zero
+		zeroResult := eitherCodec.Decode(either.Right[any](0))
+		assert.True(t, either.IsLeft(zeroResult))
+
+		// Invalid: negative
+		negativeResult := eitherCodec.Decode(either.Right[any](-5))
+		assert.True(t, either.IsLeft(negativeResult))
+	})
+
+	t.Run("validates both branches independently", func(t *testing.T) {
+		// Create codecs with specific validation rules
+		nonEmptyString := MakeType(
+			"NonEmptyString",
+			func(u any) either.Either[error, string] {
+				s, ok := u.(string)
+				if !ok || len(s) == 0 {
+					return either.Left[string](fmt.Errorf("not a non-empty string"))
+				}
+				return either.Of[error](s)
+			},
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if len(s) == 0 {
+						return validation.FailureWithMessage[string](s, "must not be empty")(c)
+					}
+					return validation.Success(s)
+				}
+			},
+			F.Identity[string],
+		)
+
+		evenInt := MakeType(
+			"EvenInt",
+			func(u any) either.Either[error, int] {
+				i, ok := u.(int)
+				if !ok || i%2 != 0 {
+					return either.Left[int](fmt.Errorf("not an even integer"))
+				}
+				return either.Of[error](i)
+			},
+			func(i int) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					if i%2 != 0 {
+						return validation.FailureWithMessage[int](i, "must be even")(c)
+					}
+					return validation.Success(i)
+				}
+			},
+			F.Identity[int],
+		)
+
+		eitherCodec := TranscodeEither(nonEmptyString, evenInt)
+
+		// Valid Left: non-empty string
+		validLeft := eitherCodec.Decode(either.Left[int]("hello"))
+		assert.True(t, either.IsRight(validLeft))
+
+		// Invalid Left: empty string
+		invalidLeft := eitherCodec.Decode(either.Left[int](""))
+		assert.True(t, either.IsLeft(invalidLeft))
+
+		// Valid Right: even integer
+		validRight := eitherCodec.Decode(either.Right[string](42))
+		assert.True(t, either.IsRight(validRight))
+
+		// Invalid Right: odd integer
+		invalidRight := eitherCodec.Decode(either.Right[string](43))
+		assert.True(t, either.IsLeft(invalidRight))
+	})
+}
+
 func TestTranscodeEitherWithTransformation(t *testing.T) {
 	// Create a codec that transforms strings to their lengths
 	stringToLength := MakeType(

v2/optics/codec/decode/OrElse_ChainLeft_explanation.md

@@ -0,0 +1,321 @@
+# ChainLeft and OrElse in the Decode Package
+
+## Overview
+
+In [`optics/codec/decode/monad.go`](monad.go:53-62), the [`ChainLeft`](monad.go:53) and [`OrElse`](monad.go:60) functions work with decoders that may fail during decoding operations.
+
+```go
+func ChainLeft[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return readert.Chain[Decode[I, A]](
+		validation.ChainLeft,
+		f,
+	)
+}
+
+func OrElse[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return ChainLeft(f)
+}
+```
+
+## Key Insight: OrElse is ChainLeft
+
+**`OrElse` is exactly the same as `ChainLeft`** - they are aliases with identical implementations and behavior. The choice between them is purely about **code readability and semantic intent**.
+
+## Understanding the Types
+
+### Decode[I, A]
+A decoder that takes input of type `I` and produces a `Validation[A]`:
+```go
+type Decode[I, A any] = func(I) Validation[A]
+```
+
+### Kleisli[I, Errors, A]
+A function that takes `Errors` and produces a `Decode[I, A]`:
+```go
+type Kleisli[I, Errors, A] = func(Errors) Decode[I, A]
+```
+
+This allows error handlers to:
+1. Access the validation errors that occurred
+2. Access the original input (via the returned Decode function)
+3. Either recover with a success value or produce new errors
+
+### Operator[I, A, A]
+A function that transforms one decoder into another:
+```go
+type Operator[I, A, A] = func(Decode[I, A]) Decode[I, A]
+```
+
+## Core Behavior
+
+Both [`ChainLeft`](monad.go:53) and [`OrElse`](monad.go:60) delegate to [`validation.ChainLeft`](../validation/monad.go:304), which provides:
+
+### 1. Error Aggregation
+When the transformation function returns a failure, **both the original errors AND the new errors are combined** using the Errors monoid:
+
+```go
+failingDecoder := func(input string) Validation[int] {
+    return either.Left[int](validation.Errors{
+        {Value: input, Messsage: "original error"},
+    })
+}
+
+handler := ChainLeft(func(errs Errors) Decode[string, int] {
+    return func(input string) Validation[int] {
+        return either.Left[int](validation.Errors{
+            {Messsage: "additional error"},
+        })
+    }
+})
+
+decoder := handler(failingDecoder)
+result := decoder("input")
+// Result contains BOTH errors: ["original error", "additional error"]
+```
+
+### 2. Success Pass-Through
+Success values pass through unchanged - the handler is never called:
+
+```go
+successDecoder := Of[string](42)
+
+handler := ChainLeft(func(errs Errors) Decode[string, int] {
+    return func(input string) Validation[int] {
+        return either.Left[int](validation.Errors{
+            {Messsage: "never called"},
+        })
+    }
+})
+
+decoder := handler(successDecoder)
+result := decoder("input")
+// Result: Success(42) - unchanged
+```
+
+### 3. Error Recovery
+The handler can recover from failures by returning a successful decoder:
+
+```go
+failingDecoder := func(input string) Validation[int] {
+    return either.Left[int](validation.Errors{
+        {Value: input, Messsage: "not found"},
+    })
+}
+
+recoverFromNotFound := ChainLeft(func(errs Errors) Decode[string, int] {
+    for _, err := range errs {
+        if err.Messsage == "not found" {
+            return Of[string](0) // recover with default
+        }
+    }
+    return func(input string) Validation[int] {
+        return either.Left[int](errs)
+    }
+})
+
+decoder := recoverFromNotFound(failingDecoder)
+result := decoder("input")
+// Result: Success(0) - recovered from failure
+```
+
+### 4. Access to Original Input
+The handler returns a `Decode[I, A]` function, giving it access to the original input:
+
+```go
+handler := ChainLeft(func(errs Errors) Decode[string, int] {
+    return func(input string) Validation[int] {
+        // Can access both errs and input here
+        if input == "special" {
+            return validation.Of(999)
+        }
+        return either.Left[int](errs)
+    }
+})
+```
+
+## Use Cases
+
+### 1. Fallback Decoding (OrElse reads better)
+
+```go
+// Primary decoder that may fail
+primaryDecoder := func(input string) Validation[int] {
+    n, err := strconv.Atoi(input)
+    if err != nil {
+        return either.Left[int](validation.Errors{
+            {Value: input, Messsage: "not a valid integer"},
+        })
+    }
+    return validation.Of(n)
+}
+
+// Use OrElse for semantic clarity - "try primary, or else use default"
+withDefault := OrElse(func(errs Errors) Decode[string, int] {
+    return Of[string](0) // default to 0 if decoding fails
+})
+
+decoder := withDefault(primaryDecoder)
+
+result1 := decoder("42")    // Success(42)
+result2 := decoder("abc")   // Success(0) - fallback
+```
+
+### 2. Error Context Addition (ChainLeft reads better)
+
+```go
+decodeUserAge := func(data map[string]any) Validation[int] {
+    age, ok := data["age"].(int)
+    if !ok {
+        return either.Left[int](validation.Errors{
+            {Value: data["age"], Messsage: "invalid type"},
+        })
+    }
+    return validation.Of(age)
+}
+
+// Use ChainLeft when emphasizing error transformation
+addContext := ChainLeft(func(errs Errors) Decode[map[string]any, int] {
+    return func(data map[string]any) Validation[int] {
+        return either.Left[int](validation.Errors{
+            {
+                Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+                Messsage: "failed to decode user age",
+            },
+        })
+    }
+})
+
+decoder := addContext(decodeUserAge)
+// Errors will include both original error and context
+```
+
+### 3. Conditional Recovery Based on Input
+
+```go
+decodePort := func(input string) Validation[int] {
+    port, err := strconv.Atoi(input)
+    if err != nil {
+        return either.Left[int](validation.Errors{
+            {Value: input, Messsage: "invalid port"},
+        })
+    }
+    return validation.Of(port)
+}
+
+// Recover with different defaults based on input
+smartDefault := OrElse(func(errs Errors) Decode[string, int] {
+    return func(input string) Validation[int] {
+        // Check input to determine appropriate default
+        if strings.Contains(input, "http") {
+            return validation.Of(80)
+        }
+        if strings.Contains(input, "https") {
+            return validation.Of(443)
+        }
+        return validation.Of(8080)
+    }
+})
+
+decoder := smartDefault(decodePort)
+result1 := decoder("http-server")   // Success(80)
+result2 := decoder("https-server")  // Success(443)
+result3 := decoder("other")         // Success(8080)
+```
+
+### 4. Pipeline Composition
+
+```go
+type Config struct {
+    DatabaseURL string
+}
+
+decodeConfig := func(data map[string]any) Validation[Config] {
+    url, ok := data["db_url"].(string)
+    if !ok {
+        return either.Left[Config](validation.Errors{
+            {Messsage: "missing db_url"},
+        })
+    }
+    return validation.Of(Config{DatabaseURL: url})
+}
+
+// Build a pipeline with multiple error handlers
+decoder := F.Pipe2(
+    decodeConfig,
+    OrElse(func(errs Errors) Decode[map[string]any, Config] {
+        // Try environment variable as fallback
+        return func(data map[string]any) Validation[Config] {
+            if url := os.Getenv("DATABASE_URL"); url != "" {
+                return validation.Of(Config{DatabaseURL: url})
+            }
+            return either.Left[Config](errs)
+        }
+    }),
+    OrElse(func(errs Errors) Decode[map[string]any, Config] {
+        // Final fallback to default
+        return Of[map[string]any](Config{
+            DatabaseURL: "localhost:5432",
+        })
+    }),
+)
+```
+
+## Comparison with validation.ChainLeft
+
+The decode package's [`ChainLeft`](monad.go:53) wraps [`validation.ChainLeft`](../validation/monad.go:304) using the Reader transformer pattern:
+
+| Aspect | validation.ChainLeft | decode.ChainLeft |
+|--------|---------------------|------------------|
+| **Input** | `Validation[A]` | `Decode[I, A]` (function) |
+| **Handler** | `func(Errors) Validation[A]` | `func(Errors) Decode[I, A]` |
+| **Output** | `Validation[A]` | `Decode[I, A]` (function) |
+| **Context** | No input access | Access to original input `I` |
+| **Use Case** | Pure validation logic | Decoding with input-dependent recovery |
+
+The key difference is that decode's version gives handlers access to the original input through the returned `Decode[I, A]` function.
+
+## When to Use Which Name
+
+### Use **OrElse** when:
+- Emphasizing fallback/alternative decoding logic
+- Providing default values on decode failure
+- The intent is "try this, or else try that"
+- Code reads more naturally with "or else"
+
+### Use **ChainLeft** when:
+- Emphasizing technical error channel transformation
+- Adding context or enriching error information
+- The focus is on error handling mechanics
+- Working with other functional programming concepts
+
+## Verification
+
+The test suite in [`monad_test.go`](monad_test.go:385) includes comprehensive tests proving that `OrElse` and `ChainLeft` are equivalent:
+
+- ✅ Identical behavior for Success values
+- ✅ Identical behavior for error recovery
+- ✅ Identical behavior for error aggregation
+- ✅ Identical behavior in pipeline composition
+- ✅ Identical behavior for multiple error scenarios
+- ✅ Both provide access to original input
+
+Run the tests:
+```bash
+go test -v -run "TestChainLeft|TestOrElse" ./optics/codec/decode
+```
+
+## Conclusion
+
+**`OrElse` is exactly the same as `ChainLeft`** in the decode package - they are aliases with identical implementations and behavior. Both:
+
+1. **Delegate to validation.ChainLeft** for error handling logic
+2. **Aggregate errors** when transformations fail
+3. **Preserve successes** unchanged
+4. **Enable recovery** from decode failures
+5. **Provide access** to the original input
+
+The choice between them is purely about **code readability and semantic intent**:
+- Use **`OrElse`** when emphasizing fallback/alternative decoding
+- Use **`ChainLeft`** when emphasizing error transformation
+
+Both maintain the critical property of **error aggregation**, ensuring all validation failures are preserved and reported together.

v2/optics/codec/decode/bind.go

@@ -0,0 +1,335 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package decode
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	A "github.com/IBM/fp-go/v2/internal/apply"
+	C "github.com/IBM/fp-go/v2/internal/chain"
+	F "github.com/IBM/fp-go/v2/internal/functor"
+	L "github.com/IBM/fp-go/v2/optics/lens"
+)
+
+// Do creates an empty context of type S to be used with the Bind operation.
+// This is the starting point for building up a context using do-notation style.
+//
+// Example:
+//
+//	type Result struct {
+//	    x int
+//	    y string
+//	}
+//	result := Do(Result{})
+func Do[I, S any](
+	empty S,
+) Decode[I, S] {
+	return Of[I](empty)
+}
+
+// Bind attaches the result of a computation to a context S1 to produce a context S2.
+// This is used in do-notation style to sequentially build up a context.
+//
+// Example:
+//
+//	type State struct { x int; y int }
+//	decoder := F.Pipe2(
+//	    Do[string](State{}),
+//	    Bind(func(x int) func(State) State {
+//	        return func(s State) State { s.x = x; return s }
+//	    }, func(s State) Decode[string, int] {
+//	        return Of[string](42)
+//	    }),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{x: 42})
+func Bind[I, S1, S2, A any](
+	setter func(A) func(S1) S2,
+	f Kleisli[I, S1, A],
+) Operator[I, S1, S2] {
+	return C.Bind(
+		Chain[I, S1, S2],
+		Map[I, A, S2],
+		setter,
+		f,
+	)
+}
+
+// Let attaches the result of a pure computation to a context S1 to produce a context S2.
+// Unlike Bind, the computation function returns a plain value, not wrapped in Decode.
+//
+// Example:
+//
+//	type State struct { x int; computed int }
+//	decoder := F.Pipe2(
+//	    Do[string](State{x: 5}),
+//	    Let[string](func(c int) func(State) State {
+//	        return func(s State) State { s.computed = c; return s }
+//	    }, func(s State) int { return s.x * 2 }),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{x: 5, computed: 10})
+func Let[I, S1, S2, B any](
+	key func(B) func(S1) S2,
+	f func(S1) B,
+) Operator[I, S1, S2] {
+	return F.Let(
+		Map[I, S1, S2],
+		key,
+		f,
+	)
+}
+
+// LetTo attaches a constant value to a context S1 to produce a context S2.
+//
+// Example:
+//
+//	type State struct { x int; name string }
+//	result := F.Pipe2(
+//	    Do(State{x: 5}),
+//	    LetTo(func(n string) func(State) State {
+//	        return func(s State) State { s.name = n; return s }
+//	    }, "example"),
+//	)
+func LetTo[I, S1, S2, B any](
+	key func(B) func(S1) S2,
+	b B,
+) Operator[I, S1, S2] {
+	return F.LetTo(
+		Map[I, S1, S2],
+		key,
+		b,
+	)
+}
+
+// BindTo initializes a new state S1 from a value T.
+// This is typically used as the first operation after creating a Decode value.
+//
+// Example:
+//
+//	type State struct { value int }
+//	decoder := F.Pipe1(
+//	    Of[string](42),
+//	    BindTo[string](func(x int) State { return State{value: x} }),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{value: 42})
+func BindTo[I, S1, T any](
+	setter func(T) S1,
+) Operator[I, T, S1] {
+	return C.BindTo(
+		Map[I, T, S1],
+		setter,
+	)
+}
+
+// ApS attaches a value to a context S1 to produce a context S2 by considering the context and the value concurrently.
+// This uses the applicative functor pattern, allowing parallel composition.
+//
+// IMPORTANT: Unlike Bind which fails fast, ApS aggregates ALL validation errors from both the context
+// and the value. If both validations fail, all errors are collected and returned together.
+// This is useful for validating multiple independent fields and reporting all errors at once.
+//
+// Example:
+//
+//	type State struct { x int; y int }
+//	decoder := F.Pipe2(
+//	    Do[string](State{}),
+//	    ApS(func(x int) func(State) State {
+//	        return func(s State) State { s.x = x; return s }
+//	    }, Of[string](42)),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{x: 42})
+//
+// Error aggregation example:
+//
+//	// Both decoders fail - errors are aggregated
+//	decoder1 := func(input string) Validation[State] {
+//	    return validation.Failures[State](/* errors */)
+//	}
+//	decoder2 := func(input string) Validation[int] {
+//	    return validation.Failures[int](/* errors */)
+//	}
+//	combined := ApS(setter, decoder2)(decoder1)
+//	result := combined("input") // Contains BOTH sets of errors
+func ApS[I, S1, S2, T any](
+	setter func(T) func(S1) S2,
+	fa Decode[I, T],
+) Operator[I, S1, S2] {
+	return A.ApS(
+		Ap[S2, I, T],
+		Map[I, S1, func(T) S2],
+		setter,
+		fa,
+	)
+}
+
+// ApSL attaches a value to a context using a lens-based setter.
+// This is a convenience function that combines ApS with a lens, allowing you to use
+// optics to update nested structures in a more composable way.
+//
+// IMPORTANT: Like ApS, this function aggregates ALL validation errors. If both the context
+// and the value fail validation, all errors are collected and returned together.
+// This enables comprehensive error reporting for complex nested structures.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// This eliminates the need to manually write setter functions.
+//
+// Example:
+//
+//	type Address struct {
+//	    Street string
+//	    City   string
+//	}
+//
+//	type Person struct {
+//	    Name    string
+//	    Address Address
+//	}
+//
+//	// Create a lens for the Address field
+//	addressLens := lens.MakeLens(
+//	    func(p Person) Address { return p.Address },
+//	    func(p Person, a Address) Person { p.Address = a; return p },
+//	)
+//
+//	// Use ApSL to update the address
+//	decoder := F.Pipe2(
+//	    Of[string](Person{Name: "Alice"}),
+//	    ApSL(
+//	        addressLens,
+//	        Of[string](Address{Street: "Main St", City: "NYC"}),
+//	    ),
+//	)
+//	result := decoder("input") // Returns validation.Success(Person{...})
+func ApSL[I, S, T any](
+	lens L.Lens[S, T],
+	fa Decode[I, T],
+) Operator[I, S, S] {
+	return ApS(lens.Set, fa)
+}
+
+// BindL attaches the result of a computation to a context using a lens-based setter.
+// This is a convenience function that combines Bind with a lens, allowing you to use
+// optics to update nested structures based on their current values.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// The computation function f receives the current value of the focused field and returns
+// a Validation that produces the new value.
+//
+// Unlike ApSL, BindL uses monadic sequencing, meaning the computation f can depend on
+// the current value of the focused field.
+//
+// Example:
+//
+//	type Counter struct {
+//	    Value int
+//	}
+//
+//	valueLens := lens.MakeLens(
+//	    func(c Counter) int { return c.Value },
+//	    func(c Counter, v int) Counter { c.Value = v; return c },
+//	)
+//
+//	// Increment the counter, but fail if it would exceed 100
+//	increment := func(v int) Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        if v >= 100 {
+//	            return validation.Failures[int](/* errors */)
+//	        }
+//	        return validation.Success(v + 1)
+//	    }
+//	}
+//
+//	decoder := F.Pipe1(
+//	    Of[string](Counter{Value: 42}),
+//	    BindL(valueLens, increment),
+//	)
+//	result := decoder("input") // Returns validation.Success(Counter{Value: 43})
+func BindL[I, S, T any](
+	lens L.Lens[S, T],
+	f Kleisli[I, T, T],
+) Operator[I, S, S] {
+	return Bind(lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetL attaches the result of a pure computation to a context using a lens-based setter.
+// This is a convenience function that combines Let with a lens, allowing you to use
+// optics to update nested structures with pure transformations.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// The transformation function f receives the current value of the focused field and returns
+// the new value directly (not wrapped in Validation).
+//
+// This is useful for pure transformations that cannot fail, such as mathematical operations,
+// string manipulations, or other deterministic updates.
+//
+// Example:
+//
+//	type Counter struct {
+//	    Value int
+//	}
+//
+//	valueLens := lens.MakeLens(
+//	    func(c Counter) int { return c.Value },
+//	    func(c Counter, v int) Counter { c.Value = v; return c },
+//	)
+//
+//	// Double the counter value
+//	double := func(v int) int { return v * 2 }
+//
+//	decoder := F.Pipe1(
+//	    Of[string](Counter{Value: 21}),
+//	    LetL(valueLens, double),
+//	)
+//	result := decoder("input") // Returns validation.Success(Counter{Value: 42})
+func LetL[I, S, T any](
+	lens L.Lens[S, T],
+	f Endomorphism[T],
+) Operator[I, S, S] {
+	return Let[I](lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetToL attaches a constant value to a context using a lens-based setter.
+// This is a convenience function that combines LetTo with a lens, allowing you to use
+// optics to set nested fields to specific values.
+//
+// The lens parameter provides the setter for a field within the structure S.
+// Unlike LetL which transforms the current value, LetToL simply replaces it with
+// the provided constant value b.
+//
+// This is useful for resetting fields, initializing values, or setting fields to
+// predetermined constants.
+//
+// Example:
+//
+//	type Config struct {
+//	    Debug   bool
+//	    Timeout int
+//	}
+//
+//	debugLens := lens.MakeLens(
+//	    func(c Config) bool { return c.Debug },
+//	    func(c Config, d bool) Config { c.Debug = d; return c },
+//	)
+//
+//	decoder := F.Pipe1(
+//	    Of[string](Config{Debug: true, Timeout: 30}),
+//	    LetToL(debugLens, false),
+//	)
+//	result := decoder("input") // Returns validation.Success(Config{Debug: false, Timeout: 30})
+func LetToL[I, S, T any](
+	lens L.Lens[S, T],
+	b T,
+) Operator[I, S, S] {
+	return LetTo[I](lens.Set, b)
+}

v2/optics/codec/decode/bind_test.go

@@ -0,0 +1,665 @@
+package decode
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	L "github.com/IBM/fp-go/v2/optics/lens"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDo(t *testing.T) {
+	t.Run("creates decoder with empty state", func(t *testing.T) {
+		type State struct {
+			x int
+			y string
+		}
+		decoder := Do[string](State{})
+		result := decoder("input")
+
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{}, value)
+	})
+
+	t.Run("creates decoder with initialized state", func(t *testing.T) {
+		type State struct {
+			x int
+			y string
+		}
+		initial := State{x: 42, y: "hello"}
+		decoder := Do[string](initial)
+		result := decoder("input")
+
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, initial, value)
+	})
+
+	t.Run("works with different input types", func(t *testing.T) {
+		intDecoder := Do[int](0)
+		assert.True(t, either.IsRight(intDecoder(42)))
+
+		strDecoder := Do[string]("")
+		assert.True(t, either.IsRight(strDecoder("test")))
+
+		type Custom struct{ Value int }
+		customDecoder := Do[[]byte](Custom{Value: 100})
+		assert.True(t, either.IsRight(customDecoder([]byte("data"))))
+	})
+}
+
+func TestBind(t *testing.T) {
+	type State struct {
+		x int
+		y int
+	}
+
+	t.Run("binds successful decode to state", func(t *testing.T) {
+		decoder := F.Pipe2(
+			Do[string](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Decode[string, int] {
+				return Of[string](42)
+			}),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Decode[string, int] {
+				return Of[string](10)
+			}),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 42, y: 10}, value)
+	})
+
+	t.Run("propagates failure", func(t *testing.T) {
+		decoder := F.Pipe2(
+			Do[string](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Decode[string, int] {
+				return Of[string](42)
+			}),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Decode[string, int] {
+				return func(input string) validation.Validation[int] {
+					return validation.Failures[int](validation.Errors{
+						&validation.ValidationError{Messsage: "y failed"},
+					})
+				}
+			}),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(State) validation.Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "y failed", errors[0].Messsage)
+	})
+
+	t.Run("can access previous state values", func(t *testing.T) {
+		decoder := F.Pipe2(
+			Do[string](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Decode[string, int] {
+				return Of[string](10)
+			}),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Decode[string, int] {
+				// y depends on x
+				return Of[string](s.x * 2)
+			}),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 10, y: 20}, value)
+	})
+
+	t.Run("can access input in decoder", func(t *testing.T) {
+		decoder := F.Pipe1(
+			Do[string](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Decode[string, int] {
+				return func(input string) validation.Validation[int] {
+					// Use input to determine value
+					if input == "large" {
+						return validation.Success(100)
+					}
+					return validation.Success(10)
+				}
+			}),
+		)
+
+		result1 := decoder("large")
+		value1 := either.MonadFold(result1,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, 100, value1.x)
+
+		result2 := decoder("small")
+		value2 := either.MonadFold(result2,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, 10, value2.x)
+	})
+}
+
+func TestLet(t *testing.T) {
+	type State struct {
+		x        int
+		computed int
+	}
+
+	t.Run("attaches pure computation result to state", func(t *testing.T) {
+		decoder := F.Pipe1(
+			Do[string](State{x: 5}),
+			Let[string](func(c int) func(State) State {
+				return func(s State) State { s.computed = c; return s }
+			}, func(s State) int { return s.x * 2 }),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 5, computed: 10}, value)
+	})
+
+	t.Run("chains multiple Let operations", func(t *testing.T) {
+		type State struct {
+			x int
+			y int
+			z int
+		}
+		decoder := F.Pipe3(
+			Do[string](State{x: 5}),
+			Let[string](func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) int { return s.x * 2 }),
+			Let[string](func(z int) func(State) State {
+				return func(s State) State { s.z = z; return s }
+			}, func(s State) int { return s.y + 10 }),
+			Let[string](func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) int { return s.z * 3 }),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 60, y: 10, z: 20}, value)
+	})
+}
+
+func TestLetTo(t *testing.T) {
+	type State struct {
+		x    int
+		name string
+	}
+
+	t.Run("attaches constant value to state", func(t *testing.T) {
+		decoder := F.Pipe1(
+			Do[string](State{x: 5}),
+			LetTo[string](func(n string) func(State) State {
+				return func(s State) State { s.name = n; return s }
+			}, "example"),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 5, name: "example"}, value)
+	})
+
+	t.Run("sets multiple constant values", func(t *testing.T) {
+		type State struct {
+			name    string
+			version int
+			active  bool
+		}
+		decoder := F.Pipe3(
+			Do[string](State{}),
+			LetTo[string](func(n string) func(State) State {
+				return func(s State) State { s.name = n; return s }
+			}, "app"),
+			LetTo[string](func(v int) func(State) State {
+				return func(s State) State { s.version = v; return s }
+			}, 2),
+			LetTo[string](func(a bool) func(State) State {
+				return func(s State) State { s.active = a; return s }
+			}, true),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{name: "app", version: 2, active: true}, value)
+	})
+}
+
+func TestBindTo(t *testing.T) {
+	type State struct {
+		value int
+	}
+
+	t.Run("initializes state from value", func(t *testing.T) {
+		decoder := F.Pipe1(
+			Of[string](42),
+			BindTo[string](func(x int) State { return State{value: x} }),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{value: 42}, value)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		type StringState struct {
+			text string
+		}
+		decoder := F.Pipe1(
+			Of[string]("hello"),
+			BindTo[string](func(s string) StringState { return StringState{text: s} }),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) StringState { return StringState{} },
+			F.Identity[StringState],
+		)
+		assert.Equal(t, StringState{text: "hello"}, value)
+	})
+}
+
+func TestApS(t *testing.T) {
+	type State struct {
+		x int
+		y int
+	}
+
+	t.Run("attaches value using applicative pattern", func(t *testing.T) {
+		decoder := F.Pipe1(
+			Do[string](State{}),
+			ApS(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, Of[string](42)),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 42}, value)
+	})
+
+	t.Run("accumulates errors from both decoders", func(t *testing.T) {
+		stateDecoder := func(input string) validation.Validation[State] {
+			return validation.Failures[State](validation.Errors{
+				&validation.ValidationError{Messsage: "state error"},
+			})
+		}
+		valueDecoder := func(input string) validation.Validation[int] {
+			return validation.Failures[int](validation.Errors{
+				&validation.ValidationError{Messsage: "value error"},
+			})
+		}
+
+		decoder := ApS(func(x int) func(State) State {
+			return func(s State) State { s.x = x; return s }
+		}, valueDecoder)(stateDecoder)
+
+		result := decoder("input")
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(State) validation.Errors { return nil },
+		)
+		assert.Len(t, errors, 2)
+		messages := []string{errors[0].Messsage, errors[1].Messsage}
+		assert.Contains(t, messages, "state error")
+		assert.Contains(t, messages, "value error")
+	})
+
+	t.Run("combines multiple ApS operations", func(t *testing.T) {
+		decoder := F.Pipe2(
+			Do[string](State{}),
+			ApS(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, Of[string](10)),
+			ApS(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, Of[string](20)),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 10, y: 20}, value)
+	})
+}
+
+func TestApSL(t *testing.T) {
+	type Address struct {
+		Street string
+		City   string
+	}
+
+	type Person struct {
+		Name    string
+		Address Address
+	}
+
+	t.Run("updates nested structure using lens", func(t *testing.T) {
+		addressLens := L.MakeLens(
+			func(p Person) Address { return p.Address },
+			func(p Person, a Address) Person { p.Address = a; return p },
+		)
+
+		decoder := F.Pipe1(
+			Of[string](Person{Name: "Alice"}),
+			ApSL(
+				addressLens,
+				Of[string](Address{Street: "Main St", City: "NYC"}),
+			),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) Person { return Person{} },
+			F.Identity[Person],
+		)
+		assert.Equal(t, "Alice", value.Name)
+		assert.Equal(t, "Main St", value.Address.Street)
+		assert.Equal(t, "NYC", value.Address.City)
+	})
+
+	t.Run("accumulates errors", func(t *testing.T) {
+		addressLens := L.MakeLens(
+			func(p Person) Address { return p.Address },
+			func(p Person, a Address) Person { p.Address = a; return p },
+		)
+
+		personDecoder := func(input string) validation.Validation[Person] {
+			return validation.Failures[Person](validation.Errors{
+				&validation.ValidationError{Messsage: "person error"},
+			})
+		}
+		addressDecoder := func(input string) validation.Validation[Address] {
+			return validation.Failures[Address](validation.Errors{
+				&validation.ValidationError{Messsage: "address error"},
+			})
+		}
+
+		decoder := ApSL(addressLens, addressDecoder)(personDecoder)
+		result := decoder("input")
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(Person) validation.Errors { return nil },
+		)
+		assert.Len(t, errors, 2)
+	})
+}
+
+func TestBindL(t *testing.T) {
+	type Counter struct {
+		Value int
+	}
+
+	valueLens := L.MakeLens(
+		func(c Counter) int { return c.Value },
+		func(c Counter, v int) Counter { c.Value = v; return c },
+	)
+
+	t.Run("updates field based on current value", func(t *testing.T) {
+		increment := func(v int) Decode[string, int] {
+			return Of[string](v + 1)
+		}
+
+		decoder := F.Pipe1(
+			Of[string](Counter{Value: 42}),
+			BindL(valueLens, increment),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) Counter { return Counter{} },
+			F.Identity[Counter],
+		)
+		assert.Equal(t, Counter{Value: 43}, value)
+	})
+
+	t.Run("fails validation based on current value", func(t *testing.T) {
+		increment := func(v int) Decode[string, int] {
+			return func(input string) validation.Validation[int] {
+				if v >= 100 {
+					return validation.Failures[int](validation.Errors{
+						&validation.ValidationError{Messsage: "exceeds limit"},
+					})
+				}
+				return validation.Success(v + 1)
+			}
+		}
+
+		decoder := F.Pipe1(
+			Of[string](Counter{Value: 100}),
+			BindL(valueLens, increment),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(Counter) validation.Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "exceeds limit", errors[0].Messsage)
+	})
+}
+
+func TestLetL(t *testing.T) {
+	type Counter struct {
+		Value int
+	}
+
+	valueLens := L.MakeLens(
+		func(c Counter) int { return c.Value },
+		func(c Counter, v int) Counter { c.Value = v; return c },
+	)
+
+	t.Run("transforms field with pure function", func(t *testing.T) {
+		double := func(v int) int { return v * 2 }
+
+		decoder := F.Pipe1(
+			Of[string](Counter{Value: 21}),
+			LetL[string](valueLens, double),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) Counter { return Counter{} },
+			F.Identity[Counter],
+		)
+		assert.Equal(t, Counter{Value: 42}, value)
+	})
+
+	t.Run("chains multiple transformations", func(t *testing.T) {
+		add10 := func(v int) int { return v + 10 }
+		double := func(v int) int { return v * 2 }
+
+		decoder := F.Pipe2(
+			Of[string](Counter{Value: 5}),
+			LetL[string](valueLens, add10),
+			LetL[string](valueLens, double),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) Counter { return Counter{} },
+			F.Identity[Counter],
+		)
+		assert.Equal(t, Counter{Value: 30}, value)
+	})
+}
+
+func TestLetToL(t *testing.T) {
+	type Config struct {
+		Debug   bool
+		Timeout int
+	}
+
+	debugLens := L.MakeLens(
+		func(c Config) bool { return c.Debug },
+		func(c Config, d bool) Config { c.Debug = d; return c },
+	)
+
+	t.Run("sets field to constant value", func(t *testing.T) {
+		decoder := F.Pipe1(
+			Of[string](Config{Debug: true, Timeout: 30}),
+			LetToL[string](debugLens, false),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) Config { return Config{} },
+			F.Identity[Config],
+		)
+		assert.Equal(t, Config{Debug: false, Timeout: 30}, value)
+	})
+
+	t.Run("sets multiple fields", func(t *testing.T) {
+		timeoutLens := L.MakeLens(
+			func(c Config) int { return c.Timeout },
+			func(c Config, t int) Config { c.Timeout = t; return c },
+		)
+
+		decoder := F.Pipe2(
+			Of[string](Config{Debug: true, Timeout: 30}),
+			LetToL[string](debugLens, false),
+			LetToL[string](timeoutLens, 60),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) Config { return Config{} },
+			F.Identity[Config],
+		)
+		assert.Equal(t, Config{Debug: false, Timeout: 60}, value)
+	})
+}
+
+func TestBindOperationsComposition(t *testing.T) {
+	type User struct {
+		Name  string
+		Age   int
+		Email string
+	}
+
+	t.Run("combines Do, Bind, Let, and LetTo", func(t *testing.T) {
+		decoder := F.Pipe4(
+			Do[string](User{}),
+			LetTo[string](func(n string) func(User) User {
+				return func(u User) User { u.Name = n; return u }
+			}, "Alice"),
+			Bind(func(a int) func(User) User {
+				return func(u User) User { u.Age = a; return u }
+			}, func(u User) Decode[string, int] {
+				// Age validation
+				if len(u.Name) > 0 {
+					return Of[string](25)
+				}
+				return func(input string) validation.Validation[int] {
+					return validation.Failures[int](validation.Errors{
+						&validation.ValidationError{Messsage: "name required"},
+					})
+				}
+			}),
+			Let[string](func(e string) func(User) User {
+				return func(u User) User { u.Email = e; return u }
+			}, func(u User) string {
+				// Derive email from name
+				return u.Name + "@example.com"
+			}),
+			Bind(func(a int) func(User) User {
+				return func(u User) User { u.Age = a; return u }
+			}, func(u User) Decode[string, int] {
+				// Validate age is positive
+				if u.Age > 0 {
+					return Of[string](u.Age)
+				}
+				return func(input string) validation.Validation[int] {
+					return validation.Failures[int](validation.Errors{
+						&validation.ValidationError{Messsage: "age must be positive"},
+					})
+				}
+			}),
+		)
+
+		result := decoder("input")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) User { return User{} },
+			F.Identity[User],
+		)
+		assert.Equal(t, "Alice", value.Name)
+		assert.Equal(t, 25, value.Age)
+		assert.Equal(t, "Alice@example.com", value.Email)
+	})
+}

v2/optics/codec/decode/monad.go

@@ -1,9 +1,10 @@
 package decode
 
 import (
+	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/readert"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
-	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/readereither"
 )
 
 // Of creates a Decode that always succeeds with the given value.
@@ -14,7 +15,82 @@ import (
 //	decoder := decode.Of[string](42)
 //	result := decoder("any input") // Always returns validation.Success(42)
 func Of[I, A any](a A) Decode[I, A] {
-	return reader.Of[I](validation.Of(a))
+	return readereither.Of[I, Errors](a)
+}
+
+// Left creates a Decode that always fails with the given validation errors.
+// This is the dual of Of - while Of lifts a success value, Left lifts failure errors
+// into the Decode context.
+//
+// Left is useful for:
+//   - Creating decoders that represent known failure states
+//   - Short-circuiting decode pipelines with specific errors
+//   - Building custom validation error responses
+//   - Testing error handling paths
+//
+// The returned decoder ignores its input and always returns a validation failure
+// containing the provided errors. This makes it the identity element for the
+// Alt/OrElse operations when used as a fallback.
+//
+// Type signature: func(Errors) Decode[I, A]
+//   - Takes validation errors
+//   - Returns a decoder that always fails with those errors
+//   - The decoder ignores its input of type I
+//   - The failure type A can be any type (phantom type)
+//
+// Example - Creating a failing decoder:
+//
+//	failDecoder := decode.Left[string, int](validation.Errors{
+//	    &validation.ValidationError{
+//	        Value:    nil,
+//	        Messsage: "operation not supported",
+//	    },
+//	})
+//	result := failDecoder("any input") // Always fails with the error
+//
+// Example - Short-circuiting with specific errors:
+//
+//	validateAge := func(age int) Decode[map[string]any, int] {
+//	    if age < 0 {
+//	        return decode.Left[map[string]any, int](validation.Errors{
+//	            &validation.ValidationError{
+//	                Value:    age,
+//	                Context:  validation.Context{{Key: "age", Type: "int"}},
+//	                Messsage: "age cannot be negative",
+//	            },
+//	        })
+//	    }
+//	    return decode.Of[map[string]any](age)
+//	}
+//
+// Example - Building error responses:
+//
+//	notFoundError := decode.Left[string, User](validation.Errors{
+//	    &validation.ValidationError{
+//	        Messsage: "user not found",
+//	    },
+//	})
+//
+//	decoder := decode.MonadAlt(
+//	    tryFindUser,
+//	    func() Decode[string, User] { return notFoundError },
+//	)
+//
+// Example - Testing error paths:
+//
+//	// Create a decoder that always fails for testing
+//	alwaysFails := decode.Left[string, int](validation.Errors{
+//	    &validation.ValidationError{Messsage: "test error"},
+//	})
+//
+//	// Test error recovery logic
+//	recovered := decode.OrElse(func(errs Errors) Decode[string, int] {
+//	    return decode.Of[string](0) // recover with default
+//	})(alwaysFails)
+//
+//	result := recovered("input") // Success(0)
+func Left[I, A any](err Errors) Decode[I, A] {
+	return readereither.Left[I, A](err)
 }
 
 // MonadChain sequences two decode operations, passing the result of the first to the second.
@@ -50,6 +126,212 @@ func Chain[I, A, B any](f Kleisli[I, A, B]) Operator[I, A, B] {
 	)
 }
 
+// ChainLeft transforms the error channel of a decoder, enabling error recovery and context addition.
+// This is the left-biased monadic chain operation that operates on validation failures.
+//
+// **Key behaviors**:
+//   - Success values pass through unchanged - the handler is never called
+//   - On failure, the handler receives the errors and can recover or add context
+//   - When the handler also fails, **both original and new errors are aggregated**
+//   - The handler returns a Decode[I, A], giving it access to the original input
+//
+// **Error Aggregation**: Unlike standard Either operations, when the transformation function
+// returns a failure, both the original errors AND the new errors are combined using the
+// Errors monoid. This ensures no validation errors are lost.
+//
+// Use cases:
+//   - Adding contextual information to validation errors
+//   - Recovering from specific error conditions
+//   - Transforming error messages while preserving original errors
+//   - Implementing conditional recovery based on error types
+//
+// Example - Error recovery:
+//
+//	failingDecoder := func(input string) Validation[int] {
+//	    return either.Left[int](validation.Errors{
+//	        {Value: input, Messsage: "not found"},
+//	    })
+//	}
+//
+//	recoverFromNotFound := ChainLeft(func(errs Errors) Decode[string, int] {
+//	    for _, err := range errs {
+//	        if err.Messsage == "not found" {
+//	            return Of[string](0) // recover with default
+//	        }
+//	    }
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](errs)
+//	    }
+//	})
+//
+//	decoder := recoverFromNotFound(failingDecoder)
+//	result := decoder("input") // Success(0) - recovered from failure
+//
+// Example - Adding context:
+//
+//	addContext := ChainLeft(func(errs Errors) Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](validation.Errors{
+//	            {
+//	                Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+//	                Messsage: "failed to decode user age",
+//	            },
+//	        })
+//	    }
+//	})
+//	// Result will contain BOTH original error and context error
+func ChainLeft[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return readert.Chain[Decode[I, A]](
+		validation.ChainLeft,
+		f,
+	)
+}
+
+// MonadChainLeft transforms the error channel of a decoder, enabling error recovery and context addition.
+// This is the uncurried version of ChainLeft, taking both the decoder and the transformation function directly.
+//
+// **Key behaviors**:
+//   - Success values pass through unchanged - the handler is never called
+//   - On failure, the handler receives the errors and can recover or add context
+//   - When the handler also fails, **both original and new errors are aggregated**
+//   - The handler returns a Decode[I, A], giving it access to the original input
+//
+// **Error Aggregation**: Unlike standard Either operations, when the transformation function
+// returns a failure, both the original errors AND the new errors are combined using the
+// Errors monoid. This ensures no validation errors are lost.
+//
+// This function is the direct, uncurried form of ChainLeft. Use ChainLeft when you need
+// a curried operator for composition pipelines, and use MonadChainLeft when you have both
+// the decoder and transformation function available at once.
+//
+// Use cases:
+//   - Adding contextual information to validation errors
+//   - Recovering from specific error conditions
+//   - Transforming error messages while preserving original errors
+//   - Implementing conditional recovery based on error types
+//
+// Example - Error recovery:
+//
+//	failingDecoder := func(input string) Validation[int] {
+//	    return either.Left[int](validation.Errors{
+//	        {Value: input, Messsage: "not found"},
+//	    })
+//	}
+//
+//	recoverFromNotFound := func(errs Errors) Decode[string, int] {
+//	    for _, err := range errs {
+//	        if err.Messsage == "not found" {
+//	            return Of[string](0) // recover with default
+//	        }
+//	    }
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](errs)
+//	    }
+//	}
+//
+//	decoder := MonadChainLeft(failingDecoder, recoverFromNotFound)
+//	result := decoder("input") // Success(0) - recovered from failure
+//
+// Example - Adding context:
+//
+//	addContext := func(errs Errors) Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](validation.Errors{
+//	            {
+//	                Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+//	                Messsage: "failed to decode user age",
+//	            },
+//	        })
+//	    }
+//	}
+//
+//	decoder := MonadChainLeft(failingDecoder, addContext)
+//	result := decoder("abc")
+//	// Result will contain BOTH original error and context error
+//
+// Example - Comparison with ChainLeft:
+//
+//	// MonadChainLeft - direct application
+//	result1 := MonadChainLeft(decoder, handler)("input")
+//
+//	// ChainLeft - curried for pipelines
+//	result2 := ChainLeft(handler)(decoder)("input")
+//
+//	// Both produce identical results
+func MonadChainLeft[I, A any](fa Decode[I, A], f Kleisli[I, Errors, A]) Decode[I, A] {
+	return readert.MonadChain(
+		validation.MonadChainLeft,
+		fa,
+		f,
+	)
+}
+
+// OrElse provides fallback decoding logic when the primary decoder fails.
+// This is an alias for ChainLeft with a more semantic name for fallback scenarios.
+//
+// **OrElse is exactly the same as ChainLeft** - they are aliases with identical implementations
+// and behavior. The choice between them is purely about code readability and semantic intent:
+//   - Use **OrElse** when emphasizing fallback/alternative decoding logic
+//   - Use **ChainLeft** when emphasizing technical error channel transformation
+//
+// **Key behaviors** (identical to ChainLeft):
+//   - Success values pass through unchanged - the handler is never called
+//   - On failure, the handler receives the errors and can provide an alternative
+//   - When the handler also fails, **both original and new errors are aggregated**
+//   - The handler returns a Decode[I, A], giving it access to the original input
+//
+// The name "OrElse" reads naturally in code: "try this decoder, or else try this alternative."
+// This makes it ideal for expressing fallback logic and default values.
+//
+// Use cases:
+//   - Providing default values when decoding fails
+//   - Trying alternative decoding strategies
+//   - Implementing fallback chains with multiple alternatives
+//   - Input-dependent recovery (using access to original input)
+//
+// Example - Simple fallback:
+//
+//	primaryDecoder := func(input string) Validation[int] {
+//	    n, err := strconv.Atoi(input)
+//	    if err != nil {
+//	        return either.Left[int](validation.Errors{
+//	            {Value: input, Messsage: "not a valid integer"},
+//	        })
+//	    }
+//	    return validation.Of(n)
+//	}
+//
+//	withDefault := OrElse(func(errs Errors) Decode[string, int] {
+//	    return Of[string](0) // default to 0 if decoding fails
+//	})
+//
+//	decoder := withDefault(primaryDecoder)
+//	result1 := decoder("42")  // Success(42)
+//	result2 := decoder("abc") // Success(0) - fallback
+//
+// Example - Input-dependent fallback:
+//
+//	smartDefault := OrElse(func(errs Errors) Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        // Access original input to determine appropriate default
+//	        if strings.Contains(input, "http") {
+//	            return validation.Of(80)
+//	        }
+//	        if strings.Contains(input, "https") {
+//	            return validation.Of(443)
+//	        }
+//	        return validation.Of(8080)
+//	    }
+//	})
+//
+//	decoder := smartDefault(decodePort)
+//	result1 := decoder("http-server")  // Success(80)
+//	result2 := decoder("https-server") // Success(443)
+//	result3 := decoder("other")        // Success(8080)
+func OrElse[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return ChainLeft(f)
+}
+
 // MonadMap transforms the decoded value using the provided function.
 // This is the functor map operation that applies a transformation to successful decode results.
 //
@@ -127,3 +409,155 @@ func Ap[B, I, A any](fa Decode[I, A]) Operator[I, func(A) B, B] {
 		fa,
 	)
 }
+
+// MonadAlt provides alternative/fallback decoding with error aggregation.
+// This is the Alternative pattern's core operation that tries the first decoder,
+// and if it fails, tries the second decoder as a fallback.
+//
+// **Key behaviors**:
+//   - If first succeeds: returns the first result (second is never evaluated)
+//   - If first fails and second succeeds: returns the second result
+//   - If both fail: **aggregates errors from both decoders**
+//
+// **Error Aggregation**: Unlike simple fallback patterns, when both decoders fail,
+// MonadAlt combines ALL errors from both attempts using the Errors monoid. This ensures
+// complete visibility into why all alternatives failed, which is crucial for debugging
+// and providing comprehensive error messages to users.
+//
+// The name "Alt" comes from the Alternative type class in functional programming,
+// which represents computations with a notion of choice and failure.
+//
+// Use cases:
+//   - Trying multiple decoding strategies for the same input
+//   - Providing fallback decoders when primary decoder fails
+//   - Building validation pipelines with multiple alternatives
+//   - Implementing "try this, or else try that" logic
+//
+// Example - Simple fallback:
+//
+//	primaryDecoder := func(input string) Validation[int] {
+//	    n, err := strconv.Atoi(input)
+//	    if err != nil {
+//	        return either.Left[int](validation.Errors{
+//	            {Value: input, Messsage: "not a valid integer"},
+//	        })
+//	    }
+//	    return validation.Of(n)
+//	}
+//
+//	fallbackDecoder := func() Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        // Try parsing as float and converting to int
+//	        f, err := strconv.ParseFloat(input, 64)
+//	        if err != nil {
+//	            return either.Left[int](validation.Errors{
+//	                {Value: input, Messsage: "not a valid number"},
+//	            })
+//	        }
+//	        return validation.Of(int(f))
+//	    }
+//	}
+//
+//	decoder := MonadAlt(primaryDecoder, fallbackDecoder)
+//	result1 := decoder("42")    // Success(42) - primary succeeds
+//	result2 := decoder("42.5")  // Success(42) - fallback succeeds
+//	result3 := decoder("abc")   // Failures with both errors aggregated
+//
+// Example - Multiple alternatives:
+//
+//	decoder1 := parseAsJSON
+//	decoder2 := func() Decode[string, Config] { return parseAsYAML }
+//	decoder3 := func() Decode[string, Config] { return parseAsINI }
+//
+//	// Try JSON, then YAML, then INI
+//	decoder := MonadAlt(MonadAlt(decoder1, decoder2), decoder3)
+//	// If all fail, errors from all three attempts are aggregated
+//
+// Example - Error aggregation:
+//
+//	failing1 := func(input string) Validation[int] {
+//	    return either.Left[int](validation.Errors{
+//	        {Messsage: "primary decoder failed"},
+//	    })
+//	}
+//	failing2 := func() Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](validation.Errors{
+//	            {Messsage: "fallback decoder failed"},
+//	        })
+//	    }
+//	}
+//
+//	decoder := MonadAlt(failing1, failing2)
+//	result := decoder("input")
+//	// Result contains BOTH errors: ["primary decoder failed", "fallback decoder failed"]
+func MonadAlt[I, A any](first Decode[I, A], second Lazy[Decode[I, A]]) Decode[I, A] {
+	return MonadChainLeft(first, function.Ignore1of1[Errors](second))
+}
+
+// Alt creates an operator that provides alternative/fallback decoding with error aggregation.
+// This is the curried version of MonadAlt, useful for composition pipelines.
+//
+// **Key behaviors** (identical to MonadAlt):
+//   - If first succeeds: returns the first result (second is never evaluated)
+//   - If first fails and second succeeds: returns the second result
+//   - If both fail: **aggregates errors from both decoders**
+//
+// The Alt operator enables building reusable fallback chains that can be applied
+// to different decoders. It reads naturally in pipelines: "apply this decoder,
+// with this alternative if it fails."
+//
+// Use cases:
+//   - Creating reusable fallback strategies
+//   - Building decoder combinators with alternatives
+//   - Composing multiple fallback layers
+//   - Implementing retry logic with different strategies
+//
+// Example - Creating a reusable fallback:
+//
+//	// Create an operator that falls back to a default value
+//	withDefault := Alt(func() Decode[string, int] {
+//	    return Of[string](0)
+//	})
+//
+//	// Apply to any decoder
+//	decoder1 := withDefault(parseInteger)
+//	decoder2 := withDefault(parseFromJSON)
+//
+//	result1 := decoder1("42")  // Success(42)
+//	result2 := decoder1("abc") // Success(0) - fallback
+//
+// Example - Composing multiple alternatives:
+//
+//	tryYAML := Alt(func() Decode[string, Config] { return parseAsYAML })
+//	tryINI := Alt(func() Decode[string, Config] { return parseAsINI })
+//	useDefault := Alt(func() Decode[string, Config] {
+//	    return Of[string](defaultConfig)
+//	})
+//
+//	// Build a pipeline: try JSON, then YAML, then INI, then default
+//	decoder := useDefault(tryINI(tryYAML(parseAsJSON)))
+//
+// Example - Error aggregation in pipeline:
+//
+//	failing1 := func(input string) Validation[int] {
+//	    return either.Left[int](validation.Errors{{Messsage: "error 1"}})
+//	}
+//	failing2 := func() Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](validation.Errors{{Messsage: "error 2"}})
+//	    }
+//	}
+//	failing3 := func() Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](validation.Errors{{Messsage: "error 3"}})
+//	    }
+//	}
+//
+//	// Chain multiple alternatives
+//	decoder := Alt(failing3)(Alt(failing2)(failing1))
+//	result := decoder("input")
+//	// Result contains ALL errors: ["error 1", "error 2", "error 3"]
+func Alt[I, A any](second Lazy[Decode[I, A]]) Operator[I, A, A] {
+	return ChainLeft(function.Ignore1of1[Errors](second))
+}

v2/optics/codec/decode/monoid.go

@@ -0,0 +1,368 @@
+package decode
+
+import "github.com/IBM/fp-go/v2/monoid"
+
+// ApplicativeMonoid creates a Monoid instance for Decode[I, A] given a Monoid for A.
+// This allows combining decoders where both the decoded values and validation errors
+// are combined according to their respective monoid operations.
+//
+// The resulting monoid enables:
+//   - Combining multiple decoders that produce monoidal values
+//   - Accumulating validation errors when any decoder fails
+//   - Building complex decoders from simpler ones through composition
+//
+// **Behavior**:
+//   - Empty: Returns a decoder that always succeeds with the empty value from the inner monoid
+//   - Concat: Combines two decoders:
+//   - Both succeed: Combines decoded values using the inner monoid
+//   - Any fails: Accumulates all validation errors using the Errors monoid
+//
+// This is particularly useful for:
+//   - Aggregating results from multiple independent decoders
+//   - Building decoders that combine partial results
+//   - Validating and combining configuration from multiple sources
+//   - Parallel validation with result accumulation
+//
+// Example - Combining string decoders:
+//
+//	import S "github.com/IBM/fp-go/v2/string"
+//
+//	// Create a monoid for decoders that produce strings
+//	m := ApplicativeMonoid[map[string]any](S.Monoid)
+//
+//	decoder1 := func(data map[string]any) Validation[string] {
+//	    if name, ok := data["firstName"].(string); ok {
+//	        return validation.Of(name)
+//	    }
+//	    return either.Left[string](validation.Errors{
+//	        {Messsage: "missing firstName"},
+//	    })
+//	}
+//
+//	decoder2 := func(data map[string]any) Validation[string] {
+//	    if name, ok := data["lastName"].(string); ok {
+//	        return validation.Of(" " + name)
+//	    }
+//	    return either.Left[string](validation.Errors{
+//	        {Messsage: "missing lastName"},
+//	    })
+//	}
+//
+//	// Combine decoders - will concatenate strings if both succeed
+//	combined := m.Concat(decoder1, decoder2)
+//	result := combined(map[string]any{
+//	    "firstName": "John",
+//	    "lastName":  "Doe",
+//	}) // Success("John Doe")
+//
+// Example - Error accumulation:
+//
+//	// If any decoder fails, errors are accumulated
+//	result := combined(map[string]any{}) // Failures with both error messages
+//
+// Example - Numeric aggregation:
+//
+//	import N "github.com/IBM/fp-go/v2/number"
+//
+//	intMonoid := monoid.MakeMonoid(N.Add[int], 0)
+//	m := ApplicativeMonoid[string](intMonoid)
+//
+//	decoder1 := func(input string) Validation[int] {
+//	    return validation.Of(10)
+//	}
+//	decoder2 := func(input string) Validation[int] {
+//	    return validation.Of(32)
+//	}
+//
+//	combined := m.Concat(decoder1, decoder2)
+//	result := combined("input") // Success(42) - values are added
+func ApplicativeMonoid[I, A any](m Monoid[A]) Monoid[Decode[I, A]] {
+	return monoid.ApplicativeMonoid(
+		Of[I, A],
+		MonadMap[I, A, Endomorphism[A]],
+		MonadAp[A, I, A],
+		m,
+	)
+}
+
+// AlternativeMonoid creates a Monoid instance for Decode[I, A] using the Alternative pattern.
+// This combines applicative error-accumulation behavior with alternative fallback behavior,
+// allowing you to both accumulate errors and provide fallback alternatives when combining decoders.
+//
+// The Alternative pattern provides two key operations:
+//   - Applicative operations (Of, Map, Ap): accumulate errors when combining decoders
+//   - Alternative operation (Alt): provide fallback when a decoder fails
+//
+// This monoid is particularly useful when you want to:
+//   - Try multiple decoding strategies and fall back to alternatives
+//   - Combine successful values using the provided monoid
+//   - Accumulate all errors from failed attempts
+//   - Build decoding pipelines with fallback logic
+//
+// **Behavior**:
+//   - Empty: Returns a decoder that always succeeds with the empty value from the inner monoid
+//   - Concat: Combines two decoders using both applicative and alternative semantics:
+//   - If first succeeds and second succeeds: combines decoded values using inner monoid
+//   - If first fails: tries second as fallback (alternative behavior)
+//   - If both fail: **accumulates all errors from both decoders**
+//
+// **Error Aggregation**: When both decoders fail, all validation errors from both attempts
+// are combined using the Errors monoid. This provides complete visibility into why all
+// alternatives failed, which is essential for debugging and user feedback.
+//
+// Type Parameters:
+//   - I: The input type being decoded
+//   - A: The output type after successful decoding
+//
+// Parameters:
+//   - m: The monoid for combining successful decoded values of type A
+//
+// Returns:
+//
+//	A Monoid[Decode[I, A]] that combines applicative and alternative behaviors
+//
+// Example - Combining successful decoders:
+//
+//	import S "github.com/IBM/fp-go/v2/string"
+//
+//	m := AlternativeMonoid[string](S.Monoid)
+//
+//	decoder1 := func(input string) Validation[string] {
+//	    return validation.Of("Hello")
+//	}
+//	decoder2 := func(input string) Validation[string] {
+//	    return validation.Of(" World")
+//	}
+//
+//	combined := m.Concat(decoder1, decoder2)
+//	result := combined("input")
+//	// Result: Success("Hello World") - values combined using string monoid
+//
+// Example - Fallback behavior:
+//
+//	m := AlternativeMonoid[string](S.Monoid)
+//
+//	failing := func(input string) Validation[string] {
+//	    return either.Left[string](validation.Errors{
+//	        {Value: input, Messsage: "primary failed"},
+//	    })
+//	}
+//	fallback := func(input string) Validation[string] {
+//	    return validation.Of("fallback value")
+//	}
+//
+//	combined := m.Concat(failing, fallback)
+//	result := combined("input")
+//	// Result: Success("fallback value") - second decoder used as fallback
+//
+// Example - Error accumulation when both fail:
+//
+//	m := AlternativeMonoid[string](S.Monoid)
+//
+//	failing1 := func(input string) Validation[string] {
+//	    return either.Left[string](validation.Errors{
+//	        {Value: input, Messsage: "error 1"},
+//	    })
+//	}
+//	failing2 := func(input string) Validation[string] {
+//	    return either.Left[string](validation.Errors{
+//	        {Value: input, Messsage: "error 2"},
+//	    })
+//	}
+//
+//	combined := m.Concat(failing1, failing2)
+//	result := combined("input")
+//	// Result: Failures with accumulated errors: ["error 1", "error 2"]
+//
+// Example - Building decoder with multiple fallbacks:
+//
+//	import N "github.com/IBM/fp-go/v2/number"
+//
+//	m := AlternativeMonoid[string](N.MonoidSum[int]())
+//
+//	// Try to parse from different formats
+//	parseJSON := func(input string) Validation[int] { /* ... */ }
+//	parseYAML := func(input string) Validation[int] { /* ... */ }
+//	parseINI := func(input string) Validation[int] { /* ... */ }
+//
+//	// Combine with fallback chain
+//	decoder := m.Concat(m.Concat(parseJSON, parseYAML), parseINI)
+//	// Uses first successful parser, or accumulates all errors if all fail
+//
+// Example - Combining multiple configuration sources:
+//
+//	type Config struct{ Port int }
+//	configMonoid := monoid.MakeMonoid(
+//	    func(a, b Config) Config {
+//	        if b.Port != 0 { return b }
+//	        return a
+//	    },
+//	    Config{Port: 0},
+//	)
+//
+//	m := AlternativeMonoid[map[string]any](configMonoid)
+//
+//	fromEnv := func(data map[string]any) Validation[Config] { /* ... */ }
+//	fromFile := func(data map[string]any) Validation[Config] { /* ... */ }
+//	fromDefault := func(data map[string]any) Validation[Config] {
+//	    return validation.Of(Config{Port: 8080})
+//	}
+//
+//	// Try env, then file, then default
+//	decoder := m.Concat(m.Concat(fromEnv, fromFile), fromDefault)
+//	// Returns first successful config, or all errors if all fail
+func AlternativeMonoid[I, A any](m Monoid[A]) Monoid[Decode[I, A]] {
+	return monoid.AlternativeMonoid(
+		Of[I, A],
+		MonadMap[I, A, func(A) A],
+		MonadAp[A, I, A],
+		MonadAlt[I, A],
+		m,
+	)
+}
+
+// AltMonoid creates a Monoid instance for Decode[I, A] using the Alt (alternative) operation.
+// This monoid provides a way to combine decoders with fallback behavior, where the second
+// decoder is used as an alternative if the first one fails.
+//
+// The Alt operation implements the "try first, fallback to second" pattern, which is useful
+// for decoding scenarios where you want to attempt multiple decoding strategies in sequence
+// and use the first one that succeeds.
+//
+// **Behavior**:
+//   - Empty: Returns the provided zero value (a lazy computation that produces a Decode[I, A])
+//   - Concat: Combines two decoders using Alt semantics:
+//   - If first succeeds: returns the first result (second is never evaluated)
+//   - If first fails: tries the second decoder as fallback
+//   - If both fail: **aggregates errors from both decoders**
+//
+// **Error Aggregation**: When both decoders fail, all validation errors from both attempts
+// are combined using the Errors monoid. This ensures complete visibility into why all
+// alternatives failed.
+//
+// This is different from [AlternativeMonoid] in that:
+//   - AltMonoid uses a custom zero value (provided by the user)
+//   - AlternativeMonoid derives the zero from an inner monoid
+//   - AltMonoid is simpler and only provides fallback behavior
+//   - AlternativeMonoid combines applicative and alternative behaviors
+//
+// Type Parameters:
+//   - I: The input type being decoded
+//   - A: The output type after successful decoding
+//
+// Parameters:
+//   - zero: A lazy computation that produces the identity/empty Decode[I, A].
+//     This is typically a decoder that always succeeds with a default value, or could be
+//     a decoder that always fails representing "no decoding attempted"
+//
+// Returns:
+//
+//	A Monoid[Decode[I, A]] that combines decoders with fallback behavior
+//
+// Example - Using default value as zero:
+//
+//	m := AltMonoid(func() Decode[string, int] {
+//	    return Of[string](0)
+//	})
+//
+//	failing := func(input string) Validation[int] {
+//	    return either.Left[int](validation.Errors{
+//	        {Value: input, Messsage: "failed"},
+//	    })
+//	}
+//	succeeding := func(input string) Validation[int] {
+//	    return validation.Of(42)
+//	}
+//
+//	combined := m.Concat(failing, succeeding)
+//	result := combined("input")
+//	// Result: Success(42) - falls back to second decoder
+//
+//	empty := m.Empty()
+//	result2 := empty("input")
+//	// Result: Success(0) - the provided zero value
+//
+// Example - Chaining multiple fallbacks:
+//
+//	m := AltMonoid(func() Decode[string, Config] {
+//	    return Of[string](defaultConfig)
+//	})
+//
+//	primary := parseFromPrimarySource    // Fails
+//	secondary := parseFromSecondarySource // Fails
+//	tertiary := parseFromTertiarySource   // Succeeds
+//
+//	// Chain fallbacks
+//	decoder := m.Concat(m.Concat(primary, secondary), tertiary)
+//	result := decoder("input")
+//	// Result: Success from tertiary - uses first successful decoder
+//
+// Example - Error aggregation when all fail:
+//
+//	m := AltMonoid(func() Decode[string, int] {
+//	    return func(input string) Validation[int] {
+//	        return either.Left[int](validation.Errors{
+//	            {Messsage: "no default available"},
+//	        })
+//	    }
+//	})
+//
+//	failing1 := func(input string) Validation[int] {
+//	    return either.Left[int](validation.Errors{
+//	        {Value: input, Messsage: "error 1"},
+//	    })
+//	}
+//	failing2 := func(input string) Validation[int] {
+//	    return either.Left[int](validation.Errors{
+//	        {Value: input, Messsage: "error 2"},
+//	    })
+//	}
+//
+//	combined := m.Concat(failing1, failing2)
+//	result := combined("input")
+//	// Result: Failures with accumulated errors: ["error 1", "error 2"]
+//
+// Example - Building a decoder pipeline with fallbacks:
+//
+//	m := AltMonoid(func() Decode[string, Config] {
+//	    return Of[string](defaultConfig)
+//	})
+//
+//	// Try multiple decoding sources in order
+//	decoders := []Decode[string, Config]{
+//	    loadFromFile("config.json"),      // Try file first
+//	    loadFromEnv,                       // Then environment
+//	    loadFromRemote("api.example.com"), // Then remote API
+//	}
+//
+//	// Fold using the monoid to get first successful config
+//	result := array.MonoidFold(m)(decoders)
+//	// Result: First successful config, or defaultConfig if all fail
+//
+// Example - Comparing with AlternativeMonoid:
+//
+//	// AltMonoid - simple fallback with custom zero
+//	altM := AltMonoid(func() Decode[string, int] {
+//	    return Of[string](0)
+//	})
+//
+//	// AlternativeMonoid - combines values when both succeed
+//	import N "github.com/IBM/fp-go/v2/number"
+//	altMonoid := AlternativeMonoid[string](N.MonoidSum[int]())
+//
+//	decoder1 := Of[string](10)
+//	decoder2 := Of[string](32)
+//
+//	// AltMonoid: returns first success (10)
+//	result1 := altM.Concat(decoder1, decoder2)("input")
+//	// Result: Success(10)
+//
+//	// AlternativeMonoid: combines both successes (10 + 32 = 42)
+//	result2 := altMonoid.Concat(decoder1, decoder2)("input")
+//	// Result: Success(42)
+func AltMonoid[I, A any](zero Lazy[Decode[I, A]]) Monoid[Decode[I, A]] {
+	return monoid.AltMonoid(
+		zero,
+		MonadAlt[I, A],
+	)
+}

v2/optics/codec/decode/monoid_test.go

@@ -0,0 +1,970 @@
+package decode
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	MO "github.com/IBM/fp-go/v2/monoid"
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	S "github.com/IBM/fp-go/v2/string"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestApplicativeMonoid(t *testing.T) {
+	t.Run("with string monoid", func(t *testing.T) {
+		m := ApplicativeMonoid[string](S.Monoid)
+
+		t.Run("empty returns decoder that succeeds with empty string", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("any input")
+
+			assert.Equal(t, validation.Of(""), result)
+		})
+
+		t.Run("concat combines successful decoders", func(t *testing.T) {
+			decoder1 := Of[string]("Hello")
+			decoder2 := Of[string](" World")
+
+			combined := m.Concat(decoder1, decoder2)
+			result := combined("input")
+
+			assert.Equal(t, validation.Of("Hello World"), result)
+		})
+
+		t.Run("concat with failure returns failure", func(t *testing.T) {
+			decoder1 := Of[string]("Hello")
+			decoder2 := func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "decode failed"},
+				})
+			}
+
+			combined := m.Concat(decoder1, decoder2)
+			result := combined("input")
+
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(string) Errors { return nil },
+			)
+			assert.Len(t, errors, 1)
+			assert.Equal(t, "decode failed", errors[0].Messsage)
+		})
+
+		t.Run("concat accumulates all errors from both failures", func(t *testing.T) {
+			decoder1 := func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+			decoder2 := func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "error 2"},
+				})
+			}
+
+			combined := m.Concat(decoder1, decoder2)
+			result := combined("input")
+
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(string) Errors { return nil },
+			)
+			assert.Len(t, errors, 2)
+			messages := []string{errors[0].Messsage, errors[1].Messsage}
+			assert.Contains(t, messages, "error 1")
+			assert.Contains(t, messages, "error 2")
+		})
+
+		t.Run("concat with empty preserves decoder", func(t *testing.T) {
+			decoder := Of[string]("test")
+			empty := m.Empty()
+
+			result1 := m.Concat(decoder, empty)("input")
+			result2 := m.Concat(empty, decoder)("input")
+
+			val1 := either.MonadFold(result1,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			val2 := either.MonadFold(result2,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+
+			assert.Equal(t, "test", val1)
+			assert.Equal(t, "test", val2)
+		})
+	})
+
+	t.Run("with int addition monoid", func(t *testing.T) {
+		intMonoid := MO.MakeMonoid(
+			func(a, b int) int { return a + b },
+			0,
+		)
+		m := ApplicativeMonoid[string](intMonoid)
+
+		t.Run("empty returns decoder with zero", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")
+
+			value := either.MonadFold(result,
+				func(Errors) int { return -1 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 0, value)
+		})
+
+		t.Run("concat adds decoded values", func(t *testing.T) {
+			decoder1 := Of[string](10)
+			decoder2 := Of[string](32)
+
+			combined := m.Concat(decoder1, decoder2)
+			result := combined("input")
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 42, value)
+		})
+
+		t.Run("multiple concat operations", func(t *testing.T) {
+			decoder1 := Of[string](1)
+			decoder2 := Of[string](2)
+			decoder3 := Of[string](3)
+			decoder4 := Of[string](4)
+
+			combined := m.Concat(m.Concat(m.Concat(decoder1, decoder2), decoder3), decoder4)
+			result := combined("input")
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 10, value)
+		})
+	})
+
+	t.Run("with map input type", func(t *testing.T) {
+		m := ApplicativeMonoid[map[string]any](S.Monoid)
+
+		t.Run("combines decoders with different inputs", func(t *testing.T) {
+			decoder1 := func(data map[string]any) Validation[string] {
+				if name, ok := data["firstName"].(string); ok {
+					return validation.Of(name)
+				}
+				return either.Left[string](validation.Errors{
+					{Messsage: "missing firstName"},
+				})
+			}
+
+			decoder2 := func(data map[string]any) Validation[string] {
+				if name, ok := data["lastName"].(string); ok {
+					return validation.Of(" " + name)
+				}
+				return either.Left[string](validation.Errors{
+					{Messsage: "missing lastName"},
+				})
+			}
+
+			combined := m.Concat(decoder1, decoder2)
+
+			// Test success case
+			result1 := combined(map[string]any{
+				"firstName": "John",
+				"lastName":  "Doe",
+			})
+			value1 := either.MonadFold(result1,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			assert.Equal(t, "John Doe", value1)
+
+			// Test failure case - both fields missing
+			result2 := combined(map[string]any{})
+			assert.True(t, either.IsLeft(result2))
+			errors := either.MonadFold(result2,
+				F.Identity[Errors],
+				func(string) Errors { return nil },
+			)
+			assert.Len(t, errors, 2)
+		})
+	})
+}
+
+func TestMonoidLaws(t *testing.T) {
+	t.Run("ApplicativeMonoid satisfies monoid laws", func(t *testing.T) {
+		m := ApplicativeMonoid[string](S.Monoid)
+
+		decoder1 := Of[string]("a")
+		decoder2 := Of[string]("b")
+
+		t.Run("left identity", func(t *testing.T) {
+			// empty + a = a
+			result := m.Concat(m.Empty(), decoder1)("input")
+			value := either.MonadFold(result,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			assert.Equal(t, "a", value)
+		})
+
+		t.Run("right identity", func(t *testing.T) {
+			// a + empty = a
+			result := m.Concat(decoder1, m.Empty())("input")
+			value := either.MonadFold(result,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			assert.Equal(t, "a", value)
+		})
+
+		t.Run("associativity", func(t *testing.T) {
+			decoder3 := Of[string]("c")
+			// (a + b) + c = a + (b + c)
+			left := m.Concat(m.Concat(decoder1, decoder2), decoder3)("input")
+			right := m.Concat(decoder1, m.Concat(decoder2, decoder3))("input")
+
+			leftVal := either.MonadFold(left,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			rightVal := either.MonadFold(right,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+
+			assert.Equal(t, "abc", leftVal)
+			assert.Equal(t, "abc", rightVal)
+		})
+	})
+}
+
+func TestApplicativeMonoidWithFailures(t *testing.T) {
+	m := ApplicativeMonoid[string](S.Monoid)
+
+	t.Run("failure propagates through concat", func(t *testing.T) {
+		decoder1 := Of[string]("a")
+		decoder2 := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "error"},
+			})
+		}
+		decoder3 := Of[string]("c")
+
+		combined := m.Concat(m.Concat(decoder1, decoder2), decoder3)
+		result := combined("input")
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(string) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+	})
+
+	t.Run("multiple failures accumulate", func(t *testing.T) {
+		decoder1 := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "error 1"},
+			})
+		}
+		decoder2 := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "error 2"},
+			})
+		}
+		decoder3 := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "error 3"},
+			})
+		}
+
+		combined := m.Concat(m.Concat(decoder1, decoder2), decoder3)
+		result := combined("input")
+
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(string) Errors { return nil },
+		)
+		assert.Len(t, errors, 3)
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "error 1")
+		assert.Contains(t, messages, "error 2")
+		assert.Contains(t, messages, "error 3")
+	})
+}
+
+func TestApplicativeMonoidEdgeCases(t *testing.T) {
+	t.Run("with custom struct monoid", func(t *testing.T) {
+		type Counter struct{ Count int }
+
+		counterMonoid := MO.MakeMonoid(
+			func(a, b Counter) Counter { return Counter{Count: a.Count + b.Count} },
+			Counter{Count: 0},
+		)
+
+		m := ApplicativeMonoid[string](counterMonoid)
+
+		decoder1 := Of[string](Counter{Count: 5})
+		decoder2 := Of[string](Counter{Count: 10})
+
+		combined := m.Concat(decoder1, decoder2)
+		result := combined("input")
+
+		value := either.MonadFold(result,
+			func(Errors) Counter { return Counter{} },
+			F.Identity[Counter],
+		)
+		assert.Equal(t, 15, value.Count)
+	})
+
+	t.Run("empty concat empty", func(t *testing.T) {
+		m := ApplicativeMonoid[string](S.Monoid)
+
+		combined := m.Concat(m.Empty(), m.Empty())
+		result := combined("input")
+
+		value := either.MonadFold(result,
+			func(Errors) string { return "ERROR" },
+			F.Identity[string],
+		)
+		assert.Equal(t, "", value)
+	})
+
+	t.Run("with different input types", func(t *testing.T) {
+		intMonoid := MO.MakeMonoid(
+			func(a, b int) int { return a + b },
+			0,
+		)
+		m := ApplicativeMonoid[int](intMonoid)
+
+		decoder1 := func(input int) Validation[int] {
+			return validation.Of(input * 2)
+		}
+		decoder2 := func(input int) Validation[int] {
+			return validation.Of(input + 10)
+		}
+
+		combined := m.Concat(decoder1, decoder2)
+		result := combined(5)
+
+		value := either.MonadFold(result,
+			func(Errors) int { return 0 },
+			F.Identity[int],
+		)
+		// (5 * 2) + (5 + 10) = 10 + 15 = 25
+		assert.Equal(t, 25, value)
+	})
+}
+
+func TestApplicativeMonoidRealWorldScenarios(t *testing.T) {
+	t.Run("combining configuration from multiple sources", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
+
+		// Monoid that combines configs (last non-empty wins for strings, sum for ints)
+		configMonoid := MO.MakeMonoid(
+			func(a, b Config) Config {
+				host := a.Host
+				if b.Host != "" {
+					host = b.Host
+				}
+				return Config{
+					Host: host,
+					Port: a.Port + b.Port,
+				}
+			},
+			Config{Host: "", Port: 0},
+		)
+
+		m := ApplicativeMonoid[map[string]any](configMonoid)
+
+		decoder1 := func(data map[string]any) Validation[Config] {
+			if host, ok := data["host"].(string); ok {
+				return validation.Of(Config{Host: host, Port: 0})
+			}
+			return either.Left[Config](validation.Errors{
+				{Messsage: "missing host"},
+			})
+		}
+
+		decoder2 := func(data map[string]any) Validation[Config] {
+			if port, ok := data["port"].(int); ok {
+				return validation.Of(Config{Host: "", Port: port})
+			}
+			return either.Left[Config](validation.Errors{
+				{Messsage: "missing port"},
+			})
+		}
+
+		combined := m.Concat(decoder1, decoder2)
+
+		// Success case
+		result := combined(map[string]any{
+			"host": "localhost",
+			"port": 8080,
+		})
+
+		config := either.MonadFold(result,
+			func(Errors) Config { return Config{} },
+			F.Identity[Config],
+		)
+		assert.Equal(t, "localhost", config.Host)
+		assert.Equal(t, 8080, config.Port)
+	})
+
+	t.Run("aggregating validation results", func(t *testing.T) {
+		intMonoid := MO.MakeMonoid(
+			func(a, b int) int { return a + b },
+			0,
+		)
+		m := ApplicativeMonoid[string](intMonoid)
+
+		// Decoder that extracts and validates a number
+		makeDecoder := func(value int, shouldFail bool) Decode[string, int] {
+			return func(input string) Validation[int] {
+				if shouldFail {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "validation failed"},
+					})
+				}
+				return validation.Of(value)
+			}
+		}
+
+		// All succeed - values are summed
+		decoder1 := makeDecoder(10, false)
+		decoder2 := makeDecoder(20, false)
+		decoder3 := makeDecoder(12, false)
+
+		combined := m.Concat(m.Concat(decoder1, decoder2), decoder3)
+		result := combined("input")
+
+		value := either.MonadFold(result,
+			func(Errors) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 42, value)
+
+		// Some fail - errors are accumulated
+		decoder4 := makeDecoder(10, true)
+		decoder5 := makeDecoder(20, true)
+
+		combinedFail := m.Concat(decoder4, decoder5)
+		resultFail := combinedFail("input")
+
+		assert.True(t, either.IsLeft(resultFail))
+		errors := either.MonadFold(resultFail,
+			F.Identity[Errors],
+			func(int) Errors { return nil },
+		)
+		assert.Len(t, errors, 2)
+	})
+}
+
+// TestAlternativeMonoid tests the AlternativeMonoid function
+func TestAlternativeMonoid(t *testing.T) {
+	t.Run("with string monoid", func(t *testing.T) {
+		m := AlternativeMonoid[string](S.Monoid)
+
+		t.Run("empty returns decoder that succeeds with empty string", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")
+
+			assert.Equal(t, validation.Of(""), result)
+		})
+
+		t.Run("concat combines successful decoders using monoid", func(t *testing.T) {
+			decoder1 := Of[string]("Hello")
+			decoder2 := Of[string](" World")
+
+			combined := m.Concat(decoder1, decoder2)
+			result := combined("input")
+
+			assert.Equal(t, validation.Of("Hello World"), result)
+		})
+
+		t.Run("concat uses second as fallback when first fails", func(t *testing.T) {
+			failing := func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "first failed"},
+				})
+			}
+			succeeding := Of[string]("fallback")
+
+			combined := m.Concat(failing, succeeding)
+			result := combined("input")
+
+			assert.Equal(t, validation.Of("fallback"), result)
+		})
+
+		t.Run("concat aggregates errors when both fail", func(t *testing.T) {
+			failing1 := func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+			failing2 := func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "error 2"},
+				})
+			}
+
+			combined := m.Concat(failing1, failing2)
+			result := combined("input")
+
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(string) Errors { return nil },
+			)
+			assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors from both decoders")
+
+			messages := make([]string, len(errors))
+			for i, err := range errors {
+				messages[i] = err.Messsage
+			}
+			assert.Contains(t, messages, "error 1")
+			assert.Contains(t, messages, "error 2")
+		})
+
+		t.Run("concat with empty preserves decoder", func(t *testing.T) {
+			decoder := Of[string]("test")
+			empty := m.Empty()
+
+			result1 := m.Concat(decoder, empty)("input")
+			result2 := m.Concat(empty, decoder)("input")
+
+			val1 := either.MonadFold(result1,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			val2 := either.MonadFold(result2,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+
+			assert.Equal(t, "test", val1)
+			assert.Equal(t, "test", val2)
+		})
+	})
+
+	t.Run("with int addition monoid", func(t *testing.T) {
+		intMonoid := MO.MakeMonoid(
+			func(a, b int) int { return a + b },
+			0,
+		)
+		m := AlternativeMonoid[string](intMonoid)
+
+		t.Run("empty returns decoder with zero", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")
+
+			value := either.MonadFold(result,
+				func(Errors) int { return -1 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 0, value)
+		})
+
+		t.Run("concat combines decoded values when both succeed", func(t *testing.T) {
+			decoder1 := Of[string](10)
+			decoder2 := Of[string](32)
+
+			combined := m.Concat(decoder1, decoder2)
+			result := combined("input")
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 42, value)
+		})
+
+		t.Run("concat uses fallback when first fails", func(t *testing.T) {
+			failing := func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "failed"},
+				})
+			}
+			succeeding := Of[string](42)
+
+			combined := m.Concat(failing, succeeding)
+			result := combined("input")
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 42, value)
+		})
+
+		t.Run("multiple concat operations", func(t *testing.T) {
+			decoder1 := Of[string](1)
+			decoder2 := Of[string](2)
+			decoder3 := Of[string](3)
+			decoder4 := Of[string](4)
+
+			combined := m.Concat(m.Concat(m.Concat(decoder1, decoder2), decoder3), decoder4)
+			result := combined("input")
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 10, value)
+		})
+	})
+
+	t.Run("satisfies monoid laws", func(t *testing.T) {
+		m := AlternativeMonoid[string](S.Monoid)
+
+		decoder1 := Of[string]("a")
+		decoder2 := Of[string]("b")
+		decoder3 := Of[string]("c")
+
+		t.Run("left identity", func(t *testing.T) {
+			result := m.Concat(m.Empty(), decoder1)("input")
+			value := either.MonadFold(result,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			assert.Equal(t, "a", value)
+		})
+
+		t.Run("right identity", func(t *testing.T) {
+			result := m.Concat(decoder1, m.Empty())("input")
+			value := either.MonadFold(result,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			assert.Equal(t, "a", value)
+		})
+
+		t.Run("associativity", func(t *testing.T) {
+			left := m.Concat(m.Concat(decoder1, decoder2), decoder3)("input")
+			right := m.Concat(decoder1, m.Concat(decoder2, decoder3))("input")
+
+			leftVal := either.MonadFold(left,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			rightVal := either.MonadFold(right,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+
+			assert.Equal(t, "abc", leftVal)
+			assert.Equal(t, "abc", rightVal)
+		})
+	})
+
+	t.Run("error aggregation with multiple failures", func(t *testing.T) {
+		m := AlternativeMonoid[string](S.Monoid)
+
+		failing1 := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "error 1"},
+			})
+		}
+		failing2 := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "error 2"},
+			})
+		}
+		failing3 := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "error 3"},
+			})
+		}
+
+		combined := m.Concat(m.Concat(failing1, failing2), failing3)
+		result := combined("input")
+
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(string) Errors { return nil },
+		)
+		assert.GreaterOrEqual(t, len(errors), 3, "Should aggregate errors from all decoders")
+
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "error 1")
+		assert.Contains(t, messages, "error 2")
+		assert.Contains(t, messages, "error 3")
+	})
+}
+
+// TestAltMonoid tests the AltMonoid function
+func TestAltMonoid(t *testing.T) {
+	t.Run("with default value as zero", func(t *testing.T) {
+		m := AltMonoid(func() Decode[string, int] {
+			return Of[string](0)
+		})
+
+		t.Run("empty returns the provided zero decoder", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")
+
+			assert.Equal(t, validation.Of(0), result)
+		})
+
+		t.Run("concat returns first decoder when it succeeds", func(t *testing.T) {
+			decoder1 := Of[string](42)
+			decoder2 := Of[string](100)
+
+			combined := m.Concat(decoder1, decoder2)
+			result := combined("input")
+
+			assert.Equal(t, validation.Of(42), result)
+		})
+
+		t.Run("concat uses second as fallback when first fails", func(t *testing.T) {
+			failing := func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "failed"},
+				})
+			}
+			succeeding := Of[string](42)
+
+			combined := m.Concat(failing, succeeding)
+			result := combined("input")
+
+			assert.Equal(t, validation.Of(42), result)
+		})
+
+		t.Run("concat aggregates errors when both fail", func(t *testing.T) {
+			failing1 := func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+			failing2 := func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 2"},
+				})
+			}
+
+			combined := m.Concat(failing1, failing2)
+			result := combined("input")
+
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(int) Errors { return nil },
+			)
+			assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors from both decoders")
+
+			messages := make([]string, len(errors))
+			for i, err := range errors {
+				messages[i] = err.Messsage
+			}
+			assert.Contains(t, messages, "error 1")
+			assert.Contains(t, messages, "error 2")
+		})
+	})
+
+	t.Run("with failing zero", func(t *testing.T) {
+		m := AltMonoid(func() Decode[string, int] {
+			return func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Messsage: "no default available"},
+				})
+			}
+		})
+
+		t.Run("empty returns the failing zero decoder", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")
+
+			assert.True(t, either.IsLeft(result))
+		})
+
+		t.Run("concat with all failures aggregates errors", func(t *testing.T) {
+			failing1 := func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+			failing2 := func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 2"},
+				})
+			}
+
+			combined := m.Concat(failing1, failing2)
+			result := combined("input")
+
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(int) Errors { return nil },
+			)
+			assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors")
+		})
+	})
+
+	t.Run("chaining multiple fallbacks", func(t *testing.T) {
+		m := AltMonoid(func() Decode[string, string] {
+			return Of[string]("default")
+		})
+
+		primary := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "primary failed"},
+			})
+		}
+		secondary := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "secondary failed"},
+			})
+		}
+		tertiary := Of[string]("tertiary value")
+
+		combined := m.Concat(m.Concat(primary, secondary), tertiary)
+		result := combined("input")
+
+		assert.Equal(t, validation.Of("tertiary value"), result)
+	})
+
+	t.Run("satisfies monoid laws", func(t *testing.T) {
+		m := AltMonoid(func() Decode[string, int] {
+			return Of[string](0)
+		})
+
+		decoder1 := Of[string](1)
+		decoder2 := Of[string](2)
+		decoder3 := Of[string](3)
+
+		t.Run("left identity", func(t *testing.T) {
+			result := m.Concat(m.Empty(), decoder1)("input")
+			value := either.MonadFold(result,
+				func(Errors) int { return -1 },
+				F.Identity[int],
+			)
+			// With AltMonoid, first success wins, so empty (0) is returned
+			assert.Equal(t, 0, value)
+		})
+
+		t.Run("right identity", func(t *testing.T) {
+			result := m.Concat(decoder1, m.Empty())("input")
+			value := either.MonadFold(result,
+				func(Errors) int { return -1 },
+				F.Identity[int],
+			)
+			// First decoder succeeds, so 1 is returned
+			assert.Equal(t, 1, value)
+		})
+
+		t.Run("associativity", func(t *testing.T) {
+			// For AltMonoid, first success wins
+			left := m.Concat(m.Concat(decoder1, decoder2), decoder3)("input")
+			right := m.Concat(decoder1, m.Concat(decoder2, decoder3))("input")
+
+			leftVal := either.MonadFold(left,
+				func(Errors) int { return -1 },
+				F.Identity[int],
+			)
+			rightVal := either.MonadFold(right,
+				func(Errors) int { return -1 },
+				F.Identity[int],
+			)
+
+			// Both should return 1 (first success)
+			assert.Equal(t, 1, leftVal)
+			assert.Equal(t, 1, rightVal)
+		})
+	})
+
+	t.Run("difference from AlternativeMonoid", func(t *testing.T) {
+		// AltMonoid - first success wins
+		altM := AltMonoid(func() Decode[string, int] {
+			return Of[string](0)
+		})
+
+		// AlternativeMonoid - combines successes
+		altMonoid := AlternativeMonoid[string](N.MonoidSum[int]())
+
+		decoder1 := Of[string](10)
+		decoder2 := Of[string](32)
+
+		// AltMonoid: returns first success (10)
+		result1 := altM.Concat(decoder1, decoder2)("input")
+		value1 := either.MonadFold(result1,
+			func(Errors) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 10, value1, "AltMonoid returns first success")
+
+		// AlternativeMonoid: combines both successes (10 + 32 = 42)
+		result2 := altMonoid.Concat(decoder1, decoder2)("input")
+		value2 := either.MonadFold(result2,
+			func(Errors) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 42, value2, "AlternativeMonoid combines successes")
+	})
+
+	t.Run("error aggregation with context", func(t *testing.T) {
+		m := AltMonoid(func() Decode[string, int] {
+			return Of[string](0)
+		})
+
+		failing1 := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{
+					Value:    input,
+					Messsage: "parse error",
+					Context:  validation.Context{{Key: "field", Type: "int"}},
+				},
+			})
+		}
+		failing2 := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{
+					Value:    input,
+					Messsage: "validation error",
+					Context:  validation.Context{{Key: "value", Type: "int"}},
+				},
+			})
+		}
+
+		combined := m.Concat(failing1, failing2)
+		result := combined("abc")
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(int) Errors { return nil },
+		)
+		assert.GreaterOrEqual(t, len(errors), 2, "Should have errors from both decoders")
+
+		// Verify that errors with context are present
+		hasParseError := false
+		hasValidationError := false
+		for _, err := range errors {
+			if err.Messsage == "parse error" {
+				hasParseError = true
+				assert.NotNil(t, err.Context)
+			}
+			if err.Messsage == "validation error" {
+				hasValidationError = true
+				assert.NotNil(t, err.Context)
+			}
+		}
+		assert.True(t, hasParseError, "Should have parse error")
+		assert.True(t, hasValidationError, "Should have validation error")
+	})
+}

v2/optics/codec/decode/types.go

@@ -1,30 +1,346 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package decode
 
 import (
+	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
 	"github.com/IBM/fp-go/v2/reader"
 )
 
 type (
+	// Errors is a collection of validation errors that occurred during decoding.
+	// This is an alias for validation.Errors, which is []*ValidationError.
+	//
+	// Errors accumulates multiple validation failures, allowing decoders to report
+	// all problems at once rather than failing on the first error. This is particularly
+	// useful for form validation, API request validation, and configuration parsing
+	// where users benefit from seeing all issues simultaneously.
+	//
+	// The Errors type forms a Semigroup and Monoid, enabling:
+	//  - Concatenation: Combining errors from multiple decoders
+	//  - Accumulation: Collecting errors through applicative operations
+	//  - Empty value: An empty slice representing no errors (success)
+	//
+	// Each error in the collection is a *ValidationError containing:
+	//  - Value: The actual value that failed validation
+	//  - Context: The path to the value in nested structures
+	//  - Message: Human-readable error description
+	//  - Cause: Optional underlying error
+	//
+	// Example:
+	//
+	//	// Multiple validation failures
+	//	errors := Errors{
+	//	    &validation.ValidationError{
+	//	        Value:    "",
+	//	        Context:  []validation.ContextEntry{{Key: "name"}},
+	//	        Messsage: "name is required",
+	//	    },
+	//	    &validation.ValidationError{
+	//	        Value:    "invalid@",
+	//	        Context:  []validation.ContextEntry{{Key: "email"}},
+	//	        Messsage: "invalid email format",
+	//	    },
+	//	}
+	//
+	//	// Create a failed validation with these errors
+	//	result := validation.Failures[User](errors)
+	//
+	//	// Errors can be combined using the monoid
+	//	moreErrors := Errors{
+	//	    &validation.ValidationError{
+	//	        Value:    -1,
+	//	        Context:  []validation.ContextEntry{{Key: "age"}},
+	//	        Messsage: "age must be positive",
+	//	    },
+	//	}
+	//	allErrors := append(errors, moreErrors...)
+	Errors = validation.Errors
 
 	// Validation represents the result of a validation operation that may contain
 	// validation errors or a successfully validated value of type A.
+	// This is an alias for validation.Validation[A], which is Either[Errors, A].
+	//
+	// In the decode context:
+	//   - Left(Errors): Decoding failed with one or more validation errors
+	//   - Right(A): Successfully decoded value of type A
+	//
+	// Example:
+	//
+	//	// Success case
+	//	valid := validation.Success(42)  // Right(42)
+	//
+	//	// Failure case
+	//	invalid := validation.Failures[int](validation.Errors{
+	//	    &validation.ValidationError{Messsage: "invalid format"},
+	//	})  // Left([...])
 	Validation[A any] = validation.Validation[A]
 
 	// Reader represents a computation that depends on an environment R and produces a value A.
+	// This is an alias for reader.Reader[R, A], which is func(R) A.
+	//
+	// In the decode context, Reader is used to access the input data being decoded.
+	// The environment R is typically the raw input (e.g., JSON, string, bytes) that
+	// needs to be decoded into a structured type A.
+	//
+	// Example:
+	//
+	//	// A reader that extracts a field from a map
+	//	getField := func(data map[string]any) string {
+	//	    return data["name"].(string)
+	//	}  // Reader[map[string]any, string]
 	Reader[R, A any] = reader.Reader[R, A]
 
 	// Decode is a function that decodes input I to type A with validation.
-	// It returns a Validation result directly.
+	// It combines the Reader pattern (for accessing input) with Validation (for error handling).
+	//
+	// Type: func(I) Validation[A]
+	//
+	// A Decode function:
+	//  1. Takes raw input of type I (e.g., JSON, string, bytes)
+	//  2. Attempts to decode/parse it into type A
+	//  3. Returns a Validation[A] with either:
+	//     - Success(A): Successfully decoded value
+	//     - Failures(Errors): Validation errors describing what went wrong
+	//
+	// This type is the foundation of the decode package, enabling composable,
+	// type-safe decoding with comprehensive error reporting.
+	//
+	// Example:
+	//
+	//	// Decode a string to an integer
+	//	decodeInt := func(input string) Validation[int] {
+	//	    n, err := strconv.Atoi(input)
+	//	    if err != nil {
+	//	        return validation.Failures[int](validation.Errors{
+	//	            &validation.ValidationError{
+	//	                Value:    input,
+	//	                Messsage: "not a valid integer",
+	//	                Cause:    err,
+	//	            },
+	//	        })
+	//	    }
+	//	    return validation.Success(n)
+	//	}  // Decode[string, int]
+	//
+	//	result := decodeInt("42")  // Success(42)
+	//	result := decodeInt("abc") // Failures([...])
 	Decode[I, A any] = Reader[I, Validation[A]]
 
 	// Kleisli represents a function from A to a decoded B given input type I.
 	// It's a Reader that takes an input A and produces a Decode[I, B] function.
 	// This enables composition of decoding operations in a functional style.
+	//
+	// Type: func(A) Decode[I, B]
+	//       which expands to: func(A) func(I) Validation[B]
+	//
+	// Kleisli arrows are the fundamental building blocks for composing decoders.
+	// They allow you to chain decoding operations where each step can:
+	//  1. Depend on the result of the previous step (the A parameter)
+	//  2. Access the original input (the I parameter via Decode)
+	//  3. Fail with validation errors (via Validation[B])
+	//
+	// This is particularly useful for:
+	//  - Conditional decoding based on previously decoded values
+	//  - Multi-stage decoding pipelines
+	//  - Dependent field validation
+	//
+	// Example:
+	//
+	//	// Decode a user, then decode their age based on their type
+	//	decodeAge := func(userType string) Decode[map[string]any, int] {
+	//	    return func(data map[string]any) Validation[int] {
+	//	        if userType == "admin" {
+	//	            // Admins must be 18+
+	//	            age := data["age"].(int)
+	//	            if age < 18 {
+	//	                return validation.Failures[int](/* error */)
+	//	            }
+	//	            return validation.Success(age)
+	//	        }
+	//	        // Regular users can be any age
+	//	        return validation.Success(data["age"].(int))
+	//	    }
+	//	}  // Kleisli[map[string]any, string, int]
+	//
+	//	// Use with Chain to compose decoders
+	//	decoder := F.Pipe2(
+	//	    decodeUserType,           // Decode[map[string]any, string]
+	//	    Chain(decodeAge),         // Chains with Kleisli
+	//	    Map(func(age int) User {  // Transform to final type
+	//	        return User{Age: age}
+	//	    }),
+	//	)
 	Kleisli[I, A, B any] = Reader[A, Decode[I, B]]
 
 	// Operator represents a decoding transformation that takes a decoded A and produces a decoded B.
 	// It's a specialized Kleisli arrow for composing decode operations where the input is already decoded.
 	// This allows chaining multiple decode transformations together.
+	//
+	// Type: func(Decode[I, A]) Decode[I, B]
+	//
+	// Operators are higher-order functions that transform one decoder into another.
+	// They are the result of partially applying functions like Map, Chain, and Ap,
+	// making them ideal for use in composition pipelines with F.Pipe.
+	//
+	// Key characteristics:
+	//  - Takes a Decode[I, A] as input
+	//  - Returns a Decode[I, B] as output
+	//  - Preserves the input type I (the raw data being decoded)
+	//  - Transforms the output type from A to B
+	//
+	// Common operators:
+	//  - Map(f): Transforms successful decode results
+	//  - Chain(f): Sequences dependent decode operations
+	//  - Ap(fa): Applies function decoders to value decoders
+	//
+	// Example:
+	//
+	//	// Create reusable operators
+	//	toString := Map(func(n int) string {
+	//	    return strconv.Itoa(n)
+	//	})  // Operator[string, int, string]
+	//
+	//	validatePositive := Chain(func(n int) Decode[string, int] {
+	//	    return func(input string) Validation[int] {
+	//	        if n <= 0 {
+	//	            return validation.Failures[int](/* error */)
+	//	        }
+	//	        return validation.Success(n)
+	//	    }
+	//	})  // Operator[string, int, int]
+	//
+	//	// Compose operators in a pipeline
+	//	decoder := F.Pipe2(
+	//	    decodeInt,          // Decode[string, int]
+	//	    validatePositive,   // Operator[string, int, int]
+	//	    toString,           // Operator[string, int, string]
+	//	)  // Decode[string, string]
+	//
+	//	result := decoder("42")  // Success("42")
+	//	result := decoder("-5")  // Failures([...])
 	Operator[I, A, B any] = Kleisli[I, Decode[I, A], B]
+
+	// Endomorphism represents a function from a type to itself: func(A) A.
+	// This is an alias for endomorphism.Endomorphism[A].
+	//
+	// In the decode context, endomorphisms are used with LetL to transform
+	// decoded values using pure functions that don't change the type.
+	//
+	// Endomorphisms are useful for:
+	//  - Normalizing data (e.g., trimming strings, rounding numbers)
+	//  - Applying business rules (e.g., clamping values to ranges)
+	//  - Data sanitization (e.g., removing special characters)
+	//
+	// Example:
+	//
+	//	// Normalize a string by trimming and lowercasing
+	//	normalize := func(s string) string {
+	//	    return strings.ToLower(strings.TrimSpace(s))
+	//	}  // Endomorphism[string]
+	//
+	//	// Clamp an integer to a range
+	//	clamp := func(n int) int {
+	//	    if n < 0 { return 0 }
+	//	    if n > 100 { return 100 }
+	//	    return n
+	//	}  // Endomorphism[int]
+	//
+	//	// Use with LetL to transform decoded values
+	//	decoder := F.Pipe1(
+	//	    decodeString,
+	//	    LetL(nameLens, normalize),
+	//	)
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	// Monoid represents an algebraic structure with an associative binary operation
+	// and an identity element. This is an alias for monoid.Monoid[A].
+	//
+	// A Monoid[A] consists of:
+	//  - Concat: func(A, A) A - An associative binary operation
+	//  - Empty: func() A - An identity element
+	//
+	// In the decode context, monoids are used to combine multiple decoders or
+	// validation results. The most common use case is combining validation errors
+	// from multiple decoders using the Errors monoid.
+	//
+	// Properties:
+	//  - Associativity: Concat(Concat(a, b), c) == Concat(a, Concat(b, c))
+	//  - Identity: Concat(Empty(), a) == a == Concat(a, Empty())
+	//
+	// Common monoid instances:
+	//  - Errors: Combines validation errors from multiple sources
+	//  - Array: Concatenates arrays of decoded values
+	//  - String: Concatenates strings
+	//
+	// Example:
+	//
+	//	// Combine validation errors from multiple decoders
+	//	errorsMonoid := validation.GetMonoid[int]()
+	//
+	//	// Decode multiple fields and combine errors
+	//	result1 := decodeField1(data)  // Validation[string]
+	//	result2 := decodeField2(data)  // Validation[int]
+	//
+	//	// If both fail, errors are combined using the monoid
+	//	combined := errorsMonoid.Concat(result1, result2)
+	//
+	//	// The monoid's Empty() provides a successful validation with no errors
+	//	empty := errorsMonoid.Empty()  // Success with no value
+	Monoid[A any] = monoid.Monoid[A]
+
+	// Lazy represents a deferred computation that produces a value of type A.
+	// This is an alias for lazy.Lazy[A], which is func() A.
+	//
+	// In the decode context, Lazy is used to defer expensive computations or
+	// recursive decoder definitions until they are actually needed. This is
+	// particularly important for:
+	//  - Recursive data structures (e.g., trees, linked lists)
+	//  - Expensive default values
+	//  - Breaking circular dependencies in decoder definitions
+	//
+	// A Lazy[A] is simply a function that takes no arguments and returns A.
+	// The computation is only executed when the function is called, allowing
+	// for lazy evaluation and recursive definitions.
+	//
+	// Example:
+	//
+	//	// Define a recursive decoder for a tree structure
+	//	type Tree struct {
+	//	    Value    int
+	//	    Children []Tree
+	//	}
+	//
+	//	// Use Lazy to break the circular dependency
+	//	var decodeTree Decode[map[string]any, Tree]
+	//	decodeTree = func(data map[string]any) Validation[Tree] {
+	//	    // Lazy evaluation allows referencing decodeTree within itself
+	//	    childrenDecoder := Array(Lazy(func() Decode[map[string]any, Tree] {
+	//	        return decodeTree
+	//	    }))
+	//	    // ... rest of decoder implementation
+	//	}
+	//
+	//	// Lazy default value that's only computed if needed
+	//	expensiveDefault := Lazy(func() Config {
+	//	    // This computation only runs if the decode fails
+	//	    return computeExpensiveDefaultConfig()
+	//	})
+	Lazy[A any] = lazy.Lazy[A]
 )

v2/optics/codec/either.go

@@ -0,0 +1,265 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/optics/codec/validate"
+)
+
+// encodeEither creates an encoder for Either[A, B] values.
+//
+// This function produces an encoder that handles both Left and Right cases of an Either value.
+// It uses the provided codecs to encode the Left (A) and Right (B) values respectively.
+//
+// # Type Parameters
+//
+//   - A: The type of the Left value
+//   - B: The type of the Right value
+//   - O: The output type after encoding
+//   - I: The input type for validation (not used in encoding)
+//
+// # Parameters
+//
+//   - leftItem: The codec for encoding Left values of type A
+//   - rightItem: The codec for encoding Right values of type B
+//
+// # Returns
+//
+// An Encode function that takes an Either[A, B] and returns O by encoding
+// either the Left or Right value using the appropriate codec.
+//
+// # Example
+//
+//	stringCodec := String()
+//	intCodec := Int()
+//	encoder := encodeEither(stringCodec, intCodec)
+//
+//	// Encode a Left value
+//	leftResult := encoder(either.Left[int]("error"))
+//	// leftResult contains the encoded string "error"
+//
+//	// Encode a Right value
+//	rightResult := encoder(either.Right[string](42))
+//	// rightResult contains the encoded int 42
+//
+// # Notes
+//
+//   - Uses either.Fold to pattern match on the Either value
+//   - Left values are encoded using leftItem.Encode
+//   - Right values are encoded using rightItem.Encode
+func encodeEither[A, B, O, I any](
+	leftItem Type[A, O, I],
+	rightItem Type[B, O, I],
+) Encode[either.Either[A, B], O] {
+	return either.Fold(
+		leftItem.Encode,
+		rightItem.Encode,
+	)
+}
+
+// validateEither creates a validator for Either[A, B] values.
+//
+// This function produces a validator that attempts to validate the input as both
+// a Left (A) and Right (B) value. The validation strategy is:
+//  1. First, try to validate as a Right value (B)
+//  2. If Right validation succeeds, return Either.Right[A](B)
+//  3. If Right validation fails, try to validate as a Left value (A)
+//  4. If Left validation succeeds, return Either.Left[B](A)
+//  5. If both validations fail, concatenate all errors from both attempts
+//
+// This approach ensures that the validator tries both branches and provides
+// comprehensive error information when both fail.
+//
+// # Type Parameters
+//
+//   - A: The type of the Left value
+//   - B: The type of the Right value
+//   - O: The output type after encoding (not used in validation)
+//   - I: The input type to validate
+//
+// # Parameters
+//
+//   - leftItem: The codec for validating Left values of type A
+//   - rightItem: The codec for validating Right values of type B
+//
+// # Returns
+//
+// A Validate function that takes an input I and returns a Decode function.
+// The Decode function takes a Context and returns a Validation[Either[A, B]].
+//
+// # Validation Logic
+//
+// The validator follows this decision tree:
+//
+//	Input I
+//	  |
+//	  +--> Validate as Right (B)
+//	         |
+//	         +-- Success --> Return Either.Right[A](B)
+//	         |
+//	         +-- Failure --> Validate as Left (A)
+//	                           |
+//	                           +-- Success --> Return Either.Left[B](A)
+//	                           |
+//	                           +-- Failure --> Return all errors (Left + Right)
+//
+// # Example
+//
+//	stringCodec := String()
+//	intCodec := Int()
+//	validator := validateEither(stringCodec, intCodec)
+//
+//	// Validate a string (will succeed as Left)
+//	result1 := validator("hello")(validation.Context{})
+//	// result1 is Success(Either.Left[int]("hello"))
+//
+//	// Validate an int (will succeed as Right)
+//	result2 := validator(42)(validation.Context{})
+//	// result2 is Success(Either.Right[string](42))
+//
+//	// Validate something that's neither (will fail with both errors)
+//	result3 := validator([]int{1, 2, 3})(validation.Context{})
+//	// result3 is Failure with errors from both string and int validation
+//
+// # Notes
+//
+//   - Prioritizes Right validation over Left validation
+//   - Accumulates errors from both branches when both fail
+//   - Uses the validation context to provide detailed error messages
+//   - The validator is lazy: it only evaluates Left if Right fails
+func validateEither[A, B, O, I any](
+	leftItem Type[A, O, I],
+	rightItem Type[B, O, I],
+) Validate[I, either.Either[A, B]] {
+
+	valRight := F.Pipe1(
+		rightItem.Validate,
+		validate.Map[I, B](either.Right[A]),
+	)
+
+	valLeft := F.Pipe1(
+		leftItem.Validate,
+		validate.Map[I, A](either.Left[B]),
+	)
+
+	return F.Pipe1(
+		valRight,
+		validate.Alt(lazy.Of(valLeft)),
+	)
+}
+
+// Either creates a codec for Either[A, B] values.
+//
+// This function constructs a complete codec that can encode, decode, and validate
+// Either values. An Either represents a value that can be one of two types: Left (A)
+// or Right (B). This is commonly used for error handling, where Left represents an
+// error and Right represents a success value.
+//
+// The codec handles both branches of the Either type using the provided codecs for
+// each branch. During validation, it attempts to validate the input as both types
+// and succeeds if either validation passes.
+//
+// # Type Parameters
+//
+//   - A: The type of the Left value
+//   - B: The type of the Right value
+//   - O: The output type after encoding
+//   - I: The input type for validation
+//
+// # Parameters
+//
+//   - leftItem: The codec for handling Left values of type A
+//   - rightItem: The codec for handling Right values of type B
+//
+// # Returns
+//
+// A Type[either.Either[A, B], O, I] that can encode, decode, and validate Either values.
+//
+// # Codec Behavior
+//
+// Encoding:
+//   - Left values are encoded using leftItem.Encode
+//   - Right values are encoded using rightItem.Encode
+//
+// Validation:
+//   - First attempts to validate as Right (B)
+//   - If Right fails, attempts to validate as Left (A)
+//   - If both fail, returns all accumulated errors
+//   - If either succeeds, returns the corresponding Either value
+//
+// Type Checking:
+//   - Uses Is[either.Either[A, B]]() to verify the value is an Either
+//
+// Naming:
+//   - The codec name is "Either[<leftName>, <rightName>]"
+//   - Example: "Either[string, int]"
+//
+// # Example
+//
+//	// Create a codec for Either[string, int]
+//	stringCodec := String()
+//	intCodec := Int()
+//	eitherCodec := Either(stringCodec, intCodec)
+//
+//	// Encode a Left value
+//	leftEncoded := eitherCodec.Encode(either.Left[int]("error"))
+//	// leftEncoded contains the encoded string
+//
+//	// Encode a Right value
+//	rightEncoded := eitherCodec.Encode(either.Right[string](42))
+//	// rightEncoded contains the encoded int
+//
+//	// Decode/validate an input
+//	result := eitherCodec.Decode("hello")
+//	// result is Success(Either.Left[int]("hello"))
+//
+//	result2 := eitherCodec.Decode(42)
+//	// result2 is Success(Either.Right[string](42))
+//
+//	// Get the codec name
+//	name := eitherCodec.Name()
+//	// name is "Either[string, int]"
+//
+// # Use Cases
+//
+//   - Error handling: Either[Error, Value]
+//   - Alternative values: Either[DefaultValue, CustomValue]
+//   - Union types: Either[TypeA, TypeB]
+//   - Validation results: Either[ValidationError, ValidatedValue]
+//
+// # Notes
+//
+//   - The codec prioritizes Right validation over Left validation
+//   - Both branches must have compatible encoding output types (O)
+//   - Both branches must have compatible validation input types (I)
+//   - The codec name includes the names of both branch codecs
+//   - This is a building block for more complex sum types
+func Either[A, B, O, I any](
+	leftItem Type[A, O, I],
+	rightItem Type[B, O, I],
+) Type[either.Either[A, B], O, I] {
+	return MakeType(
+		fmt.Sprintf("Either[%s, %s]", leftItem.Name(), rightItem.Name()),
+		Is[either.Either[A, B]](),
+		validateEither(leftItem, rightItem),
+		encodeEither(leftItem, rightItem),
+	)
+}

v2/optics/codec/either_test.go

@@ -0,0 +1,368 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+	"strconv"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestEitherWithIdentityCodecs tests the Either function with identity codecs
+// where both branches have the same output and input types
+func TestEitherWithIdentityCodecs(t *testing.T) {
+	t.Run("creates codec with correct name", func(t *testing.T) {
+		// The Either function is designed for cases where both branches encode to the same type
+		// For example, both encode to string or both encode to JSON
+
+		// Create codecs that both encode to string
+		stringToString := Id[string]()
+		intToString := IntFromString()
+
+		eitherCodec := Either(stringToString, intToString)
+
+		assert.Equal(t, "Either[string, IntFromString]", eitherCodec.Name())
+	})
+}
+
+// TestEitherEncode tests encoding of Either values
+func TestEitherEncode(t *testing.T) {
+	// Create codecs that both encode to string
+	stringToString := Id[string]()
+	intToString := IntFromString()
+
+	eitherCodec := Either(stringToString, intToString)
+
+	t.Run("encodes Left value", func(t *testing.T) {
+		leftValue := either.Left[int]("hello")
+		encoded := eitherCodec.Encode(leftValue)
+
+		assert.Equal(t, "hello", encoded)
+	})
+
+	t.Run("encodes Right value", func(t *testing.T) {
+		rightValue := either.Right[string](42)
+		encoded := eitherCodec.Encode(rightValue)
+
+		assert.Equal(t, "42", encoded)
+	})
+}
+
+// TestEitherDecode tests decoding/validation of Either values
+func TestEitherDecode(t *testing.T) {
+	getOrElseNull := either.GetOrElse(reader.Of[validation.Errors, either.Either[string, int]](either.Left[int]("")))
+
+	// Create codecs that both work with string input
+	stringCodec := Id[string]()
+	intFromString := IntFromString()
+
+	eitherCodec := Either(stringCodec, intFromString)
+
+	t.Run("decodes integer string as Right", func(t *testing.T) {
+		result := eitherCodec.Decode("42")
+
+		assert.True(t, either.IsRight(result), "should successfully decode integer string")
+
+		value := getOrElseNull(result)
+		assert.True(t, either.IsRight(value), "should be Right")
+
+		rightValue := either.MonadFold(value,
+			func(string) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 42, rightValue)
+	})
+
+	t.Run("decodes non-integer string as Left", func(t *testing.T) {
+		result := eitherCodec.Decode("hello")
+
+		assert.True(t, either.IsRight(result), "should successfully decode string")
+
+		value := getOrElseNull(result)
+		assert.True(t, either.IsLeft(value), "should be Left")
+
+		leftValue := either.MonadFold(value,
+			F.Identity[string],
+			func(int) string { return "" },
+		)
+		assert.Equal(t, "hello", leftValue)
+	})
+}
+
+// TestEitherValidation tests validation behavior
+func TestEitherValidation(t *testing.T) {
+	t.Run("validates with custom codecs", func(t *testing.T) {
+		// Create a codec that only accepts non-empty strings
+		nonEmptyString := MakeType(
+			"NonEmptyString",
+			func(u any) either.Either[error, string] {
+				s, ok := u.(string)
+				if !ok || len(s) == 0 {
+					return either.Left[string](fmt.Errorf("not a non-empty string"))
+				}
+				return either.Of[error](s)
+			},
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if len(s) == 0 {
+						return validation.FailureWithMessage[string](s, "must not be empty")(c)
+					}
+					return validation.Success(s)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Create a codec that only accepts positive integers from strings
+		positiveIntFromString := MakeType(
+			"PositiveInt",
+			func(u any) either.Either[error, int] {
+				i, ok := u.(int)
+				if !ok || i <= 0 {
+					return either.Left[int](fmt.Errorf("not a positive integer"))
+				}
+				return either.Of[error](i)
+			},
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					var n int
+					_, err := fmt.Sscanf(s, "%d", &n)
+					if err != nil {
+						return validation.FailureWithError[int](s, "expected integer string")(err)(c)
+					}
+					if n <= 0 {
+						return validation.FailureWithMessage[int](n, "must be positive")(c)
+					}
+					return validation.Success(n)
+				}
+			},
+			func(n int) string {
+				return fmt.Sprintf("%d", n)
+			},
+		)
+
+		eitherCodec := Either(nonEmptyString, positiveIntFromString)
+
+		// Valid non-empty string
+		validLeft := eitherCodec.Decode("hello")
+		assert.True(t, either.IsRight(validLeft))
+
+		// Valid positive integer
+		validRight := eitherCodec.Decode("42")
+		assert.True(t, either.IsRight(validRight))
+
+		// Invalid empty string - should fail both validations
+		invalidEmpty := eitherCodec.Decode("")
+		assert.True(t, either.IsLeft(invalidEmpty))
+
+		// Invalid zero - should fail Right validation, succeed as Left
+		zeroResult := eitherCodec.Decode("0")
+		// "0" is a valid non-empty string, so it should succeed as Left
+		assert.True(t, either.IsRight(zeroResult))
+	})
+}
+
+// TestEitherRoundTrip tests encoding and decoding round trips
+func TestEitherRoundTrip(t *testing.T) {
+	stringCodec := Id[string]()
+	intFromString := IntFromString()
+
+	eitherCodec := Either(stringCodec, intFromString)
+
+	t.Run("round-trip Left value", func(t *testing.T) {
+		original := "hello"
+
+		// Decode
+		decodeResult := eitherCodec.Decode(original)
+		require.True(t, either.IsRight(decodeResult))
+
+		decoded := either.MonadFold(decodeResult,
+			func(validation.Errors) either.Either[string, int] { return either.Left[int]("") },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Encode
+		encoded := eitherCodec.Encode(decoded)
+
+		// Verify
+		assert.Equal(t, original, encoded)
+	})
+
+	t.Run("round-trip Right value", func(t *testing.T) {
+		original := "42"
+
+		// Decode
+		decodeResult := eitherCodec.Decode(original)
+		require.True(t, either.IsRight(decodeResult))
+
+		decoded := either.MonadFold(decodeResult,
+			func(validation.Errors) either.Either[string, int] { return either.Right[string](0) },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Encode
+		encoded := eitherCodec.Encode(decoded)
+
+		// Verify
+		assert.Equal(t, original, encoded)
+	})
+}
+
+// TestEitherPrioritization tests that Right validation is prioritized over Left
+func TestEitherPrioritization(t *testing.T) {
+	stringCodec := Id[string]()
+	intFromString := IntFromString()
+
+	eitherCodec := Either(stringCodec, intFromString)
+
+	t.Run("prioritizes Right over Left when both could succeed", func(t *testing.T) {
+		// "42" can be validated as both string (Left) and int (Right)
+		// The codec should prioritize Right
+		result := eitherCodec.Decode("42")
+
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) either.Either[string, int] { return either.Left[int]("") },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Should be Right because int validation succeeds and is prioritized
+		assert.True(t, either.IsRight(value))
+
+		rightValue := either.MonadFold(value,
+			func(string) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 42, rightValue)
+	})
+
+	t.Run("falls back to Left when Right fails", func(t *testing.T) {
+		// "hello" can only be validated as string (Left), not as int (Right)
+		result := eitherCodec.Decode("hello")
+
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) either.Either[string, int] { return either.Left[int]("") },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Should be Left because int validation failed
+		assert.True(t, either.IsLeft(value))
+
+		leftValue := either.MonadFold(value,
+			F.Identity[string],
+			func(int) string { return "" },
+		)
+		assert.Equal(t, "hello", leftValue)
+	})
+}
+
+// TestEitherErrorAccumulation tests that errors from both branches are accumulated
+func TestEitherErrorAccumulation(t *testing.T) {
+	// Create codecs with specific validation rules that will both fail
+	nonEmptyString := MakeType(
+		"NonEmptyString",
+		func(u any) either.Either[error, string] {
+			s, ok := u.(string)
+			if !ok || len(s) == 0 {
+				return either.Left[string](fmt.Errorf("not a non-empty string"))
+			}
+			return either.Of[error](s)
+		},
+		func(s string) Decode[Context, string] {
+			return func(c Context) Validation[string] {
+				if len(s) == 0 {
+					return validation.FailureWithMessage[string](s, "must not be empty")(c)
+				}
+				return validation.Success(s)
+			}
+		},
+		F.Identity[string],
+	)
+
+	positiveIntFromString := MakeType(
+		"PositiveInt",
+		func(u any) either.Either[error, int] {
+			i, ok := u.(int)
+			if !ok || i <= 0 {
+				return either.Left[int](fmt.Errorf("not a positive integer"))
+			}
+			return either.Of[error](i)
+		},
+		func(s string) Decode[Context, int] {
+			return func(c Context) Validation[int] {
+				var n int
+				_, err := fmt.Sscanf(s, "%d", &n)
+				if err != nil {
+					return validation.FailureWithError[int](s, "expected integer string")(err)(c)
+				}
+				if n <= 0 {
+					return validation.FailureWithMessage[int](n, "must be positive")(c)
+				}
+				return validation.Success(n)
+			}
+		},
+		strconv.Itoa,
+	)
+
+	eitherCodec := Either(nonEmptyString, positiveIntFromString)
+
+	t.Run("accumulates errors from both branches when both fail", func(t *testing.T) {
+		// Empty string will fail both validations
+		result := eitherCodec.Decode("")
+
+		assert.True(t, either.IsLeft(result))
+
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(either.Either[string, int]) validation.Errors { return nil },
+		)
+
+		require.NotNil(t, errors)
+		// Should have errors from both string and int validation attempts
+		assert.GreaterOrEqual(t, len(errors), 2, "Should have at least 2 errors (one from Right validation, one from Left validation)")
+
+		// Verify we have errors from both validation attempts
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+
+		// Check that we have errors related to both validations
+		hasIntError := false
+		hasStringError := false
+		for _, msg := range messages {
+			if msg == "expected integer string" || msg == "must be positive" {
+				hasIntError = true
+			}
+			if msg == "must not be empty" {
+				hasStringError = true
+			}
+		}
+
+		assert.True(t, hasIntError, "Should have error from integer validation (Right branch)")
+		assert.True(t, hasStringError, "Should have error from string validation (Left branch)")
+	})
+}

v2/optics/codec/types.go

@@ -4,6 +4,7 @@ import (
 	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/internal/formatting"
 	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/codec/decode"
 	"github.com/IBM/fp-go/v2/optics/codec/validate"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
@@ -40,6 +41,27 @@ type (
 
 	// Codec combines a Decoder and an Encoder for bidirectional transformations.
 	// It can decode input I to type A and encode type A to output O.
+	//
+	// This is a simple struct that pairs a decoder with an encoder, providing
+	// the basic building blocks for bidirectional data transformation. Unlike
+	// the Type interface, Codec is a concrete struct without validation context
+	// or type checking capabilities.
+	//
+	// Type Parameters:
+	//   - I: The input type to decode from
+	//   - O: The output type to encode to
+	//   - A: The intermediate type (decoded to, encoded from)
+	//
+	// Fields:
+	//   - Decode: A decoder that transforms I to A
+	//   - Encode: An encoder that transforms A to O
+	//
+	// Example:
+	//   A Codec[string, string, int] can decode strings to integers and
+	//   encode integers back to strings.
+	//
+	// Note: For most use cases, prefer using the Type interface which provides
+	// additional validation and type checking capabilities.
 	Codec[I, O, A any] struct {
 		Decode decoder.Decoder[I, A]
 		Encode encoder.Encoder[O, A]
@@ -55,16 +77,82 @@ type (
 
 	// Validate is a function that validates input I to produce type A.
 	// It takes an input and returns a Reader that depends on the validation Context.
+	//
+	// The Validate type is the core validation abstraction, defined as:
+	//   Reader[I, Decode[Context, A]]
+	//
+	// This means:
+	//  1. It takes an input of type I
+	//  2. Returns a Reader that depends on validation Context
+	//  3. That Reader produces a Validation[A] (Either[Errors, A])
+	//
+	// This layered structure allows validators to:
+	//   - Access the input value
+	//   - Track validation context (path in nested structures)
+	//   - Accumulate multiple validation errors
+	//   - Compose with other validators
+	//
+	// Example:
+	//   A Validate[string, int] takes a string and returns a context-aware
+	//   function that validates and converts it to an integer.
 	Validate[I, A any] = validate.Validate[I, A]
 
 	// Decode is a function that decodes input I to type A with validation.
 	// It returns a Validation result directly.
+	//
+	// The Decode type is defined as:
+	//   Reader[I, Validation[A]]
+	//
+	// This is simpler than Validate as it doesn't require explicit context passing.
+	// The context is typically created automatically when the decoder is invoked.
+	//
+	// Decode is used when:
+	//   - You don't need to manually manage validation context
+	//   - You want a simpler API for basic validation
+	//   - You're working at the top level of validation
+	//
+	// Example:
+	//   A Decode[string, int] takes a string and returns a Validation[int]
+	//   which is Either[Errors, int].
 	Decode[I, A any] = decode.Decode[I, A]
 
 	// Encode is a function that encodes type A to output O.
+	//
+	// Encode is simply a Reader[A, O], which is a function from A to O.
+	// Encoders are pure functions with no error handling - they assume
+	// the input is valid.
+	//
+	// Encoding is the inverse of decoding:
+	//   - Decoding: I -> Validation[A] (may fail)
+	//   - Encoding: A -> O (always succeeds)
+	//
+	// Example:
+	//   An Encode[int, string] takes an integer and returns its string
+	//   representation.
 	Encode[A, O any] = Reader[A, O]
 
 	// Decoder is an interface for types that can decode and validate input.
+	//
+	// A Decoder transforms input of type I into a validated value of type A,
+	// providing detailed error information when validation fails. It supports
+	// both context-aware validation (via Validate) and direct decoding (via Decode).
+	//
+	// Type Parameters:
+	//   - I: The input type to decode from
+	//   - A: The target type to decode to
+	//
+	// Methods:
+	//   - Name(): Returns a descriptive name for this decoder (used in error messages)
+	//   - Validate(I): Returns a context-aware validation function that can track
+	//     the path through nested structures
+	//   - Decode(I): Directly decodes input to a Validation result with a fresh context
+	//
+	// The Validate method is more flexible as it returns a Reader that can be called
+	// with different contexts, while Decode is a convenience method that creates a
+	// new context automatically.
+	//
+	// Example:
+	//   A Decoder[string, int] can decode strings to integers with validation.
 	Decoder[I, A any] interface {
 		Name() string
 		Validate(I) Decode[Context, A]
@@ -72,13 +160,76 @@ type (
 	}
 
 	// Encoder is an interface for types that can encode values.
+	//
+	// An Encoder transforms values of type A into output format O. This is the
+	// inverse operation of decoding, allowing bidirectional transformations.
+	//
+	// Type Parameters:
+	//   - A: The source type to encode from
+	//   - O: The output type to encode to
+	//
+	// Methods:
+	//   - Encode(A): Transforms a value of type A into output format O
+	//
+	// Encoders are pure functions with no validation or error handling - they
+	// assume the input is valid. Validation should be performed during decoding.
+	//
+	// Example:
+	//   An Encoder[int, string] can encode integers to their string representation.
 	Encoder[A, O any] interface {
 		// Encode transforms a value of type A into output format O.
 		Encode(A) O
 	}
+
 	// Type is a bidirectional codec that combines encoding, decoding, validation,
 	// and type checking capabilities. It represents a complete specification of
 	// how to work with a particular type.
+	//
+	// Type is the central abstraction in the codec package, providing:
+	//   - Decoding: Transform input I to validated type A
+	//   - Encoding: Transform type A to output O
+	//   - Validation: Context-aware validation with detailed error reporting
+	//   - Type Checking: Runtime type verification via Is()
+	//   - Formatting: Human-readable type descriptions via Name()
+	//
+	// Type Parameters:
+	//   - A: The target type (what we decode to and encode from)
+	//   - O: The output type (what we encode to)
+	//   - I: The input type (what we decode from)
+	//
+	// Common patterns:
+	//   - Type[A, A, A]: Identity codec (no transformation)
+	//   - Type[A, string, string]: String-based serialization
+	//   - Type[A, any, any]: Generic codec accepting any input/output
+	//   - Type[A, JSON, JSON]: JSON codec
+	//
+	// Methods:
+	//   - Name(): Returns the codec's descriptive name
+	//   - Validate(I): Returns context-aware validation function
+	//   - Decode(I): Decodes input with automatic context creation
+	//   - Encode(A): Encodes value to output format
+	//   - AsDecoder(): Returns this Type as a Decoder interface
+	//   - AsEncoder(): Returns this Type as an Encoder interface
+	//   - Is(any): Checks if a value can be converted to type A
+	//
+	// Example usage:
+	//   intCodec := codec.Int()                    // Type[int, int, any]
+	//   stringCodec := codec.String()              // Type[string, string, any]
+	//   intFromString := codec.IntFromString()     // Type[int, string, string]
+	//
+	//   // Decode
+	//   result := intFromString.Decode("42")       // Validation[int]
+	//
+	//   // Encode
+	//   str := intFromString.Encode(42)            // "42"
+	//
+	//   // Type check
+	//   isInt := intCodec.Is(42)                   // Right(42)
+	//   notInt := intCodec.Is("42")                // Left(error)
+	//
+	// Composition:
+	//   Types can be composed using operators like Alt, Map, Chain, and Pipe
+	//   to build complex codecs from simpler ones.
 	Type[A, O, I any] interface {
 		Formattable
 		Decoder[I, A]
@@ -99,6 +250,92 @@ type (
 	// contain a value of type A. It provides a way to preview and review values.
 	Prism[S, A any] = prism.Prism[S, A]
 
-	// Refinement represents the concept that B is a specialized type of A
+	// Refinement represents the concept that B is a specialized type of A.
+	// It's an alias for Prism[A, B], providing a semantic name for type refinement operations.
+	//
+	// A refinement allows you to:
+	//   - Preview: Try to extract a B from an A (may fail if A is not a B)
+	//   - Review: Inject a B back into an A
+	//
+	// This is useful for working with subtypes, validated types, or constrained types.
+	//
+	// Example:
+	//   - Refinement[int, PositiveInt] - refines int to positive integers only
+	//   - Refinement[string, NonEmptyString] - refines string to non-empty strings
+	//   - Refinement[any, User] - refines any to User type
 	Refinement[A, B any] = Prism[A, B]
+
+	// Kleisli represents a Kleisli arrow in the codec context.
+	// It's a function that takes a value of type A and returns a codec Type[B, O, I].
+	//
+	// This is the fundamental building block for codec transformations and compositions.
+	// Kleisli arrows allow you to:
+	//   - Chain codec operations
+	//   - Build dependent codecs (where the next codec depends on the previous result)
+	//   - Create codec pipelines
+	//
+	// Type Parameters:
+	//   - A: The input type to the function
+	//   - B: The target type that the resulting codec decodes to
+	//   - O: The output type that the resulting codec encodes to
+	//   - I: The input type that the resulting codec decodes from
+	//
+	// Example:
+	//   A Kleisli[string, int, string, string] takes a string and returns a codec
+	//   that can decode strings to ints and encode ints to strings.
+	Kleisli[A, B, O, I any] = Reader[A, Type[B, O, I]]
+
+	// Operator is a specialized Kleisli arrow that transforms codecs.
+	// It takes a codec Type[A, O, I] and returns a new codec Type[B, O, I].
+	//
+	// Operators are the primary way to build codec transformation pipelines.
+	// They enable functional composition of codec transformations using F.Pipe.
+	//
+	// Type Parameters:
+	//   - A: The source type that the input codec decodes to
+	//   - B: The target type that the output codec decodes to
+	//   - O: The output type (same for both input and output codecs)
+	//   - I: The input type (same for both input and output codecs)
+	//
+	// Common operators include:
+	//   - Map: Transforms the decoded value
+	//   - Chain: Sequences dependent codec operations
+	//   - Alt: Provides alternative fallback codecs
+	//   - Refine: Adds validation constraints
+	//
+	// Example:
+	//   An Operator[int, PositiveInt, int, any] transforms a codec that decodes
+	//   to int into a codec that decodes to PositiveInt (with validation).
+	//
+	// Usage with F.Pipe:
+	//   codec := F.Pipe2(
+	//       baseCodec,
+	//       operator1,  // Operator[A, B, O, I]
+	//       operator2,  // Operator[B, C, O, I]
+	//   )
+	Operator[A, B, O, I any] = Kleisli[Type[A, O, I], B, O, I]
+
+	// Monoid represents an algebraic structure with an associative binary operation
+	// and an identity element.
+	//
+	// A Monoid[A] provides:
+	//   - Empty(): Returns the identity element
+	//   - Concat(A, A): Combines two values associatively
+	//
+	// Monoid laws:
+	//   1. Left Identity: Concat(Empty(), a) = a
+	//   2. Right Identity: Concat(a, Empty()) = a
+	//   3. Associativity: Concat(Concat(a, b), c) = Concat(a, Concat(b, c))
+	//
+	// In the codec context, monoids are used to:
+	//   - Combine multiple codecs with specific semantics
+	//   - Build codec chains with fallback behavior (AltMonoid)
+	//   - Aggregate validation results (ApplicativeMonoid)
+	//   - Compose codec transformations
+	//
+	// Example monoids for codecs:
+	//   - AltMonoid: First success wins (alternative semantics)
+	//   - ApplicativeMonoid: Combines successful results using inner monoid
+	//   - AlternativeMonoid: Combines applicative and alternative behaviors
+	Monoid[A any] = monoid.Monoid[A]
 )

v2/optics/codec/validate/bind.go

@@ -0,0 +1,335 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package validate
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	A "github.com/IBM/fp-go/v2/internal/apply"
+	C "github.com/IBM/fp-go/v2/internal/chain"
+	F "github.com/IBM/fp-go/v2/internal/functor"
+	L "github.com/IBM/fp-go/v2/optics/lens"
+)
+
+// Do creates an empty context of type S to be used with the Bind operation.
+// This is the starting point for building up a context using do-notation style.
+//
+// Example:
+//
+//	type Result struct {
+//	    x int
+//	    y string
+//	}
+//	result := Do(Result{})
+func Do[I, S any](
+	empty S,
+) Validate[I, S] {
+	return Of[I](empty)
+}
+
+// Bind attaches the result of a computation to a context S1 to produce a context S2.
+// This is used in do-notation style to sequentially build up a context.
+//
+// Example:
+//
+//	type State struct { x int; y int }
+//	decoder := F.Pipe2(
+//	    Do[string](State{}),
+//	    Bind(func(x int) func(State) State {
+//	        return func(s State) State { s.x = x; return s }
+//	    }, func(s State) Validate[string, int] {
+//	        return Of[string](42)
+//	    }),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{x: 42})
+func Bind[I, S1, S2, A any](
+	setter func(A) func(S1) S2,
+	f Kleisli[I, S1, A],
+) Operator[I, S1, S2] {
+	return C.Bind(
+		Chain[I, S1, S2],
+		Map[I, A, S2],
+		setter,
+		f,
+	)
+}
+
+// Let attaches the result of a pure computation to a context S1 to produce a context S2.
+// Unlike Bind, the computation function returns a plain value, not wrapped in Validate.
+//
+// Example:
+//
+//	type State struct { x int; computed int }
+//	decoder := F.Pipe2(
+//	    Do[string](State{x: 5}),
+//	    Let[string](func(c int) func(State) State {
+//	        return func(s State) State { s.computed = c; return s }
+//	    }, func(s State) int { return s.x * 2 }),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{x: 5, computed: 10})
+func Let[I, S1, S2, B any](
+	key func(B) func(S1) S2,
+	f func(S1) B,
+) Operator[I, S1, S2] {
+	return F.Let(
+		Map[I, S1, S2],
+		key,
+		f,
+	)
+}
+
+// LetTo attaches a constant value to a context S1 to produce a context S2.
+//
+// Example:
+//
+//	type State struct { x int; name string }
+//	result := F.Pipe2(
+//	    Do(State{x: 5}),
+//	    LetTo(func(n string) func(State) State {
+//	        return func(s State) State { s.name = n; return s }
+//	    }, "example"),
+//	)
+func LetTo[I, S1, S2, B any](
+	key func(B) func(S1) S2,
+	b B,
+) Operator[I, S1, S2] {
+	return F.LetTo(
+		Map[I, S1, S2],
+		key,
+		b,
+	)
+}
+
+// BindTo initializes a new state S1 from a value T.
+// This is typically used as the first operation after creating a Validate value.
+//
+// Example:
+//
+//	type State struct { value int }
+//	decoder := F.Pipe1(
+//	    Of[string](42),
+//	    BindTo[string](func(x int) State { return State{value: x} }),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{value: 42})
+func BindTo[I, S1, T any](
+	setter func(T) S1,
+) Operator[I, T, S1] {
+	return C.BindTo(
+		Map[I, T, S1],
+		setter,
+	)
+}
+
+// ApS attaches a value to a context S1 to produce a context S2 by considering the context and the value concurrently.
+// This uses the applicative functor pattern, allowing parallel composition.
+//
+// IMPORTANT: Unlike Bind which fails fast, ApS aggregates ALL validation errors from both the context
+// and the value. If both validations fail, all errors are collected and returned together.
+// This is useful for validating multiple independent fields and reporting all errors at once.
+//
+// Example:
+//
+//	type State struct { x int; y int }
+//	decoder := F.Pipe2(
+//	    Do[string](State{}),
+//	    ApS(func(x int) func(State) State {
+//	        return func(s State) State { s.x = x; return s }
+//	    }, Of[string](42)),
+//	)
+//	result := decoder("input") // Returns validation.Success(State{x: 42})
+//
+// Error aggregation example:
+//
+//	// Both decoders fail - errors are aggregated
+//	decoder1 := func(input string) Validation[State] {
+//	    return validation.Failures[State](/* errors */)
+//	}
+//	decoder2 := func(input string) Validation[int] {
+//	    return validation.Failures[int](/* errors */)
+//	}
+//	combined := ApS(setter, decoder2)(decoder1)
+//	result := combined("input") // Contains BOTH sets of errors
+func ApS[I, S1, S2, T any](
+	setter func(T) func(S1) S2,
+	fa Validate[I, T],
+) Operator[I, S1, S2] {
+	return A.ApS(
+		Ap[S2, I, T],
+		Map[I, S1, func(T) S2],
+		setter,
+		fa,
+	)
+}
+
+// ApSL attaches a value to a context using a lens-based setter.
+// This is a convenience function that combines ApS with a lens, allowing you to use
+// optics to update nested structures in a more composable way.
+//
+// IMPORTANT: Like ApS, this function aggregates ALL validation errors. If both the context
+// and the value fail validation, all errors are collected and returned together.
+// This enables comprehensive error reporting for complex nested structures.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// This eliminates the need to manually write setter functions.
+//
+// Example:
+//
+//	type Address struct {
+//	    Street string
+//	    City   string
+//	}
+//
+//	type Person struct {
+//	    Name    string
+//	    Address Address
+//	}
+//
+//	// Create a lens for the Address field
+//	addressLens := lens.MakeLens(
+//	    func(p Person) Address { return p.Address },
+//	    func(p Person, a Address) Person { p.Address = a; return p },
+//	)
+//
+//	// Use ApSL to update the address
+//	decoder := F.Pipe2(
+//	    Of[string](Person{Name: "Alice"}),
+//	    ApSL(
+//	        addressLens,
+//	        Of[string](Address{Street: "Main St", City: "NYC"}),
+//	    ),
+//	)
+//	result := decoder("input") // Returns validation.Success(Person{...})
+func ApSL[I, S, T any](
+	lens L.Lens[S, T],
+	fa Validate[I, T],
+) Operator[I, S, S] {
+	return ApS(lens.Set, fa)
+}
+
+// BindL attaches the result of a computation to a context using a lens-based setter.
+// This is a convenience function that combines Bind with a lens, allowing you to use
+// optics to update nested structures based on their current values.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// The computation function f receives the current value of the focused field and returns
+// a Validation that produces the new value.
+//
+// Unlike ApSL, BindL uses monadic sequencing, meaning the computation f can depend on
+// the current value of the focused field.
+//
+// Example:
+//
+//	type Counter struct {
+//	    Value int
+//	}
+//
+//	valueLens := lens.MakeLens(
+//	    func(c Counter) int { return c.Value },
+//	    func(c Counter, v int) Counter { c.Value = v; return c },
+//	)
+//
+//	// Increment the counter, but fail if it would exceed 100
+//	increment := func(v int) Validate[string, int] {
+//	    return func(input string) Validation[int] {
+//	        if v >= 100 {
+//	            return validation.Failures[int](/* errors */)
+//	        }
+//	        return validation.Success(v + 1)
+//	    }
+//	}
+//
+//	decoder := F.Pipe1(
+//	    Of[string](Counter{Value: 42}),
+//	    BindL(valueLens, increment),
+//	)
+//	result := decoder("input") // Returns validation.Success(Counter{Value: 43})
+func BindL[I, S, T any](
+	lens L.Lens[S, T],
+	f Kleisli[I, T, T],
+) Operator[I, S, S] {
+	return Bind(lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetL attaches the result of a pure computation to a context using a lens-based setter.
+// This is a convenience function that combines Let with a lens, allowing you to use
+// optics to update nested structures with pure transformations.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// The transformation function f receives the current value of the focused field and returns
+// the new value directly (not wrapped in Validation).
+//
+// This is useful for pure transformations that cannot fail, such as mathematical operations,
+// string manipulations, or other deterministic updates.
+//
+// Example:
+//
+//	type Counter struct {
+//	    Value int
+//	}
+//
+//	valueLens := lens.MakeLens(
+//	    func(c Counter) int { return c.Value },
+//	    func(c Counter, v int) Counter { c.Value = v; return c },
+//	)
+//
+//	// Double the counter value
+//	double := func(v int) int { return v * 2 }
+//
+//	decoder := F.Pipe1(
+//	    Of[string](Counter{Value: 21}),
+//	    LetL(valueLens, double),
+//	)
+//	result := decoder("input") // Returns validation.Success(Counter{Value: 42})
+func LetL[I, S, T any](
+	lens L.Lens[S, T],
+	f Endomorphism[T],
+) Operator[I, S, S] {
+	return Let[I](lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetToL attaches a constant value to a context using a lens-based setter.
+// This is a convenience function that combines LetTo with a lens, allowing you to use
+// optics to set nested fields to specific values.
+//
+// The lens parameter provides the setter for a field within the structure S.
+// Unlike LetL which transforms the current value, LetToL simply replaces it with
+// the provided constant value b.
+//
+// This is useful for resetting fields, initializing values, or setting fields to
+// predetermined constants.
+//
+// Example:
+//
+//	type Config struct {
+//	    Debug   bool
+//	    Timeout int
+//	}
+//
+//	debugLens := lens.MakeLens(
+//	    func(c Config) bool { return c.Debug },
+//	    func(c Config, d bool) Config { c.Debug = d; return c },
+//	)
+//
+//	decoder := F.Pipe1(
+//	    Of[string](Config{Debug: true, Timeout: 30}),
+//	    LetToL(debugLens, false),
+//	)
+//	result := decoder("input") // Returns validation.Success(Config{Debug: false, Timeout: 30})
+func LetToL[I, S, T any](
+	lens L.Lens[S, T],
+	b T,
+) Operator[I, S, S] {
+	return LetTo[I](lens.Set, b)
+}

v2/optics/codec/validate/bind_test.go

@@ -0,0 +1,733 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package validate
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	L "github.com/IBM/fp-go/v2/optics/lens"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDo(t *testing.T) {
+	t.Run("creates successful validation with empty state", func(t *testing.T) {
+		type State struct {
+			x int
+			y string
+		}
+		validator := Do[string](State{})
+		result := validator("input")(nil)
+
+		assert.Equal(t, either.Of[Errors](State{}), result)
+	})
+
+	t.Run("creates successful validation with initialized state", func(t *testing.T) {
+		type State struct {
+			x int
+			y string
+		}
+		initial := State{x: 42, y: "hello"}
+		validator := Do[string](initial)
+		result := validator("input")(nil)
+
+		assert.Equal(t, either.Of[Errors](initial), result)
+	})
+
+	t.Run("works with different input types", func(t *testing.T) {
+		intValidator := Do[int](0)
+		assert.Equal(t, either.Of[Errors](0), intValidator(42)(nil))
+
+		strValidator := Do[string]("")
+		assert.Equal(t, either.Of[Errors](""), strValidator("test")(nil))
+
+		type Custom struct{ Value int }
+		customValidator := Do[[]byte](Custom{Value: 100})
+		assert.Equal(t, either.Of[Errors](Custom{Value: 100}), customValidator([]byte("data"))(nil))
+	})
+}
+
+func TestBind(t *testing.T) {
+	type State struct {
+		x int
+		y int
+	}
+
+	t.Run("binds successful validation to state", func(t *testing.T) {
+		validator := F.Pipe2(
+			Do[string](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Validate[string, int] {
+				return Of[string](42)
+			}),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Validate[string, int] {
+				return Of[string](10)
+			}),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](State{x: 42, y: 10}), result)
+	})
+
+	t.Run("propagates failure", func(t *testing.T) {
+		validator := F.Pipe2(
+			Do[string](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Validate[string, int] {
+				return Of[string](42)
+			}),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Validate[string, int] {
+				return func(input string) Reader[Context, Validation[int]] {
+					return func(ctx Context) Validation[int] {
+						return validation.Failures[int](Errors{&validation.ValidationError{Messsage: "y failed"}})
+					}
+				}
+			}),
+		)
+
+		result := validator("input")(nil)
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "y failed", errors[0].Messsage)
+	})
+
+	t.Run("can access previous state values", func(t *testing.T) {
+		validator := F.Pipe2(
+			Do[string](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Validate[string, int] {
+				return Of[string](10)
+			}),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Validate[string, int] {
+				// y depends on x
+				return Of[string](s.x * 2)
+			}),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](State{x: 10, y: 20}), result)
+	})
+
+	t.Run("can access input value", func(t *testing.T) {
+		validator := F.Pipe1(
+			Do[int](State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Validate[int, int] {
+				return func(input int) Reader[Context, Validation[int]] {
+					return func(ctx Context) Validation[int] {
+						return validation.Success(input * 2)
+					}
+				}
+			}),
+		)
+
+		result := validator(21)(nil)
+		assert.Equal(t, either.Of[Errors](State{x: 42}), result)
+	})
+}
+
+func TestLet(t *testing.T) {
+	type State struct {
+		x        int
+		computed int
+	}
+
+	t.Run("attaches pure computation result to state", func(t *testing.T) {
+		validator := F.Pipe1(
+			Do[string](State{x: 5}),
+			Let[string](func(c int) func(State) State {
+				return func(s State) State { s.computed = c; return s }
+			}, func(s State) int { return s.x * 2 }),
+		)
+
+		result := validator("input")(nil)
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 5, computed: 10}, value)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := func(input string) Reader[Context, Validation[State]] {
+			return func(ctx Context) Validation[State] {
+				return validation.Failures[State](Errors{&validation.ValidationError{Messsage: "error"}})
+			}
+		}
+		validator := Let[string](func(c int) func(State) State {
+			return func(s State) State { s.computed = c; return s }
+		}, func(s State) int { return s.x * 2 })
+
+		result := validator(failure)("input")(nil)
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "error", errors[0].Messsage)
+	})
+
+	t.Run("chains multiple Let operations", func(t *testing.T) {
+		type State struct {
+			x int
+			y int
+			z int
+		}
+		validator := F.Pipe3(
+			Do[string](State{x: 5}),
+			Let[string](func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) int { return s.x * 2 }),
+			Let[string](func(z int) func(State) State {
+				return func(s State) State { s.z = z; return s }
+			}, func(s State) int { return s.y + 10 }),
+			Let[string](func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) int { return s.z * 3 }),
+		)
+
+		result := validator("input")(nil)
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 60, y: 10, z: 20}, value)
+	})
+}
+
+func TestLetTo(t *testing.T) {
+	type State struct {
+		x    int
+		name string
+	}
+
+	t.Run("attaches constant value to state", func(t *testing.T) {
+		validator := F.Pipe1(
+			Do[string](State{x: 5}),
+			LetTo[string](func(n string) func(State) State {
+				return func(s State) State { s.name = n; return s }
+			}, "example"),
+		)
+
+		result := validator("input")(nil)
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{x: 5, name: "example"}, value)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := func(input string) Reader[Context, Validation[State]] {
+			return func(ctx Context) Validation[State] {
+				return validation.Failures[State](Errors{&validation.ValidationError{Messsage: "error"}})
+			}
+		}
+		validator := LetTo[string](func(n string) func(State) State {
+			return func(s State) State { s.name = n; return s }
+		}, "example")
+
+		result := validator(failure)("input")(nil)
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("sets multiple constant values", func(t *testing.T) {
+		type State struct {
+			name    string
+			version int
+			active  bool
+		}
+		validator := F.Pipe3(
+			Do[string](State{}),
+			LetTo[string](func(n string) func(State) State {
+				return func(s State) State { s.name = n; return s }
+			}, "app"),
+			LetTo[string](func(v int) func(State) State {
+				return func(s State) State { s.version = v; return s }
+			}, 2),
+			LetTo[string](func(a bool) func(State) State {
+				return func(s State) State { s.active = a; return s }
+			}, true),
+		)
+
+		result := validator("input")(nil)
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{name: "app", version: 2, active: true}, value)
+	})
+}
+
+func TestBindTo(t *testing.T) {
+	type State struct {
+		value int
+	}
+
+	t.Run("initializes state from value", func(t *testing.T) {
+		validator := F.Pipe1(
+			Of[string](42),
+			BindTo[string](func(x int) State { return State{value: x} }),
+		)
+
+		result := validator("input")(nil)
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(Errors) State { return State{} },
+			F.Identity[State],
+		)
+		assert.Equal(t, State{value: 42}, value)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return validation.Failures[int](Errors{&validation.ValidationError{Messsage: "error"}})
+			}
+		}
+		validator := BindTo[string](func(x int) State { return State{value: x} })
+
+		result := validator(failure)("input")(nil)
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "error", errors[0].Messsage)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		type StringState struct {
+			text string
+		}
+		validator := F.Pipe1(
+			Of[int]("hello"),
+			BindTo[int](func(s string) StringState { return StringState{text: s} }),
+		)
+
+		result := validator(42)(nil)
+		assert.Equal(t, either.Of[Errors](StringState{text: "hello"}), result)
+	})
+}
+
+func TestApS(t *testing.T) {
+	type State struct {
+		x int
+		y int
+	}
+
+	t.Run("attaches value using applicative pattern", func(t *testing.T) {
+		validator := F.Pipe1(
+			Do[string](State{}),
+			ApS(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, Of[string](42)),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](State{x: 42}), result)
+	})
+
+	t.Run("accumulates errors from both validations", func(t *testing.T) {
+		stateFailure := func(input string) Reader[Context, Validation[State]] {
+			return func(ctx Context) Validation[State] {
+				return validation.Failures[State](Errors{&validation.ValidationError{Messsage: "state error"}})
+			}
+		}
+		valueFailure := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return validation.Failures[int](Errors{&validation.ValidationError{Messsage: "value error"}})
+			}
+		}
+
+		validator := ApS(func(x int) func(State) State {
+			return func(s State) State { s.x = x; return s }
+		}, valueFailure)
+
+		result := validator(stateFailure)("input")(nil)
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 2)
+		messages := []string{errors[0].Messsage, errors[1].Messsage}
+		assert.Contains(t, messages, "state error")
+		assert.Contains(t, messages, "value error")
+	})
+
+	t.Run("combines multiple ApS operations", func(t *testing.T) {
+		validator := F.Pipe2(
+			Do[string](State{}),
+			ApS(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, Of[string](10)),
+			ApS(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, Of[string](20)),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](State{x: 10, y: 20}), result)
+	})
+}
+
+func TestApSL(t *testing.T) {
+	type Address struct {
+		Street string
+		City   string
+	}
+
+	type Person struct {
+		Name    string
+		Address Address
+	}
+
+	t.Run("updates nested structure using lens", func(t *testing.T) {
+		addressLens := L.MakeLens(
+			func(p Person) Address { return p.Address },
+			func(p Person, a Address) Person { p.Address = a; return p },
+		)
+
+		validator := F.Pipe1(
+			Of[string](Person{Name: "Alice"}),
+			ApSL(
+				addressLens,
+				Of[string](Address{Street: "Main St", City: "NYC"}),
+			),
+		)
+
+		result := validator("input")(nil)
+		expected := Person{
+			Name:    "Alice",
+			Address: Address{Street: "Main St", City: "NYC"},
+		}
+		assert.Equal(t, either.Of[Errors](expected), result)
+	})
+
+	t.Run("accumulates errors", func(t *testing.T) {
+		addressLens := L.MakeLens(
+			func(p Person) Address { return p.Address },
+			func(p Person, a Address) Person { p.Address = a; return p },
+		)
+
+		personFailure := func(input string) Reader[Context, Validation[Person]] {
+			return func(ctx Context) Validation[Person] {
+				return validation.Failures[Person](Errors{&validation.ValidationError{Messsage: "person error"}})
+			}
+		}
+		addressFailure := func(input string) Reader[Context, Validation[Address]] {
+			return func(ctx Context) Validation[Address] {
+				return validation.Failures[Address](Errors{&validation.ValidationError{Messsage: "address error"}})
+			}
+		}
+
+		validator := ApSL(addressLens, addressFailure)
+		result := validator(personFailure)("input")(nil)
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(Person) Errors { return nil },
+		)
+		assert.Len(t, errors, 2)
+	})
+}
+
+func TestBindL(t *testing.T) {
+	type Counter struct {
+		Value int
+	}
+
+	valueLens := L.MakeLens(
+		func(c Counter) int { return c.Value },
+		func(c Counter, v int) Counter { c.Value = v; return c },
+	)
+
+	t.Run("updates field based on current value", func(t *testing.T) {
+		increment := func(v int) Validate[string, int] {
+			return Of[string](v + 1)
+		}
+
+		validator := F.Pipe1(
+			Of[string](Counter{Value: 42}),
+			BindL(valueLens, increment),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](Counter{Value: 43}), result)
+	})
+
+	t.Run("fails validation based on current value", func(t *testing.T) {
+		increment := func(v int) Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					if v >= 100 {
+						return validation.Failures[int](Errors{&validation.ValidationError{Messsage: "exceeds limit"}})
+					}
+					return validation.Success(v + 1)
+				}
+			}
+		}
+
+		validator := F.Pipe1(
+			Of[string](Counter{Value: 100}),
+			BindL(valueLens, increment),
+		)
+
+		result := validator("input")(nil)
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(Counter) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "exceeds limit", errors[0].Messsage)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		increment := func(v int) Validate[string, int] {
+			return Of[string](v + 1)
+		}
+
+		failure := func(input string) Reader[Context, Validation[Counter]] {
+			return func(ctx Context) Validation[Counter] {
+				return validation.Failures[Counter](Errors{&validation.ValidationError{Messsage: "error"}})
+			}
+		}
+		validator := BindL(valueLens, increment)
+		result := validator(failure)("input")(nil)
+
+		assert.True(t, either.IsLeft(result))
+	})
+}
+
+func TestLetL(t *testing.T) {
+	type Counter struct {
+		Value int
+	}
+
+	valueLens := L.MakeLens(
+		func(c Counter) int { return c.Value },
+		func(c Counter, v int) Counter { c.Value = v; return c },
+	)
+
+	t.Run("transforms field with pure function", func(t *testing.T) {
+		double := func(v int) int { return v * 2 }
+
+		validator := F.Pipe1(
+			Of[string](Counter{Value: 21}),
+			LetL[string](valueLens, double),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](Counter{Value: 42}), result)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		double := func(v int) int { return v * 2 }
+
+		failure := func(input string) Reader[Context, Validation[Counter]] {
+			return func(ctx Context) Validation[Counter] {
+				return validation.Failures[Counter](Errors{&validation.ValidationError{Messsage: "error"}})
+			}
+		}
+		validator := LetL[string](valueLens, double)
+		result := validator(failure)("input")(nil)
+
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("chains multiple transformations", func(t *testing.T) {
+		add10 := func(v int) int { return v + 10 }
+		double := func(v int) int { return v * 2 }
+
+		validator := F.Pipe2(
+			Of[string](Counter{Value: 5}),
+			LetL[string](valueLens, add10),
+			LetL[string](valueLens, double),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](Counter{Value: 30}), result)
+	})
+}
+
+func TestLetToL(t *testing.T) {
+	type Config struct {
+		Debug   bool
+		Timeout int
+	}
+
+	debugLens := L.MakeLens(
+		func(c Config) bool { return c.Debug },
+		func(c Config, d bool) Config { c.Debug = d; return c },
+	)
+
+	t.Run("sets field to constant value", func(t *testing.T) {
+		validator := F.Pipe1(
+			Of[string](Config{Debug: true, Timeout: 30}),
+			LetToL[string](debugLens, false),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](Config{Debug: false, Timeout: 30}), result)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := func(input string) Reader[Context, Validation[Config]] {
+			return func(ctx Context) Validation[Config] {
+				return validation.Failures[Config](Errors{&validation.ValidationError{Messsage: "error"}})
+			}
+		}
+		validator := LetToL[string](debugLens, false)
+		result := validator(failure)("input")(nil)
+
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("sets multiple fields", func(t *testing.T) {
+		timeoutLens := L.MakeLens(
+			func(c Config) int { return c.Timeout },
+			func(c Config, t int) Config { c.Timeout = t; return c },
+		)
+
+		validator := F.Pipe2(
+			Of[string](Config{Debug: true, Timeout: 30}),
+			LetToL[string](debugLens, false),
+			LetToL[string](timeoutLens, 60),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, either.Of[Errors](Config{Debug: false, Timeout: 60}), result)
+	})
+}
+
+func TestBindOperationsComposition(t *testing.T) {
+	type User struct {
+		Name  string
+		Age   int
+		Email string
+	}
+
+	t.Run("combines Do, Bind, Let, and LetTo", func(t *testing.T) {
+		validator := F.Pipe4(
+			Do[string](User{}),
+			LetTo[string](func(n string) func(User) User {
+				return func(u User) User { u.Name = n; return u }
+			}, "Alice"),
+			Bind(func(a int) func(User) User {
+				return func(u User) User { u.Age = a; return u }
+			}, func(u User) Validate[string, int] {
+				// Age validation
+				if len(u.Name) > 0 {
+					return Of[string](25)
+				}
+				return func(input string) Reader[Context, Validation[int]] {
+					return func(ctx Context) Validation[int] {
+						return validation.Failures[int](Errors{&validation.ValidationError{Messsage: "name required"}})
+					}
+				}
+			}),
+			Let[string](func(e string) func(User) User {
+				return func(u User) User { u.Email = e; return u }
+			}, func(u User) string {
+				// Derive email from name
+				return u.Name + "@example.com"
+			}),
+			Bind(func(a int) func(User) User {
+				return func(u User) User { u.Age = a; return u }
+			}, func(u User) Validate[string, int] {
+				// Validate age is positive
+				if u.Age > 0 {
+					return Of[string](u.Age)
+				}
+				return func(input string) Reader[Context, Validation[int]] {
+					return func(ctx Context) Validation[int] {
+						return validation.Failures[int](Errors{&validation.ValidationError{Messsage: "age must be positive"}})
+					}
+				}
+			}),
+		)
+
+		result := validator("input")(nil)
+		expected := User{
+			Name:  "Alice",
+			Age:   25,
+			Email: "Alice@example.com",
+		}
+		assert.Equal(t, either.Of[Errors](expected), result)
+	})
+
+	t.Run("validates with input-dependent logic", func(t *testing.T) {
+		type Config struct {
+			MaxValue int
+			Value    int
+		}
+
+		validator := F.Pipe2(
+			Do[int](Config{}),
+			Bind(func(max int) func(Config) Config {
+				return func(c Config) Config { c.MaxValue = max; return c }
+			}, func(c Config) Validate[int, int] {
+				// Extract max from input
+				return func(input int) Reader[Context, Validation[int]] {
+					return func(ctx Context) Validation[int] {
+						return validation.Success(input)
+					}
+				}
+			}),
+			Bind(func(val int) func(Config) Config {
+				return func(c Config) Config { c.Value = val; return c }
+			}, func(c Config) Validate[int, int] {
+				// Validate value against max
+				return func(input int) Reader[Context, Validation[int]] {
+					return func(ctx Context) Validation[int] {
+						if input/2 <= c.MaxValue {
+							return validation.Success(input / 2)
+						}
+						return validation.Failures[int](Errors{&validation.ValidationError{Messsage: "value exceeds max"}})
+					}
+				}
+			}),
+		)
+
+		result := validator(100)(nil)
+		assert.Equal(t, either.Of[Errors](Config{MaxValue: 100, Value: 50}), result)
+	})
+}

v2/optics/codec/validate/monad_test.go

@@ -0,0 +1,661 @@
+package validate
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestMonadChainLeft tests the MonadChainLeft function
+func TestMonadChainLeft(t *testing.T) {
+	t.Run("transforms failures while preserving successes", func(t *testing.T) {
+		// Create a failing validator
+		failingValidator := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "validation failed"},
+				})
+			}
+		}
+
+		// Handler that recovers from specific errors
+		handler := func(errs Errors) Validate[string, int] {
+			for _, err := range errs {
+				if err.Messsage == "validation failed" {
+					return Of[string, int](0) // recover with default
+				}
+			}
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](errs)
+				}
+			}
+		}
+
+		validator := MonadChainLeft(failingValidator, handler)
+		res := validator("input")(nil)
+
+		assert.Equal(t, validation.Of(0), res, "Should recover from failure")
+	})
+
+	t.Run("preserves success values unchanged", func(t *testing.T) {
+		successValidator := Of[string, int](42)
+
+		handler := func(errs Errors) Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Messsage: "should not be called"},
+					})
+				}
+			}
+		}
+
+		validator := MonadChainLeft(successValidator, handler)
+		res := validator("input")(nil)
+
+		assert.Equal(t, validation.Of(42), res, "Success should pass through unchanged")
+	})
+
+	t.Run("aggregates errors when transformation also fails", func(t *testing.T) {
+		failingValidator := func(input string) Reader[Context, Validation[string]] {
+			return func(ctx Context) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "original error"},
+				})
+			}
+		}
+
+		handler := func(errs Errors) Validate[string, string] {
+			return func(input string) Reader[Context, Validation[string]] {
+				return func(ctx Context) Validation[string] {
+					return either.Left[string](validation.Errors{
+						{Messsage: "additional error"},
+					})
+				}
+			}
+		}
+
+		validator := MonadChainLeft(failingValidator, handler)
+		res := validator("input")(nil)
+
+		assert.True(t, either.IsLeft(res))
+		errors := either.MonadFold(res,
+			reader.Ask[Errors](),
+			func(string) Errors { return nil },
+		)
+		assert.Len(t, errors, 2, "Should aggregate both errors")
+
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "original error")
+		assert.Contains(t, messages, "additional error")
+	})
+
+	t.Run("adds context to errors", func(t *testing.T) {
+		failingValidator := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "invalid format"},
+				})
+			}
+		}
+
+		addContext := func(errs Errors) Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{
+							Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+							Messsage: "failed to validate user age",
+						},
+					})
+				}
+			}
+		}
+
+		validator := MonadChainLeft(failingValidator, addContext)
+		res := validator("abc")(nil)
+
+		assert.True(t, either.IsLeft(res))
+		errors := either.MonadFold(res,
+			reader.Ask[Errors](),
+			func(int) Errors { return nil },
+		)
+		assert.Len(t, errors, 2, "Should have both original and context errors")
+	})
+
+	t.Run("works with different input types", func(t *testing.T) {
+		type Config struct {
+			Port int
+		}
+
+		failingValidator := func(cfg Config) Reader[Context, Validation[string]] {
+			return func(ctx Context) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: cfg.Port, Messsage: "invalid port"},
+				})
+			}
+		}
+
+		handler := func(errs Errors) Validate[Config, string] {
+			return Of[Config, string]("default-value")
+		}
+
+		validator := MonadChainLeft(failingValidator, handler)
+		res := validator(Config{Port: 9999})(nil)
+
+		assert.Equal(t, validation.Of("default-value"), res)
+	})
+
+	t.Run("handler can access original input", func(t *testing.T) {
+		failingValidator := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "parse failed"},
+				})
+			}
+		}
+
+		handler := func(errs Errors) Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					// Handler can use the original input to make decisions
+					if input == "special" {
+						return validation.Of(999)
+					}
+					return validation.Of(0)
+				}
+			}
+		}
+
+		validator := MonadChainLeft(failingValidator, handler)
+
+		res1 := validator("special")(nil)
+		assert.Equal(t, validation.Of(999), res1)
+
+		res2 := validator("other")(nil)
+		assert.Equal(t, validation.Of(0), res2)
+	})
+
+	t.Run("is equivalent to ChainLeft", func(t *testing.T) {
+		failingValidator := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error"},
+				})
+			}
+		}
+
+		handler := func(errs Errors) Validate[string, int] {
+			return Of[string, int](42)
+		}
+
+		// MonadChainLeft - direct application
+		result1 := MonadChainLeft(failingValidator, handler)("input")(nil)
+
+		// ChainLeft - curried for pipelines
+		result2 := ChainLeft(handler)(failingValidator)("input")(nil)
+
+		assert.Equal(t, result1, result2, "MonadChainLeft and ChainLeft should produce identical results")
+	})
+
+	t.Run("chains multiple error transformations", func(t *testing.T) {
+		failingValidator := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error1"},
+				})
+			}
+		}
+
+		handler1 := func(errs Errors) Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Messsage: "error2"},
+					})
+				}
+			}
+		}
+
+		handler2 := func(errs Errors) Validate[string, int] {
+			// Check if we can recover
+			for _, err := range errs {
+				if err.Messsage == "error1" {
+					return Of[string, int](100) // recover
+				}
+			}
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](errs)
+				}
+			}
+		}
+
+		// Chain handlers
+		validator := MonadChainLeft(MonadChainLeft(failingValidator, handler1), handler2)
+		res := validator("input")(nil)
+
+		// Should recover because error1 is present
+		assert.Equal(t, validation.Of(100), res)
+	})
+
+	t.Run("does not call handler on success", func(t *testing.T) {
+		successValidator := Of[string, int](42)
+		handlerCalled := false
+
+		handler := func(errs Errors) Validate[string, int] {
+			handlerCalled = true
+			return Of[string, int](0)
+		}
+
+		validator := MonadChainLeft(successValidator, handler)
+		res := validator("input")(nil)
+
+		assert.Equal(t, validation.Of(42), res)
+		assert.False(t, handlerCalled, "Handler should not be called on success")
+	})
+}
+
+// TestMonadAlt tests the MonadAlt function
+func TestMonadAlt(t *testing.T) {
+	t.Run("returns first validator when it succeeds", func(t *testing.T) {
+		validator1 := Of[string, int](42)
+		validator2 := func() Validate[string, int] {
+			return Of[string, int](100)
+		}
+
+		result := MonadAlt(validator1, validator2)("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+	})
+
+	t.Run("returns second validator when first fails", func(t *testing.T) {
+		failing := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "first failed"},
+				})
+			}
+		}
+		fallback := func() Validate[string, int] {
+			return Of[string, int](42)
+		}
+
+		result := MonadAlt(failing, fallback)("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+	})
+
+	t.Run("aggregates errors when both fail", func(t *testing.T) {
+		failing1 := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+		}
+		failing2 := func() Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
+			}
+		}
+
+		result := MonadAlt(failing1, failing2)("input")(nil)
+		assert.True(t, either.IsLeft(result))
+
+		errors := either.MonadFold(result,
+			reader.Ask[Errors](),
+			func(int) Errors { return nil },
+		)
+		assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors from both validators")
+
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "error 1", "Should contain error from first validator")
+		assert.Contains(t, messages, "error 2", "Should contain error from second validator")
+	})
+
+	t.Run("does not evaluate second validator when first succeeds", func(t *testing.T) {
+		validator1 := Of[string, int](42)
+		evaluated := false
+		validator2 := func() Validate[string, int] {
+			evaluated = true
+			return Of[string, int](100)
+		}
+
+		result := MonadAlt(validator1, validator2)("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+		assert.False(t, evaluated, "Second validator should not be evaluated")
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		failing := func(input string) Reader[Context, Validation[string]] {
+			return func(ctx Context) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "failed"},
+				})
+			}
+		}
+		fallback := func() Validate[string, string] {
+			return Of[string, string]("fallback")
+		}
+
+		result := MonadAlt(failing, fallback)("input")(nil)
+		assert.Equal(t, validation.Of("fallback"), result)
+	})
+
+	t.Run("chains multiple alternatives", func(t *testing.T) {
+		failing1 := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+		}
+		failing2 := func() Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
+			}
+		}
+		succeeding := func() Validate[string, int] {
+			return Of[string, int](42)
+		}
+
+		// Chain: try failing1, then failing2, then succeeding
+		result := MonadAlt(MonadAlt(failing1, failing2), succeeding)("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+	})
+
+	t.Run("works with complex input types", func(t *testing.T) {
+		type Config struct {
+			Port int
+		}
+
+		failing := func(cfg Config) Reader[Context, Validation[string]] {
+			return func(ctx Context) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: cfg.Port, Messsage: "invalid port"},
+				})
+			}
+		}
+		fallback := func() Validate[Config, string] {
+			return Of[Config, string]("default")
+		}
+
+		result := MonadAlt(failing, fallback)(Config{Port: 9999})(nil)
+		assert.Equal(t, validation.Of("default"), result)
+	})
+
+	t.Run("preserves error context", func(t *testing.T) {
+		failing1 := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{
+						Value:    input,
+						Messsage: "parse error",
+						Context:  validation.Context{{Key: "field", Type: "int"}},
+					},
+				})
+			}
+		}
+		failing2 := func() Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{
+							Value:    input,
+							Messsage: "validation error",
+							Context:  validation.Context{{Key: "value", Type: "int"}},
+						},
+					})
+				}
+			}
+		}
+
+		result := MonadAlt(failing1, failing2)("abc")(nil)
+		assert.True(t, either.IsLeft(result))
+
+		errors := either.MonadFold(result,
+			reader.Ask[Errors](),
+			func(int) Errors { return nil },
+		)
+		assert.GreaterOrEqual(t, len(errors), 2, "Should have errors from both validators")
+
+		// Verify that errors with context are present
+		hasParseError := false
+		hasValidationError := false
+		for _, err := range errors {
+			if err.Messsage == "parse error" {
+				hasParseError = true
+				assert.NotNil(t, err.Context)
+			}
+			if err.Messsage == "validation error" {
+				hasValidationError = true
+				assert.NotNil(t, err.Context)
+			}
+		}
+		assert.True(t, hasParseError, "Should have parse error")
+		assert.True(t, hasValidationError, "Should have validation error")
+	})
+}
+
+// TestAlt tests the Alt function
+func TestAlt(t *testing.T) {
+	t.Run("returns first validator when it succeeds", func(t *testing.T) {
+		validator1 := Of[string, int](42)
+		validator2 := func() Validate[string, int] {
+			return Of[string, int](100)
+		}
+
+		withAlt := Alt(validator2)
+		result := withAlt(validator1)("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+	})
+
+	t.Run("returns second validator when first fails", func(t *testing.T) {
+		failing := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "first failed"},
+				})
+			}
+		}
+		fallback := func() Validate[string, int] {
+			return Of[string, int](42)
+		}
+
+		withAlt := Alt(fallback)
+		result := withAlt(failing)("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+	})
+
+	t.Run("aggregates errors when both fail", func(t *testing.T) {
+		failing1 := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+		}
+		failing2 := func() Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
+			}
+		}
+
+		withAlt := Alt(failing2)
+		result := withAlt(failing1)("input")(nil)
+		assert.True(t, either.IsLeft(result))
+
+		errors := either.MonadFold(result,
+			reader.Ask[Errors](),
+			func(int) Errors { return nil },
+		)
+		assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors from both validators")
+
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "error 1")
+		assert.Contains(t, messages, "error 2")
+	})
+
+	t.Run("does not evaluate second validator when first succeeds", func(t *testing.T) {
+		validator1 := Of[string, int](42)
+		evaluated := false
+		validator2 := func() Validate[string, int] {
+			evaluated = true
+			return Of[string, int](100)
+		}
+
+		withAlt := Alt(validator2)
+		result := withAlt(validator1)("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+		assert.False(t, evaluated, "Second validator should not be evaluated")
+	})
+
+	t.Run("can be used in pipelines", func(t *testing.T) {
+		failing1 := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+		}
+		failing2 := func() Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
+			}
+		}
+		succeeding := func() Validate[string, int] {
+			return Of[string, int](42)
+		}
+
+		// Use F.Pipe to chain alternatives
+		validator := F.Pipe2(
+			failing1,
+			Alt(failing2),
+			Alt(succeeding),
+		)
+
+		result := validator("input")(nil)
+		assert.Equal(t, validation.Of(42), result)
+	})
+
+	t.Run("is equivalent to MonadAlt", func(t *testing.T) {
+		failing := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error"},
+				})
+			}
+		}
+		fallback := func() Validate[string, int] {
+			return Of[string, int](42)
+		}
+
+		// Alt - curried for pipelines
+		result1 := Alt(fallback)(failing)("input")(nil)
+
+		// MonadAlt - direct application
+		result2 := MonadAlt(failing, fallback)("input")(nil)
+
+		assert.Equal(t, result1, result2, "Alt and MonadAlt should produce identical results")
+	})
+}
+
+// TestMonadAltAndAltEquivalence tests that MonadAlt and Alt are equivalent
+func TestMonadAltAndAltEquivalence(t *testing.T) {
+	t.Run("both produce same results for success", func(t *testing.T) {
+		validator1 := Of[string, int](42)
+		validator2 := func() Validate[string, int] {
+			return Of[string, int](100)
+		}
+
+		resultMonadAlt := MonadAlt(validator1, validator2)("input")(nil)
+		resultAlt := Alt(validator2)(validator1)("input")(nil)
+
+		assert.Equal(t, resultMonadAlt, resultAlt)
+	})
+
+	t.Run("both produce same results for fallback", func(t *testing.T) {
+		failing := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "failed"},
+				})
+			}
+		}
+		fallback := func() Validate[string, int] {
+			return Of[string, int](42)
+		}
+
+		resultMonadAlt := MonadAlt(failing, fallback)("input")(nil)
+		resultAlt := Alt(fallback)(failing)("input")(nil)
+
+		assert.Equal(t, resultMonadAlt, resultAlt)
+	})
+
+	t.Run("both produce same results for error aggregation", func(t *testing.T) {
+		failing1 := func(input string) Reader[Context, Validation[int]] {
+			return func(ctx Context) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Value: input, Messsage: "error 1"},
+				})
+			}
+		}
+		failing2 := func() Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
+			}
+		}
+
+		resultMonadAlt := MonadAlt(failing1, failing2)("input")(nil)
+		resultAlt := Alt(failing2)(failing1)("input")(nil)
+
+		// Both should fail
+		assert.True(t, either.IsLeft(resultMonadAlt))
+		assert.True(t, either.IsLeft(resultAlt))
+
+		// Both should have same errors
+		errorsMonadAlt := either.MonadFold(resultMonadAlt,
+			reader.Ask[Errors](),
+			func(int) Errors { return nil },
+		)
+		errorsAlt := either.MonadFold(resultAlt,
+			reader.Ask[Errors](),
+			func(int) Errors { return nil },
+		)
+
+		assert.Equal(t, len(errorsMonadAlt), len(errorsAlt))
+	})
+}

v2/optics/codec/validate/monoid.go

@@ -122,3 +122,268 @@ func ApplicativeMonoid[I, A any](m Monoid[A]) Monoid[Validate[I, A]] {
 		m,
 	)
 }
+
+// AlternativeMonoid creates a Monoid instance for Validate[I, A] that combines both
+// applicative and alternative semantics.
+//
+// This function creates a monoid that:
+//  1. When both validators succeed: Combines their results using the provided monoid operation
+//  2. When one validator fails: Uses the successful validator's result (alternative behavior)
+//  3. When both validators fail: Aggregates all errors from both validators
+//
+// This is a hybrid approach that combines:
+//   - ApplicativeMonoid: Combines successful results using the monoid operation
+//   - AltMonoid: Provides fallback behavior when validators fail
+//
+// # Type Parameters
+//
+//   - I: The input type that validators accept
+//   - A: The output type that validators produce (must have a Monoid instance)
+//
+// # Parameters
+//
+//   - m: A Monoid[A] that defines how to combine values of type A
+//
+// # Returns
+//
+// A Monoid[Validate[I, A]] that combines validators using both applicative and alternative semantics.
+//
+// # Behavior Details
+//
+// The AlternativeMonoid differs from ApplicativeMonoid in how it handles mixed success/failure:
+//
+//   - **Both succeed**: Results are combined using the monoid operation (like ApplicativeMonoid)
+//   - **First succeeds, second fails**: Returns the first result (alternative fallback)
+//   - **First fails, second succeeds**: Returns the second result (alternative fallback)
+//   - **Both fail**: Aggregates errors from both validators
+//
+// # Example: String Concatenation with Fallback
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec/validate"
+//	    "github.com/IBM/fp-go/v2/optics/codec/validation"
+//	    S "github.com/IBM/fp-go/v2/string"
+//	)
+//
+//	m := validate.AlternativeMonoid[string, string](S.Monoid)
+//
+//	// Both succeed - results are concatenated
+//	validator1 := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.Success("Hello")
+//	    }
+//	}
+//	validator2 := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.Success(" World")
+//	    }
+//	}
+//	combined := m.Concat(validator1, validator2)
+//	result := combined("input")(nil)
+//	// result is validation.Success("Hello World")
+//
+// # Example: Fallback Behavior
+//
+//	// First fails, second succeeds - uses second result
+//	failing := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.FailureWithMessage[string](input, "first failed")(ctx)
+//	    }
+//	}
+//	succeeding := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.Success("fallback")
+//	    }
+//	}
+//	combined := m.Concat(failing, succeeding)
+//	result := combined("input")(nil)
+//	// result is validation.Success("fallback")
+//
+// # Example: Error Aggregation
+//
+//	// Both fail - errors are aggregated
+//	failing1 := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.FailureWithMessage[string](input, "error 1")(ctx)
+//	    }
+//	}
+//	failing2 := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.FailureWithMessage[string](input, "error 2")(ctx)
+//	    }
+//	}
+//	combined := m.Concat(failing1, failing2)
+//	result := combined("input")(nil)
+//	// result contains both "error 1" and "error 2"
+//
+// # Comparison with Other Monoids
+//
+//   - **ApplicativeMonoid**: Always combines results when both succeed, fails if either fails
+//   - **AlternativeMonoid**: Combines results when both succeed, provides fallback when one fails
+//   - **AltMonoid**: Always uses first success, never combines results
+//
+// # Use Cases
+//
+//   - Validation with fallback strategies and result combination
+//   - Building validators that accumulate results but provide alternatives
+//   - Configuration loading with multiple sources and merging
+//   - Data aggregation with error recovery
+//
+// # Notes
+//
+//   - Both validators receive the same input value I
+//   - The empty element of the monoid serves as the identity for the Concat operation
+//   - Error aggregation ensures no validation failures are lost
+//   - This follows both applicative and alternative functor laws
+//
+// # See Also
+//
+//   - ApplicativeMonoid: For pure applicative combination without fallback
+//   - AltMonoid: For pure alternative behavior without result combination
+//   - MonadAlt: The underlying alternative operation
+func AlternativeMonoid[I, A any](m Monoid[A]) Monoid[Validate[I, A]] {
+	return monoid.AlternativeMonoid(
+		Of[I, A],
+		MonadMap[I, A, func(A) A],
+		MonadAp[A, I, A],
+		MonadAlt[I, A],
+		m,
+	)
+}
+
+// AltMonoid creates a Monoid instance for Validate[I, A] using alternative semantics
+// with a provided zero/default validator.
+//
+// This function creates a monoid where:
+//  1. The first successful validator wins (no result combination)
+//  2. If the first fails, the second is tried as a fallback
+//  3. If both fail, errors are aggregated
+//  4. The provided zero validator serves as the identity element
+//
+// Unlike AlternativeMonoid, AltMonoid does NOT combine successful results - it always
+// returns the first success. This makes it ideal for fallback chains and default values.
+//
+// # Type Parameters
+//
+//   - I: The input type that validators accept
+//   - A: The output type that validators produce
+//
+// # Parameters
+//
+//   - zero: A lazy Validate[I, A] that serves as the identity element. This is typically
+//     a validator that always succeeds with a default value, but can also be a failing
+//     validator if no default is appropriate.
+//
+// # Returns
+//
+// A Monoid[Validate[I, A]] that combines validators using alternative semantics where
+// the first success wins.
+//
+// # Behavior Details
+//
+// The AltMonoid implements a "first success wins" strategy:
+//
+//   - **First succeeds**: Returns the first result, second is never evaluated
+//   - **First fails, second succeeds**: Returns the second result
+//   - **Both fail**: Aggregates errors from both validators
+//   - **Concat with Empty**: The zero validator is used as fallback
+//
+// # Example: Default Value Fallback
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec/validate"
+//	)
+//
+//	// Create a monoid with a default value of 0
+//	m := validate.AltMonoid(func() validate.Validate[string, int] {
+//	    return validate.Of[string, int](0)
+//	})
+//
+//	// First validator succeeds - returns 42, second is not evaluated
+//	validator1 := validate.Of[string, int](42)
+//	validator2 := validate.Of[string, int](100)
+//	combined := m.Concat(validator1, validator2)
+//	result := combined("input")(nil)
+//	// result is validation.Success(42)
+//
+// # Example: Fallback Chain
+//
+//	// Try primary, then fallback, then default
+//	m := validate.AltMonoid(func() validate.Validate[string, string] {
+//	    return validate.Of[string, string]("default")
+//	})
+//
+//	primary := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.FailureWithMessage[string](input, "primary failed")(ctx)
+//	    }
+//	}
+//	secondary := func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        return validation.Success("secondary value")
+//	    }
+//	}
+//
+//	// Chain: try primary, then secondary, then default
+//	combined := m.Concat(m.Concat(primary, secondary), m.Empty())
+//	result := combined("input")(nil)
+//	// result is validation.Success("secondary value")
+//
+// # Example: Error Aggregation
+//
+//	// Both fail - errors are aggregated
+//	m := validate.AltMonoid(func() validate.Validate[string, int] {
+//	    return func(input string) validate.Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            return validation.FailureWithMessage[int](input, "no default")(ctx)
+//	        }
+//	    }
+//	})
+//
+//	failing1 := func(input string) validate.Reader[validation.Context, validation.Validation[int]] {
+//	    return func(ctx validation.Context) validation.Validation[int] {
+//	        return validation.FailureWithMessage[int](input, "error 1")(ctx)
+//	    }
+//	}
+//	failing2 := func(input string) validate.Reader[validation.Context, validation.Validation[int]] {
+//	    return func(ctx validation.Context) validation.Validation[int] {
+//	        return validation.FailureWithMessage[int](input, "error 2")(ctx)
+//	    }
+//	}
+//
+//	combined := m.Concat(failing1, failing2)
+//	result := combined("input")(nil)
+//	// result contains both "error 1" and "error 2"
+//
+// # Comparison with Other Monoids
+//
+//   - **ApplicativeMonoid**: Combines results when both succeed using monoid operation
+//   - **AlternativeMonoid**: Combines results when both succeed, provides fallback when one fails
+//   - **AltMonoid**: First success wins, never combines results (pure alternative)
+//
+// # Use Cases
+//
+//   - Configuration loading with fallback sources (try file, then env, then default)
+//   - Validation with default values
+//   - Parser combinators with alternative branches
+//   - Error recovery with multiple strategies
+//
+// # Notes
+//
+//   - The zero validator is lazily evaluated, only when needed
+//   - First success short-circuits evaluation (second validator not called)
+//   - Error aggregation ensures all validation failures are reported
+//   - This follows the alternative functor laws
+//
+// # See Also
+//
+//   - AlternativeMonoid: For combining results when both succeed
+//   - ApplicativeMonoid: For pure applicative combination
+//   - MonadAlt: The underlying alternative operation
+//   - Alt: The curried version for pipeline composition
+func AltMonoid[I, A any](zero Lazy[Validate[I, A]]) Monoid[Validate[I, A]] {
+	return monoid.AltMonoid(
+		zero,
+		MonadAlt[I, A],
+	)
+}

v2/optics/codec/validate/monoid_test.go

@@ -1,475 +1,397 @@
-// Copyright (c) 2023 - 2025 IBM Corp.
-// All rights reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
 package validate
 
 import (
 	"testing"
 
-	E "github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	MO "github.com/IBM/fp-go/v2/monoid"
 	N "github.com/IBM/fp-go/v2/number"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
 	S "github.com/IBM/fp-go/v2/string"
 	"github.com/stretchr/testify/assert"
 )
 
-var (
-	intAddMonoid = N.MonoidSum[int]()
-	strMonoid    = S.Monoid
-)
+// TestAlternativeMonoid tests the AlternativeMonoid function
+func TestAlternativeMonoid(t *testing.T) {
+	t.Run("with string monoid", func(t *testing.T) {
+		m := AlternativeMonoid[string, string](S.Monoid)
 
-// Helper function to create a successful validator
-func successValidator[I, A any](value A) Validate[I, A] {
-	return func(input I) Reader[validation.Context, validation.Validation[A]] {
-		return func(ctx validation.Context) validation.Validation[A] {
-			return validation.Success(value)
-		}
-	}
-}
+		t.Run("empty returns validator that succeeds with empty string", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")(nil)
 
-// Helper function to create a failing validator
-func failureValidator[I, A any](message string) Validate[I, A] {
-	return func(input I) Reader[validation.Context, validation.Validation[A]] {
-		return validation.FailureWithMessage[A](input, message)
-	}
-}
+			assert.Equal(t, validation.Of(""), result)
+		})
 
-// Helper function to create a validator that uses the input
-func inputDependentValidator[A any](f func(A) A) Validate[A, A] {
-	return func(input A) Reader[validation.Context, validation.Validation[A]] {
-		return func(ctx validation.Context) validation.Validation[A] {
-			return validation.Success(f(input))
-		}
-	}
-}
+		t.Run("concat combines successful validators using monoid", func(t *testing.T) {
+			validator1 := Of[string, string]("Hello")
+			validator2 := Of[string, string](" World")
 
-// TestApplicativeMonoid_EmptyElement tests the empty element of the monoid
-func TestApplicativeMonoid_EmptyElement(t *testing.T) {
-	t.Run("int addition monoid", func(t *testing.T) {
-		m := ApplicativeMonoid[string](intAddMonoid)
-		empty := m.Empty()
+			combined := m.Concat(validator1, validator2)
+			result := combined("input")(nil)
 
-		result := empty("test")(nil)
+			assert.Equal(t, validation.Of("Hello World"), result)
+		})
 
-		assert.Equal(t, validation.Of(0), result)
-	})
-
-	t.Run("string concatenation monoid", func(t *testing.T) {
-		m := ApplicativeMonoid[int](strMonoid)
-		empty := m.Empty()
-
-		result := empty(42)(nil)
-
-		assert.Equal(t, validation.Of(""), result)
-	})
-}
-
-// TestApplicativeMonoid_ConcatSuccesses tests concatenating two successful validators
-func TestApplicativeMonoid_ConcatSuccesses(t *testing.T) {
-	t.Run("int addition", func(t *testing.T) {
-		m := ApplicativeMonoid[string](intAddMonoid)
-
-		v1 := successValidator[string](5)
-		v2 := successValidator[string](3)
-
-		combined := m.Concat(v1, v2)
-		result := combined("input")(nil)
-
-		assert.Equal(t, validation.Of(8), result)
-	})
-
-	t.Run("string concatenation", func(t *testing.T) {
-		m := ApplicativeMonoid[int](strMonoid)
-
-		v1 := successValidator[int]("Hello")
-		v2 := successValidator[int](" World")
-
-		combined := m.Concat(v1, v2)
-		result := combined(42)(nil)
-
-		assert.Equal(t, validation.Of("Hello World"), result)
-	})
-}
-
-// TestApplicativeMonoid_ConcatWithFailure tests concatenating validators where one fails
-func TestApplicativeMonoid_ConcatWithFailure(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	t.Run("left failure", func(t *testing.T) {
-		v1 := failureValidator[string, int]("left error")
-		v2 := successValidator[string](5)
-
-		combined := m.Concat(v1, v2)
-		result := combined("input")(nil)
-
-		assert.True(t, E.IsLeft(result))
-		_, errors := E.Unwrap(result)
-		assert.Len(t, errors, 1)
-		assert.Equal(t, "left error", errors[0].Messsage)
-	})
-
-	t.Run("right failure", func(t *testing.T) {
-		v1 := successValidator[string](5)
-		v2 := failureValidator[string, int]("right error")
-
-		combined := m.Concat(v1, v2)
-		result := combined("input")(nil)
-
-		assert.True(t, E.IsLeft(result))
-		_, errors := E.Unwrap(result)
-		assert.Len(t, errors, 1)
-		assert.Equal(t, "right error", errors[0].Messsage)
-	})
-
-	t.Run("both failures", func(t *testing.T) {
-		v1 := failureValidator[string, int]("left error")
-		v2 := failureValidator[string, int]("right error")
-
-		combined := m.Concat(v1, v2)
-		result := combined("input")(nil)
-
-		assert.True(t, E.IsLeft(result))
-		_, errors := E.Unwrap(result)
-		// Note: The current implementation returns the first error encountered
-		assert.GreaterOrEqual(t, len(errors), 1)
-		// At least one of the errors should be present
-		hasError := false
-		for _, err := range errors {
-			if err.Messsage == "left error" || err.Messsage == "right error" {
-				hasError = true
-				break
+		t.Run("concat uses second as fallback when first fails", func(t *testing.T) {
+			failing := func(input string) Reader[Context, Validation[string]] {
+				return func(ctx Context) Validation[string] {
+					return either.Left[string](validation.Errors{
+						{Value: input, Messsage: "first failed"},
+					})
+				}
 			}
-		}
-		assert.True(t, hasError, "Should contain at least one validation error")
-	})
-}
+			succeeding := Of[string, string]("fallback")
 
-// TestApplicativeMonoid_LeftIdentity tests the left identity law
-func TestApplicativeMonoid_LeftIdentity(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
+			combined := m.Concat(failing, succeeding)
+			result := combined("input")(nil)
 
-	v := successValidator[string](42)
+			assert.Equal(t, validation.Of("fallback"), result)
+		})
 
-	// empty <> v == v
-	combined := m.Concat(m.Empty(), v)
-
-	resultCombined := combined("test")(nil)
-	resultOriginal := v("test")(nil)
-
-	assert.Equal(t, resultOriginal, resultCombined)
-}
-
-// TestApplicativeMonoid_RightIdentity tests the right identity law
-func TestApplicativeMonoid_RightIdentity(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	v := successValidator[string](42)
-
-	// v <> empty == v
-	combined := m.Concat(v, m.Empty())
-
-	resultCombined := combined("test")(nil)
-	resultOriginal := v("test")(nil)
-
-	assert.Equal(t, resultOriginal, resultCombined)
-}
-
-// TestApplicativeMonoid_Associativity tests the associativity law
-func TestApplicativeMonoid_Associativity(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	v1 := successValidator[string](1)
-	v2 := successValidator[string](2)
-	v3 := successValidator[string](3)
-
-	// (v1 <> v2) <> v3 == v1 <> (v2 <> v3)
-	left := m.Concat(m.Concat(v1, v2), v3)
-	right := m.Concat(v1, m.Concat(v2, v3))
-
-	resultLeft := left("test")(nil)
-	resultRight := right("test")(nil)
-
-	assert.Equal(t, resultRight, resultLeft)
-
-	// Both should equal 6
-	assert.Equal(t, validation.Of(6), resultLeft)
-}
-
-// TestApplicativeMonoid_AssociativityWithFailures tests associativity with failures
-func TestApplicativeMonoid_AssociativityWithFailures(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	v1 := successValidator[string](1)
-	v2 := failureValidator[string, int]("error 2")
-	v3 := successValidator[string](3)
-
-	// (v1 <> v2) <> v3 == v1 <> (v2 <> v3)
-	left := m.Concat(m.Concat(v1, v2), v3)
-	right := m.Concat(v1, m.Concat(v2, v3))
-
-	resultLeft := left("test")(nil)
-	resultRight := right("test")(nil)
-
-	// Both should fail with the same error
-	assert.True(t, E.IsLeft(resultLeft))
-	assert.True(t, E.IsLeft(resultRight))
-
-	_, errorsLeft := E.Unwrap(resultLeft)
-	_, errorsRight := E.Unwrap(resultRight)
-
-	assert.Len(t, errorsLeft, 1)
-	assert.Len(t, errorsRight, 1)
-	assert.Equal(t, "error 2", errorsLeft[0].Messsage)
-	assert.Equal(t, "error 2", errorsRight[0].Messsage)
-}
-
-// TestApplicativeMonoid_MultipleValidators tests combining multiple validators
-func TestApplicativeMonoid_MultipleValidators(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	v1 := successValidator[string](10)
-	v2 := successValidator[string](20)
-	v3 := successValidator[string](30)
-	v4 := successValidator[string](40)
-
-	// Chain multiple concat operations
-	combined := m.Concat(
-		m.Concat(
-			m.Concat(v1, v2),
-			v3,
-		),
-		v4,
-	)
-
-	result := combined("test")(nil)
-
-	assert.Equal(t, validation.Of(100), result)
-}
-
-// TestApplicativeMonoid_InputDependent tests validators that depend on input
-func TestApplicativeMonoid_InputDependent(t *testing.T) {
-	m := ApplicativeMonoid[int](intAddMonoid)
-
-	// Validator that doubles the input
-	v1 := inputDependentValidator(N.Mul(2))
-	// Validator that adds 10 to the input
-	v2 := inputDependentValidator(N.Add(10))
-
-	combined := m.Concat(v1, v2)
-	result := combined(5)(nil)
-
-	// (5 * 2) + (5 + 10) = 10 + 15 = 25
-	assert.Equal(t, validation.Of(25), result)
-}
-
-// TestApplicativeMonoid_ContextPropagation tests that context is properly propagated
-func TestApplicativeMonoid_ContextPropagation(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	// Create a validator that captures the context
-	var capturedContext validation.Context
-	v1 := func(input string) Reader[validation.Context, validation.Validation[int]] {
-		return func(ctx validation.Context) validation.Validation[int] {
-			capturedContext = ctx
-			return validation.Success(5)
-		}
-	}
-
-	v2 := successValidator[string](3)
-
-	combined := m.Concat(v1, v2)
-
-	// Create a context with some entries
-	ctx := validation.Context{
-		{Key: "field1", Type: "int"},
-		{Key: "field2", Type: "string"},
-	}
-
-	result := combined("test")(ctx)
-
-	assert.True(t, E.IsRight(result))
-	assert.Equal(t, ctx, capturedContext)
-}
-
-// TestApplicativeMonoid_ErrorAccumulation tests that errors are accumulated
-func TestApplicativeMonoid_ErrorAccumulation(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	v1 := failureValidator[string, int]("error 1")
-	v2 := failureValidator[string, int]("error 2")
-	v3 := failureValidator[string, int]("error 3")
-
-	combined := m.Concat(m.Concat(v1, v2), v3)
-	result := combined("test")(nil)
-
-	assert.True(t, E.IsLeft(result))
-	_, errors := E.Unwrap(result)
-
-	// Note: The current implementation returns the first error encountered
-	// At least one error should be present
-	assert.GreaterOrEqual(t, len(errors), 1)
-	hasError := false
-	for _, err := range errors {
-		if err.Messsage == "error 1" || err.Messsage == "error 2" || err.Messsage == "error 3" {
-			hasError = true
-			break
-		}
-	}
-	assert.True(t, hasError, "Should contain at least one validation error")
-}
-
-// TestApplicativeMonoid_MixedSuccessFailure tests mixing successes and failures
-func TestApplicativeMonoid_MixedSuccessFailure(t *testing.T) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	v1 := successValidator[string](10)
-	v2 := failureValidator[string, int]("error in v2")
-	v3 := successValidator[string](20)
-	v4 := failureValidator[string, int]("error in v4")
-
-	combined := m.Concat(
-		m.Concat(
-			m.Concat(v1, v2),
-			v3,
-		),
-		v4,
-	)
-
-	result := combined("test")(nil)
-
-	assert.True(t, E.IsLeft(result))
-	_, errors := E.Unwrap(result)
-
-	// Note: The current implementation returns the first error encountered
-	// At least one error should be present
-	assert.GreaterOrEqual(t, len(errors), 1)
-	hasError := false
-	for _, err := range errors {
-		if err.Messsage == "error in v2" || err.Messsage == "error in v4" {
-			hasError = true
-			break
-		}
-	}
-	assert.True(t, hasError, "Should contain at least one validation error")
-}
-
-// TestApplicativeMonoid_DifferentInputTypes tests with different input types
-func TestApplicativeMonoid_DifferentInputTypes(t *testing.T) {
-	t.Run("struct input", func(t *testing.T) {
-		type Config struct {
-			Port    int
-			Timeout int
-		}
-
-		m := ApplicativeMonoid[Config](intAddMonoid)
-
-		v1 := func(cfg Config) Reader[validation.Context, validation.Validation[int]] {
-			return func(ctx validation.Context) validation.Validation[int] {
-				return validation.Success(cfg.Port)
+		t.Run("concat aggregates errors when both fail", func(t *testing.T) {
+			failing1 := func(input string) Reader[Context, Validation[string]] {
+				return func(ctx Context) Validation[string] {
+					return either.Left[string](validation.Errors{
+						{Value: input, Messsage: "error 1"},
+					})
+				}
 			}
-		}
-
-		v2 := func(cfg Config) Reader[validation.Context, validation.Validation[int]] {
-			return func(ctx validation.Context) validation.Validation[int] {
-				return validation.Success(cfg.Timeout)
+			failing2 := func(input string) Reader[Context, Validation[string]] {
+				return func(ctx Context) Validation[string] {
+					return either.Left[string](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
 			}
-		}
 
-		combined := m.Concat(v1, v2)
-		result := combined(Config{Port: 8080, Timeout: 30})(nil)
+			combined := m.Concat(failing1, failing2)
+			result := combined("input")(nil)
 
-		assert.Equal(t, validation.Of(8110), result) // 8080 + 30
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(string) Errors { return nil },
+			)
+			assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors from both validators")
+
+			messages := make([]string, len(errors))
+			for i, err := range errors {
+				messages[i] = err.Messsage
+			}
+			assert.Contains(t, messages, "error 1")
+			assert.Contains(t, messages, "error 2")
+		})
+
+		t.Run("concat with empty preserves validator", func(t *testing.T) {
+			validator := Of[string, string]("test")
+			empty := m.Empty()
+
+			result1 := m.Concat(validator, empty)("input")(nil)
+			result2 := m.Concat(empty, validator)("input")(nil)
+
+			val1 := either.MonadFold(result1,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			val2 := either.MonadFold(result2,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+
+			assert.Equal(t, "test", val1)
+			assert.Equal(t, "test", val2)
+		})
 	})
-}
 
-// TestApplicativeMonoid_StringConcatenation tests string concatenation scenarios
-func TestApplicativeMonoid_StringConcatenation(t *testing.T) {
-	m := ApplicativeMonoid[string](strMonoid)
-
-	t.Run("build sentence", func(t *testing.T) {
-		v1 := successValidator[string]("The")
-		v2 := successValidator[string](" quick")
-		v3 := successValidator[string](" brown")
-		v4 := successValidator[string](" fox")
-
-		combined := m.Concat(
-			m.Concat(
-				m.Concat(v1, v2),
-				v3,
-			),
-			v4,
+	t.Run("with int addition monoid", func(t *testing.T) {
+		intMonoid := MO.MakeMonoid(
+			func(a, b int) int { return a + b },
+			0,
 		)
+		m := AlternativeMonoid[string, int](intMonoid)
 
-		result := combined("input")(nil)
+		t.Run("empty returns validator with zero", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")(nil)
 
-		assert.Equal(t, validation.Of("The quick brown fox"), result)
+			value := either.MonadFold(result,
+				func(Errors) int { return -1 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 0, value)
+		})
+
+		t.Run("concat combines decoded values when both succeed", func(t *testing.T) {
+			validator1 := Of[string, int](10)
+			validator2 := Of[string, int](32)
+
+			combined := m.Concat(validator1, validator2)
+			result := combined("input")(nil)
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 42, value)
+		})
+
+		t.Run("concat uses fallback when first fails", func(t *testing.T) {
+			failing := func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "failed"},
+					})
+				}
+			}
+			succeeding := Of[string, int](42)
+
+			combined := m.Concat(failing, succeeding)
+			result := combined("input")(nil)
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 42, value)
+		})
+
+		t.Run("multiple concat operations", func(t *testing.T) {
+			validator1 := Of[string, int](1)
+			validator2 := Of[string, int](2)
+			validator3 := Of[string, int](3)
+			validator4 := Of[string, int](4)
+
+			combined := m.Concat(m.Concat(m.Concat(validator1, validator2), validator3), validator4)
+			result := combined("input")(nil)
+
+			value := either.MonadFold(result,
+				func(Errors) int { return 0 },
+				F.Identity[int],
+			)
+			assert.Equal(t, 10, value)
+		})
 	})
 
-	t.Run("with empty strings", func(t *testing.T) {
-		v1 := successValidator[string]("Hello")
-		v2 := successValidator[string]("")
-		v3 := successValidator[string]("World")
+	t.Run("satisfies monoid laws", func(t *testing.T) {
+		m := AlternativeMonoid[string, string](S.Monoid)
 
-		combined := m.Concat(m.Concat(v1, v2), v3)
-		result := combined("input")(nil)
+		validator1 := Of[string, string]("a")
+		validator2 := Of[string, string]("b")
+		validator3 := Of[string, string]("c")
 
-		assert.Equal(t, validation.Of("HelloWorld"), result)
+		t.Run("left identity", func(t *testing.T) {
+			result := m.Concat(m.Empty(), validator1)("input")(nil)
+			value := either.MonadFold(result,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			assert.Equal(t, "a", value)
+		})
+
+		t.Run("right identity", func(t *testing.T) {
+			result := m.Concat(validator1, m.Empty())("input")(nil)
+			value := either.MonadFold(result,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			assert.Equal(t, "a", value)
+		})
+
+		t.Run("associativity", func(t *testing.T) {
+			left := m.Concat(m.Concat(validator1, validator2), validator3)("input")(nil)
+			right := m.Concat(validator1, m.Concat(validator2, validator3))("input")(nil)
+
+			leftVal := either.MonadFold(left,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+			rightVal := either.MonadFold(right,
+				func(Errors) string { return "" },
+				F.Identity[string],
+			)
+
+			assert.Equal(t, "abc", leftVal)
+			assert.Equal(t, "abc", rightVal)
+		})
 	})
 }
 
-// Benchmark tests
-func BenchmarkApplicativeMonoid_ConcatSuccesses(b *testing.B) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-	v1 := successValidator[string](5)
-	v2 := successValidator[string](3)
-	combined := m.Concat(v1, v2)
+// TestAltMonoid tests the AltMonoid function
+func TestAltMonoid(t *testing.T) {
+	t.Run("with default value as zero", func(t *testing.T) {
+		m := AltMonoid(func() Validate[string, int] {
+			return Of[string, int](0)
+		})
 
-	b.ResetTimer()
-	for range b.N {
-		_ = combined("test")(nil)
-	}
-}
-
-func BenchmarkApplicativeMonoid_ConcatFailures(b *testing.B) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-	v1 := failureValidator[string, int]("error 1")
-	v2 := failureValidator[string, int]("error 2")
-	combined := m.Concat(v1, v2)
-
-	b.ResetTimer()
-	for range b.N {
-		_ = combined("test")(nil)
-	}
-}
-
-func BenchmarkApplicativeMonoid_MultipleConcat(b *testing.B) {
-	m := ApplicativeMonoid[string](intAddMonoid)
-
-	validators := make([]Validate[string, int], 10)
-	for i := range validators {
-		validators[i] = successValidator[string](i)
-	}
-
-	// Chain all validators
-	combined := validators[0]
-	for i := 1; i < len(validators); i++ {
-		combined = m.Concat(combined, validators[i])
-	}
-
-	b.ResetTimer()
-	for range b.N {
-		_ = combined("test")(nil)
-	}
+		t.Run("empty returns the provided zero validator", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")(nil)
+
+			assert.Equal(t, validation.Of(0), result)
+		})
+
+		t.Run("concat returns first validator when it succeeds", func(t *testing.T) {
+			validator1 := Of[string, int](42)
+			validator2 := Of[string, int](100)
+
+			combined := m.Concat(validator1, validator2)
+			result := combined("input")(nil)
+
+			assert.Equal(t, validation.Of(42), result)
+		})
+
+		t.Run("concat uses second as fallback when first fails", func(t *testing.T) {
+			failing := func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "failed"},
+					})
+				}
+			}
+			succeeding := Of[string, int](42)
+
+			combined := m.Concat(failing, succeeding)
+			result := combined("input")(nil)
+
+			assert.Equal(t, validation.Of(42), result)
+		})
+
+		t.Run("concat aggregates errors when both fail", func(t *testing.T) {
+			failing1 := func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 1"},
+					})
+				}
+			}
+			failing2 := func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
+			}
+
+			combined := m.Concat(failing1, failing2)
+			result := combined("input")(nil)
+
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(int) Errors { return nil },
+			)
+			assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors from both validators")
+
+			messages := make([]string, len(errors))
+			for i, err := range errors {
+				messages[i] = err.Messsage
+			}
+			assert.Contains(t, messages, "error 1")
+			assert.Contains(t, messages, "error 2")
+		})
+	})
+
+	t.Run("with failing zero", func(t *testing.T) {
+		m := AltMonoid(func() Validate[string, int] {
+			return func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Messsage: "no default available"},
+					})
+				}
+			}
+		})
+
+		t.Run("empty returns the failing zero validator", func(t *testing.T) {
+			empty := m.Empty()
+			result := empty("input")(nil)
+
+			assert.True(t, either.IsLeft(result))
+		})
+
+		t.Run("concat with all failures aggregates errors", func(t *testing.T) {
+			failing1 := func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 1"},
+					})
+				}
+			}
+			failing2 := func(input string) Reader[Context, Validation[int]] {
+				return func(ctx Context) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Value: input, Messsage: "error 2"},
+					})
+				}
+			}
+
+			combined := m.Concat(failing1, failing2)
+			result := combined("input")(nil)
+
+			assert.True(t, either.IsLeft(result))
+			errors := either.MonadFold(result,
+				F.Identity[Errors],
+				func(int) Errors { return nil },
+			)
+			assert.GreaterOrEqual(t, len(errors), 2, "Should aggregate errors")
+		})
+	})
+
+	t.Run("chaining multiple fallbacks", func(t *testing.T) {
+		m := AltMonoid(func() Validate[string, string] {
+			return Of[string, string]("default")
+		})
+
+		primary := func(input string) Reader[Context, Validation[string]] {
+			return func(ctx Context) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "primary failed"},
+				})
+			}
+		}
+		secondary := func(input string) Reader[Context, Validation[string]] {
+			return func(ctx Context) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Value: input, Messsage: "secondary failed"},
+				})
+			}
+		}
+		tertiary := Of[string, string]("tertiary value")
+
+		combined := m.Concat(m.Concat(primary, secondary), tertiary)
+		result := combined("input")(nil)
+
+		assert.Equal(t, validation.Of("tertiary value"), result)
+	})
+
+	t.Run("difference from AlternativeMonoid", func(t *testing.T) {
+		// AltMonoid - first success wins
+		altM := AltMonoid(func() Validate[string, int] {
+			return Of[string, int](0)
+		})
+
+		// AlternativeMonoid - combines successes
+		altMonoid := AlternativeMonoid[string, int](N.MonoidSum[int]())
+
+		validator1 := Of[string, int](10)
+		validator2 := Of[string, int](32)
+
+		// AltMonoid: returns first success (10)
+		result1 := altM.Concat(validator1, validator2)("input")(nil)
+		value1 := either.MonadFold(result1,
+			func(Errors) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 10, value1, "AltMonoid returns first success")
+
+		// AlternativeMonoid: combines both successes (10 + 32 = 42)
+		result2 := altMonoid.Concat(validator1, validator2)("input")(nil)
+		value2 := either.MonadFold(result2,
+			func(Errors) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 42, value2, "AlternativeMonoid combines successes")
+	})
 }

v2/optics/codec/validate/types.go

@@ -16,6 +16,8 @@
 package validate
 
 import (
+	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/codec/decode"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
@@ -90,25 +92,72 @@ type (
 	//   "at user.address.zipCode: expected string, got number"
 	Context = validation.Context
 
-	Decode[I, A any] = decode.Decode[I, A]
-
-	// Validate is a function that validates input I to produce type A with full context tracking.
+	// Decode represents a decoding operation that transforms input I into output A
+	// within a validation context.
 	//
 	// Type structure:
-	//   Validate[I, A] = Reader[I, Decode[Context, A]]
+	//   Decode[I, A] = Reader[Context, Validation[A]]
 	//
 	// This means:
-	//  1. Takes an input of type I
-	//  2. Returns a Reader that depends on validation Context
-	//  3. That Reader produces a Validation[A] (Either[Errors, A])
+	//   1. Takes a validation Context (path through nested structures)
+	//   2. Returns a Validation[A] (Either[Errors, A])
 	//
-	// The layered structure enables:
-	//   - Access to the input value being validated
-	//   - Context tracking through nested structures
-	//   - Error accumulation with detailed paths
-	//   - Composition with other validators
+	// Decode is used as the foundation for validation operations, providing:
+	//   - Context-aware error reporting with detailed paths
+	//   - Error accumulation across multiple validations
+	//   - Composable validation logic
+	//
+	// The Decode type is typically not used directly but through the Validate type,
+	// which adds an additional Reader layer for accessing the input value.
+	//
+	// Example:
+	//   decoder := func(ctx Context) Validation[int] {
+	//     // Perform validation and return result
+	//     return validation.Success(42)
+	//   }
+	//   // decoder is a Decode[any, int]
+	Decode[I, A any] = decode.Decode[I, A]
+
+	// Validate represents a composable validator that transforms input I to output A
+	// with comprehensive error tracking and context propagation.
+	//
+	// # Type Structure
+	//
+	//   Validate[I, A] = Reader[I, Decode[Context, A]]
+	//                  = Reader[I, Reader[Context, Validation[A]]]
+	//                  = func(I) func(Context) Either[Errors, A]
+	//
+	// This three-layer structure provides:
+	//   1. Input access: The outer Reader[I, ...] gives access to the input value I
+	//   2. Context tracking: The middle Reader[Context, ...] tracks the validation path
+	//   3. Error handling: The inner Validation[A] accumulates errors or produces value A
+	//
+	// # Purpose
+	//
+	// Validate is the core type for building type-safe, composable validators that:
+	//   - Transform and validate data from one type to another
+	//   - Track the path through nested structures for detailed error messages
+	//   - Accumulate multiple validation errors instead of failing fast
+	//   - Compose with other validators using functional patterns
+	//
+	// # Key Features
+	//
+	//   - Context-aware: Automatically tracks validation path (e.g., "user.address.zipCode")
+	//   - Error accumulation: Collects all validation errors, not just the first one
+	//   - Type-safe: Leverages Go's type system to ensure correctness
+	//   - Composable: Validators can be combined using Map, Chain, Ap, and other operators
+	//
+	// # Algebraic Structure
+	//
+	// Validate forms several algebraic structures:
+	//   - Functor: Transform successful results with Map
+	//   - Applicative: Combine independent validators in parallel with Ap
+	//   - Monad: Chain dependent validators sequentially with Chain
+	//
+	// # Example Usage
+	//
+	// Basic validator:
 	//
-	// Example usage:
 	//   validatePositive := func(n int) Reader[Context, Validation[int]] {
 	//     return func(ctx Context) Validation[int] {
 	//       if n > 0 {
@@ -119,10 +168,33 @@ type (
 	//   }
 	//   // validatePositive is a Validate[int, int]
 	//
-	// The Validate type forms:
-	//   - A Functor: Can map over successful results
-	//   - An Applicative: Can combine validators in parallel
-	//   - A Monad: Can chain dependent validations
+	// Composing validators:
+	//
+	//   // Transform the result of a validator
+	//   doubled := Map[int, int, int](func(x int) int { return x * 2 })(validatePositive)
+	//
+	//   // Chain dependent validations
+	//   validateRange := func(n int) Validate[int, int] {
+	//     return func(input int) Reader[Context, Validation[int]] {
+	//       return func(ctx Context) Validation[int] {
+	//         if n <= 100 {
+	//           return validation.Success(n)
+	//         }
+	//         return validation.FailureWithMessage[int](n, "must be <= 100")(ctx)
+	//       }
+	//     }
+	//   }
+	//   combined := Chain(validateRange)(validatePositive)
+	//
+	// # Integration
+	//
+	// Validate integrates with the broader optics/codec ecosystem:
+	//   - Works with Decode for decoding operations
+	//   - Uses Validation for error handling
+	//   - Leverages Context for detailed error reporting
+	//   - Composes with other codec types for complete encode/decode pipelines
+	//
+	// See the package documentation for more examples and patterns.
 	Validate[I, A any] = Reader[I, Decode[Context, A]]
 
 	// Errors is a collection of validation errors that occurred during validation.
@@ -174,4 +246,32 @@ type (
 	//   // toUpper is an Operator[string, string, string]
 	//   // It can be applied to any string validator to uppercase the result
 	Operator[I, A, B any] = Kleisli[I, Validate[I, A], B]
+
+	// Endomorphism represents a function from a type to itself.
+	//
+	// Type: Endomorphism[A] = func(A) A
+	//
+	// An endomorphism is a morphism (structure-preserving map) where the source
+	// and target are the same type. In simpler terms, it's a function that takes
+	// a value of type A and returns a value of the same type A.
+	//
+	// Endomorphisms are useful for:
+	//   - Transformations that preserve type (e.g., string normalization)
+	//   - Composable updates and modifications
+	//   - Building pipelines of same-type transformations
+	//   - Implementing the Monoid pattern (composition as binary operation)
+	//
+	// Endomorphisms form a Monoid under function composition:
+	//   - Identity: func(a A) A { return a }
+	//   - Concat: func(f, g Endomorphism[A]) Endomorphism[A] {
+	//       return func(a A) A { return f(g(a)) }
+	//     }
+	//
+	// Example:
+	//   trim := strings.TrimSpace      // Endomorphism[string]
+	//   lower := strings.ToLower       // Endomorphism[string]
+	//   normalize := compose(trim, lower)  // Endomorphism[string]
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	Lazy[A any] = lazy.Lazy[A]
 )

v2/optics/codec/validate/validate.go

@@ -119,6 +119,7 @@
 package validate
 
 import (
+	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/readert"
 	"github.com/IBM/fp-go/v2/optics/codec/decode"
 	"github.com/IBM/fp-go/v2/reader"
@@ -309,6 +310,364 @@ func Chain[I, A, B any](f Kleisli[I, A, B]) Operator[I, A, B] {
 	)
 }
 
+// ChainLeft sequences a computation on the failure (Left) channel of a validation.
+//
+// This function operates on the error path of validation, allowing you to transform,
+// enrich, or recover from validation failures. It's the dual of Chain - while Chain
+// operates on success values, ChainLeft operates on error values.
+//
+// # Key Behavior
+//
+// **Critical difference from standard Either operations**: This validation-specific
+// implementation **aggregates errors** using the Errors monoid. When the transformation
+// function returns a failure, both the original errors AND the new errors are combined,
+// ensuring comprehensive error reporting.
+//
+//  1. **Success Pass-Through**: If validation succeeds, the handler is never called and
+//     the success value passes through unchanged.
+//
+//  2. **Error Recovery**: The handler can recover from failures by returning a successful
+//     validation, converting Left to Right.
+//
+//  3. **Error Aggregation**: When the handler also returns a failure, both the original
+//     errors and the new errors are combined using the Errors monoid.
+//
+//  4. **Input Access**: The handler returns a Validate[I, A] function, giving it access
+//     to the original input value I for context-aware error handling.
+//
+// # Type Parameters
+//
+//   - I: The input type
+//   - A: The type of the validation result
+//
+// # Parameters
+//
+//   - f: A Kleisli arrow that takes Errors and returns a Validate[I, A]. This function
+//     is called only when validation fails, receiving the accumulated errors.
+//
+// # Returns
+//
+// An Operator[I, A, A] that transforms validators by handling their error cases.
+//
+// # Example: Error Recovery
+//
+//	// Validator that may fail
+//	validatePositive := func(n int) Reader[validation.Context, validation.Validation[int]] {
+//	    return func(ctx validation.Context) validation.Validation[int] {
+//	        if n > 0 {
+//	            return validation.Success(n)
+//	        }
+//	        return validation.FailureWithMessage[int](n, "must be positive")(ctx)
+//	    }
+//	}
+//
+//	// Recover from specific errors with a default value
+//	withDefault := ChainLeft(func(errs Errors) Validate[int, int] {
+//	    for _, err := range errs {
+//	        if err.Messsage == "must be positive" {
+//	            return Of[int](0) // recover with default
+//	        }
+//	    }
+//	    return func(input int) Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            return either.Left[int](errs)
+//	        }
+//	    }
+//	})
+//
+//	validator := withDefault(validatePositive)
+//	result := validator(-5)(nil)
+//	// Result: Success(0) - recovered from failure
+//
+// # Example: Error Context Addition
+//
+//	// Add contextual information to errors
+//	addContext := ChainLeft(func(errs Errors) Validate[string, int] {
+//	    return func(input string) Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            return either.Left[int](validation.Errors{
+//	                {
+//	                    Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+//	                    Messsage: "failed to validate user age",
+//	                },
+//	            })
+//	        }
+//	    }
+//	})
+//
+//	validator := addContext(someValidator)
+//	// Errors will include both original error and context
+//
+// # Example: Input-Dependent Recovery
+//
+//	// Recover with different defaults based on input
+//	smartDefault := ChainLeft(func(errs Errors) Validate[string, int] {
+//	    return func(input string) Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            // Use input to determine appropriate default
+//	            if strings.Contains(input, "http") {
+//	                return validation.Of(80)
+//	            }
+//	            if strings.Contains(input, "https") {
+//	                return validation.Of(443)
+//	            }
+//	            return validation.Of(8080)
+//	        }
+//	    }
+//	})
+//
+// # Notes
+//
+//   - Errors are accumulated, not replaced - this ensures no validation failures are lost
+//   - The handler has access to both the errors and the original input
+//   - Success values bypass the handler completely
+//   - This enables sophisticated error handling strategies including recovery, enrichment, and transformation
+//   - Use OrElse as a semantic alias when emphasizing fallback/alternative logic
+func ChainLeft[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return readert.Chain[Validate[I, A]](
+		decode.ChainLeft,
+		f,
+	)
+}
+
+// MonadChainLeft sequences a computation on the failure (Left) channel of a validation.
+//
+// This is the direct application version of ChainLeft. It operates on the error path
+// of validation, allowing you to transform, enrich, or recover from validation failures.
+// It's the dual of Chain - while Chain operates on success values, MonadChainLeft
+// operates on error values.
+//
+// # Key Behavior
+//
+// **Critical difference from standard Either operations**: This validation-specific
+// implementation **aggregates errors** using the Errors monoid. When the transformation
+// function returns a failure, both the original errors AND the new errors are combined,
+// ensuring comprehensive error reporting.
+//
+//  1. **Success Pass-Through**: If validation succeeds, the handler is never called and
+//     the success value passes through unchanged.
+//
+//  2. **Error Recovery**: The handler can recover from failures by returning a successful
+//     validation, converting Left to Right.
+//
+//  3. **Error Aggregation**: When the handler also returns a failure, both the original
+//     errors and the new errors are combined using the Errors monoid.
+//
+//  4. **Input Access**: The handler returns a Validate[I, A] function, giving it access
+//     to the original input value I for context-aware error handling.
+//
+// # Type Parameters
+//
+//   - I: The input type
+//   - A: The type of the validation result
+//
+// # Parameters
+//
+//   - fa: The Validate[I, A] to transform
+//   - f: A Kleisli arrow that takes Errors and returns a Validate[I, A]. This function
+//     is called only when validation fails, receiving the accumulated errors.
+//
+// # Returns
+//
+// A Validate[I, A] that handles error cases according to the provided function.
+//
+// # Example: Error Recovery
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec/validate"
+//	    "github.com/IBM/fp-go/v2/optics/codec/validation"
+//	)
+//
+//	// Validator that may fail
+//	validatePositive := func(n int) validate.Reader[validation.Context, validation.Validation[int]] {
+//	    return func(ctx validation.Context) validation.Validation[int] {
+//	        if n > 0 {
+//	            return validation.Success(n)
+//	        }
+//	        return validation.FailureWithMessage[int](n, "must be positive")(ctx)
+//	    }
+//	}
+//
+//	// Recover from specific errors with a default value
+//	withDefault := func(errs validation.Errors) validate.Validate[int, int] {
+//	    for _, err := range errs {
+//	        if err.Messsage == "must be positive" {
+//	            return validate.Of[int](0) // recover with default
+//	        }
+//	    }
+//	    // Propagate other errors
+//	    return func(input int) validate.Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            return either.Left[int](errs)
+//	        }
+//	    }
+//	}
+//
+//	validator := validate.MonadChainLeft(validatePositive, withDefault)
+//	result := validator(-5)(nil)
+//	// Result: Success(0) - recovered from failure
+//
+// # Example: Error Context Addition
+//
+//	// Add contextual information to errors
+//	addContext := func(errs validation.Errors) validate.Validate[string, int] {
+//	    return func(input string) validate.Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            // Add context error (will be aggregated with original)
+//	            return either.Left[int](validation.Errors{
+//	                {
+//	                    Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+//	                    Messsage: "failed to validate user age",
+//	                },
+//	            })
+//	        }
+//	    }
+//	}
+//
+//	validator := validate.MonadChainLeft(someValidator, addContext)
+//	// Errors will include both original error and context
+//
+// # Example: Input-Dependent Recovery
+//
+//	// Recover with different defaults based on input
+//	smartDefault := func(errs validation.Errors) validate.Validate[string, int] {
+//	    return func(input string) validate.Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            // Use input to determine appropriate default
+//	            if strings.Contains(input, "http:") {
+//	                return validation.Success(80)
+//	            }
+//	            if strings.Contains(input, "https:") {
+//	                return validation.Success(443)
+//	            }
+//	            return validation.Success(8080)
+//	        }
+//	    }
+//	}
+//
+//	validator := validate.MonadChainLeft(parsePort, smartDefault)
+//
+// # Notes
+//
+//   - Errors are accumulated, not replaced - this ensures no validation failures are lost
+//   - The handler has access to both the errors and the original input
+//   - Success values bypass the handler completely
+//   - This is the direct application version of ChainLeft
+//   - This enables sophisticated error handling strategies including recovery, enrichment, and transformation
+//
+// # See Also
+//
+//   - ChainLeft: The curried, point-free version
+//   - OrElse: Semantic alias for ChainLeft emphasizing fallback logic
+//   - MonadAlt: Simplified alternative that ignores error details
+//   - Alt: Curried version of MonadAlt
+func MonadChainLeft[I, A any](fa Validate[I, A], f Kleisli[I, Errors, A]) Validate[I, A] {
+	return readert.MonadChain(
+		decode.MonadChainLeft,
+		fa,
+		f,
+	)
+}
+
+// OrElse provides an alternative validation when the primary validation fails.
+//
+// This is a semantic alias for ChainLeft with identical behavior. The name "OrElse"
+// emphasizes the intent of providing fallback or alternative validation logic, making
+// code more readable when that's the primary use case.
+//
+// # Relationship to ChainLeft
+//
+// **OrElse and ChainLeft are functionally identical** - they produce exactly the same
+// results for all inputs. The choice between them is purely about code readability:
+//
+//   - Use **OrElse** when emphasizing fallback/alternative validation logic
+//   - Use **ChainLeft** when emphasizing technical error channel transformation
+//
+// Both maintain the critical property of **error aggregation**, ensuring all validation
+// failures are preserved and reported together.
+//
+// # Type Parameters
+//
+//   - I: The input type
+//   - A: The type of the validation result
+//
+// # Parameters
+//
+//   - f: A Kleisli arrow that takes Errors and returns a Validate[I, A]. This function
+//     is called only when validation fails, receiving the accumulated errors.
+//
+// # Returns
+//
+// An Operator[I, A, A] that transforms validators by providing alternative validation.
+//
+// # Example: Fallback Validation
+//
+//	// Primary validator that may fail
+//	validateFromConfig := func(key string) Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        // Try to get value from config
+//	        if value, ok := config[key]; ok {
+//	            return validation.Success(value)
+//	        }
+//	        return validation.FailureWithMessage[string](key, "not found in config")(ctx)
+//	    }
+//	}
+//
+//	// Use OrElse for semantic clarity - "try config, or else use environment"
+//	withEnvFallback := OrElse(func(errs Errors) Validate[string, string] {
+//	    return func(key string) Reader[validation.Context, validation.Validation[string]] {
+//	        return func(ctx validation.Context) validation.Validation[string] {
+//	            if value := os.Getenv(key); value != "" {
+//	                return validation.Success(value)
+//	            }
+//	            return either.Left[string](errs) // propagate original errors
+//	        }
+//	    }
+//	})
+//
+//	validator := withEnvFallback(validateFromConfig)
+//	result := validator("DATABASE_URL")(nil)
+//	// Tries config first, falls back to environment variable
+//
+// # Example: Default Value on Failure
+//
+//	// Provide a default value when validation fails
+//	withDefault := OrElse(func(errs Errors) Validate[int, int] {
+//	    return Of[int](0) // default to 0 on any failure
+//	})
+//
+//	validator := withDefault(someValidator)
+//	result := validator(input)(nil)
+//	// Always succeeds, using default value if validation fails
+//
+// # Example: Pipeline with Multiple Fallbacks
+//
+//	// Build a validation pipeline with multiple fallback strategies
+//	validator := F.Pipe2(
+//	    validateFromDatabase,
+//	    OrElse(func(errs Errors) Validate[string, Config] {
+//	        // Try cache as first fallback
+//	        return validateFromCache
+//	    }),
+//	    OrElse(func(errs Errors) Validate[string, Config] {
+//	        // Use default config as final fallback
+//	        return Of[string](defaultConfig)
+//	    }),
+//	)
+//	// Tries database, then cache, then default
+//
+// # Notes
+//
+//   - Identical behavior to ChainLeft - they are aliases
+//   - Errors are accumulated when transformations fail
+//   - Success values pass through unchanged
+//   - The handler has access to both errors and original input
+//   - Choose OrElse for better readability when providing alternatives
+//   - See ChainLeft documentation for detailed behavior and additional examples
+func OrElse[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return ChainLeft(f)
+}
+
 // MonadAp applies a validator containing a function to a validator containing a value.
 //
 // This is the applicative apply operation for Validate. It allows you to apply
@@ -409,3 +768,218 @@ func Ap[B, I, A any](fa Validate[I, A]) Operator[I, func(A) B, B] {
 		fa,
 	)
 }
+
+// Alt provides an alternative validator when the primary validator fails.
+//
+// This is the curried, point-free version of MonadAlt. It creates an operator that
+// transforms a validator by adding a fallback alternative. When the first validator
+// fails, the second (lazily evaluated) validator is tried. If both fail, errors are
+// aggregated.
+//
+// Alt implements the Alternative typeclass pattern, providing a way to express
+// "try this, or else try that" logic in a composable way.
+//
+// # Type Parameters
+//
+//   - I: The input type
+//   - A: The type of the validation result
+//
+// # Parameters
+//
+//   - second: A lazy Validate[I, A] that serves as the fallback. It's only evaluated
+//     if the first validator fails.
+//
+// # Returns
+//
+// An Operator[I, A, A] that transforms validators by adding alternative fallback logic.
+//
+// # Behavior
+//
+//   - **First succeeds**: Returns the first result, second is never evaluated
+//   - **First fails, second succeeds**: Returns the second result
+//   - **Both fail**: Aggregates errors from both validators
+//
+// # Example: Fallback Validation
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    "github.com/IBM/fp-go/v2/optics/codec/validate"
+//	    "github.com/IBM/fp-go/v2/optics/codec/validation"
+//	)
+//
+//	// Primary validator that may fail
+//	validateFromConfig := func(key string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        // Try to get value from config
+//	        if value, ok := config[key]; ok {
+//	            return validation.Success(value)
+//	        }
+//	        return validation.FailureWithMessage[string](key, "not in config")(ctx)
+//	    }
+//	}
+//
+//	// Fallback to environment variable
+//	validateFromEnv := func(key string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        if value := os.Getenv(key); value != "" {
+//	            return validation.Success(value)
+//	        }
+//	        return validation.FailureWithMessage[string](key, "not in env")(ctx)
+//	    }
+//	}
+//
+//	// Use Alt to add fallback - point-free style
+//	withFallback := validate.Alt(func() validate.Validate[string, string] {
+//	    return validateFromEnv
+//	})
+//
+//	validator := withFallback(validateFromConfig)
+//	result := validator("DATABASE_URL")(nil)
+//	// Tries config first, falls back to environment variable
+//
+// # Example: Pipeline with Multiple Alternatives
+//
+//	// Chain multiple alternatives using function composition
+//	validator := F.Pipe2(
+//	    validateFromDatabase,
+//	    validate.Alt(func() validate.Validate[string, Config] {
+//	        return validateFromCache
+//	    }),
+//	    validate.Alt(func() validate.Validate[string, Config] {
+//	        return validate.Of[string](defaultConfig)
+//	    }),
+//	)
+//	// Tries database, then cache, then default
+//
+// # Notes
+//
+//   - The second validator is lazily evaluated for efficiency
+//   - First success short-circuits evaluation
+//   - Errors are aggregated when both fail
+//   - This is the point-free version of MonadAlt
+//   - Useful for building validation pipelines with F.Pipe
+//
+// # See Also
+//
+//   - MonadAlt: The direct application version
+//   - ChainLeft: The more general error transformation operator
+//   - OrElse: Semantic alias for ChainLeft
+//   - AltMonoid: For combining multiple alternatives with monoid structure
+func Alt[I, A any](second Lazy[Validate[I, A]]) Operator[I, A, A] {
+	return ChainLeft(function.Ignore1of1[Errors](second))
+}
+
+// MonadAlt provides an alternative validator when the primary validator fails.
+//
+// This is the direct application version of Alt. It takes two validators and returns
+// a new validator that tries the first, and if it fails, tries the second. If both
+// fail, errors from both are aggregated.
+//
+// MonadAlt implements the Alternative typeclass pattern, enabling "try this, or else
+// try that" logic with comprehensive error reporting.
+//
+// # Type Parameters
+//
+//   - I: The input type
+//   - A: The type of the validation result
+//
+// # Parameters
+//
+//   - first: The primary Validate[I, A] to try first
+//   - second: A lazy Validate[I, A] that serves as the fallback. It's only evaluated
+//     if the first validator fails.
+//
+// # Returns
+//
+// A Validate[I, A] that tries the first validator, falling back to the second if needed.
+//
+// # Behavior
+//
+//   - **First succeeds**: Returns the first result, second is never evaluated
+//   - **First fails, second succeeds**: Returns the second result
+//   - **Both fail**: Aggregates errors from both validators
+//
+// # Example: Configuration with Fallback
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec/validate"
+//	    "github.com/IBM/fp-go/v2/optics/codec/validation"
+//	)
+//
+//	// Primary validator
+//	validateFromConfig := func(key string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        if value, ok := config[key]; ok {
+//	            return validation.Success(value)
+//	        }
+//	        return validation.FailureWithMessage[string](key, "not in config")(ctx)
+//	    }
+//	}
+//
+//	// Fallback validator
+//	validateFromEnv := func(key string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        if value := os.Getenv(key); value != "" {
+//	            return validation.Success(value)
+//	        }
+//	        return validation.FailureWithMessage[string](key, "not in env")(ctx)
+//	    }
+//	}
+//
+//	// Combine with MonadAlt
+//	validator := validate.MonadAlt(
+//	    validateFromConfig,
+//	    func() validate.Validate[string, string] { return validateFromEnv },
+//	)
+//	result := validator("DATABASE_URL")(nil)
+//	// Tries config first, falls back to environment variable
+//
+// # Example: Multiple Fallbacks
+//
+//	// Chain multiple alternatives
+//	validator := validate.MonadAlt(
+//	    validate.MonadAlt(
+//	        validateFromDatabase,
+//	        func() validate.Validate[string, Config] { return validateFromCache },
+//	    ),
+//	    func() validate.Validate[string, Config] { return validate.Of[string](defaultConfig) },
+//	)
+//	// Tries database, then cache, then default
+//
+// # Example: Error Aggregation
+//
+//	failing1 := func(input string) validate.Reader[validation.Context, validation.Validation[int]] {
+//	    return func(ctx validation.Context) validation.Validation[int] {
+//	        return validation.FailureWithMessage[int](input, "error 1")(ctx)
+//	    }
+//	}
+//	failing2 := func(input string) validate.Reader[validation.Context, validation.Validation[int]] {
+//	    return func(ctx validation.Context) validation.Validation[int] {
+//	        return validation.FailureWithMessage[int](input, "error 2")(ctx)
+//	    }
+//	}
+//
+//	validator := validate.MonadAlt(
+//	    failing1,
+//	    func() validate.Validate[string, int] { return failing2 },
+//	)
+//	result := validator("input")(nil)
+//	// result contains both "error 1" and "error 2"
+//
+// # Notes
+//
+//   - The second validator is lazily evaluated for efficiency
+//   - First success short-circuits evaluation (second not called)
+//   - Errors are aggregated when both fail
+//   - This is equivalent to Alt but with direct application
+//   - Both validators receive the same input value
+//
+// # See Also
+//
+//   - Alt: The curried, point-free version
+//   - MonadChainLeft: The underlying error transformation operation
+//   - OrElse: Semantic alias for ChainLeft
+//   - AltMonoid: For combining multiple alternatives with monoid structure
+func MonadAlt[I, A any](first Validate[I, A], second Lazy[Validate[I, A]]) Validate[I, A] {
+	return MonadChainLeft(first, function.Ignore1of1[Errors](second))
+}

v2/optics/codec/validate/validate_test.go

@@ -849,3 +849,428 @@ func TestFunctorLaws(t *testing.T) {
 		}
 	})
 }
+
+// TestChainLeft tests the ChainLeft function
+func TestChainLeft(t *testing.T) {
+	t.Run("transforms failures while preserving successes", func(t *testing.T) {
+		// Create a failing validator
+		failingValidator := func(n int) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](n, "validation failed")(ctx)
+			}
+		}
+
+		// Handler that recovers from specific errors
+		handler := ChainLeft(func(errs Errors) Validate[int, int] {
+			for _, err := range errs {
+				if err.Messsage == "validation failed" {
+					return Of[int](0) // recover with default
+				}
+			}
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					return E.Left[int](errs)
+				}
+			}
+		})
+
+		validator := handler(failingValidator)
+		result := validator(-5)(nil)
+
+		assert.Equal(t, validation.Of(0), result, "Should recover from failure")
+	})
+
+	t.Run("preserves success values unchanged", func(t *testing.T) {
+		successValidator := Of[int](42)
+
+		handler := ChainLeft(func(errs Errors) Validate[int, int] {
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					return validation.FailureWithMessage[int](input, "should not be called")(ctx)
+				}
+			}
+		})
+
+		validator := handler(successValidator)
+		result := validator(100)(nil)
+
+		assert.Equal(t, validation.Of(42), result, "Success should pass through unchanged")
+	})
+
+	t.Run("aggregates errors when transformation also fails", func(t *testing.T) {
+		failingValidator := func(s string) Reader[validation.Context, validation.Validation[string]] {
+			return func(ctx validation.Context) validation.Validation[string] {
+				return validation.FailureWithMessage[string](s, "original error")(ctx)
+			}
+		}
+
+		handler := ChainLeft(func(errs Errors) Validate[string, string] {
+			return func(input string) Reader[validation.Context, validation.Validation[string]] {
+				return func(ctx validation.Context) validation.Validation[string] {
+					return validation.FailureWithMessage[string](input, "additional error")(ctx)
+				}
+			}
+		})
+
+		validator := handler(failingValidator)
+		result := validator("test")(nil)
+
+		assert.True(t, E.IsLeft(result))
+		_, errors := E.Unwrap(result)
+		assert.Len(t, errors, 2, "Should aggregate both errors")
+
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "original error")
+		assert.Contains(t, messages, "additional error")
+	})
+
+	t.Run("adds context to errors", func(t *testing.T) {
+		failingValidator := func(n int) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](n, "invalid value")(ctx)
+			}
+		}
+
+		addContext := ChainLeft(func(errs Errors) Validate[int, int] {
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					return E.Left[int](validation.Errors{
+						{
+							Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+							Messsage: "failed to validate user age",
+						},
+					})
+				}
+			}
+		})
+
+		validator := addContext(failingValidator)
+		result := validator(150)(nil)
+
+		assert.True(t, E.IsLeft(result))
+		_, errors := E.Unwrap(result)
+		assert.Len(t, errors, 2, "Should have both original and context errors")
+	})
+
+	t.Run("can be composed in pipeline", func(t *testing.T) {
+		failingValidator := func(n int) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](n, "error1")(ctx)
+			}
+		}
+
+		handler1 := ChainLeft(func(errs Errors) Validate[int, int] {
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					return validation.FailureWithMessage[int](input, "error2")(ctx)
+				}
+			}
+		})
+
+		handler2 := ChainLeft(func(errs Errors) Validate[int, int] {
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					return validation.FailureWithMessage[int](input, "error3")(ctx)
+				}
+			}
+		})
+
+		validator := handler2(handler1(failingValidator))
+		result := validator(42)(nil)
+
+		assert.True(t, E.IsLeft(result))
+		_, errors := E.Unwrap(result)
+		assert.GreaterOrEqual(t, len(errors), 2, "Should accumulate errors through pipeline")
+	})
+
+	t.Run("provides access to original input", func(t *testing.T) {
+		failingValidator := func(n int) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](n, "failed")(ctx)
+			}
+		}
+
+		// Handler uses input to determine recovery strategy
+		handler := ChainLeft(func(errs Errors) Validate[int, int] {
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					// Use input value to decide on recovery
+					if input < 0 {
+						return validation.Of(0)
+					}
+					if input > 100 {
+						return validation.Of(100)
+					}
+					return E.Left[int](errs)
+				}
+			}
+		})
+
+		validator := handler(failingValidator)
+
+		result1 := validator(-10)(nil)
+		assert.Equal(t, validation.Of(0), result1, "Should recover negative to 0")
+
+		result2 := validator(150)(nil)
+		assert.Equal(t, validation.Of(100), result2, "Should recover large to 100")
+	})
+
+	t.Run("works with different input and output types", func(t *testing.T) {
+		// Validator that converts string to int
+		parseValidator := func(s string) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](s, "parse failed")(ctx)
+			}
+		}
+
+		// Handler that provides default based on input string
+		handler := ChainLeft(func(errs Errors) Validate[string, int] {
+			return func(input string) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					if input == "default" {
+						return validation.Of(42)
+					}
+					return E.Left[int](errs)
+				}
+			}
+		})
+
+		validator := handler(parseValidator)
+		result := validator("default")(nil)
+
+		assert.Equal(t, validation.Of(42), result)
+	})
+}
+
+// TestOrElse tests the OrElse function
+func TestOrElse(t *testing.T) {
+	t.Run("provides fallback for failing validation", func(t *testing.T) {
+		// Primary validator that fails
+		primaryValidator := func(s string) Reader[validation.Context, validation.Validation[string]] {
+			return func(ctx validation.Context) validation.Validation[string] {
+				return validation.FailureWithMessage[string](s, "not found")(ctx)
+			}
+		}
+
+		// Use OrElse to provide fallback
+		withFallback := OrElse(func(errs Errors) Validate[string, string] {
+			return Of[string]("default value")
+		})
+
+		validator := withFallback(primaryValidator)
+		result := validator("missing")(nil)
+
+		assert.Equal(t, validation.Of("default value"), result)
+	})
+
+	t.Run("preserves success values unchanged", func(t *testing.T) {
+		successValidator := Of[string]("success")
+
+		withFallback := OrElse(func(errs Errors) Validate[string, string] {
+			return Of[string]("fallback")
+		})
+
+		validator := withFallback(successValidator)
+		result := validator("input")(nil)
+
+		assert.Equal(t, validation.Of("success"), result, "Should not use fallback for success")
+	})
+
+	t.Run("aggregates errors when fallback also fails", func(t *testing.T) {
+		failingValidator := func(n int) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](n, "primary failed")(ctx)
+			}
+		}
+
+		withFallback := OrElse(func(errs Errors) Validate[int, int] {
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					return validation.FailureWithMessage[int](input, "fallback failed")(ctx)
+				}
+			}
+		})
+
+		validator := withFallback(failingValidator)
+		result := validator(42)(nil)
+
+		assert.True(t, E.IsLeft(result))
+		_, errors := E.Unwrap(result)
+		assert.Len(t, errors, 2, "Should aggregate both errors")
+
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "primary failed")
+		assert.Contains(t, messages, "fallback failed")
+	})
+
+	t.Run("supports multiple fallback strategies", func(t *testing.T) {
+		failingValidator := func(s string) Reader[validation.Context, validation.Validation[string]] {
+			return func(ctx validation.Context) validation.Validation[string] {
+				return validation.FailureWithMessage[string](s, "not in database")(ctx)
+			}
+		}
+
+		// First fallback: try cache
+		tryCache := OrElse(func(errs Errors) Validate[string, string] {
+			return func(input string) Reader[validation.Context, validation.Validation[string]] {
+				return func(ctx validation.Context) validation.Validation[string] {
+					if input == "cached" {
+						return validation.Of("from cache")
+					}
+					return E.Left[string](errs)
+				}
+			}
+		})
+
+		// Second fallback: use default
+		useDefault := OrElse(func(errs Errors) Validate[string, string] {
+			return Of[string]("default")
+		})
+
+		// Compose fallbacks
+		validator := useDefault(tryCache(failingValidator))
+
+		// Test with cached value
+		result1 := validator("cached")(nil)
+		assert.Equal(t, validation.Of("from cache"), result1)
+
+		// Test with non-cached value (should use default)
+		result2 := validator("other")(nil)
+		assert.Equal(t, validation.Of("default"), result2)
+	})
+
+	t.Run("provides input-dependent fallback", func(t *testing.T) {
+		failingValidator := func(s string) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](s, "parse failed")(ctx)
+			}
+		}
+
+		// Fallback with different defaults based on input
+		smartFallback := OrElse(func(errs Errors) Validate[string, int] {
+			return func(input string) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					// Provide context-aware defaults
+					if input == "http" {
+						return validation.Of(80)
+					}
+					if input == "https" {
+						return validation.Of(443)
+					}
+					return validation.Of(8080)
+				}
+			}
+		})
+
+		validator := smartFallback(failingValidator)
+
+		result1 := validator("http")(nil)
+		assert.Equal(t, validation.Of(80), result1)
+
+		result2 := validator("https")(nil)
+		assert.Equal(t, validation.Of(443), result2)
+
+		result3 := validator("other")(nil)
+		assert.Equal(t, validation.Of(8080), result3)
+	})
+
+	t.Run("is equivalent to ChainLeft", func(t *testing.T) {
+		// Create identical handlers
+		handler := func(errs Errors) Validate[int, int] {
+			return func(input int) Reader[validation.Context, validation.Validation[int]] {
+				return func(ctx validation.Context) validation.Validation[int] {
+					if input < 0 {
+						return validation.Of(0)
+					}
+					return E.Left[int](errs)
+				}
+			}
+		}
+
+		failingValidator := func(n int) Reader[validation.Context, validation.Validation[int]] {
+			return func(ctx validation.Context) validation.Validation[int] {
+				return validation.FailureWithMessage[int](n, "failed")(ctx)
+			}
+		}
+
+		// Apply with ChainLeft
+		withChainLeft := ChainLeft(handler)(failingValidator)
+
+		// Apply with OrElse
+		withOrElse := OrElse(handler)(failingValidator)
+
+		// Test with same inputs
+		inputs := []int{-10, 0, 10, -5, 100}
+		for _, input := range inputs {
+			result1 := withChainLeft(input)(nil)
+			result2 := withOrElse(input)(nil)
+
+			// Results should be identical
+			assert.Equal(t, E.IsLeft(result1), E.IsLeft(result2))
+			if E.IsRight(result1) {
+				val1, _ := E.Unwrap(result1)
+				val2, _ := E.Unwrap(result2)
+				assert.Equal(t, val1, val2, "OrElse and ChainLeft should produce identical results")
+			}
+		}
+	})
+
+	t.Run("works in complex validation pipeline", func(t *testing.T) {
+		type Config struct {
+			Port int
+			Host string
+		}
+
+		// Validator that tries to parse config
+		parseConfig := func(s string) Reader[validation.Context, validation.Validation[Config]] {
+			return func(ctx validation.Context) validation.Validation[Config] {
+				return validation.FailureWithMessage[Config](s, "invalid config")(ctx)
+			}
+		}
+
+		// Fallback to environment variables
+		tryEnv := OrElse(func(errs Errors) Validate[string, Config] {
+			return func(input string) Reader[validation.Context, validation.Validation[Config]] {
+				return func(ctx validation.Context) validation.Validation[Config] {
+					// Simulate env var lookup
+					if input == "from_env" {
+						return validation.Of(Config{Port: 8080, Host: "localhost"})
+					}
+					return E.Left[Config](errs)
+				}
+			}
+		})
+
+		// Final fallback to defaults
+		useDefaults := OrElse(func(errs Errors) Validate[string, Config] {
+			return Of[string](Config{Port: 3000, Host: "0.0.0.0"})
+		})
+
+		// Build pipeline
+		validator := useDefaults(tryEnv(parseConfig))
+
+		// Test with env fallback
+		result1 := validator("from_env")(nil)
+		assert.True(t, E.IsRight(result1))
+		if E.IsRight(result1) {
+			cfg, _ := E.Unwrap(result1)
+			assert.Equal(t, 8080, cfg.Port)
+			assert.Equal(t, "localhost", cfg.Host)
+		}
+
+		// Test with default fallback
+		result2 := validator("other")(nil)
+		assert.True(t, E.IsRight(result2))
+		if E.IsRight(result2) {
+			cfg, _ := E.Unwrap(result2)
+			assert.Equal(t, 3000, cfg.Port)
+			assert.Equal(t, "0.0.0.0", cfg.Host)
+		}
+	})
+}

v2/optics/codec/validation/OrElse_explanation.md

@@ -0,0 +1,162 @@
+# OrElse is Equivalent to ChainLeft
+
+## Overview
+
+In [`optics/codec/validation/monad.go`](monad.go:474-476), the [`OrElse`](monad.go:474) function is defined as a simple alias for [`ChainLeft`](monad.go:304):
+
+```go
+//go:inline
+func OrElse[A any](f Kleisli[Errors, A]) Operator[A, A] {
+	return ChainLeft(f)
+}
+```
+
+This means **`OrElse` and `ChainLeft` are functionally identical** - they produce exactly the same results for all inputs.
+
+## Why Have Both?
+
+While they are technically the same, they serve different **semantic purposes**:
+
+### ChainLeft - Technical Perspective
+[`ChainLeft`](monad.go:304-309) emphasizes the **technical operation**: it chains a computation on the Left (failure) channel of the Either/Validation monad. This name comes from category theory and functional programming terminology.
+
+### OrElse - Semantic Perspective  
+[`OrElse`](monad.go:474-476) emphasizes the **intent**: it provides an alternative or fallback when validation fails. The name reads naturally in code: "try this validation, **or else** try this alternative."
+
+## Key Behavior
+
+Both functions share the same critical behavior that distinguishes them from standard Either operations:
+
+### Error Aggregation
+When the transformation function returns a failure, **both the original errors AND the new errors are combined** using the Errors monoid. This ensures no validation errors are lost.
+
+```go
+// Example: Error aggregation
+result := OrElse(func(errs Errors) Validation[string] {
+    return Failures[string](Errors{
+        &ValidationError{Messsage: "additional error"},
+    })
+})(Failures[string](Errors{
+    &ValidationError{Messsage: "original error"},
+}))
+
+// Result contains BOTH errors: ["original error", "additional error"]
+```
+
+### Success Pass-Through
+Success values pass through unchanged - the function is never called:
+
+```go
+result := OrElse(func(errs Errors) Validation[int] {
+    return Failures[int](Errors{
+        &ValidationError{Messsage: "never called"},
+    })
+})(Success(42))
+
+// Result: Success(42) - unchanged
+```
+
+### Error Recovery
+The function can recover from failures by returning a Success:
+
+```go
+recoverFromNotFound := OrElse(func(errs Errors) Validation[int] {
+    for _, err := range errs {
+        if err.Messsage == "not found" {
+            return Success(0) // recover with default
+        }
+    }
+    return Failures[int](errs)
+})
+
+result := recoverFromNotFound(Failures[int](Errors{
+    &ValidationError{Messsage: "not found"},
+}))
+
+// Result: Success(0) - recovered from failure
+```
+
+## Use Cases
+
+### 1. Fallback Validation (OrElse reads better)
+```go
+validatePositive := func(x int) Validation[int] {
+    if x > 0 {
+        return Success(x)
+    }
+    return Failures[int](Errors{
+        &ValidationError{Messsage: "must be positive"},
+    })
+}
+
+// Use OrElse for semantic clarity
+withDefault := OrElse(func(errs Errors) Validation[int] {
+    return Success(1) // default to 1 if validation fails
+})
+
+result := F.Pipe1(validatePositive(-5), withDefault)
+// Result: Success(1)
+```
+
+### 2. Error Context Addition (ChainLeft reads better)
+```go
+addContext := ChainLeft(func(errs Errors) Validation[string] {
+    return Failures[string](Errors{
+        &ValidationError{
+            Messsage: "validation failed in user.email field",
+        },
+    })
+})
+
+result := F.Pipe1(
+    Failures[string](Errors{
+        &ValidationError{Messsage: "invalid format"},
+    }),
+    addContext,
+)
+// Result contains: ["invalid format", "validation failed in user.email field"]
+```
+
+### 3. Pipeline Composition
+Both can be used in pipelines, with errors accumulating at each step:
+
+```go
+result := F.Pipe2(
+    Failures[int](Errors{
+        &ValidationError{Messsage: "database error"},
+    }),
+    OrElse(func(errs Errors) Validation[int] {
+        return Failures[int](Errors{
+            &ValidationError{Messsage: "context added"},
+        })
+    }),
+    OrElse(func(errs Errors) Validation[int] {
+        return Failures[int](errs) // propagate
+    }),
+)
+// Errors accumulate at each step in the pipeline
+```
+
+## Verification
+
+The test suite in [`monad_test.go`](monad_test.go:1698) includes comprehensive tests proving that `OrElse` and `ChainLeft` are equivalent:
+
+- ✅ Identical behavior for Success values
+- ✅ Identical behavior for error recovery
+- ✅ Identical behavior for error aggregation
+- ✅ Identical behavior in pipeline composition
+- ✅ Identical behavior for multiple error scenarios
+
+Run the tests:
+```bash
+go test -v -run TestOrElse ./optics/codec/validation
+```
+
+## Conclusion
+
+**`OrElse` is exactly the same as `ChainLeft`** - they are aliases with identical implementations and behavior. The choice between them is purely about **code readability and semantic intent**:
+
+- Use **`OrElse`** when emphasizing fallback/alternative validation logic
+- Use **`ChainLeft`** when emphasizing technical error channel transformation
+
+Both maintain the critical validation property of **error aggregation**, ensuring all validation failures are preserved and reported together.

v2/optics/codec/validation/bind.go

@@ -0,0 +1,318 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package validation
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	A "github.com/IBM/fp-go/v2/internal/apply"
+	C "github.com/IBM/fp-go/v2/internal/chain"
+	F "github.com/IBM/fp-go/v2/internal/functor"
+	L "github.com/IBM/fp-go/v2/optics/lens"
+)
+
+// Do creates an empty context of type S to be used with the Bind operation.
+// This is the starting point for building up a context using do-notation style.
+//
+// Example:
+//
+//	type Result struct {
+//	    x int
+//	    y string
+//	}
+//	result := Do(Result{})
+func Do[S any](
+	empty S,
+) Validation[S] {
+	return Of(empty)
+}
+
+// Bind attaches the result of a computation to a context S1 to produce a context S2.
+// This is used in do-notation style to sequentially build up a context.
+//
+// Example:
+//
+//	type State struct { x int; y int }
+//	result := F.Pipe2(
+//	    Do(State{}),
+//	    Bind(func(x int) func(State) State {
+//	        return func(s State) State { s.x = x; return s }
+//	    }, func(s State) Validation[int] { return Success(42) }),
+//	)
+func Bind[S1, S2, A any](
+	setter func(A) func(S1) S2,
+	f Kleisli[S1, A],
+) Operator[S1, S2] {
+	return C.Bind(
+		Chain[S1, S2],
+		Map[A, S2],
+		setter,
+		f,
+	)
+}
+
+// Let attaches the result of a pure computation to a context S1 to produce a context S2.
+// Unlike Bind, the computation function returns a plain value, not an Option.
+//
+// Example:
+//
+//	type State struct { x int; computed int }
+//	result := F.Pipe2(
+//	    Do(State{x: 5}),
+//	    Let(func(c int) func(State) State {
+//	        return func(s State) State { s.computed = c; return s }
+//	    }, func(s State) int { return s.x * 2 }),
+//	)
+func Let[S1, S2, B any](
+	key func(B) func(S1) S2,
+	f func(S1) B,
+) Operator[S1, S2] {
+	return F.Let(
+		Map[S1, S2],
+		key,
+		f,
+	)
+}
+
+// LetTo attaches a constant value to a context S1 to produce a context S2.
+//
+// Example:
+//
+//	type State struct { x int; name string }
+//	result := F.Pipe2(
+//	    Do(State{x: 5}),
+//	    LetTo(func(n string) func(State) State {
+//	        return func(s State) State { s.name = n; return s }
+//	    }, "example"),
+//	)
+func LetTo[S1, S2, B any](
+	key func(B) func(S1) S2,
+	b B,
+) Operator[S1, S2] {
+	return F.LetTo(
+		Map[S1, S2],
+		key,
+		b,
+	)
+}
+
+// BindTo initializes a new state S1 from a value T.
+// This is typically used as the first operation after creating a Validation value.
+//
+// Example:
+//
+//	type State struct { value int }
+//	result := F.Pipe1(
+//	    Success(42),
+//	    BindTo(func(x int) State { return State{value: x} }),
+//	)
+func BindTo[S1, T any](
+	setter func(T) S1,
+) Operator[T, S1] {
+	return C.BindTo(
+		Map[T, S1],
+		setter,
+	)
+}
+
+// ApS attaches a value to a context S1 to produce a context S2 by considering the context and the value concurrently.
+// This uses the applicative functor pattern, allowing parallel composition.
+//
+// IMPORTANT: Unlike Bind which fails fast, ApS aggregates ALL validation errors from both the context
+// and the value. If both validations fail, all errors are collected and returned together.
+// This is useful for validating multiple independent fields and reporting all errors at once.
+//
+// Example:
+//
+//	type State struct { x int; y int }
+//	result := F.Pipe2(
+//	    Do(State{}),
+//	    ApS(func(x int) func(State) State {
+//	        return func(s State) State { s.x = x; return s }
+//	    }, Success(42)),
+//	)
+//
+// Error aggregation example:
+//
+//	stateFailure := Failures[State](Errors{&ValidationError{Messsage: "state error"}})
+//	valueFailure := Failures[int](Errors{&ValidationError{Messsage: "value error"}})
+//	result := ApS(setter, valueFailure)(stateFailure)
+//	// Result contains BOTH errors: ["state error", "value error"]
+func ApS[S1, S2, T any](
+	setter func(T) func(S1) S2,
+	fa Validation[T],
+) Operator[S1, S2] {
+	return A.ApS(
+		Ap[S2, T],
+		Map[S1, func(T) S2],
+		setter,
+		fa,
+	)
+}
+
+// ApSL attaches a value to a context using a lens-based setter.
+// This is a convenience function that combines ApS with a lens, allowing you to use
+// optics to update nested structures in a more composable way.
+//
+// IMPORTANT: Like ApS, this function aggregates ALL validation errors. If both the context
+// and the value fail validation, all errors are collected and returned together.
+// This enables comprehensive error reporting for complex nested structures.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// This eliminates the need to manually write setter functions.
+//
+// Example:
+//
+//	type Address struct {
+//	    Street string
+//	    City   string
+//	}
+//
+//	type Person struct {
+//	    Name    string
+//	    Address Address
+//	}
+//
+//	// Create a lens for the Address field
+//	addressLens := lens.MakeLens(
+//	    func(p Person) Address { return p.Address },
+//	    func(p Person, a Address) Person { p.Address = a; return p },
+//	)
+//
+//	// Use ApSL to update the address
+//	result := F.Pipe2(
+//	    Success(Person{Name: "Alice"}),
+//	    ApSL(
+//	        addressLens,
+//	        Success(Address{Street: "Main St", City: "NYC"}),
+//	    ),
+//	)
+func ApSL[S, T any](
+	lens L.Lens[S, T],
+	fa Validation[T],
+) Operator[S, S] {
+	return ApS(lens.Set, fa)
+}
+
+// BindL attaches the result of a computation to a context using a lens-based setter.
+// This is a convenience function that combines Bind with a lens, allowing you to use
+// optics to update nested structures based on their current values.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// The computation function f receives the current value of the focused field and returns
+// a Validation that produces the new value.
+//
+// Unlike ApSL, BindL uses monadic sequencing, meaning the computation f can depend on
+// the current value of the focused field.
+//
+// Example:
+//
+//	type Counter struct {
+//	    Value int
+//	}
+//
+//	valueLens := lens.MakeLens(
+//	    func(c Counter) int { return c.Value },
+//	    func(c Counter, v int) Counter { c.Value = v; return c },
+//	)
+//
+//	// Increment the counter, but fail if it would exceed 100
+//	increment := func(v int) Validation[int] {
+//	    if v >= 100 {
+//	        return Failures[int](Errors{&ValidationError{Messsage: "exceeds limit"}})
+//	    }
+//	    return Success(v + 1)
+//	}
+//
+//	result := F.Pipe1(
+//	    Success(Counter{Value: 42}),
+//	    BindL(valueLens, increment),
+//	) // Success(Counter{Value: 43})
+func BindL[S, T any](
+	lens L.Lens[S, T],
+	f Kleisli[T, T],
+) Operator[S, S] {
+	return Bind(lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetL attaches the result of a pure computation to a context using a lens-based setter.
+// This is a convenience function that combines Let with a lens, allowing you to use
+// optics to update nested structures with pure transformations.
+//
+// The lens parameter provides both the getter and setter for a field within the structure S.
+// The transformation function f receives the current value of the focused field and returns
+// the new value directly (not wrapped in Validation).
+//
+// This is useful for pure transformations that cannot fail, such as mathematical operations,
+// string manipulations, or other deterministic updates.
+//
+// Example:
+//
+//	type Counter struct {
+//	    Value int
+//	}
+//
+//	valueLens := lens.MakeLens(
+//	    func(c Counter) int { return c.Value },
+//	    func(c Counter, v int) Counter { c.Value = v; return c },
+//	)
+//
+//	// Double the counter value
+//	double := func(v int) int { return v * 2 }
+//
+//	result := F.Pipe1(
+//	    Success(Counter{Value: 21}),
+//	    LetL(valueLens, double),
+//	) // Success(Counter{Value: 42})
+func LetL[S, T any](
+	lens L.Lens[S, T],
+	f Endomorphism[T],
+) Operator[S, S] {
+	return Let(lens.Set, function.Flow2(lens.Get, f))
+}
+
+// LetToL attaches a constant value to a context using a lens-based setter.
+// This is a convenience function that combines LetTo with a lens, allowing you to use
+// optics to set nested fields to specific values.
+//
+// The lens parameter provides the setter for a field within the structure S.
+// Unlike LetL which transforms the current value, LetToL simply replaces it with
+// the provided constant value b.
+//
+// This is useful for resetting fields, initializing values, or setting fields to
+// predetermined constants.
+//
+// Example:
+//
+//	type Config struct {
+//	    Debug   bool
+//	    Timeout int
+//	}
+//
+//	debugLens := lens.MakeLens(
+//	    func(c Config) bool { return c.Debug },
+//	    func(c Config, d bool) Config { c.Debug = d; return c },
+//	)
+//
+//	result := F.Pipe1(
+//	    Success(Config{Debug: true, Timeout: 30}),
+//	    LetToL(debugLens, false),
+//	) // Success(Config{Debug: false, Timeout: 30})
+func LetToL[S, T any](
+	lens L.Lens[S, T],
+	b T,
+) Operator[S, S] {
+	return LetTo(lens.Set, b)
+}

v2/optics/codec/validation/bind_test.go

@@ -0,0 +1,540 @@
+package validation
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	L "github.com/IBM/fp-go/v2/optics/lens"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDo(t *testing.T) {
+	t.Run("creates successful validation with empty state", func(t *testing.T) {
+		type State struct {
+			x int
+			y string
+		}
+		result := Do(State{})
+
+		assert.Equal(t, Of(State{}), result)
+	})
+
+	t.Run("creates successful validation with initialized state", func(t *testing.T) {
+		type State struct {
+			x int
+			y string
+		}
+		initial := State{x: 42, y: "hello"}
+		result := Do(initial)
+
+		assert.Equal(t, Of(initial), result)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		intResult := Do(0)
+		assert.Equal(t, Of(0), intResult)
+
+		strResult := Do("")
+		assert.Equal(t, Of(""), strResult)
+
+		type Custom struct{ Value int }
+		customResult := Do(Custom{Value: 100})
+		assert.Equal(t, Of(Custom{Value: 100}), customResult)
+	})
+}
+
+func TestBind(t *testing.T) {
+	type State struct {
+		x int
+		y int
+	}
+
+	t.Run("binds successful validation to state", func(t *testing.T) {
+		result := F.Pipe2(
+			Do(State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Validation[int] { return Success(42) }),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Validation[int] { return Success(10) }),
+		)
+
+		assert.Equal(t, Of(State{x: 42, y: 10}), result)
+	})
+
+	t.Run("propagates failure", func(t *testing.T) {
+		result := F.Pipe2(
+			Do(State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Validation[int] { return Success(42) }),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Validation[int] {
+				return Failures[int](Errors{&ValidationError{Messsage: "y failed"}})
+			}),
+		)
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "y failed", errors[0].Messsage)
+	})
+
+	t.Run("can access previous state values", func(t *testing.T) {
+		result := F.Pipe2(
+			Do(State{}),
+			Bind(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) Validation[int] { return Success(10) }),
+			Bind(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) Validation[int] {
+				// y depends on x
+				return Success(s.x * 2)
+			}),
+		)
+
+		assert.Equal(t, Success(State{x: 10, y: 20}), result)
+	})
+}
+
+func TestLet(t *testing.T) {
+	type State struct {
+		x        int
+		computed int
+	}
+
+	t.Run("attaches pure computation result to state", func(t *testing.T) {
+		result := F.Pipe1(
+			Do(State{x: 5}),
+			Let(func(c int) func(State) State {
+				return func(s State) State { s.computed = c; return s }
+			}, func(s State) int { return s.x * 2 }),
+		)
+
+		assert.Equal(t, Of(State{x: 5, computed: 10}), result)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := Failures[State](Errors{&ValidationError{Messsage: "error"}})
+		result := Let(func(c int) func(State) State {
+			return func(s State) State { s.computed = c; return s }
+		}, func(s State) int { return s.x * 2 })(failure)
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "error", errors[0].Messsage)
+	})
+
+	t.Run("chains multiple Let operations", func(t *testing.T) {
+		type State struct {
+			x int
+			y int
+			z int
+		}
+		result := F.Pipe3(
+			Do(State{x: 5}),
+			Let(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, func(s State) int { return s.x * 2 }),
+			Let(func(z int) func(State) State {
+				return func(s State) State { s.z = z; return s }
+			}, func(s State) int { return s.y + 10 }),
+			Let(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, func(s State) int { return s.z * 3 }),
+		)
+
+		assert.Equal(t, Of(State{x: 60, y: 10, z: 20}), result)
+	})
+}
+
+func TestLetTo(t *testing.T) {
+	type State struct {
+		x    int
+		name string
+	}
+
+	t.Run("attaches constant value to state", func(t *testing.T) {
+		result := F.Pipe1(
+			Do(State{x: 5}),
+			LetTo(func(n string) func(State) State {
+				return func(s State) State { s.name = n; return s }
+			}, "example"),
+		)
+
+		assert.Equal(t, Of(State{x: 5, name: "example"}), result)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := Failures[State](Errors{&ValidationError{Messsage: "error"}})
+		result := LetTo(func(n string) func(State) State {
+			return func(s State) State { s.name = n; return s }
+		}, "example")(failure)
+
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("sets multiple constant values", func(t *testing.T) {
+		type State struct {
+			name    string
+			version int
+			active  bool
+		}
+		result := F.Pipe3(
+			Do(State{}),
+			LetTo(func(n string) func(State) State {
+				return func(s State) State { s.name = n; return s }
+			}, "app"),
+			LetTo(func(v int) func(State) State {
+				return func(s State) State { s.version = v; return s }
+			}, 2),
+			LetTo(func(a bool) func(State) State {
+				return func(s State) State { s.active = a; return s }
+			}, true),
+		)
+
+		assert.Equal(t, Of(State{name: "app", version: 2, active: true}), result)
+	})
+}
+
+func TestBindTo(t *testing.T) {
+	type State struct {
+		value int
+	}
+
+	t.Run("initializes state from value", func(t *testing.T) {
+		result := F.Pipe1(
+			Success(42),
+			BindTo(func(x int) State { return State{value: x} }),
+		)
+
+		assert.Equal(t, Of(State{value: 42}), result)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := Failures[int](Errors{&ValidationError{Messsage: "error"}})
+		result := BindTo(func(x int) State { return State{value: x} })(failure)
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "error", errors[0].Messsage)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		type StringState struct {
+			text string
+		}
+		result := F.Pipe1(
+			Success("hello"),
+			BindTo(func(s string) StringState { return StringState{text: s} }),
+		)
+
+		assert.Equal(t, Of(StringState{text: "hello"}), result)
+	})
+}
+
+func TestApS(t *testing.T) {
+	type State struct {
+		x int
+		y int
+	}
+
+	t.Run("attaches value using applicative pattern", func(t *testing.T) {
+		result := F.Pipe1(
+			Do(State{}),
+			ApS(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, Success(42)),
+		)
+
+		assert.Equal(t, Of(State{x: 42}), result)
+	})
+
+	t.Run("accumulates errors from both validations", func(t *testing.T) {
+		stateFailure := Failures[State](Errors{&ValidationError{Messsage: "state error"}})
+		valueFailure := Failures[int](Errors{&ValidationError{Messsage: "value error"}})
+
+		result := ApS(func(x int) func(State) State {
+			return func(s State) State { s.x = x; return s }
+		}, valueFailure)(stateFailure)
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(State) Errors { return nil },
+		)
+		assert.Len(t, errors, 2)
+		messages := []string{errors[0].Messsage, errors[1].Messsage}
+		assert.Contains(t, messages, "state error")
+		assert.Contains(t, messages, "value error")
+	})
+
+	t.Run("combines multiple ApS operations", func(t *testing.T) {
+		result := F.Pipe2(
+			Do(State{}),
+			ApS(func(x int) func(State) State {
+				return func(s State) State { s.x = x; return s }
+			}, Success(10)),
+			ApS(func(y int) func(State) State {
+				return func(s State) State { s.y = y; return s }
+			}, Success(20)),
+		)
+
+		assert.Equal(t, Of(State{x: 10, y: 20}), result)
+	})
+}
+
+func TestApSL(t *testing.T) {
+	type Address struct {
+		Street string
+		City   string
+	}
+
+	type Person struct {
+		Name    string
+		Address Address
+	}
+
+	t.Run("updates nested structure using lens", func(t *testing.T) {
+		addressLens := L.MakeLens(
+			func(p Person) Address { return p.Address },
+			func(p Person, a Address) Person { p.Address = a; return p },
+		)
+
+		result := F.Pipe1(
+			Success(Person{Name: "Alice"}),
+			ApSL(
+				addressLens,
+				Success(Address{Street: "Main St", City: "NYC"}),
+			),
+		)
+
+		expected := Person{
+			Name:    "Alice",
+			Address: Address{Street: "Main St", City: "NYC"},
+		}
+		assert.Equal(t, Of(expected), result)
+	})
+
+	t.Run("accumulates errors", func(t *testing.T) {
+		addressLens := L.MakeLens(
+			func(p Person) Address { return p.Address },
+			func(p Person, a Address) Person { p.Address = a; return p },
+		)
+
+		personFailure := Failures[Person](Errors{&ValidationError{Messsage: "person error"}})
+		addressFailure := Failures[Address](Errors{&ValidationError{Messsage: "address error"}})
+
+		result := ApSL(addressLens, addressFailure)(personFailure)
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(Person) Errors { return nil },
+		)
+		assert.Len(t, errors, 2)
+	})
+}
+
+func TestBindL(t *testing.T) {
+	type Counter struct {
+		Value int
+	}
+
+	valueLens := L.MakeLens(
+		func(c Counter) int { return c.Value },
+		func(c Counter, v int) Counter { c.Value = v; return c },
+	)
+
+	t.Run("updates field based on current value", func(t *testing.T) {
+		increment := func(v int) Validation[int] {
+			return Success(v + 1)
+		}
+
+		result := F.Pipe1(
+			Success(Counter{Value: 42}),
+			BindL(valueLens, increment),
+		)
+
+		assert.Equal(t, Of(Counter{Value: 43}), result)
+	})
+
+	t.Run("fails validation based on current value", func(t *testing.T) {
+		increment := func(v int) Validation[int] {
+			if v >= 100 {
+				return Failures[int](Errors{&ValidationError{Messsage: "exceeds limit"}})
+			}
+			return Success(v + 1)
+		}
+
+		result := F.Pipe1(
+			Success(Counter{Value: 100}),
+			BindL(valueLens, increment),
+		)
+
+		assert.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result,
+			F.Identity[Errors],
+			func(Counter) Errors { return nil },
+		)
+		assert.Len(t, errors, 1)
+		assert.Equal(t, "exceeds limit", errors[0].Messsage)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		increment := func(v int) Validation[int] {
+			return Success(v + 1)
+		}
+
+		failure := Failures[Counter](Errors{&ValidationError{Messsage: "error"}})
+		result := BindL(valueLens, increment)(failure)
+
+		assert.True(t, either.IsLeft(result))
+	})
+}
+
+func TestLetL(t *testing.T) {
+	type Counter struct {
+		Value int
+	}
+
+	valueLens := L.MakeLens(
+		func(c Counter) int { return c.Value },
+		func(c Counter, v int) Counter { c.Value = v; return c },
+	)
+
+	t.Run("transforms field with pure function", func(t *testing.T) {
+		double := func(v int) int { return v * 2 }
+
+		result := F.Pipe1(
+			Success(Counter{Value: 21}),
+			LetL(valueLens, double),
+		)
+
+		assert.Equal(t, Of(Counter{Value: 42}), result)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		double := func(v int) int { return v * 2 }
+
+		failure := Failures[Counter](Errors{&ValidationError{Messsage: "error"}})
+		result := LetL(valueLens, double)(failure)
+
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("chains multiple transformations", func(t *testing.T) {
+		add10 := func(v int) int { return v + 10 }
+		double := func(v int) int { return v * 2 }
+
+		result := F.Pipe2(
+			Success(Counter{Value: 5}),
+			LetL(valueLens, add10),
+			LetL(valueLens, double),
+		)
+
+		assert.Equal(t, Of(Counter{Value: 30}), result)
+	})
+}
+
+func TestLetToL(t *testing.T) {
+	type Config struct {
+		Debug   bool
+		Timeout int
+	}
+
+	debugLens := L.MakeLens(
+		func(c Config) bool { return c.Debug },
+		func(c Config, d bool) Config { c.Debug = d; return c },
+	)
+
+	t.Run("sets field to constant value", func(t *testing.T) {
+		result := F.Pipe1(
+			Success(Config{Debug: true, Timeout: 30}),
+			LetToL(debugLens, false),
+		)
+
+		assert.Equal(t, Of(Config{Debug: false, Timeout: 30}), result)
+	})
+
+	t.Run("preserves failure", func(t *testing.T) {
+		failure := Failures[Config](Errors{&ValidationError{Messsage: "error"}})
+		result := LetToL(debugLens, false)(failure)
+
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("sets multiple fields", func(t *testing.T) {
+		timeoutLens := L.MakeLens(
+			func(c Config) int { return c.Timeout },
+			func(c Config, t int) Config { c.Timeout = t; return c },
+		)
+
+		result := F.Pipe2(
+			Success(Config{Debug: true, Timeout: 30}),
+			LetToL(debugLens, false),
+			LetToL(timeoutLens, 60),
+		)
+
+		assert.Equal(t, Of(Config{Debug: false, Timeout: 60}), result)
+	})
+}
+
+func TestBindOperationsComposition(t *testing.T) {
+	type User struct {
+		Name  string
+		Age   int
+		Email string
+	}
+
+	t.Run("combines Do, Bind, Let, and LetTo", func(t *testing.T) {
+		result := F.Pipe4(
+			Do(User{}),
+			LetTo(func(n string) func(User) User {
+				return func(u User) User { u.Name = n; return u }
+			}, "Alice"),
+			Bind(func(a int) func(User) User {
+				return func(u User) User { u.Age = a; return u }
+			}, func(u User) Validation[int] {
+				// Age validation
+				if len(u.Name) > 0 {
+					return Success(25)
+				}
+				return Failures[int](Errors{&ValidationError{Messsage: "name required"}})
+			}),
+			Let(func(e string) func(User) User {
+				return func(u User) User { u.Email = e; return u }
+			}, func(u User) string {
+				// Derive email from name
+				return u.Name + "@example.com"
+			}),
+			Bind(func(a int) func(User) User {
+				return func(u User) User { u.Age = a; return u }
+			}, func(u User) Validation[int] {
+				// Validate age is positive
+				if u.Age > 0 {
+					return Success(u.Age)
+				}
+				return Failures[int](Errors{&ValidationError{Messsage: "age must be positive"}})
+			}),
+		)
+
+		expected := User{Name: "Alice", Age: 25, Email: "Alice@example.com"}
+		assert.Equal(t, Of(expected), result)
+	})
+}

v2/optics/codec/validation/monad.go

@@ -1,10 +1,14 @@
 package validation
 
 import (
+	"github.com/IBM/fp-go/v2/array"
 	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/applicative"
 )
 
+var errorsMonoid = ErrorsMonoid()
+
 // Of creates a successful validation result containing the given value.
 // This is the pure/return operation for the Validation monad.
 //
@@ -28,37 +32,376 @@ func Of[A any](a A) Validation[A] {
 //		return func(age int) User { return User{name, age} }
 //	}))(validateName))(validateAge)
 func Ap[B, A any](fa Validation[A]) Operator[func(A) B, B] {
-	return either.ApV[B, A](ErrorsMonoid())(fa)
+	return either.ApV[B, A](errorsMonoid)(fa)
 }
 
+// MonadAp applies a validation containing a function to a validation containing a value.
+// This is the applicative apply operation that **accumulates errors** from both validations.
+//
+// **Key behavior**: Unlike Either's MonadAp which fails fast (returns first error),
+// this validation-specific implementation **accumulates all errors** using the Errors monoid.
+// When both the function validation and value validation fail, all errors from both are combined.
+//
+// This error accumulation is the defining characteristic of the Validation applicative,
+// making it ideal for scenarios where you want to collect all validation failures at once
+// rather than stopping at the first error.
+//
+// Behavior:
+//   - Both succeed: applies the function to the value → Success(result)
+//   - Function fails, value succeeds: returns function's errors → Failure(func errors)
+//   - Function succeeds, value fails: returns value's errors → Failure(value errors)
+//   - Both fail: **combines all errors** → Failure(func errors + value errors)
+//
+// This is particularly useful for:
+//   - Form validation: collect all field errors at once
+//   - Configuration validation: report all invalid settings together
+//   - Data validation: accumulate all constraint violations
+//   - Multi-field validation: validate independent fields in parallel
+//
+// Example - Both succeed:
+//
+//	double := func(x int) int { return x * 2 }
+//	result := MonadAp(Of(double), Of(21))
+//	// Result: Success(42)
+//
+// Example - Error accumulation (key feature):
+//
+//	funcValidation := Failures[func(int) int](Errors{
+//	    &ValidationError{Messsage: "function error"},
+//	})
+//	valueValidation := Failures[int](Errors{
+//	    &ValidationError{Messsage: "value error"},
+//	})
+//	result := MonadAp(funcValidation, valueValidation)
+//	// Result: Failure with BOTH errors: ["function error", "value error"]
+//
+// Example - Validating multiple fields:
+//
+//	type User struct {
+//	    Name string
+//	    Age  int
+//	}
+//
+//	makeUser := func(name string) func(int) User {
+//	    return func(age int) User { return User{name, age} }
+//	}
+//
+//	nameValidation := validateName("ab")  // Fails: too short
+//	ageValidation := validateAge(16)      // Fails: too young
+//
+//	// First apply name
+//	step1 := MonadAp(Of(makeUser), nameValidation)
+//	// Then apply age
+//	result := MonadAp(step1, ageValidation)
+//	// Result contains ALL validation errors from both fields
 func MonadAp[B, A any](fab Validation[func(A) B], fa Validation[A]) Validation[B] {
-	return either.MonadApV[B, A](ErrorsMonoid())(fab, fa)
+	return either.MonadApV[B, A](errorsMonoid)(fab, fa)
 }
 
 // Map transforms the value inside a successful validation using the provided function.
 // If the validation is a failure, the errors are preserved unchanged.
 // This is the functor map operation for Validation.
 //
-// Example:
+// Map is used for transforming successful values without changing the validation context.
+// It's the most basic operation for working with validated values and forms the foundation
+// for more complex validation pipelines.
+//
+// Behavior:
+//   - Success: applies function to value → Success(f(value))
+//   - Failure: preserves errors unchanged → Failure(same errors)
+//
+// This is useful for:
+//   - Type transformations: converting validated values to different types
+//   - Value transformations: normalizing, formatting, or computing derived values
+//   - Pipeline composition: chaining multiple transformations
+//   - Preserving validation context: errors pass through unchanged
+//
+// Example - Transform successful value:
 //
 //	doubled := Map(func(x int) int { return x * 2 })(Of(21))
 //	// Result: Success(42)
+//
+// Example - Failure preserved:
+//
+//	result := Map(func(x int) int { return x * 2 })(
+//	    Failures[int](Errors{&ValidationError{Messsage: "invalid"}}),
+//	)
+//	// Result: Failure with same error: ["invalid"]
+//
+// Example - Type transformation:
+//
+//	toString := Map(func(x int) string { return fmt.Sprintf("%d", x) })
+//	result := toString(Of(42))
+//	// Result: Success("42")
+//
+// Example - Chaining transformations:
+//
+//	result := F.Pipe3(
+//	    Of(5),
+//	    Map(func(x int) int { return x + 10 }),  // 15
+//	    Map(func(x int) int { return x * 2 }),   // 30
+//	    Map(func(x int) string { return fmt.Sprintf("%d", x) }),  // "30"
+//	)
+//	// Result: Success("30")
 func Map[A, B any](f func(A) B) Operator[A, B] {
 	return either.Map[Errors](f)
 }
 
+// MonadMap transforms the value inside a successful validation using the provided function.
+// If the validation is a failure, the errors are preserved unchanged.
+// This is the non-curried version of [Map].
+//
+// MonadMap is useful when you have both the validation and the transformation function
+// available at the same time, rather than needing to create a reusable operator.
+//
+// Behavior:
+//   - Success: applies function to value → Success(f(value))
+//   - Failure: preserves errors unchanged → Failure(same errors)
+//
+// Example - Transform successful value:
+//
+//	result := MonadMap(Of(21), func(x int) int { return x * 2 })
+//	// Result: Success(42)
+//
+// Example - Failure preserved:
+//
+//	result := MonadMap(
+//	    Failures[int](Errors{&ValidationError{Messsage: "invalid"}}),
+//	    func(x int) int { return x * 2 },
+//	)
+//	// Result: Failure with same error: ["invalid"]
+//
+// Example - Type transformation:
+//
+//	result := MonadMap(Of(42), func(x int) string {
+//	    return fmt.Sprintf("Value: %d", x)
+//	})
+//	// Result: Success("Value: 42")
+//
+// Example - Computing derived values:
+//
+//	type User struct { FirstName, LastName string }
+//	result := MonadMap(
+//	    Of(User{"John", "Doe"}),
+//	    func(u User) string { return u.FirstName + " " + u.LastName },
+//	)
+//	// Result: Success("John Doe")
 func MonadMap[A, B any](fa Validation[A], f func(A) B) Validation[B] {
 	return either.MonadMap(fa, f)
 }
 
+// Chain is the curried version of [MonadChain].
+// Sequences two validation computations where the second depends on the first.
+//
+// Example:
+//
+//	validatePositive := func(x int) Validation[int] {
+//	    if x > 0 { return Success(x) }
+//	    return Failure("must be positive")
+//	}
+//	result := Chain(validatePositive)(Success(42)) // Success(42)
 func Chain[A, B any](f Kleisli[A, B]) Operator[A, B] {
 	return either.Chain(f)
 }
 
+// MonadChain sequences two validation computations where the second depends on the first.
+// If the first validation fails, returns the failure without executing the second.
+// This is the monadic bind operation for Validation.
+//
+// Example:
+//
+//	result := MonadChain(
+//	    Success(42),
+//	    func(x int) Validation[string] {
+//	        return Success(fmt.Sprintf("Value: %d", x))
+//	    },
+//	) // Success("Value: 42")
 func MonadChain[A, B any](fa Validation[A], f Kleisli[A, B]) Validation[B] {
 	return either.MonadChain(fa, f)
 }
 
+// chainErrors is an internal helper that chains error transformations while accumulating errors.
+// When the transformation function f returns a failure, it concatenates the original errors (e1)
+// with the new errors (e2) using the Errors monoid, ensuring all validation errors are preserved.
+func chainErrors[A any](f Kleisli[Errors, A]) func(Errors) Validation[A] {
+	return func(e1 Errors) Validation[A] {
+		return either.MonadFold(
+			f(e1),
+			function.Flow2(array.Concat(e1), either.Left[A]),
+			Of[A],
+		)
+	}
+}
+
+// ChainLeft is the curried version of [MonadChainLeft].
+// Returns a function that transforms validation failures while preserving successes.
+//
+// Unlike the standard Either ChainLeft which replaces errors, this validation-specific
+// implementation **aggregates errors** using the Errors monoid. When the transformation
+// function returns a failure, both the original errors and the new errors are combined,
+// ensuring no validation errors are lost.
+//
+// This is particularly useful for:
+//   - Error recovery with fallback validation
+//   - Adding contextual information to existing errors
+//   - Transforming error types while preserving all error details
+//   - Building error handling pipelines that accumulate failures
+//
+// Key behavior:
+//   - Success values pass through unchanged
+//   - When transforming failures, if the transformation also fails, **all errors are aggregated**
+//   - If the transformation succeeds, it recovers from the original failure
+//
+// Example - Error recovery with aggregation:
+//
+//	recoverFromNotFound := ChainLeft(func(errs Errors) Validation[int] {
+//	    // Check if this is a "not found" error
+//	    for _, err := range errs {
+//	        if err.Messsage == "not found" {
+//	            return Success(0) // recover with default
+//	        }
+//	    }
+//	    // Add context to existing errors
+//	    return Failures[int](Errors{
+//	        &ValidationError{Messsage: "recovery failed"},
+//	    })
+//	    // Result will contain BOTH original errors AND "recovery failed"
+//	})
+//
+//	result := recoverFromNotFound(Failures[int](Errors{
+//	    &ValidationError{Messsage: "database error"},
+//	}))
+//	// Result contains: ["database error", "recovery failed"]
+//
+// Example - Adding context to errors:
+//
+//	addContext := ChainLeft(func(errs Errors) Validation[string] {
+//	    // Add contextual information
+//	    return Failures[string](Errors{
+//	        &ValidationError{
+//	            Messsage: "validation failed in user.email field",
+//	        },
+//	    })
+//	    // Original errors are preserved and new context is added
+//	})
+//
+//	result := F.Pipe1(
+//	    Failures[string](Errors{
+//	        &ValidationError{Messsage: "invalid format"},
+//	    }),
+//	    addContext,
+//	)
+//	// Result contains: ["invalid format", "validation failed in user.email field"]
+//
+// Example - Success values pass through:
+//
+//	handler := ChainLeft(func(errs Errors) Validation[int] {
+//	    return Failures[int](Errors{
+//	        &ValidationError{Messsage: "never called"},
+//	    })
+//	})
+//	result := handler(Success(42)) // Success(42) - unchanged
+func ChainLeft[A any](f Kleisli[Errors, A]) Operator[A, A] {
+	return either.Fold(
+		chainErrors(f),
+		Of[A],
+	)
+}
+
+// MonadChainLeft sequences a computation on the failure (Left) channel of a Validation.
+// If the Validation is a failure, applies the function to transform or recover from the errors.
+// If the Validation is a success, returns the success value unchanged.
+//
+// **Critical difference from Either.MonadChainLeft**: This validation-specific implementation
+// **aggregates errors** using the Errors monoid. When the transformation function returns a
+// failure, both the original errors and the new errors are combined, ensuring comprehensive
+// error reporting.
+//
+// This is the dual of [MonadChain] - while Chain operates on success values, ChainLeft
+// operates on failure values. It's particularly useful for:
+//   - Error recovery: converting specific errors into successful values
+//   - Error enrichment: adding context or transforming error messages
+//   - Fallback logic: providing alternative validations when the first fails
+//   - Error aggregation: combining multiple validation failures
+//
+// The function parameter receives the collection of validation errors and must return
+// a new Validation[A]. This allows you to:
+//   - Recover by returning Success(value)
+//   - Transform errors by returning Failures(newErrors) - **original errors are preserved**
+//   - Implement conditional error handling based on error content
+//
+// Example - Error recovery:
+//
+//	result := MonadChainLeft(
+//	    Failures[int](Errors{
+//	        &ValidationError{Messsage: "not found"},
+//	    }),
+//	    func(errs Errors) Validation[int] {
+//	        // Check if we can recover
+//	        for _, err := range errs {
+//	            if err.Messsage == "not found" {
+//	                return Success(0) // recover with default value
+//	            }
+//	        }
+//	        return Failures[int](errs) // propagate errors
+//	    },
+//	) // Success(0)
+//
+// Example - Error aggregation (key feature):
+//
+//	result := MonadChainLeft(
+//	    Failures[string](Errors{
+//	        &ValidationError{Messsage: "error 1"},
+//	        &ValidationError{Messsage: "error 2"},
+//	    }),
+//	    func(errs Errors) Validation[string] {
+//	        // Transformation also fails
+//	        return Failures[string](Errors{
+//	            &ValidationError{Messsage: "error 3"},
+//	        })
+//	    },
+//	)
+//	// Result contains ALL errors: ["error 1", "error 2", "error 3"]
+//	// This is different from Either.MonadChainLeft which would only keep "error 3"
+//
+// Example - Adding context to errors:
+//
+//	result := MonadChainLeft(
+//	    Failures[int](Errors{
+//	        &ValidationError{Value: "abc", Messsage: "invalid number"},
+//	    }),
+//	    func(errs Errors) Validation[int] {
+//	        // Add contextual information
+//	        contextErrors := Errors{
+//	            &ValidationError{
+//	                Context:  []ContextEntry{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+//	                Messsage: "failed to parse user age",
+//	            },
+//	        }
+//	        return Failures[int](contextErrors)
+//	    },
+//	)
+//	// Result contains both original error and context:
+//	// ["invalid number", "failed to parse user age"]
+//
+// Example - Success values pass through:
+//
+//	result := MonadChainLeft(
+//	    Success(42),
+//	    func(errs Errors) Validation[int] {
+//	        return Failures[int](Errors{
+//	            &ValidationError{Messsage: "never called"},
+//	        })
+//	    },
+//	) // Success(42) - unchanged
+func MonadChainLeft[A any](fa Validation[A], f Kleisli[Errors, A]) Validation[A] {
+	return either.MonadFold(
+		fa,
+		chainErrors(f),
+		Of[A],
+	)
+}
+
 // Applicative creates an Applicative instance for Validation with error accumulation.
 //
 // This returns a lawful Applicative that accumulates validation errors using the Errors monoid.
@@ -123,6 +466,176 @@ func MonadChain[A, B any](fa Validation[A], f Kleisli[A, B]) Validation[B] {
 //	An Applicative instance with Of, Map, and Ap operations that accumulate errors
 func Applicative[A, B any]() applicative.Applicative[A, B, Validation[A], Validation[B], Validation[func(A) B]] {
 	return either.ApplicativeV[Errors, A, B](
-		ErrorsMonoid(),
+		errorsMonoid,
 	)
 }
+
+//go:inline
+func OrElse[A any](f Kleisli[Errors, A]) Operator[A, A] {
+	return ChainLeft(f)
+}
+
+// MonadAlt implements the Alternative operation for Validation, providing fallback behavior.
+// If the first validation fails, it evaluates and returns the second validation as an alternative.
+// If the first validation succeeds, it returns the first validation without evaluating the second.
+//
+// This is the fundamental operation for the Alt typeclass, enabling "try first, fallback to second"
+// semantics. It's particularly useful for:
+//   - Providing default values when validation fails
+//   - Trying multiple validation strategies in sequence
+//   - Building validation pipelines with fallback logic
+//   - Implementing optional validation with defaults
+//
+// **Key behavior**: When both validations fail, MonadAlt DOES accumulate errors from both
+// validations using the Errors monoid. This is different from standard Either Alt behavior.
+// The error accumulation happens through the underlying ChainLeft/chainErrors mechanism.
+//
+// The second parameter is lazy (Lazy[Validation[A]]) to avoid unnecessary computation when
+// the first validation succeeds. The second validation is only evaluated if needed.
+//
+// Behavior:
+//   - First succeeds: returns first validation (second is not evaluated)
+//   - First fails, second succeeds: returns second validation
+//   - Both fail: aggregates errors from both validations
+//
+// This is useful for:
+//   - Fallback values: provide defaults when primary validation fails
+//   - Alternative strategies: try different validation approaches
+//   - Optional validation: make validation optional with a default
+//   - Chaining attempts: try multiple sources until one succeeds
+//
+// Type Parameters:
+//   - A: The type of the successful value
+//
+// Parameters:
+//   - first: The primary validation to try
+//   - second: A lazy computation producing the fallback validation (only evaluated if first fails)
+//
+// Returns:
+//
+//	The first validation if it succeeds, otherwise the second validation
+//
+// Example - Fallback to default:
+//
+//	primary := parseConfig("config.json")  // Fails
+//	fallback := func() Validation[Config] {
+//	    return Success(defaultConfig)
+//	}
+//	result := MonadAlt(primary, fallback)
+//	// Result: Success(defaultConfig)
+//
+// Example - First succeeds (second not evaluated):
+//
+//	primary := Success(42)
+//	fallback := func() Validation[int] {
+//	    panic("never called") // This won't execute
+//	}
+//	result := MonadAlt(primary, fallback)
+//	// Result: Success(42)
+//
+// Example - Chaining multiple alternatives:
+//
+//	result := MonadAlt(
+//	    parseFromEnv("API_KEY"),
+//	    func() Validation[string] {
+//	        return MonadAlt(
+//	            parseFromFile(".env"),
+//	            func() Validation[string] {
+//	                return Success("default-key")
+//	            },
+//	        )
+//	    },
+//	)
+//	// Tries: env var → file → default (uses first that succeeds)
+//
+// Example - Error accumulation when both fail:
+//
+//	v1 := Failures[int](Errors{
+//	    &ValidationError{Messsage: "error 1"},
+//	    &ValidationError{Messsage: "error 2"},
+//	})
+//	v2 := func() Validation[int] {
+//	    return Failures[int](Errors{
+//	        &ValidationError{Messsage: "error 3"},
+//	    })
+//	}
+//	result := MonadAlt(v1, v2)
+//	// Result: Failures with ALL errors ["error 1", "error 2", "error 3"]
+//	// The errors from v1 are aggregated with errors from v2
+func MonadAlt[A any](first Validation[A], second Lazy[Validation[A]]) Validation[A] {
+	return MonadChainLeft(first, function.Ignore1of1[Errors](second))
+}
+
+// Alt is the curried version of [MonadAlt].
+// Returns a function that provides fallback behavior for a Validation.
+//
+// This is useful for creating reusable fallback operators that can be applied
+// to multiple validations, or for use in function composition pipelines.
+//
+// The returned function takes a validation and returns either that validation
+// (if successful) or the provided alternative (if the validation fails).
+//
+// Type Parameters:
+//   - A: The type of the successful value
+//
+// Parameters:
+//   - second: A lazy computation producing the fallback validation
+//
+// Returns:
+//
+//	A function that takes a Validation[A] and returns a Validation[A] with fallback behavior
+//
+// Example - Creating a reusable fallback operator:
+//
+//	withDefault := Alt(func() Validation[int] {
+//	    return Success(0)
+//	})
+//
+//	result1 := withDefault(parseNumber("42"))    // Success(42)
+//	result2 := withDefault(parseNumber("abc"))   // Success(0) - fallback
+//	result3 := withDefault(parseNumber("123"))   // Success(123)
+//
+// Example - Using in a pipeline:
+//
+//	import F "github.com/IBM/fp-go/v2/function"
+//
+//	result := F.Pipe2(
+//	    parseFromEnv("CONFIG_PATH"),
+//	    Alt(func() Validation[string] {
+//	        return parseFromFile("config.json")
+//	    }),
+//	    Alt(func() Validation[string] {
+//	        return Success("./default-config.json")
+//	    }),
+//	)
+//	// Tries: env var → file → default path
+//
+// Example - Combining with Map:
+//
+//	import F "github.com/IBM/fp-go/v2/function"
+//
+//	result := F.Pipe2(
+//	    validatePositive(-5),  // Fails
+//	    Alt(func() Validation[int] { return Success(1) }),
+//	    Map(func(x int) int { return x * 2 }),
+//	)
+//	// Result: Success(2) - uses fallback value 1, then doubles it
+//
+// Example - Multiple fallback layers:
+//
+//	primaryFallback := Alt(func() Validation[Config] {
+//	    return loadFromFile("backup.json")
+//	})
+//	secondaryFallback := Alt(func() Validation[Config] {
+//	    return Success(defaultConfig)
+//	})
+//
+//	result := F.Pipe2(
+//	    loadFromFile("config.json"),
+//	    primaryFallback,
+//	    secondaryFallback,
+//	)
+//	// Tries: config.json → backup.json → default
+func Alt[A any](second Lazy[Validation[A]]) Operator[A, A] {
+	return ChainLeft(function.Ignore1of1[Errors](second))
+}

v2/optics/codec/validation/monoid.go

@@ -52,3 +52,177 @@ func ApplicativeMonoid[A any](m Monoid[A]) Monoid[Validation[A]] {
 		m,
 	)
 }
+
+// AlternativeMonoid creates a Monoid instance for Validation[A] using the Alternative pattern.
+// This combines the applicative error-accumulation behavior with the alternative fallback behavior,
+// allowing you to both accumulate errors and provide fallback alternatives.
+//
+// The Alternative pattern provides two key operations:
+//   - Applicative operations (Of, Map, Ap): accumulate errors when combining validations
+//   - Alternative operation (Alt): provide fallback when a validation fails
+//
+// This monoid is particularly useful when you want to:
+//   - Try multiple validation strategies and fall back to alternatives
+//   - Combine successful values using the provided monoid
+//   - Accumulate all errors from failed attempts
+//   - Build validation pipelines with fallback logic
+//
+// The resulting monoid:
+//   - Empty: Returns a successful validation with the empty value from the inner monoid
+//   - Concat: Combines two validations using both applicative and alternative semantics:
+//   - If first succeeds and second succeeds: combines values using inner monoid
+//   - If first fails: tries second as fallback (alternative behavior)
+//   - If both fail: accumulates all errors
+//
+// Type Parameters:
+//   - A: The type of the successful value
+//
+// Parameters:
+//   - m: The monoid for combining successful values of type A
+//
+// Returns:
+//
+//	A Monoid[Validation[A]] that combines applicative and alternative behaviors
+//
+// Example - Combining successful validations:
+//
+//	import "github.com/IBM/fp-go/v2/string"
+//
+//	m := AlternativeMonoid(string.Monoid)
+//	v1 := Success("Hello")
+//	v2 := Success(" World")
+//	result := m.Concat(v1, v2)
+//	// Result: Success("Hello World")
+//
+// Example - Fallback behavior:
+//
+//	m := AlternativeMonoid(string.Monoid)
+//	v1 := Failures[string](Errors{&ValidationError{Messsage: "first failed"}})
+//	v2 := Success("fallback value")
+//	result := m.Concat(v1, v2)
+//	// Result: Success("fallback value") - second validation used as fallback
+//
+// Example - Error accumulation when both fail:
+//
+//	m := AlternativeMonoid(string.Monoid)
+//	v1 := Failures[string](Errors{&ValidationError{Messsage: "error 1"}})
+//	v2 := Failures[string](Errors{&ValidationError{Messsage: "error 2"}})
+//	result := m.Concat(v1, v2)
+//	// Result: Failures with accumulated errors: ["error 1", "error 2"]
+//
+// Example - Building validation with fallbacks:
+//
+//	import N "github.com/IBM/fp-go/v2/number"
+//
+//	m := AlternativeMonoid(N.MonoidSum[int]())
+//
+//	// Try to parse from different sources
+//	fromEnv := parseFromEnv()      // Fails
+//	fromConfig := parseFromConfig() // Succeeds with 42
+//	fromDefault := Success(0)       // Default fallback
+//
+//	result := m.Concat(m.Concat(fromEnv, fromConfig), fromDefault)
+//	// Result: Success(42) - uses first successful validation
+func AlternativeMonoid[A any](m Monoid[A]) Monoid[Validation[A]] {
+	return M.AlternativeMonoid(
+		Of[A],
+		MonadMap[A, func(A) A],
+		MonadAp[A, A],
+		MonadAlt[A],
+		m,
+	)
+}
+
+// AltMonoid creates a Monoid instance for Validation[A] using the Alt (alternative) operation.
+// This monoid provides a way to combine validations with fallback behavior, where the second
+// validation is used as an alternative if the first one fails.
+//
+// The Alt operation implements the "try first, fallback to second" pattern, which is useful
+// for validation scenarios where you want to attempt multiple validation strategies in sequence
+// and use the first one that succeeds.
+//
+// The resulting monoid:
+//   - Empty: Returns the provided zero value (a lazy computation that produces a Validation[A])
+//   - Concat: Combines two validations using Alt semantics:
+//   - If first succeeds: returns the first validation (ignores second)
+//   - If first fails: returns the second validation as fallback
+//
+// This is different from [AlternativeMonoid] in that:
+//   - AltMonoid uses a custom zero value (provided by the user)
+//   - AlternativeMonoid derives the zero from an inner monoid
+//   - AltMonoid is simpler and only provides fallback behavior
+//   - AlternativeMonoid combines applicative and alternative behaviors
+//
+// Type Parameters:
+//   - A: The type of the successful value
+//
+// Parameters:
+//   - zero: A lazy computation that produces the identity/empty Validation[A].
+//     This is typically a successful validation with a default value, or could be
+//     a failure representing "no validation attempted"
+//
+// Returns:
+//
+//	A Monoid[Validation[A]] that combines validations with fallback behavior
+//
+// Example - Using default value as zero:
+//
+//	m := AltMonoid(func() Validation[int] { return Success(0) })
+//
+//	v1 := Failures[int](Errors{&ValidationError{Messsage: "failed"}})
+//	v2 := Success(42)
+//
+//	result := m.Concat(v1, v2)
+//	// Result: Success(42) - falls back to second validation
+//
+//	empty := m.Empty()
+//	// Result: Success(0) - the provided zero value
+//
+// Example - Chaining multiple fallbacks:
+//
+//	m := AltMonoid(func() Validation[string] {
+//	    return Success("default")
+//	})
+//
+//	primary := parseFromPrimarySource()   // Fails
+//	secondary := parseFromSecondary()     // Fails
+//	tertiary := parseFromTertiary()       // Succeeds with "value"
+//
+//	result := m.Concat(m.Concat(primary, secondary), tertiary)
+//	// Result: Success("value") - uses first successful validation
+//
+// Example - All validations fail:
+//
+//	m := AltMonoid(func() Validation[int] {
+//	    return Failures[int](Errors{&ValidationError{Messsage: "no default"}})
+//	})
+//
+//	v1 := Failures[int](Errors{&ValidationError{Messsage: "error 1"}})
+//	v2 := Failures[int](Errors{&ValidationError{Messsage: "error 2"}})
+//
+//	result := m.Concat(v1, v2)
+//	// Result: Failures with errors from v2: ["error 2"]
+//	// Note: Unlike AlternativeMonoid, errors are NOT accumulated
+//
+// Example - Building a validation pipeline with fallbacks:
+//
+//	m := AltMonoid(func() Validation[Config] {
+//	    return Success(defaultConfig)
+//	})
+//
+//	// Try multiple configuration sources in order
+//	configs := []Validation[Config]{
+//	    loadFromFile("config.json"),      // Try file first
+//	    loadFromEnv(),                     // Then environment
+//	    loadFromRemote("api.example.com"), // Then remote API
+//	}
+//
+//	// Fold using the monoid to get first successful config
+//	result := A.MonoidFold(m)(configs)
+//	// Result: First successful config, or defaultConfig if all fail
+func AltMonoid[A any](zero Lazy[Validation[A]]) Monoid[Validation[A]] {
+	return M.AltMonoid(
+		zero,
+		MonadAlt[A],
+	)
+}

v2/optics/codec/validation/monoid_test.go

@@ -74,12 +74,7 @@ func TestApplicativeMonoid(t *testing.T) {
 		t.Run("empty returns successful validation with empty string", func(t *testing.T) {
 			empty := m.Empty()
 
-			assert.True(t, either.IsRight(empty))
-			value := either.MonadFold(empty,
-				func(Errors) string { return "ERROR" },
-				F.Identity[string],
-			)
-			assert.Equal(t, "", value)
+			assert.Equal(t, Success(""), empty)
 		})
 
 		t.Run("concat combines successful validations", func(t *testing.T) {
@@ -88,12 +83,7 @@ func TestApplicativeMonoid(t *testing.T) {
 
 			result := m.Concat(v1, v2)
 
-			assert.True(t, either.IsRight(result))
-			value := either.MonadFold(result,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-			assert.Equal(t, "Hello World", value)
+			assert.Equal(t, Success("Hello World"), result)
 		})
 
 		t.Run("concat with failure returns failure", func(t *testing.T) {
@@ -141,17 +131,8 @@ func TestApplicativeMonoid(t *testing.T) {
 			result1 := m.Concat(v, empty)
 			result2 := m.Concat(empty, v)
 
-			val1 := either.MonadFold(result1,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-			val2 := either.MonadFold(result2,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-
-			assert.Equal(t, "test", val1)
-			assert.Equal(t, "test", val2)
+			assert.Equal(t, Of("test"), result1)
+			assert.Equal(t, Of("test"), result2)
 		})
 	})
 
@@ -166,11 +147,7 @@ func TestApplicativeMonoid(t *testing.T) {
 		t.Run("empty returns zero", func(t *testing.T) {
 			empty := m.Empty()
 
-			value := either.MonadFold(empty,
-				func(Errors) int { return -1 },
-				F.Identity[int],
-			)
-			assert.Equal(t, 0, value)
+			assert.Equal(t, Of(0), empty)
 		})
 
 		t.Run("concat adds values", func(t *testing.T) {
@@ -179,11 +156,7 @@ func TestApplicativeMonoid(t *testing.T) {
 
 			result := m.Concat(v1, v2)
 
-			value := either.MonadFold(result,
-				func(Errors) int { return 0 },
-				F.Identity[int],
-			)
-			assert.Equal(t, 42, value)
+			assert.Equal(t, Of(42), result)
 		})
 
 		t.Run("multiple concat operations", func(t *testing.T) {
@@ -194,11 +167,7 @@ func TestApplicativeMonoid(t *testing.T) {
 
 			result := m.Concat(m.Concat(m.Concat(v1, v2), v3), v4)
 
-			value := either.MonadFold(result,
-				func(Errors) int { return 0 },
-				F.Identity[int],
-			)
-			assert.Equal(t, 10, value)
+			assert.Equal(t, Of(10), result)
 		})
 	})
 }
@@ -245,21 +214,13 @@ func TestMonoidLaws(t *testing.T) {
 		t.Run("left identity", func(t *testing.T) {
 			// empty + a = a
 			result := m.Concat(m.Empty(), v1)
-			value := either.MonadFold(result,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-			assert.Equal(t, "a", value)
+			assert.Equal(t, Of("a"), result)
 		})
 
 		t.Run("right identity", func(t *testing.T) {
 			// a + empty = a
 			result := m.Concat(v1, m.Empty())
-			value := either.MonadFold(result,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-			assert.Equal(t, "a", value)
+			assert.Equal(t, Of("a"), result)
 		})
 
 		t.Run("associativity", func(t *testing.T) {
@@ -268,17 +229,8 @@ func TestMonoidLaws(t *testing.T) {
 			left := m.Concat(m.Concat(v1, v2), v3)
 			right := m.Concat(v1, m.Concat(v2, v3))
 
-			leftVal := either.MonadFold(left,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-			rightVal := either.MonadFold(right,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-
-			assert.Equal(t, "abc", leftVal)
-			assert.Equal(t, "abc", rightVal)
+			assert.Equal(t, Of("abc"), left)
+			assert.Equal(t, Of("abc"), right)
 		})
 	})
 }
@@ -332,11 +284,7 @@ func TestApplicativeMonoidEdgeCases(t *testing.T) {
 
 		result := m.Concat(v1, v2)
 
-		value := either.MonadFold(result,
-			func(Errors) Counter { return Counter{} },
-			F.Identity[Counter],
-		)
-		assert.Equal(t, 15, value.Count)
+		assert.Equal(t, Of(Counter{Count: 15}), result)
 	})
 
 	t.Run("empty concat empty", func(t *testing.T) {
@@ -344,10 +292,6 @@ func TestApplicativeMonoidEdgeCases(t *testing.T) {
 
 		result := m.Concat(m.Empty(), m.Empty())
 
-		value := either.MonadFold(result,
-			func(Errors) string { return "ERROR" },
-			F.Identity[string],
-		)
-		assert.Equal(t, "", value)
+		assert.Equal(t, Of(""), result)
 	})
 }

v2/optics/codec/validation/types.go

@@ -1,7 +1,24 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package validation
 
 import (
 	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/reader"
 	"github.com/IBM/fp-go/v2/result"
@@ -9,13 +26,35 @@ import (
 
 type (
 	// Result represents a computation that may succeed with a value of type A or fail with an error.
+	// This is an alias for result.Result[A], which is Either[error, A].
+	//
+	// Used for converting validation results to standard Go error handling patterns.
 	Result[A any] = result.Result[A]
 
 	// Either represents a value that can be one of two types: Left (error) or Right (success).
+	// This is an alias for either.Either[E, A], a disjoint union type.
+	//
+	// In the validation context:
+	//   - Left[E]: Contains error information of type E
+	//   - Right[A]: Contains a successfully validated value of type A
 	Either[E, A any] = either.Either[E, A]
 
 	// ContextEntry represents a single entry in the validation context path.
-	// It tracks the location and type information during nested validation.
+	// It tracks the location and type information during nested validation,
+	// enabling precise error reporting with full path information.
+	//
+	// Fields:
+	//   - Key: The key or field name (e.g., "email", "address", "items[0]")
+	//   - Type: The expected type name (e.g., "string", "int", "User")
+	//   - Actual: The actual value being validated (for error reporting)
+	//
+	// Example:
+	//
+	//	entry := ContextEntry{
+	//	    Key:    "user.email",
+	//	    Type:   "string",
+	//	    Actual: 12345,
+	//	}
 	ContextEntry struct {
 		Key    string // The key or field name (for objects/maps)
 		Type   string // The expected type name
@@ -23,42 +62,202 @@ type (
 	}
 
 	// Context is a stack of ContextEntry values representing the path through
-	// nested structures during validation. Used to provide detailed error messages.
+	// nested structures during validation. Used to provide detailed error messages
+	// that show exactly where in a nested structure a validation failure occurred.
+	//
+	// The context builds up as validation descends into nested structures:
+	//   - [] - root level
+	//   - [{Key: "user"}] - inside user object
+	//   - [{Key: "user"}, {Key: "address"}] - inside user.address
+	//   - [{Key: "user"}, {Key: "address"}, {Key: "zipCode"}] - at user.address.zipCode
+	//
+	// Example:
+	//
+	//	ctx := Context{
+	//	    {Key: "user", Type: "User"},
+	//	    {Key: "address", Type: "Address"},
+	//	    {Key: "zipCode", Type: "string"},
+	//	}
+	//	// Represents path: user.address.zipCode
 	Context = []ContextEntry
 
-	// ValidationError represents a single validation failure with context.
+	// ValidationError represents a single validation failure with full context information.
+	// It implements the error interface and provides detailed information about what failed,
+	// where it failed, and why it failed.
+	//
+	// Fields:
+	//   - Value: The actual value that failed validation
+	//   - Context: The path to the value in nested structures (e.g., user.address.zipCode)
+	//   - Messsage: Human-readable error description
+	//   - Cause: Optional underlying error that caused the validation failure
+	//
+	// The ValidationError type implements:
+	//   - error interface: For standard Go error handling
+	//   - fmt.Formatter: For custom formatting with %v, %+v
+	//   - slog.LogValuer: For structured logging with slog
+	//
+	// Example:
+	//
+	//	err := &ValidationError{
+	//	    Value:    "not-an-email",
+	//	    Context:  []ContextEntry{{Key: "user"}, {Key: "email"}},
+	//	    Messsage: "invalid email format",
+	//	    Cause:    nil,
+	//	}
+	//	fmt.Printf("%v", err)   // at user.email: invalid email format
+	//	fmt.Printf("%+v", err)  // at user.email: invalid email format
+	//	                        //   value: "not-an-email"
 	ValidationError struct {
 		Value    any     // The value that failed validation
 		Context  Context // The path to the value in nested structures
 		Messsage string  // Human-readable error message
-		Cause    error
+		Cause    error   // Optional underlying error cause
 	}
 
 	// Errors is a collection of validation errors.
+	// This type is used to accumulate multiple validation failures,
+	// allowing all errors to be reported at once rather than failing fast.
+	//
+	// Example:
+	//
+	//	errors := Errors{
+	//	    &ValidationError{Value: "", Messsage: "name is required"},
+	//	    &ValidationError{Value: "invalid", Messsage: "invalid email"},
+	//	    &ValidationError{Value: -1, Messsage: "age must be positive"},
+	//	}
 	Errors = []*ValidationError
 
 	// validationErrors wraps a collection of validation errors with an optional root cause.
-	// It provides structured error information for validation failures.
+	// It provides structured error information for validation failures and implements
+	// the error interface for integration with standard Go error handling.
+	//
+	// This type is internal and created via MakeValidationErrors.
+	// It implements:
+	//   - error interface: For standard Go error handling
+	//   - fmt.Formatter: For custom formatting with %v, %+v
+	//   - slog.LogValuer: For structured logging with slog
+	//
+	// Fields:
+	//   - errors: The collection of individual validation errors
+	//   - cause: Optional root cause error
 	validationErrors struct {
 		errors Errors
 		cause  error
 	}
 
 	// Validation represents the result of a validation operation.
-	// Left contains validation errors, Right contains the successfully validated value.
+	// It's an Either type where:
+	//   - Left(Errors): Validation failed with one or more errors
+	//   - Right(A): Successfully validated value of type A
+	//
+	// This type supports applicative operations, allowing multiple validations
+	// to be combined while accumulating all errors rather than failing fast.
+	//
+	// Example:
+	//
+	//	// Success case
+	//	valid := Success(42)  // Right(42)
+	//
+	//	// Failure case
+	//	invalid := Failures[int](Errors{
+	//	    &ValidationError{Messsage: "must be positive"},
+	//	})  // Left([...])
+	//
+	//	// Combining validations (accumulates all errors)
+	//	result := Ap(Ap(Of(func(x int) func(y int) int {
+	//	    return func(y int) int { return x + y }
+	//	}))(validateX))(validateY)
 	Validation[A any] = Either[Errors, A]
 
 	// Reader represents a computation that depends on an environment R and produces a value A.
+	// This is an alias for reader.Reader[R, A], which is func(R) A.
+	//
+	// In the validation context, Reader is used for context-dependent validation operations
+	// where the validation logic needs access to the current validation context path.
+	//
+	// Example:
+	//
+	//	validateWithContext := func(ctx Context) Validation[int] {
+	//	    // Use ctx to provide detailed error messages
+	//	    return Success(42)
+	//	}
 	Reader[R, A any] = reader.Reader[R, A]
 
 	// Kleisli represents a function from A to a validated B.
 	// It's a Reader that takes an input A and produces a Validation[B].
+	// This is the fundamental building block for composable validation operations.
+	//
+	// Type: func(A) Validation[B]
+	//
+	// Kleisli arrows can be composed using Chain/Bind operations to build
+	// complex validation pipelines from simple validation functions.
+	//
+	// Example:
+	//
+	//	validatePositive := func(x int) Validation[int] {
+	//	    if x > 0 {
+	//	        return Success(x)
+	//	    }
+	//	    return Failures[int](/* error */)
+	//	}
+	//
+	//	validateEven := func(x int) Validation[int] {
+	//	    if x%2 == 0 {
+	//	        return Success(x)
+	//	    }
+	//	    return Failures[int](/* error */)
+	//	}
+	//
+	//	// Compose validations
+	//	validatePositiveEven := Chain(validateEven)(Success(42))
 	Kleisli[A, B any] = Reader[A, Validation[B]]
 
 	// Operator represents a validation transformation that takes a validated A and produces a validated B.
-	// It's a specialized Kleisli arrow for composing validation operations.
+	// It's a specialized Kleisli arrow for composing validation operations where the input
+	// is already a Validation[A].
+	//
+	// Type: func(Validation[A]) Validation[B]
+	//
+	// Operators are used to transform and compose validation results, enabling
+	// functional composition of validation pipelines.
+	//
+	// Example:
+	//
+	//	// Transform a validated int to a validated string
+	//	intToString := Map(func(x int) string {
+	//	    return strconv.Itoa(x)
+	//	})  // Operator[int, string]
+	//
+	//	result := intToString(Success(42))  // Success("42")
 	Operator[A, B any] = Kleisli[Validation[A], B]
 
 	// Monoid represents an algebraic structure with an associative binary operation and an identity element.
+	// This is an alias for monoid.Monoid[A].
+	//
+	// In the validation context, monoids are used to combine validation results:
+	//   - ApplicativeMonoid: Combines successful validations using the monoid operation
+	//   - AlternativeMonoid: Provides fallback behavior for failed validations
+	//
+	// Example:
+	//
+	//	import N "github.com/IBM/fp-go/v2/number"
+	//
+	//	intAdd := N.MonoidSum[int]()
+	//	m := ApplicativeMonoid(intAdd)
+	//	result := m.Concat(Success(5), Success(3))  // Success(8)
 	Monoid[A any] = monoid.Monoid[A]
+
+	// Endomorphism represents a function from a type to itself: func(A) A.
+	// This is an alias for endomorphism.Endomorphism[A].
+	//
+	// In the validation context, endomorphisms are used with LetL to transform
+	// values within a validation context using pure functions.
+	//
+	// Example:
+	//
+	//	double := func(x int) int { return x * 2 }  // Endomorphism[int]
+	//	result := LetL(lens, double)(Success(21))   // Success(42)
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	Lazy[A any] = lazy.Lazy[A]
 )

v2/optics/codec/validation/validation_test.go

@@ -186,12 +186,7 @@ func TestSuccess(t *testing.T) {
 	t.Run("creates right either with value", func(t *testing.T) {
 		result := Success(42)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(Errors) int { return 0 },
-			F.Identity[int],
-		)
-		assert.Equal(t, 42, value)
+		assert.Equal(t, Success(42), result)
 	})
 
 	t.Run("works with different types", func(t *testing.T) {
@@ -327,7 +322,7 @@ func TestValidationIntegration(t *testing.T) {
 			&ValidationError{Value: "bad", Messsage: "error"},
 		})
 
-		assert.True(t, either.IsRight(success))
+		assert.Equal(t, Success(42), success)
 		assert.True(t, either.IsLeft(failure))
 	})
 
@@ -512,12 +507,7 @@ func TestToResult(t *testing.T) {
 
 		result := ToResult(validation)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(error) int { return 0 },
-			F.Identity[int],
-		)
-		assert.Equal(t, 42, value)
+		assert.Equal(t, either.Of[error](42), result)
 	})
 
 	t.Run("converts failed validation to result with error", func(t *testing.T) {
@@ -569,23 +559,18 @@ func TestToResult(t *testing.T) {
 		// String type
 		strValidation := Success("hello")
 		strResult := ToResult(strValidation)
-		assert.True(t, either.IsRight(strResult))
+		assert.Equal(t, either.Of[error]("hello"), strResult)
 
 		// Bool type
 		boolValidation := Success(true)
 		boolResult := ToResult(boolValidation)
-		assert.True(t, either.IsRight(boolResult))
+		assert.Equal(t, either.Of[error](true), boolResult)
 
 		// Struct type
 		type User struct{ Name string }
 		userValidation := Success(User{Name: "Alice"})
 		userResult := ToResult(userValidation)
-		assert.True(t, either.IsRight(userResult))
-		user := either.MonadFold(userResult,
-			func(error) User { return User{} },
-			F.Identity[User],
-		)
-		assert.Equal(t, "Alice", user.Name)
+		assert.Equal(t, either.Of[error](User{Name: "Alice"}), userResult)
 	})
 
 	t.Run("preserves error context in result", func(t *testing.T) {

v2/optics/iso/prism/compose.go

@@ -0,0 +1,153 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prism
+
+import (
+	"fmt"
+
+	F "github.com/IBM/fp-go/v2/function"
+	P "github.com/IBM/fp-go/v2/optics/prism"
+)
+
+// Compose creates a Kleisli arrow that composes a prism with an isomorphism.
+//
+// This function takes a Prism[A, B] and returns a Kleisli arrow that can transform
+// any Iso[S, A] into a Prism[S, B]. The resulting prism changes the source type from
+// A to S using the bidirectional transformation provided by the isomorphism, while
+// maintaining the same focus type B.
+//
+// The composition works as follows:
+//   - GetOption: First transforms S to A using the iso's Get, then extracts B from A using the prism's GetOption
+//   - ReverseGet: First constructs A from B using the prism's ReverseGet, then transforms A to S using the iso's ReverseGet
+//
+// This is the dual operation of optics/prism/iso.Compose:
+//   - optics/prism/iso.Compose: Transforms the focus type (A → B) while keeping source type (S) constant
+//   - optics/iso/prism.Compose: Transforms the source type (A → S) while keeping focus type (B) constant
+//
+// This is particularly useful when you have a prism that works with one type but you
+// need to adapt it to work with a different source type that has a lossless bidirectional
+// transformation to the original type.
+//
+// Type Parameters:
+//   - S: The new source type after applying the isomorphism
+//   - A: The original source type of the prism
+//   - B: The focus type (remains constant through composition)
+//
+// Parameters:
+//   - ab: A prism that extracts B from A
+//
+// Returns:
+//   - A Kleisli arrow (function) that takes an Iso[S, A] and returns a Prism[S, B]
+//
+// Laws:
+// The composed prism must satisfy the prism laws:
+//  1. GetOption(ReverseGet(b)) == Some(b) for all b: B
+//  2. If GetOption(s) == Some(a), then GetOption(ReverseGet(a)) == Some(a)
+//
+// These laws are preserved because:
+//   - The isomorphism satisfies: ia.ReverseGet(ia.Get(s)) == s and ia.Get(ia.ReverseGet(a)) == a
+//   - The original prism satisfies the prism laws
+//
+// Haskell Equivalent:
+// This corresponds to the (.) operator for composing optics in Haskell's lens library,
+// specifically when composing an Iso with a Prism:
+//
+//	iso . prism :: Iso s a -> Prism a b -> Prism s b
+//
+// In Haskell's lens library, this is part of the general optic composition mechanism.
+// See: https://hackage.haskell.org/package/lens/docs/Control-Lens-Iso.html
+//
+// Example - Composing with Either prism:
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/either"
+//	    "github.com/IBM/fp-go/v2/optics/iso"
+//	    "github.com/IBM/fp-go/v2/optics/prism"
+//	    IP "github.com/IBM/fp-go/v2/optics/iso/prism"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	)
+//
+//	// Create a prism that extracts Right values from Either[error, string]
+//	rightPrism := prism.FromEither[error, string]()
+//
+//	// Create an isomorphism between []byte and string
+//	bytesStringIso := iso.MakeIso(
+//	    func(b []byte) string { return string(b) },
+//	    func(s string) []byte { return []byte(s) },
+//	)
+//
+//	// Compose them to get a prism that works with []byte as source
+//	bytesPrism := IP.Compose(rightPrism)(bytesStringIso)
+//
+//	// Use the composed prism
+//	// First converts []byte to string via iso, then extracts Right value
+//	bytes := []byte("hello")
+//	either := either.Right[error](string(bytes))
+//	result := bytesPrism.GetOption(bytes)  // Extracts "hello" if Right
+//
+//	// Construct []byte from string
+//	constructed := bytesPrism.ReverseGet("world")
+//	// Returns []byte("world") wrapped in Right
+//
+// Example - Composing with custom types:
+//
+//	type JSON []byte
+//	type Config struct {
+//	    Host string
+//	    Port int
+//	}
+//
+//	// Isomorphism between JSON and []byte
+//	jsonIso := iso.MakeIso(
+//	    func(j JSON) []byte { return []byte(j) },
+//	    func(b []byte) JSON { return JSON(b) },
+//	)
+//
+//	// Prism that extracts Config from []byte (via JSON parsing)
+//	configPrism := prism.MakePrism(
+//	    func(b []byte) option.Option[Config] {
+//	        var cfg Config
+//	        if err := json.Unmarshal(b, &cfg); err != nil {
+//	            return option.None[Config]()
+//	        }
+//	        return option.Some(cfg)
+//	    },
+//	    func(cfg Config) []byte {
+//	        b, _ := json.Marshal(cfg)
+//	        return b
+//	    },
+//	)
+//
+//	// Compose to work with JSON type instead of []byte
+//	jsonConfigPrism := IP.Compose(configPrism)(jsonIso)
+//
+//	jsonData := JSON(`{"host":"localhost","port":8080}`)
+//	config := jsonConfigPrism.GetOption(jsonData)
+//	// config is Some(Config{Host: "localhost", Port: 8080})
+//
+// See also:
+//   - github.com/IBM/fp-go/v2/optics/iso for isomorphism operations
+//   - github.com/IBM/fp-go/v2/optics/prism for prism operations
+//   - github.com/IBM/fp-go/v2/optics/prism/iso for the dual composition (transforming focus type)
+func Compose[S, A, B any](ab Prism[A, B]) P.Kleisli[S, Iso[S, A], B] {
+	return func(ia Iso[S, A]) Prism[S, B] {
+		return P.MakePrismWithName(
+			F.Flow2(ia.Get, ab.GetOption),
+			F.Flow2(ab.ReverseGet, ia.ReverseGet),
+			fmt.Sprintf("IsoCompose[%s -> %s]", ia, ab),
+		)
+	}
+}

v2/optics/iso/prism/compose_test.go

@@ -0,0 +1,435 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prism
+
+import (
+	"encoding/json"
+	"strconv"
+	"testing"
+
+	E "github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	I "github.com/IBM/fp-go/v2/optics/iso"
+	P "github.com/IBM/fp-go/v2/optics/prism"
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestComposeWithEitherPrism tests composing a prism with an isomorphism using Either
+func TestComposeWithEitherPrism(t *testing.T) {
+	// Create a prism that extracts Right values from Either[error, string]
+	rightPrism := P.FromEither[error, string]()
+
+	// Create an isomorphism between []byte and Either[error, string]
+	bytesEitherIso := I.MakeIso(
+		func(b []byte) E.Either[error, string] {
+			return E.Right[error](string(b))
+		},
+		func(e E.Either[error, string]) []byte {
+			return []byte(E.GetOrElse(func(error) string { return "" })(e))
+		},
+	)
+
+	// Compose them: Prism[Either, string] with Iso[[]byte, Either] -> Prism[[]byte, string]
+	bytesPrism := Compose[[]byte](rightPrism)(bytesEitherIso)
+
+	t.Run("GetOption extracts string from []byte", func(t *testing.T) {
+		bytes := []byte("hello")
+		result := bytesPrism.GetOption(bytes)
+
+		assert.True(t, O.IsSome(result))
+		str := O.GetOrElse(F.Constant(""))(result)
+		assert.Equal(t, "hello", str)
+	})
+
+	t.Run("ReverseGet constructs []byte from string", func(t *testing.T) {
+		value := "world"
+		result := bytesPrism.ReverseGet(value)
+
+		assert.Equal(t, []byte("world"), result)
+	})
+
+	t.Run("Round-trip through GetOption and ReverseGet", func(t *testing.T) {
+		original := "test"
+
+		// ReverseGet to create []byte
+		bytes := bytesPrism.ReverseGet(original)
+
+		// GetOption to extract string back
+		result := bytesPrism.GetOption(bytes)
+
+		assert.True(t, O.IsSome(result))
+		extracted := O.GetOrElse(F.Constant(""))(result)
+		assert.Equal(t, original, extracted)
+	})
+}
+
+// TestComposeWithOptionPrism tests composing a prism with an isomorphism using Option
+func TestComposeWithOptionPrism(t *testing.T) {
+	// Create a prism that extracts Some values from Option[int]
+	somePrism := P.FromOption[int]()
+
+	// Create an isomorphism between string and Option[int]
+	stringOptionIso := I.MakeIso(
+		func(s string) O.Option[int] {
+			i, err := strconv.Atoi(s)
+			if err != nil {
+				return O.None[int]()
+			}
+			return O.Some(i)
+		},
+		func(opt O.Option[int]) string {
+			return strconv.Itoa(O.GetOrElse(F.Constant(0))(opt))
+		},
+	)
+
+	// Compose them: Prism[Option, int] with Iso[string, Option] -> Prism[string, int]
+	stringPrism := Compose[string](somePrism)(stringOptionIso)
+
+	t.Run("GetOption extracts int from valid string", func(t *testing.T) {
+		result := stringPrism.GetOption("42")
+
+		assert.True(t, O.IsSome(result))
+		num := O.GetOrElse(F.Constant(0))(result)
+		assert.Equal(t, 42, num)
+	})
+
+	t.Run("GetOption returns None for invalid string", func(t *testing.T) {
+		result := stringPrism.GetOption("invalid")
+
+		assert.True(t, O.IsNone(result))
+	})
+
+	t.Run("ReverseGet constructs string from int", func(t *testing.T) {
+		result := stringPrism.ReverseGet(100)
+
+		assert.Equal(t, "100", result)
+	})
+}
+
+// Custom types for testing
+type Celsius float64
+type Fahrenheit float64
+
+type Temperature interface {
+	isTemperature()
+}
+
+type CelsiusTemp struct {
+	Value Celsius
+}
+
+func (c CelsiusTemp) isTemperature() {}
+
+type FahrenheitTemp struct {
+	Value Fahrenheit
+}
+
+func (f FahrenheitTemp) isTemperature() {}
+
+// TestComposeWithCustomPrism tests composing with custom types
+func TestComposeWithCustomPrism(t *testing.T) {
+	// Prism that extracts Celsius from Temperature
+	celsiusPrism := P.MakePrism(
+		func(t Temperature) O.Option[Celsius] {
+			if ct, ok := t.(CelsiusTemp); ok {
+				return O.Some(ct.Value)
+			}
+			return O.None[Celsius]()
+		},
+		func(c Celsius) Temperature {
+			return CelsiusTemp{Value: c}
+		},
+	)
+
+	// Isomorphism between Fahrenheit and Temperature
+	fahrenheitTempIso := I.MakeIso(
+		func(f Fahrenheit) Temperature {
+			celsius := Celsius((f - 32) * 5 / 9)
+			return CelsiusTemp{Value: celsius}
+		},
+		func(t Temperature) Fahrenheit {
+			if ct, ok := t.(CelsiusTemp); ok {
+				return Fahrenheit(ct.Value*9/5 + 32)
+			}
+			return 0
+		},
+	)
+
+	// Compose: Prism[Temperature, Celsius] with Iso[Fahrenheit, Temperature] -> Prism[Fahrenheit, Celsius]
+	fahrenheitPrism := Compose[Fahrenheit](celsiusPrism)(fahrenheitTempIso)
+
+	t.Run("GetOption extracts Celsius from Fahrenheit", func(t *testing.T) {
+		fahrenheit := Fahrenheit(68)
+		result := fahrenheitPrism.GetOption(fahrenheit)
+
+		assert.True(t, O.IsSome(result))
+		celsius := O.GetOrElse(F.Constant(Celsius(0)))(result)
+		assert.InDelta(t, 20.0, float64(celsius), 0.01)
+	})
+
+	t.Run("ReverseGet constructs Fahrenheit from Celsius", func(t *testing.T) {
+		celsius := Celsius(20)
+		result := fahrenheitPrism.ReverseGet(celsius)
+
+		assert.InDelta(t, 68.0, float64(result), 0.01)
+	})
+
+	t.Run("Round-trip preserves value", func(t *testing.T) {
+		original := Celsius(25)
+
+		// ReverseGet to create Fahrenheit
+		fahrenheit := fahrenheitPrism.ReverseGet(original)
+
+		// GetOption to extract Celsius back
+		result := fahrenheitPrism.GetOption(fahrenheit)
+
+		assert.True(t, O.IsSome(result))
+		extracted := O.GetOrElse(F.Constant(Celsius(0)))(result)
+		assert.InDelta(t, float64(original), float64(extracted), 0.01)
+	})
+}
+
+// TestComposeIdentityIso tests composing with an identity isomorphism
+func TestComposeIdentityIso(t *testing.T) {
+	// Prism that extracts Right values
+	rightPrism := P.FromEither[error, string]()
+
+	// Identity isomorphism on Either
+	idIso := I.Id[E.Either[error, string]]()
+
+	// Compose with identity should not change behavior
+	composedPrism := Compose[E.Either[error, string]](rightPrism)(idIso)
+
+	t.Run("Composed prism behaves like original prism", func(t *testing.T) {
+		either := E.Right[error]("test")
+
+		// Original prism
+		originalResult := rightPrism.GetOption(either)
+
+		// Composed prism
+		composedResult := composedPrism.GetOption(either)
+
+		assert.Equal(t, originalResult, composedResult)
+	})
+
+	t.Run("ReverseGet produces same result", func(t *testing.T) {
+		value := "test"
+
+		// Original prism
+		originalResult := rightPrism.ReverseGet(value)
+
+		// Composed prism
+		composedResult := composedPrism.ReverseGet(value)
+
+		assert.Equal(t, originalResult, composedResult)
+	})
+}
+
+// TestComposeChaining tests chaining multiple compositions
+func TestComposeChaining(t *testing.T) {
+	// Prism: extracts Right values from Either[error, int]
+	rightPrism := P.FromEither[error, int]()
+
+	// Iso 1: string to Either[error, int]
+	stringEitherIso := I.MakeIso(
+		func(s string) E.Either[error, int] {
+			i, err := strconv.Atoi(s)
+			if err != nil {
+				return E.Left[int](err)
+			}
+			return E.Right[error](i)
+		},
+		func(e E.Either[error, int]) string {
+			return strconv.Itoa(E.GetOrElse(func(error) int { return 0 })(e))
+		},
+	)
+
+	// Iso 2: []byte to string
+	bytesStringIso := I.MakeIso(
+		func(b []byte) string { return string(b) },
+		func(s string) []byte { return []byte(s) },
+	)
+
+	// First composition: Prism[Either, int] with Iso[string, Either] -> Prism[string, int]
+	step1 := Compose[string](rightPrism)(stringEitherIso)
+
+	// Second composition: Prism[string, int] with Iso[[]byte, string] -> Prism[[]byte, int]
+	step2 := Compose[[]byte](step1)(bytesStringIso)
+
+	t.Run("Chained composition extracts correctly", func(t *testing.T) {
+		bytes := []byte("42")
+		result := step2.GetOption(bytes)
+
+		assert.True(t, O.IsSome(result))
+		num := O.GetOrElse(F.Constant(0))(result)
+		assert.Equal(t, 42, num)
+	})
+
+	t.Run("Chained composition ReverseGet works correctly", func(t *testing.T) {
+		num := 100
+		result := step2.ReverseGet(num)
+
+		assert.Equal(t, []byte("100"), result)
+	})
+}
+
+// TestComposePrismLaws verifies that the composed prism satisfies prism laws
+func TestComposePrismLaws(t *testing.T) {
+	// Create a prism
+	prism := P.FromEither[error, int]()
+
+	// Create an isomorphism from string to Either[error, int]
+	iso := I.MakeIso(
+		func(s string) E.Either[error, int] {
+			i, err := strconv.Atoi(s)
+			if err != nil {
+				return E.Left[int](err)
+			}
+			return E.Right[error](i)
+		},
+		func(e E.Either[error, int]) string {
+			return strconv.Itoa(E.GetOrElse(func(error) int { return 0 })(e))
+		},
+	)
+
+	// Compose them
+	composed := Compose[string](prism)(iso)
+
+	t.Run("Law 1: GetOption(ReverseGet(b)) == Some(b)", func(t *testing.T) {
+		value := 42
+
+		// ReverseGet then GetOption should return Some(value)
+		source := composed.ReverseGet(value)
+		result := composed.GetOption(source)
+
+		assert.True(t, O.IsSome(result))
+		extracted := O.GetOrElse(F.Constant(0))(result)
+		assert.Equal(t, value, extracted)
+	})
+
+	t.Run("Law 2: If GetOption(s) == Some(a), then GetOption(ReverseGet(a)) == Some(a)", func(t *testing.T) {
+		source := "100"
+
+		// First GetOption
+		firstResult := composed.GetOption(source)
+		assert.True(t, O.IsSome(firstResult))
+
+		// Extract the value
+		value := O.GetOrElse(F.Constant(0))(firstResult)
+
+		// ReverseGet then GetOption again
+		reconstructed := composed.ReverseGet(value)
+		secondResult := composed.GetOption(reconstructed)
+
+		assert.True(t, O.IsSome(secondResult))
+		finalValue := O.GetOrElse(F.Constant(0))(secondResult)
+		assert.Equal(t, value, finalValue)
+	})
+}
+
+// TestComposeWithJSON tests a practical example with JSON parsing
+func TestComposeWithJSON(t *testing.T) {
+	type Config struct {
+		Host string `json:"host"`
+		Port int    `json:"port"`
+	}
+
+	// Prism that extracts Config from []byte (via JSON parsing)
+	configPrism := P.MakePrism(
+		func(b []byte) O.Option[Config] {
+			var cfg Config
+			if err := json.Unmarshal(b, &cfg); err != nil {
+				return O.None[Config]()
+			}
+			return O.Some(cfg)
+		},
+		func(cfg Config) []byte {
+			b, _ := json.Marshal(cfg)
+			return b
+		},
+	)
+
+	// Isomorphism between string and []byte
+	stringBytesIso := I.MakeIso(
+		func(s string) []byte { return []byte(s) },
+		func(b []byte) string { return string(b) },
+	)
+
+	// Compose: Prism[[]byte, Config] with Iso[string, []byte] -> Prism[string, Config]
+	stringConfigPrism := Compose[string](configPrism)(stringBytesIso)
+
+	t.Run("GetOption parses valid JSON string", func(t *testing.T) {
+		jsonStr := `{"host":"localhost","port":8080}`
+		result := stringConfigPrism.GetOption(jsonStr)
+
+		assert.True(t, O.IsSome(result))
+		cfg := O.GetOrElse(F.Constant(Config{}))(result)
+		assert.Equal(t, "localhost", cfg.Host)
+		assert.Equal(t, 8080, cfg.Port)
+	})
+
+	t.Run("GetOption returns None for invalid JSON", func(t *testing.T) {
+		invalidJSON := `{invalid json}`
+		result := stringConfigPrism.GetOption(invalidJSON)
+
+		assert.True(t, O.IsNone(result))
+	})
+
+	t.Run("ReverseGet creates JSON string from Config", func(t *testing.T) {
+		cfg := Config{Host: "example.com", Port: 443}
+		result := stringConfigPrism.ReverseGet(cfg)
+
+		// Parse it back to verify
+		var parsed Config
+		err := json.Unmarshal([]byte(result), &parsed)
+		assert.NoError(t, err)
+		assert.Equal(t, cfg, parsed)
+	})
+}
+
+// TestComposeWithEmptyValues tests edge cases with empty/zero values
+func TestComposeWithEmptyValues(t *testing.T) {
+	// Prism that extracts Right values
+	prism := P.FromEither[error, []byte]()
+
+	// Isomorphism between string and Either[error, []byte]
+	iso := I.MakeIso(
+		func(s string) E.Either[error, []byte] {
+			return E.Right[error]([]byte(s))
+		},
+		func(e E.Either[error, []byte]) string {
+			return string(E.GetOrElse(func(error) []byte { return []byte{} })(e))
+		},
+	)
+
+	composed := Compose[string](prism)(iso)
+
+	t.Run("Empty string is handled correctly", func(t *testing.T) {
+		result := composed.GetOption("")
+
+		assert.True(t, O.IsSome(result))
+		bytes := O.GetOrElse(F.Constant([]byte("default")))(result)
+		assert.Equal(t, []byte{}, bytes)
+	})
+
+	t.Run("ReverseGet with empty bytes", func(t *testing.T) {
+		bytes := []byte{}
+		result := composed.ReverseGet(bytes)
+
+		assert.Equal(t, "", result)
+	})
+}

v2/optics/iso/prism/types.go

@@ -0,0 +1,99 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package prism provides utilities for composing prisms with isomorphisms.
+//
+// This package enables the composition of prisms (optics for sum types) with
+// isomorphisms (bidirectional transformations), allowing you to transform the
+// source type of a prism using an isomorphism. This is the inverse operation
+// of optics/prism/iso, where we transform the focus type instead of the source type.
+//
+// # Key Concepts
+//
+// A Prism[S, A] is an optic that focuses on a specific variant within a sum type S,
+// extracting values of type A. An Iso[S, A] represents a bidirectional transformation
+// between types S and A without loss of information.
+//
+// When you compose a Prism[A, B] with an Iso[S, A], you get a Prism[S, B] that:
+//   - Transforms S to A using the isomorphism's Get
+//   - Extracts values of type B from A (using the prism)
+//   - Can construct S from B by first using the prism's ReverseGet to get A, then the iso's ReverseGet
+//
+// # Example Usage
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/iso"
+//	    "github.com/IBM/fp-go/v2/optics/prism"
+//	    IP "github.com/IBM/fp-go/v2/optics/iso/prism"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	)
+//
+//	// Create an isomorphism between []byte and string
+//	bytesStringIso := iso.MakeIso(
+//	    func(b []byte) string { return string(b) },
+//	    func(s string) []byte { return []byte(s) },
+//	)
+//
+//	// Create a prism that extracts Right values from Either[error, string]
+//	rightPrism := prism.FromEither[error, string]()
+//
+//	// Compose them to get a prism that works with []byte as source
+//	bytesPrism := IP.Compose(rightPrism)(bytesStringIso)
+//
+//	// Use the composed prism
+//	bytes := []byte(`{"status":"ok"}`)
+//	// First converts bytes to string via iso, then extracts Right value
+//	result := bytesPrism.GetOption(either.Right[error](string(bytes)))
+//
+// # Comparison with optics/prism/iso
+//
+// This package (optics/iso/prism) is the dual of optics/prism/iso:
+//   - optics/prism/iso: Composes Iso[A, B] with Prism[S, A] → Prism[S, B] (transforms focus type)
+//   - optics/iso/prism: Composes Prism[A, B] with Iso[S, A] → Prism[S, B] (transforms source type)
+//
+// # Type Aliases
+//
+// This package re-exports key types from the iso and prism packages for convenience:
+//   - Iso[S, A]: An isomorphism between types S and A
+//   - Prism[S, A]: A prism focusing on type A within sum type S
+package prism
+
+import (
+	I "github.com/IBM/fp-go/v2/optics/iso"
+	P "github.com/IBM/fp-go/v2/optics/prism"
+)
+
+type (
+	// Iso represents an isomorphism between types S and A.
+	// It is a bidirectional transformation that converts between two types
+	// without any loss of information.
+	//
+	// Type Parameters:
+	//   - S: The source type
+	//   - A: The target type
+	//
+	// See github.com/IBM/fp-go/v2/optics/iso for the full Iso API.
+	Iso[S, A any] = I.Iso[S, A]
+
+	// Prism is an optic used to select part of a sum type (tagged union).
+	// It provides operations to extract and construct values within sum types.
+	//
+	// Type Parameters:
+	//   - S: The source type (sum type)
+	//   - A: The focus type (variant within the sum type)
+	//
+	// See github.com/IBM/fp-go/v2/optics/prism for the full Prism API.
+	Prism[S, A any] = P.Prism[S, A]
+)

v2/optics/prism/iso/compose.go

@@ -0,0 +1,156 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iso
+
+import (
+	"fmt"
+
+	F "github.com/IBM/fp-go/v2/function"
+	P "github.com/IBM/fp-go/v2/optics/prism"
+	O "github.com/IBM/fp-go/v2/option"
+)
+
+// Compose creates an operator that composes an isomorphism with a prism.
+//
+// This function takes an isomorphism Iso[A, B] and returns an operator that can
+// transform any Prism[S, A] into a Prism[S, B]. The resulting prism maintains
+// the same source type S but changes the focus type from A to B using the
+// bidirectional transformation provided by the isomorphism.
+//
+// The composition works as follows:
+//   - GetOption: First extracts A from S using the prism, then transforms A to B using the iso's Get
+//   - ReverseGet: First transforms B to A using the iso's ReverseGet, then constructs S using the prism's ReverseGet
+//
+// This is particularly useful when you have a prism that focuses on one type but
+// you need to work with a different type that has a lossless bidirectional
+// transformation to the original type.
+//
+// Haskell Equivalent:
+// This corresponds to the (.) operator for composing optics in Haskell's lens library,
+// specifically when composing a Prism with an Iso:
+//
+//	prism . iso :: Prism s a -> Iso a b -> Prism s b
+//
+// In Haskell's lens library, this is part of the general optic composition mechanism.
+// See: https://hackage.haskell.org/package/lens/docs/Control-Lens-Prism.html
+//
+// Type Parameters:
+//   - S: The source type (sum type) that the prism operates on
+//   - A: The original focus type of the prism
+//   - B: The new focus type after applying the isomorphism
+//
+// Parameters:
+//   - ab: An isomorphism between types A and B that defines the bidirectional transformation
+//
+// Returns:
+//   - An Operator[S, A, B] that transforms Prism[S, A] into Prism[S, B]
+//
+// Laws:
+// The composed prism must satisfy the prism laws:
+//  1. GetOption(ReverseGet(b)) == Some(b) for all b: B
+//  2. If GetOption(s) == Some(a), then GetOption(ReverseGet(a)) == Some(a)
+//
+// These laws are preserved because:
+//   - The isomorphism satisfies: ab.ReverseGet(ab.Get(a)) == a and ab.Get(ab.ReverseGet(b)) == b
+//   - The original prism satisfies the prism laws
+//
+// Example - Composing string/bytes isomorphism with Either prism:
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/either"
+//	    "github.com/IBM/fp-go/v2/optics/iso"
+//	    "github.com/IBM/fp-go/v2/optics/prism"
+//	    PI "github.com/IBM/fp-go/v2/optics/prism/iso"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	)
+//
+//	// Create an isomorphism between string and []byte
+//	stringBytesIso := iso.MakeIso(
+//	    func(s string) []byte { return []byte(s) },
+//	    func(b []byte) string { return string(b) },
+//	)
+//
+//	// Create a prism that extracts Right values from Either[error, string]
+//	rightPrism := prism.FromEither[error, string]()
+//
+//	// Compose them to get a prism that works with []byte instead of string
+//	bytesPrism := PI.Compose(stringBytesIso)(rightPrism)
+//
+//	// Extract bytes from a Right value
+//	success := either.Right[error]("hello")
+//	result := bytesPrism.GetOption(success)
+//	// result is Some([]byte("hello"))
+//
+//	// Extract from a Left value returns None
+//	failure := either.Left[string](errors.New("error"))
+//	result = bytesPrism.GetOption(failure)
+//	// result is None
+//
+//	// Construct an Either from bytes
+//	constructed := bytesPrism.ReverseGet([]byte("world"))
+//	// constructed is Right("world")
+//
+// Example - Composing with custom types:
+//
+//	type Celsius float64
+//	type Fahrenheit float64
+//
+//	// Isomorphism between Celsius and Fahrenheit
+//	tempIso := iso.MakeIso(
+//	    func(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) },
+//	    func(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) },
+//	)
+//
+//	// Prism that extracts temperature from a weather report
+//	type WeatherReport struct {
+//	    Temperature Celsius
+//	    Condition   string
+//	}
+//	tempPrism := prism.MakePrism(
+//	    func(w WeatherReport) option.Option[Celsius] {
+//	        return option.Some(w.Temperature)
+//	    },
+//	    func(c Celsius) WeatherReport {
+//	        return WeatherReport{Temperature: c}
+//	    },
+//	)
+//
+//	// Compose to work with Fahrenheit instead
+//	fahrenheitPrism := PI.Compose(tempIso)(tempPrism)
+//
+//	report := WeatherReport{Temperature: 20, Condition: "sunny"}
+//	temp := fahrenheitPrism.GetOption(report)
+//	// temp is Some(68.0) in Fahrenheit
+//
+// See also:
+//   - github.com/IBM/fp-go/v2/optics/iso for isomorphism operations
+//   - github.com/IBM/fp-go/v2/optics/prism for prism operations
+//   - Operator for the type signature of the returned function
+func Compose[S, A, B any](ab Iso[A, B]) Operator[S, A, B] {
+	return func(pa Prism[S, A]) Prism[S, B] {
+		return P.MakePrismWithName(
+			F.Flow2(
+				pa.GetOption,
+				O.Map(ab.Get),
+			),
+			F.Flow2(
+				ab.ReverseGet,
+				pa.ReverseGet,
+			),
+			fmt.Sprintf("PrismCompose[%s -> %s]", pa, ab),
+		)
+	}
+}

v2/optics/prism/iso/compose_test.go

@@ -0,0 +1,369 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iso
+
+import (
+	"errors"
+	"strconv"
+	"testing"
+
+	E "github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	I "github.com/IBM/fp-go/v2/optics/iso"
+	P "github.com/IBM/fp-go/v2/optics/prism"
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestComposeWithEitherPrism tests composing an isomorphism with an Either prism
+func TestComposeWithEitherPrism(t *testing.T) {
+	// Create an isomorphism between string and []byte
+	stringBytesIso := I.MakeIso(
+		func(s string) []byte { return []byte(s) },
+		func(b []byte) string { return string(b) },
+	)
+
+	// Create a prism that extracts Right values from Either[error, string]
+	rightPrism := P.FromEither[error, string]()
+
+	// Compose them
+	bytesPrism := Compose[E.Either[error, string]](stringBytesIso)(rightPrism)
+
+	t.Run("GetOption extracts and transforms Right value", func(t *testing.T) {
+		success := E.Right[error]("hello")
+		result := bytesPrism.GetOption(success)
+
+		assert.True(t, O.IsSome(result))
+		bytes := O.GetOrElse(F.Constant([]byte{}))(result)
+		assert.Equal(t, []byte("hello"), bytes)
+	})
+
+	t.Run("GetOption returns None for Left value", func(t *testing.T) {
+		failure := E.Left[string](errors.New("error"))
+		result := bytesPrism.GetOption(failure)
+
+		assert.True(t, O.IsNone(result))
+	})
+
+	t.Run("ReverseGet constructs Either from transformed value", func(t *testing.T) {
+		bytes := []byte("world")
+		result := bytesPrism.ReverseGet(bytes)
+
+		assert.True(t, E.IsRight(result))
+		str := E.GetOrElse(func(error) string { return "" })(result)
+		assert.Equal(t, "world", str)
+	})
+
+	t.Run("Round-trip through GetOption and ReverseGet", func(t *testing.T) {
+		// Start with bytes
+		original := []byte("test")
+
+		// ReverseGet to create Either
+		either := bytesPrism.ReverseGet(original)
+
+		// GetOption to extract bytes back
+		result := bytesPrism.GetOption(either)
+
+		assert.True(t, O.IsSome(result))
+		extracted := O.GetOrElse(F.Constant([]byte{}))(result)
+		assert.Equal(t, original, extracted)
+	})
+}
+
+// TestComposeWithOptionPrism tests composing an isomorphism with an Option prism
+func TestComposeWithOptionPrism(t *testing.T) {
+	// Create an isomorphism between int and string
+	intStringIso := I.MakeIso(
+		func(i int) string { return strconv.Itoa(i) },
+		func(s string) int {
+			i, _ := strconv.Atoi(s)
+			return i
+		},
+	)
+
+	// Create a prism that extracts Some values from Option[int]
+	somePrism := P.FromOption[int]()
+
+	// Compose them
+	stringPrism := Compose[O.Option[int]](intStringIso)(somePrism)
+
+	t.Run("GetOption extracts and transforms Some value", func(t *testing.T) {
+		some := O.Some(42)
+		result := stringPrism.GetOption(some)
+
+		assert.True(t, O.IsSome(result))
+		str := O.GetOrElse(F.Constant(""))(result)
+		assert.Equal(t, "42", str)
+	})
+
+	t.Run("GetOption returns None for None value", func(t *testing.T) {
+		none := O.None[int]()
+		result := stringPrism.GetOption(none)
+
+		assert.True(t, O.IsNone(result))
+	})
+
+	t.Run("ReverseGet constructs Option from transformed value", func(t *testing.T) {
+		str := "100"
+		result := stringPrism.ReverseGet(str)
+
+		assert.True(t, O.IsSome(result))
+		num := O.GetOrElse(F.Constant(0))(result)
+		assert.Equal(t, 100, num)
+	})
+}
+
+// TestComposeWithCustomPrism tests composing with a custom prism
+// Custom types for TestComposeWithCustomPrism
+type Celsius float64
+type Fahrenheit float64
+
+type Temperature interface {
+	isTemperature()
+}
+
+type CelsiusTemp struct {
+	Value Celsius
+}
+
+func (c CelsiusTemp) isTemperature() {}
+
+type KelvinTemp struct {
+	Value float64
+}
+
+func (k KelvinTemp) isTemperature() {}
+
+func TestComposeWithCustomPrism(t *testing.T) {
+	// Isomorphism between Celsius and Fahrenheit
+	tempIso := I.MakeIso(
+		func(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) },
+		func(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) },
+	)
+
+	// Prism that extracts Celsius from Temperature
+	celsiusPrism := P.MakePrism(
+		func(t Temperature) O.Option[Celsius] {
+			if ct, ok := t.(CelsiusTemp); ok {
+				return O.Some(ct.Value)
+			}
+			return O.None[Celsius]()
+		},
+		func(c Celsius) Temperature {
+			return CelsiusTemp{Value: c}
+		},
+	)
+
+	// Compose to work with Fahrenheit
+	fahrenheitPrism := Compose[Temperature](tempIso)(celsiusPrism)
+
+	t.Run("GetOption extracts and converts Celsius to Fahrenheit", func(t *testing.T) {
+		temp := CelsiusTemp{Value: 0}
+		result := fahrenheitPrism.GetOption(temp)
+
+		assert.True(t, O.IsSome(result))
+		fahrenheit := O.GetOrElse(F.Constant(Fahrenheit(0)))(result)
+		assert.InDelta(t, 32.0, float64(fahrenheit), 0.01)
+	})
+
+	t.Run("GetOption returns None for non-Celsius temperature", func(t *testing.T) {
+		temp := KelvinTemp{Value: 273.15}
+		result := fahrenheitPrism.GetOption(temp)
+
+		assert.True(t, O.IsNone(result))
+	})
+
+	t.Run("ReverseGet constructs Temperature from Fahrenheit", func(t *testing.T) {
+		fahrenheit := Fahrenheit(68)
+		result := fahrenheitPrism.ReverseGet(fahrenheit)
+
+		celsiusTemp, ok := result.(CelsiusTemp)
+		assert.True(t, ok)
+		assert.InDelta(t, 20.0, float64(celsiusTemp.Value), 0.01)
+	})
+
+	t.Run("Round-trip preserves value", func(t *testing.T) {
+		original := Fahrenheit(100)
+
+		// ReverseGet to create Temperature
+		temp := fahrenheitPrism.ReverseGet(original)
+
+		// GetOption to extract Fahrenheit back
+		result := fahrenheitPrism.GetOption(temp)
+
+		assert.True(t, O.IsSome(result))
+		extracted := O.GetOrElse(F.Constant(Fahrenheit(0)))(result)
+		assert.InDelta(t, float64(original), float64(extracted), 0.01)
+	})
+}
+
+// TestComposeIdentityIso tests composing with an identity isomorphism
+func TestComposeIdentityIso(t *testing.T) {
+	// Identity isomorphism (no transformation)
+	idIso := I.Id[string]()
+
+	// Prism that extracts Right values
+	rightPrism := P.FromEither[error, string]()
+
+	// Compose with identity should not change behavior
+	composedPrism := Compose[E.Either[error, string]](idIso)(rightPrism)
+
+	t.Run("Composed prism behaves like original prism", func(t *testing.T) {
+		success := E.Right[error]("test")
+
+		// Original prism
+		originalResult := rightPrism.GetOption(success)
+
+		// Composed prism
+		composedResult := composedPrism.GetOption(success)
+
+		assert.Equal(t, originalResult, composedResult)
+	})
+
+	t.Run("ReverseGet produces same result", func(t *testing.T) {
+		value := "test"
+
+		// Original prism
+		originalEither := rightPrism.ReverseGet(value)
+
+		// Composed prism
+		composedEither := composedPrism.ReverseGet(value)
+
+		assert.Equal(t, originalEither, composedEither)
+	})
+}
+
+// TestComposeChaining tests chaining multiple compositions
+func TestComposeChaining(t *testing.T) {
+	// First isomorphism: int to string
+	intStringIso := I.MakeIso(
+		func(i int) string { return strconv.Itoa(i) },
+		func(s string) int {
+			i, _ := strconv.Atoi(s)
+			return i
+		},
+	)
+
+	// Second isomorphism: string to []byte
+	stringBytesIso := I.MakeIso(
+		func(s string) []byte { return []byte(s) },
+		func(b []byte) string { return string(b) },
+	)
+
+	// Prism that extracts Right values
+	rightPrism := P.FromEither[error, int]()
+
+	// Chain compositions: Either[error, int] -> int -> string -> []byte
+	step1 := Compose[E.Either[error, int]](intStringIso)(rightPrism) // Prism[Either[error, int], string]
+	step2 := Compose[E.Either[error, int]](stringBytesIso)(step1)    // Prism[Either[error, int], []byte]
+
+	t.Run("Chained composition extracts and transforms correctly", func(t *testing.T) {
+		either := E.Right[error](42)
+		result := step2.GetOption(either)
+
+		assert.True(t, O.IsSome(result))
+		bytes := O.GetOrElse(F.Constant([]byte{}))(result)
+		assert.Equal(t, []byte("42"), bytes)
+	})
+
+	t.Run("Chained composition ReverseGet works correctly", func(t *testing.T) {
+		bytes := []byte("100")
+		result := step2.ReverseGet(bytes)
+
+		assert.True(t, E.IsRight(result))
+		num := E.GetOrElse(func(error) int { return 0 })(result)
+		assert.Equal(t, 100, num)
+	})
+}
+
+// TestComposePrismLaws verifies that the composed prism satisfies prism laws
+func TestComposePrismLaws(t *testing.T) {
+	// Create an isomorphism
+	iso := I.MakeIso(
+		func(i int) string { return strconv.Itoa(i) },
+		func(s string) int {
+			i, _ := strconv.Atoi(s)
+			return i
+		},
+	)
+
+	// Create a prism
+	prism := P.FromEither[error, int]()
+
+	// Compose them
+	composed := Compose[E.Either[error, int]](iso)(prism)
+
+	t.Run("Law 1: GetOption(ReverseGet(b)) == Some(b)", func(t *testing.T) {
+		value := "42"
+
+		// ReverseGet then GetOption should return Some(value)
+		either := composed.ReverseGet(value)
+		result := composed.GetOption(either)
+
+		assert.True(t, O.IsSome(result))
+		extracted := O.GetOrElse(F.Constant(""))(result)
+		assert.Equal(t, value, extracted)
+	})
+
+	t.Run("Law 2: If GetOption(s) == Some(a), then GetOption(ReverseGet(a)) == Some(a)", func(t *testing.T) {
+		either := E.Right[error](100)
+
+		// First GetOption
+		firstResult := composed.GetOption(either)
+		assert.True(t, O.IsSome(firstResult))
+
+		// Extract the value
+		value := O.GetOrElse(F.Constant(""))(firstResult)
+
+		// ReverseGet then GetOption again
+		reconstructed := composed.ReverseGet(value)
+		secondResult := composed.GetOption(reconstructed)
+
+		assert.True(t, O.IsSome(secondResult))
+		finalValue := O.GetOrElse(F.Constant(""))(secondResult)
+		assert.Equal(t, value, finalValue)
+	})
+}
+
+// TestComposeWithEmptyValues tests edge cases with empty/zero values
+func TestComposeWithEmptyValues(t *testing.T) {
+	// Isomorphism that handles empty strings
+	iso := I.MakeIso(
+		func(s string) []byte { return []byte(s) },
+		func(b []byte) string { return string(b) },
+	)
+
+	prism := P.FromEither[error, string]()
+	composed := Compose[E.Either[error, string]](iso)(prism)
+
+	t.Run("Empty string is handled correctly", func(t *testing.T) {
+		either := E.Right[error]("")
+		result := composed.GetOption(either)
+
+		assert.True(t, O.IsSome(result))
+		bytes := O.GetOrElse(F.Constant([]byte("default")))(result)
+		assert.Equal(t, []byte{}, bytes)
+	})
+
+	t.Run("ReverseGet with empty bytes", func(t *testing.T) {
+		bytes := []byte{}
+		result := composed.ReverseGet(bytes)
+
+		assert.True(t, E.IsRight(result))
+		str := E.GetOrElse(func(error) string { return "default" })(result)
+		assert.Equal(t, "", str)
+	})
+}

v2/optics/prism/iso/types.go

@@ -0,0 +1,112 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package iso provides utilities for composing isomorphisms with prisms.
+//
+// This package enables the composition of isomorphisms (bidirectional transformations)
+// with prisms (optics for sum types), allowing you to transform the focus type of a prism
+// using an isomorphism. This is particularly useful when you need to work with prisms
+// that focus on a type that can be bidirectionally converted to another type.
+//
+// # Key Concepts
+//
+// An Iso[S, A] represents a bidirectional transformation between types S and A without
+// loss of information. A Prism[S, A] is an optic that focuses on a specific variant
+// within a sum type S, extracting values of type A.
+//
+// When you compose an Iso[A, B] with a Prism[S, A], you get a Prism[S, B] that:
+//   - Extracts values of type A from S (using the prism)
+//   - Transforms them to type B (using the isomorphism's Get)
+//   - Can construct S from B by reversing the transformation (using the isomorphism's ReverseGet)
+//
+// # Example Usage
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/iso"
+//	    "github.com/IBM/fp-go/v2/optics/prism"
+//	    PI "github.com/IBM/fp-go/v2/optics/prism/iso"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	)
+//
+//	// Create an isomorphism between string and []byte
+//	stringBytesIso := iso.MakeIso(
+//	    func(s string) []byte { return []byte(s) },
+//	    func(b []byte) string { return string(b) },
+//	)
+//
+//	// Create a prism that extracts Right values from Either[error, string]
+//	rightPrism := prism.FromEither[error, string]()
+//
+//	// Compose them to get a prism that extracts Right values as []byte
+//	bytesPrism := PI.Compose(stringBytesIso)(rightPrism)
+//
+//	// Use the composed prism
+//	either := either.Right[error]("hello")
+//	result := bytesPrism.GetOption(either)  // Some([]byte("hello"))
+//
+// # Type Aliases
+//
+// This package re-exports key types from the iso and prism packages for convenience:
+//   - Iso[S, A]: An isomorphism between types S and A
+//   - Prism[S, A]: A prism focusing on type A within sum type S
+//   - Operator[S, A, B]: A function that transforms Prism[S, A] to Prism[S, B]
+package iso
+
+import (
+	I "github.com/IBM/fp-go/v2/optics/iso"
+	P "github.com/IBM/fp-go/v2/optics/prism"
+)
+
+type (
+	// Iso represents an isomorphism between types S and A.
+	// It is a bidirectional transformation that converts between two types
+	// without any loss of information.
+	//
+	// Type Parameters:
+	//   - S: The source type
+	//   - A: The target type
+	//
+	// See github.com/IBM/fp-go/v2/optics/iso for the full Iso API.
+	Iso[S, A any] = I.Iso[S, A]
+
+	// Prism is an optic used to select part of a sum type (tagged union).
+	// It provides operations to extract and construct values within sum types.
+	//
+	// Type Parameters:
+	//   - S: The source type (sum type)
+	//   - A: The focus type (variant within the sum type)
+	//
+	// See github.com/IBM/fp-go/v2/optics/prism for the full Prism API.
+	Prism[S, A any] = P.Prism[S, A]
+
+	// Operator represents a function that transforms one prism into another.
+	// It takes a Prism[S, A] and returns a Prism[S, B], allowing for prism transformations.
+	//
+	// This is commonly used with the Compose function to create operators that
+	// transform the focus type of a prism using an isomorphism.
+	//
+	// Type Parameters:
+	//   - S: The source type (remains constant)
+	//   - A: The original focus type
+	//   - B: The new focus type
+	//
+	// Example:
+	//
+	//	// Create an operator that transforms string prisms to []byte prisms
+	//	stringToBytesOp := Compose(stringBytesIso)
+	//	// Apply it to a prism
+	//	bytesPrism := stringToBytesOp(stringPrism)
+	Operator[S, A, B any] = P.Operator[S, A, B]
+)

v2/optics/prism/prisms.go

@@ -23,8 +23,10 @@ import (
 	"strconv"
 	"time"
 
+	"github.com/IBM/fp-go/v2/array"
 	"github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
+	J "github.com/IBM/fp-go/v2/json"
 	"github.com/IBM/fp-go/v2/option"
 	S "github.com/IBM/fp-go/v2/string"
 )
@@ -322,6 +324,50 @@ func FromEither[E, T any]() Prism[Either[E, T], T] {
 	return MakePrismWithName(either.ToOption[E, T], either.Of[E, T], "PrismFromEither")
 }
 
+// FromResult creates a prism for extracting values from Result types.
+// It provides a safe way to work with Result values (which are Either[error, T]),
+// focusing on the success case and handling errors gracefully through the Option type.
+//
+// This is a convenience function that is equivalent to FromEither[error, T]().
+//
+// The prism's GetOption attempts to extract the success value from a Result.
+// If the Result is successful, it returns Some(value); if it's an error, it returns None.
+//
+// The prism's ReverseGet always succeeds, wrapping a value into a successful Result.
+//
+// Type Parameters:
+//   - T: The value type contained in the Result
+//
+// Returns:
+//   - A Prism[Result[T], T] that safely extracts success values
+//
+// Example:
+//
+//	// Create a prism for extracting successful results
+//	resultPrism := FromResult[int]()
+//
+//	// Extract from successful result
+//	success := result.Of[int](42)
+//	value := resultPrism.GetOption(success)  // Some(42)
+//
+//	// Extract from error result
+//	failure := result.Error[int](errors.New("failed"))
+//	value = resultPrism.GetOption(failure)  // None[int]()
+//
+//	// Wrap value into successful Result
+//	wrapped := resultPrism.ReverseGet(100)  // Result containing 100
+//
+//	// Use with Set to update successful results
+//	setter := Set[Result[int], int](200)
+//	result := setter(resultPrism)(success)  // Result containing 200
+//	result = setter(resultPrism)(failure)   // Error result (unchanged)
+//
+// Common use cases:
+//   - Extracting successful values from Result types
+//   - Filtering out errors in data pipelines
+//   - Working with fallible operations that return Result
+//   - Composing with other prisms for complex error handling
+//
 //go:inline
 func FromResult[T any]() Prism[Result[T], T] {
 	return FromEither[error, T]()
@@ -1261,3 +1307,71 @@ func MakeURLPrisms() URLPrisms {
 		RawFragment: _prismRawFragment,
 	}
 }
+
+// ParseJSON creates a prism for parsing and marshaling JSON data.
+// It provides a safe way to convert between JSON bytes and Go types,
+// handling parsing and marshaling errors gracefully through the Option type.
+//
+// The prism's GetOption attempts to unmarshal JSON bytes into type A.
+// If unmarshaling succeeds, it returns Some(A); if it fails (e.g., invalid JSON
+// or type mismatch), it returns None.
+//
+// The prism's ReverseGet marshals a value of type A into JSON bytes.
+// If marshaling fails (which is rare), it returns an empty byte slice.
+//
+// Type Parameters:
+//   - A: The Go type to unmarshal JSON into
+//
+// Returns:
+//   - A Prism[[]byte, A] that safely handles JSON parsing/marshaling
+//
+// Example:
+//
+//	// Define a struct type
+//	type Person struct {
+//	    Name string `json:"name"`
+//	    Age  int    `json:"age"`
+//	}
+//
+//	// Create a JSON parsing prism
+//	jsonPrism := ParseJSON[Person]()
+//
+//	// Parse valid JSON
+//	jsonData := []byte(`{"name":"Alice","age":30}`)
+//	person := jsonPrism.GetOption(jsonData)
+//	// Some(Person{Name: "Alice", Age: 30})
+//
+//	// Parse invalid JSON
+//	invalidJSON := []byte(`{invalid json}`)
+//	result := jsonPrism.GetOption(invalidJSON)  // None[Person]()
+//
+//	// Marshal to JSON
+//	p := Person{Name: "Bob", Age: 25}
+//	jsonBytes := jsonPrism.ReverseGet(p)
+//	// []byte(`{"name":"Bob","age":25}`)
+//
+//	// Use with Set to update JSON data
+//	newPerson := Person{Name: "Charlie", Age: 35}
+//	setter := Set[[]byte, Person](newPerson)
+//	updated := setter(jsonPrism)(jsonData)
+//	// []byte(`{"name":"Charlie","age":35}`)
+//
+// Common use cases:
+//   - Parsing JSON configuration files
+//   - Working with JSON API responses
+//   - Validating and transforming JSON data in pipelines
+//   - Type-safe JSON deserialization
+//   - Converting between JSON and Go structs
+func ParseJSON[A any]() Prism[[]byte, A] {
+	return MakePrismWithName(
+		F.Flow2(
+			J.Unmarshal[A],
+			either.ToOption[error, A],
+		),
+		F.Flow2(
+			J.Marshal[A],
+			either.GetOrElse(F.Constant1[error](array.Empty[byte]())),
+		),
+		"JSON",
+	)
+}

v2/optics/prism/prisms_test.go

@@ -16,11 +16,14 @@
 package prism
 
 import (
+	"errors"
 	"regexp"
 	"testing"
 
+	E "github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
 	O "github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/result"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -1396,3 +1399,403 @@ func TestNonEmptyStringValidation(t *testing.T) {
 		assert.Equal(t, []string{"hello", "world", "test"}, nonEmpty)
 	})
 }
+
+// TestFromResult tests the FromResult prism with Result types
+func TestFromResult(t *testing.T) {
+	t.Run("extract from successful result", func(t *testing.T) {
+		prism := FromResult[int]()
+
+		success := result.Of[int](42)
+		extracted := prism.GetOption(success)
+
+		assert.True(t, O.IsSome(extracted))
+		assert.Equal(t, 42, O.GetOrElse(F.Constant(-1))(extracted))
+	})
+
+	t.Run("extract from error result", func(t *testing.T) {
+		prism := FromResult[int]()
+
+		failure := E.Left[int](errors.New("test error"))
+		extracted := prism.GetOption(failure)
+
+		assert.True(t, O.IsNone(extracted))
+	})
+
+	t.Run("ReverseGet wraps value in successful result", func(t *testing.T) {
+		prism := FromResult[int]()
+
+		wrapped := prism.ReverseGet(100)
+
+		// Verify it's a successful result
+		extracted := prism.GetOption(wrapped)
+		assert.True(t, O.IsSome(extracted))
+		assert.Equal(t, 100, O.GetOrElse(F.Constant(-1))(extracted))
+	})
+
+	t.Run("works with string type", func(t *testing.T) {
+		prism := FromResult[string]()
+
+		success := result.Of[string]("hello")
+		extracted := prism.GetOption(success)
+
+		assert.True(t, O.IsSome(extracted))
+		assert.Equal(t, "hello", O.GetOrElse(F.Constant(""))(extracted))
+	})
+
+	t.Run("works with struct type", func(t *testing.T) {
+		type Person struct {
+			Name string
+			Age  int
+		}
+
+		prism := FromResult[Person]()
+
+		person := Person{Name: "Alice", Age: 30}
+		success := result.Of[Person](person)
+		extracted := prism.GetOption(success)
+
+		assert.True(t, O.IsSome(extracted))
+		result := O.GetOrElse(F.Constant(Person{}))(extracted)
+		assert.Equal(t, "Alice", result.Name)
+		assert.Equal(t, 30, result.Age)
+	})
+}
+
+// TestFromResultWithSet tests using Set with FromResult prism
+func TestFromResultWithSet(t *testing.T) {
+	t.Run("set on successful result", func(t *testing.T) {
+		prism := FromResult[int]()
+		setter := Set[result.Result[int], int](200)
+
+		success := result.Of[int](42)
+		updated := setter(prism)(success)
+
+		// Verify the value was updated
+		extracted := prism.GetOption(updated)
+		assert.True(t, O.IsSome(extracted))
+		assert.Equal(t, 200, O.GetOrElse(F.Constant(-1))(extracted))
+	})
+
+	t.Run("set on error result leaves it unchanged", func(t *testing.T) {
+		prism := FromResult[int]()
+		setter := Set[result.Result[int], int](200)
+
+		failure := E.Left[int](errors.New("test error"))
+		updated := setter(prism)(failure)
+
+		// Verify it's still an error
+		extracted := prism.GetOption(updated)
+		assert.True(t, O.IsNone(extracted))
+	})
+}
+
+// TestFromResultPrismLaws tests that FromResult satisfies prism laws
+func TestFromResultPrismLaws(t *testing.T) {
+	prism := FromResult[int]()
+
+	t.Run("law 1: GetOption(ReverseGet(a)) == Some(a)", func(t *testing.T) {
+		value := 42
+		wrapped := prism.ReverseGet(value)
+		extracted := prism.GetOption(wrapped)
+
+		assert.True(t, O.IsSome(extracted))
+		assert.Equal(t, value, O.GetOrElse(F.Constant(-1))(extracted))
+	})
+
+	t.Run("law 2: ReverseGet is consistent", func(t *testing.T) {
+		value := 42
+		result1 := prism.ReverseGet(value)
+		result2 := prism.ReverseGet(value)
+
+		// Both should extract the same value
+		extracted1 := prism.GetOption(result1)
+		extracted2 := prism.GetOption(result2)
+
+		val1 := O.GetOrElse(F.Constant(-1))(extracted1)
+		val2 := O.GetOrElse(F.Constant(-1))(extracted2)
+		assert.Equal(t, val1, val2)
+	})
+}
+
+// TestFromResultComposition tests composing FromResult with other prisms
+func TestFromResultComposition(t *testing.T) {
+	t.Run("compose with predicate prism", func(t *testing.T) {
+		// Create a prism that only matches positive numbers
+		positivePrism := FromPredicate(func(n int) bool { return n > 0 })
+
+		// Compose: Result[int] -> int -> positive int
+		composed := Compose[result.Result[int]](positivePrism)(FromResult[int]())
+
+		// Test with positive number
+		success := result.Of[int](42)
+		extracted := composed.GetOption(success)
+		assert.True(t, O.IsSome(extracted))
+		assert.Equal(t, 42, O.GetOrElse(F.Constant(-1))(extracted))
+
+		// Test with negative number
+		negativeSuccess := result.Of[int](-5)
+		extracted = composed.GetOption(negativeSuccess)
+		assert.True(t, O.IsNone(extracted))
+
+		// Test with error
+		failure := E.Left[int](errors.New("test error"))
+		extracted = composed.GetOption(failure)
+		assert.True(t, O.IsNone(extracted))
+	})
+}
+
+// TestParseJSON tests the ParseJSON prism with various JSON data
+func TestParseJSON(t *testing.T) {
+	type Person struct {
+		Name string `json:"name"`
+		Age  int    `json:"age"`
+	}
+
+	t.Run("parse valid JSON", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		jsonData := []byte(`{"name":"Alice","age":30}`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, "Alice", person.Name)
+		assert.Equal(t, 30, person.Age)
+	})
+
+	t.Run("parse invalid JSON", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		invalidJSON := []byte(`{invalid json}`)
+		parsed := prism.GetOption(invalidJSON)
+
+		assert.True(t, O.IsNone(parsed))
+	})
+
+	t.Run("parse JSON with missing fields", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		// Missing age field - should use zero value
+		jsonData := []byte(`{"name":"Bob"}`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, "Bob", person.Name)
+		assert.Equal(t, 0, person.Age)
+	})
+
+	t.Run("parse JSON with extra fields", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		// Extra field should be ignored
+		jsonData := []byte(`{"name":"Charlie","age":25,"extra":"ignored"}`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, "Charlie", person.Name)
+		assert.Equal(t, 25, person.Age)
+	})
+
+	t.Run("ReverseGet marshals to JSON", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		person := Person{Name: "David", Age: 35}
+		jsonBytes := prism.ReverseGet(person)
+
+		// Parse it back to verify
+		parsed := prism.GetOption(jsonBytes)
+		assert.True(t, O.IsSome(parsed))
+		result := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, "David", result.Name)
+		assert.Equal(t, 35, result.Age)
+	})
+
+	t.Run("works with primitive types", func(t *testing.T) {
+		prism := ParseJSON[int]()
+
+		jsonData := []byte(`42`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		assert.Equal(t, 42, O.GetOrElse(F.Constant(-1))(parsed))
+	})
+
+	t.Run("works with arrays", func(t *testing.T) {
+		prism := ParseJSON[[]string]()
+
+		jsonData := []byte(`["hello","world","test"]`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		arr := O.GetOrElse(F.Constant([]string{}))(parsed)
+		assert.Equal(t, []string{"hello", "world", "test"}, arr)
+	})
+
+	t.Run("works with maps", func(t *testing.T) {
+		prism := ParseJSON[map[string]int]()
+
+		jsonData := []byte(`{"a":1,"b":2,"c":3}`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		m := O.GetOrElse(F.Constant(map[string]int{}))(parsed)
+		assert.Equal(t, 1, m["a"])
+		assert.Equal(t, 2, m["b"])
+		assert.Equal(t, 3, m["c"])
+	})
+
+	t.Run("works with nested structures", func(t *testing.T) {
+		type Address struct {
+			Street string `json:"street"`
+			City   string `json:"city"`
+		}
+		type PersonWithAddress struct {
+			Name    string  `json:"name"`
+			Address Address `json:"address"`
+		}
+
+		prism := ParseJSON[PersonWithAddress]()
+
+		jsonData := []byte(`{"name":"Eve","address":{"street":"123 Main St","city":"NYC"}}`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(PersonWithAddress{}))(parsed)
+		assert.Equal(t, "Eve", person.Name)
+		assert.Equal(t, "123 Main St", person.Address.Street)
+		assert.Equal(t, "NYC", person.Address.City)
+	})
+
+	t.Run("parse empty JSON object", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		jsonData := []byte(`{}`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, "", person.Name)
+		assert.Equal(t, 0, person.Age)
+	})
+
+	t.Run("parse null JSON", func(t *testing.T) {
+		prism := ParseJSON[*Person]()
+
+		jsonData := []byte(`null`)
+		parsed := prism.GetOption(jsonData)
+
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(&Person{}))(parsed)
+		assert.Nil(t, person)
+	})
+}
+
+// TestParseJSONWithSet tests using Set with ParseJSON prism
+func TestParseJSONWithSet(t *testing.T) {
+	type Person struct {
+		Name string `json:"name"`
+		Age  int    `json:"age"`
+	}
+
+	t.Run("set updates JSON data", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		originalJSON := []byte(`{"name":"Alice","age":30}`)
+		newPerson := Person{Name: "Bob", Age: 25}
+
+		setter := Set[[]byte, Person](newPerson)
+		updatedJSON := setter(prism)(originalJSON)
+
+		// Parse the updated JSON
+		parsed := prism.GetOption(updatedJSON)
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, "Bob", person.Name)
+		assert.Equal(t, 25, person.Age)
+	})
+
+	t.Run("set on invalid JSON returns original unchanged", func(t *testing.T) {
+		prism := ParseJSON[Person]()
+
+		invalidJSON := []byte(`{invalid}`)
+		newPerson := Person{Name: "Charlie", Age: 35}
+
+		setter := Set[[]byte, Person](newPerson)
+		result := setter(prism)(invalidJSON)
+
+		// Should return original unchanged since it couldn't be parsed
+		assert.Equal(t, invalidJSON, result)
+	})
+}
+
+// TestParseJSONPrismLaws tests that ParseJSON satisfies prism laws
+func TestParseJSONPrismLaws(t *testing.T) {
+	type Person struct {
+		Name string `json:"name"`
+		Age  int    `json:"age"`
+	}
+
+	prism := ParseJSON[Person]()
+
+	t.Run("law 1: GetOption(ReverseGet(a)) == Some(a)", func(t *testing.T) {
+		person := Person{Name: "Alice", Age: 30}
+		jsonBytes := prism.ReverseGet(person)
+		parsed := prism.GetOption(jsonBytes)
+
+		assert.True(t, O.IsSome(parsed))
+		result := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, person.Name, result.Name)
+		assert.Equal(t, person.Age, result.Age)
+	})
+
+	t.Run("law 2: ReverseGet is consistent", func(t *testing.T) {
+		person := Person{Name: "Bob", Age: 25}
+		json1 := prism.ReverseGet(person)
+		json2 := prism.ReverseGet(person)
+
+		// Both should parse to the same value
+		parsed1 := prism.GetOption(json1)
+		parsed2 := prism.GetOption(json2)
+
+		result1 := O.GetOrElse(F.Constant(Person{}))(parsed1)
+		result2 := O.GetOrElse(F.Constant(Person{}))(parsed2)
+
+		assert.Equal(t, result1.Name, result2.Name)
+		assert.Equal(t, result1.Age, result2.Age)
+	})
+}
+
+// TestParseJSONComposition tests composing ParseJSON with other prisms
+func TestParseJSONComposition(t *testing.T) {
+	type Person struct {
+		Name string `json:"name"`
+		Age  int    `json:"age"`
+	}
+
+	t.Run("compose with predicate prism", func(t *testing.T) {
+		// Create a prism that only matches adults (age >= 18)
+		adultPrism := FromPredicate(func(p Person) bool { return p.Age >= 18 })
+
+		// Compose: []byte -> Person -> Adult
+		composed := Compose[[]byte](adultPrism)(ParseJSON[Person]())
+
+		// Test with adult
+		adultJSON := []byte(`{"name":"Alice","age":30}`)
+		parsed := composed.GetOption(adultJSON)
+		assert.True(t, O.IsSome(parsed))
+		person := O.GetOrElse(F.Constant(Person{}))(parsed)
+		assert.Equal(t, "Alice", person.Name)
+
+		// Test with minor
+		minorJSON := []byte(`{"name":"Bob","age":15}`)
+		parsed = composed.GetOption(minorJSON)
+		assert.True(t, O.IsNone(parsed))
+
+		// Test with invalid JSON
+		invalidJSON := []byte(`{invalid}`)
+		parsed = composed.GetOption(invalidJSON)
+		assert.True(t, O.IsNone(parsed))
+	})
+}

v2/readerio/consumer.go

@@ -3,7 +3,7 @@ package readerio
 import "github.com/IBM/fp-go/v2/io"
 
 //go:inline
-func ChainConsumer[R, A any](c Consumer[A]) Operator[R, A, struct{}] {
+func ChainConsumer[R, A any](c Consumer[A]) Operator[R, A, Void] {
 	return ChainIOK[R](io.FromConsumer(c))
 }
 

v2/readerio/types.go

@@ -18,6 +18,7 @@ package readerio
 import (
 	"github.com/IBM/fp-go/v2/consumer"
 	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/predicate"
 	"github.com/IBM/fp-go/v2/reader"
@@ -66,4 +67,6 @@ type (
 	// Predicate represents a function that tests a value of type A and returns a boolean.
 	// It's commonly used for filtering and conditional operations.
 	Predicate[A any] = predicate.Predicate[A]
+
+	Void = function.Void
 )

v2/readerioeither/reader.go

@@ -1104,6 +1104,8 @@ func After[R, E, A any](timestamp time.Time) Operator[R, E, A, A] {
 // If the ReaderIOEither is Left, it applies the provided function to the error value,
 // which returns a new ReaderIOEither that replaces the original.
 //
+// Note: OrElse is identical to [ChainLeft] - both provide the same functionality for error recovery.
+//
 // This is useful for error recovery, fallback logic, or chaining alternative IO computations
 // that need access to configuration or dependencies. The error type can be widened from E1 to E2.
 //

v2/readerioeither/reader_test.go

@@ -531,3 +531,205 @@ func TestReadIO(t *testing.T) {
 		assert.Equal(t, E.Right[error](25), result)
 	})
 }
+
+// TestChainLeftIdenticalToOrElse proves that ChainLeft and OrElse are identical functions.
+// This test verifies that both functions produce the same results for all scenarios with reader context:
+// - Left values with error recovery using reader context
+// - Left values with error transformation
+// - Right values passing through unchanged
+// - Error type widening
+func TestChainLeftIdenticalToOrElse(t *testing.T) {
+	type Config struct {
+		fallbackValue int
+		retryEnabled  bool
+	}
+
+	// Test 1: Left value with error recovery using reader context
+	t.Run("Left value recovery with reader - ChainLeft equals OrElse", func(t *testing.T) {
+		recoveryFn := func(e string) ReaderIOEither[Config, string, int] {
+			if e == "recoverable" {
+				return func(cfg Config) IOE.IOEither[string, int] {
+					return IOE.Right[string](cfg.fallbackValue)
+				}
+			}
+			return Left[Config, int](e)
+		}
+
+		input := Left[Config, int]("recoverable")
+		cfg := Config{fallbackValue: 42, retryEnabled: true}
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(recoveryFn)(input)(cfg)()
+
+		// Using OrElse
+		resultOrElse := OrElse(recoveryFn)(input)(cfg)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](42), resultChainLeft)
+	})
+
+	// Test 2: Left value with error transformation
+	t.Run("Left value transformation - ChainLeft equals OrElse", func(t *testing.T) {
+		transformFn := func(e string) ReaderIOEither[Config, string, int] {
+			return Left[Config, int]("transformed: " + e)
+		}
+
+		input := Left[Config, int]("original error")
+		cfg := Config{fallbackValue: 0, retryEnabled: false}
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(transformFn)(input)(cfg)()
+
+		// Using OrElse
+		resultOrElse := OrElse(transformFn)(input)(cfg)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Left[int]("transformed: original error"), resultChainLeft)
+	})
+
+	// Test 3: Right value - both should pass through unchanged
+	t.Run("Right value passthrough - ChainLeft equals OrElse", func(t *testing.T) {
+		handlerFn := func(e string) ReaderIOEither[Config, string, int] {
+			return Left[Config, int]("should not be called")
+		}
+
+		input := Right[Config, string](100)
+		cfg := Config{fallbackValue: 0, retryEnabled: false}
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(handlerFn)(input)(cfg)()
+
+		// Using OrElse
+		resultOrElse := OrElse(handlerFn)(input)(cfg)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](100), resultChainLeft)
+	})
+
+	// Test 4: Error type widening
+	t.Run("Error type widening - ChainLeft equals OrElse", func(t *testing.T) {
+		widenFn := func(e string) ReaderIOEither[Config, int, int] {
+			return Left[Config, int](404)
+		}
+
+		input := Left[Config, int]("not found")
+		cfg := Config{fallbackValue: 0, retryEnabled: false}
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(widenFn)(input)(cfg)()
+
+		// Using OrElse
+		resultOrElse := OrElse(widenFn)(input)(cfg)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Left[int](404), resultChainLeft)
+	})
+
+	// Test 5: Composition in pipeline with reader context
+	t.Run("Pipeline composition with reader - ChainLeft equals OrElse", func(t *testing.T) {
+		recoveryFn := func(e string) ReaderIOEither[Config, string, int] {
+			if e == "network error" {
+				return func(cfg Config) IOE.IOEither[string, int] {
+					if cfg.retryEnabled {
+						return IOE.Right[string](cfg.fallbackValue)
+					}
+					return IOE.Left[int]("retry disabled")
+				}
+			}
+			return Left[Config, int](e)
+		}
+
+		input := Left[Config, int]("network error")
+		cfg := Config{fallbackValue: 99, retryEnabled: true}
+
+		// Using ChainLeft in pipeline
+		resultChainLeft := F.Pipe1(input, ChainLeft(recoveryFn))(cfg)()
+
+		// Using OrElse in pipeline
+		resultOrElse := F.Pipe1(input, OrElse(recoveryFn))(cfg)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](99), resultChainLeft)
+	})
+
+	// Test 6: Multiple chained operations with reader context
+	t.Run("Multiple operations with reader - ChainLeft equals OrElse", func(t *testing.T) {
+		handler1 := func(e string) ReaderIOEither[Config, string, int] {
+			if e == "error1" {
+				return Right[Config, string](1)
+			}
+			return Left[Config, int](e)
+		}
+
+		handler2 := func(e string) ReaderIOEither[Config, string, int] {
+			if e == "error2" {
+				return func(cfg Config) IOE.IOEither[string, int] {
+					return IOE.Right[string](cfg.fallbackValue)
+				}
+			}
+			return Left[Config, int](e)
+		}
+
+		input := Left[Config, int]("error2")
+		cfg := Config{fallbackValue: 2, retryEnabled: false}
+
+		// Using ChainLeft
+		resultChainLeft := F.Pipe2(
+			input,
+			ChainLeft(handler1),
+			ChainLeft(handler2),
+		)(cfg)()
+
+		// Using OrElse
+		resultOrElse := F.Pipe2(
+			input,
+			OrElse(handler1),
+			OrElse(handler2),
+		)(cfg)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](2), resultChainLeft)
+	})
+
+	// Test 7: Reader context is properly threaded through both functions
+	t.Run("Reader context threading - ChainLeft equals OrElse", func(t *testing.T) {
+		var chainLeftCfg, orElseCfg *Config
+
+		recoveryFn := func(e string) ReaderIOEither[Config, string, int] {
+			return func(cfg Config) IOE.IOEither[string, int] {
+				// Capture the config to verify it's passed correctly
+				if chainLeftCfg == nil {
+					chainLeftCfg = &cfg
+				} else {
+					orElseCfg = &cfg
+				}
+				return IOE.Right[string](cfg.fallbackValue)
+			}
+		}
+
+		input := Left[Config, int]("error")
+		cfg := Config{fallbackValue: 123, retryEnabled: true}
+
+		// Using ChainLeft
+		resultChainLeft := ChainLeft(recoveryFn)(input)(cfg)()
+
+		// Using OrElse
+		resultOrElse := OrElse(recoveryFn)(input)(cfg)()
+
+		// Both should produce identical results
+		assert.Equal(t, resultOrElse, resultChainLeft)
+		assert.Equal(t, E.Right[string](123), resultChainLeft)
+
+		// Verify both received the same config
+		assert.NotNil(t, chainLeftCfg)
+		assert.NotNil(t, orElseCfg)
+		assert.Equal(t, *chainLeftCfg, *orElseCfg)
+		assert.Equal(t, cfg, *chainLeftCfg)
+	})
+}

v2/readerioresult/consumer.go

@@ -1,14 +1,107 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package readerioresult
 
 import (
 	"github.com/IBM/fp-go/v2/readerioeither"
 )
 
+// ChainConsumer chains a consumer (side-effect function) into a ReaderIOResult computation,
+// replacing the success value with Void (empty struct).
+//
+// This is useful for performing side effects (like logging, printing, or writing to a file)
+// where you don't need to preserve the original value. The consumer is only executed if the
+// computation succeeds; if it fails with an error, the consumer is skipped.
+//
+// Type parameters:
+//   - R: The context/environment type
+//   - A: The value type to consume
+//
+// Parameters:
+//   - c: A consumer function that performs a side effect on the value
+//
+// Returns:
+//
+//	An Operator that executes the consumer and returns Void on success
+//
+// Example:
+//
+//	import (
+//	    "context"
+//	    "fmt"
+//	    RIO "github.com/IBM/fp-go/v2/readerioresult"
+//	)
+//
+//	// Log a value and discard it
+//	logValue := RIO.ChainConsumer[context.Context](func(x int) {
+//	    fmt.Printf("Value: %d\n", x)
+//	})
+//
+//	computation := F.Pipe1(
+//	    RIO.Of[context.Context](42),
+//	    logValue,
+//	)
+//	// Prints "Value: 42" and returns result.Of(struct{}{})
+//
 //go:inline
-func ChainConsumer[R, A any](c Consumer[A]) Operator[R, A, struct{}] {
+func ChainConsumer[R, A any](c Consumer[A]) Operator[R, A, Void] {
 	return readerioeither.ChainConsumer[R, error](c)
 }
 
+// ChainFirstConsumer chains a consumer into a ReaderIOResult computation while preserving
+// the original value.
+//
+// This is useful for performing side effects (like logging, printing, or metrics collection)
+// where you want to keep the original value for further processing. The consumer is only
+// executed if the computation succeeds; if it fails with an error, the consumer is skipped
+// and the error is propagated.
+//
+// Type parameters:
+//   - R: The context/environment type
+//   - A: The value type to consume and preserve
+//
+// Parameters:
+//   - c: A consumer function that performs a side effect on the value
+//
+// Returns:
+//
+//	An Operator that executes the consumer and returns the original value on success
+//
+// Example:
+//
+//	import (
+//	    "context"
+//	    "fmt"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    N "github.com/IBM/fp-go/v2/number"
+//	    RIO "github.com/IBM/fp-go/v2/readerioresult"
+//	)
+//
+//	// Log a value but keep it for further processing
+//	logValue := RIO.ChainFirstConsumer[context.Context](func(x int) {
+//	    fmt.Printf("Processing: %d\n", x)
+//	})
+//
+//	computation := F.Pipe2(
+//	    RIO.Of[context.Context](10),
+//	    logValue,
+//	    RIO.Map[context.Context](N.Mul(2)),
+//	)
+//	// Prints "Processing: 10" and returns result.Of(20)
+//
 //go:inline
 func ChainFirstConsumer[R, A any](c Consumer[A]) Operator[R, A, A] {
 	return readerioeither.ChainFirstConsumer[R, error](c)

v2/readerioresult/consumer_test.go

@@ -0,0 +1,360 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package readerioresult
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	F "github.com/IBM/fp-go/v2/function"
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/result"
+	S "github.com/IBM/fp-go/v2/string"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestChainConsumer_Success tests that ChainConsumer executes the consumer
+// and returns Void when the computation succeeds
+func TestChainConsumer_Success(t *testing.T) {
+	// Track if consumer was called
+	var consumed int
+	consumer := func(x int) {
+		consumed = x
+	}
+
+	// Create a successful computation and chain the consumer
+	computation := F.Pipe1(
+		Of[context.Context](42),
+		ChainConsumer[context.Context](consumer),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer was called with correct value
+	assert.Equal(t, 42, consumed)
+
+	// Verify result is successful with Void
+	assert.True(t, result.IsRight(res))
+	if result.IsRight(res) {
+		val := result.GetOrElse(func(error) Void { return Void{} })(res)
+		assert.Equal(t, Void{}, val)
+	}
+}
+
+// TestChainConsumer_Failure tests that ChainConsumer does not execute
+// the consumer when the computation fails
+func TestChainConsumer_Failure(t *testing.T) {
+	// Track if consumer was called
+	consumerCalled := false
+	consumer := func(x int) {
+		consumerCalled = true
+	}
+
+	// Create a failing computation
+	expectedErr := errors.New("test error")
+	computation := F.Pipe1(
+		Left[context.Context, int](expectedErr),
+		ChainConsumer[context.Context](consumer),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer was NOT called
+	assert.False(t, consumerCalled)
+
+	// Verify result is an error
+	assert.True(t, result.IsLeft(res))
+}
+
+// TestChainConsumer_MultipleOperations tests chaining multiple operations
+// with ChainConsumer in a pipeline
+func TestChainConsumer_MultipleOperations(t *testing.T) {
+	// Track consumer calls
+	var values []int
+	consumer := func(x int) {
+		values = append(values, x)
+	}
+
+	// Create a pipeline with multiple operations
+	computation := F.Pipe2(
+		Of[context.Context](10),
+		Map[context.Context](N.Mul(2)),
+		ChainConsumer[context.Context](consumer),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer was called with transformed value
+	assert.Equal(t, []int{20}, values)
+
+	// Verify result is successful
+	assert.True(t, result.IsRight(res))
+}
+
+// TestChainFirstConsumer_Success tests that ChainFirstConsumer executes
+// the consumer and preserves the original value
+func TestChainFirstConsumer_Success(t *testing.T) {
+	// Track if consumer was called
+	var consumed int
+	consumer := func(x int) {
+		consumed = x
+	}
+
+	// Create a successful computation and chain the consumer
+	computation := F.Pipe1(
+		Of[context.Context](42),
+		ChainFirstConsumer[context.Context](consumer),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer was called with correct value
+	assert.Equal(t, 42, consumed)
+
+	// Verify result is successful and preserves original value
+	assert.True(t, result.IsRight(res))
+	if result.IsRight(res) {
+		val := result.GetOrElse(func(error) int { return 0 })(res)
+		assert.Equal(t, 42, val)
+	}
+}
+
+// TestChainFirstConsumer_Failure tests that ChainFirstConsumer does not
+// execute the consumer when the computation fails
+func TestChainFirstConsumer_Failure(t *testing.T) {
+	// Track if consumer was called
+	consumerCalled := false
+	consumer := func(x int) {
+		consumerCalled = true
+	}
+
+	// Create a failing computation
+	expectedErr := errors.New("test error")
+	computation := F.Pipe1(
+		Left[context.Context, int](expectedErr),
+		ChainFirstConsumer[context.Context](consumer),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer was NOT called
+	assert.False(t, consumerCalled)
+
+	// Verify result is an error
+	assert.True(t, result.IsLeft(res))
+}
+
+// TestChainFirstConsumer_PreservesValue tests that ChainFirstConsumer
+// preserves the value for further processing
+func TestChainFirstConsumer_PreservesValue(t *testing.T) {
+	// Track consumer calls
+	var logged []int
+	logger := func(x int) {
+		logged = append(logged, x)
+	}
+
+	// Create a pipeline that logs intermediate values
+	computation := F.Pipe3(
+		Of[context.Context](10),
+		ChainFirstConsumer[context.Context](logger),
+		Map[context.Context](N.Mul(2)),
+		ChainFirstConsumer[context.Context](logger),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer was called at each step
+	assert.Equal(t, []int{10, 20}, logged)
+
+	// Verify final result
+	assert.True(t, result.IsRight(res))
+	if result.IsRight(res) {
+		val := result.GetOrElse(func(error) int { return 0 })(res)
+		assert.Equal(t, 20, val)
+	}
+}
+
+// TestChainFirstConsumer_WithMap tests combining ChainFirstConsumer with Map
+func TestChainFirstConsumer_WithMap(t *testing.T) {
+	// Track intermediate values
+	var intermediate int
+	consumer := func(x int) {
+		intermediate = x
+	}
+
+	// Create a pipeline with logging and transformation
+	computation := F.Pipe2(
+		Of[context.Context](5),
+		ChainFirstConsumer[context.Context](consumer),
+		Map[context.Context](N.Mul(3)),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer saw original value
+	assert.Equal(t, 5, intermediate)
+
+	// Verify final result is transformed
+	assert.True(t, result.IsRight(res))
+	if result.IsRight(res) {
+		val := result.GetOrElse(func(error) int { return 0 })(res)
+		assert.Equal(t, 15, val)
+	}
+}
+
+// TestChainConsumer_WithContext tests that consumers work with context
+func TestChainConsumer_WithContext(t *testing.T) {
+	type Config struct {
+		Multiplier int
+	}
+
+	// Track consumer calls
+	var consumed int
+	consumer := func(x int) {
+		consumed = x
+	}
+
+	// Create a computation that uses context
+	computation := F.Pipe2(
+		Of[Config](10),
+		Map[Config](N.Mul(2)),
+		ChainConsumer[Config](consumer),
+	)
+
+	// Execute with context
+	cfg := Config{Multiplier: 3}
+	res := computation(cfg)()
+
+	// Verify consumer was called
+	assert.Equal(t, 20, consumed)
+
+	// Verify result is successful
+	assert.True(t, result.IsRight(res))
+}
+
+// TestChainFirstConsumer_SideEffects tests that ChainFirstConsumer
+// can be used for side effects like logging
+func TestChainFirstConsumer_SideEffects(t *testing.T) {
+	// Simulate a logging side effect
+	var logs []string
+	logValue := func(x string) {
+		logs = append(logs, "Processing: "+x)
+	}
+
+	// Create a pipeline with logging
+	computation := F.Pipe3(
+		Of[context.Context]("hello"),
+		ChainFirstConsumer[context.Context](logValue),
+		Map[context.Context](S.Append(" world")),
+		ChainFirstConsumer[context.Context](logValue),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify logs were created
+	assert.Equal(t, []string{
+		"Processing: hello",
+		"Processing: hello world",
+	}, logs)
+
+	// Verify final result
+	assert.True(t, result.IsRight(res))
+	if result.IsRight(res) {
+		val := result.GetOrElse(func(error) string { return "" })(res)
+		assert.Equal(t, "hello world", val)
+	}
+}
+
+// TestChainConsumer_ComplexType tests consumers with complex types
+func TestChainConsumer_ComplexType(t *testing.T) {
+	type User struct {
+		Name string
+		Age  int
+	}
+
+	// Track consumed user
+	var consumedUser *User
+	consumer := func(u User) {
+		consumedUser = &u
+	}
+
+	// Create a computation with a complex type
+	user := User{Name: "Alice", Age: 30}
+	computation := F.Pipe1(
+		Of[context.Context](user),
+		ChainConsumer[context.Context](consumer),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer received the user
+	assert.NotNil(t, consumedUser)
+	assert.Equal(t, "Alice", consumedUser.Name)
+	assert.Equal(t, 30, consumedUser.Age)
+
+	// Verify result is successful
+	assert.True(t, result.IsRight(res))
+}
+
+// TestChainFirstConsumer_ComplexType tests ChainFirstConsumer with complex types
+func TestChainFirstConsumer_ComplexType(t *testing.T) {
+	type Product struct {
+		ID    int
+		Name  string
+		Price float64
+	}
+
+	// Track consumed products
+	var consumedProducts []Product
+	consumer := func(p Product) {
+		consumedProducts = append(consumedProducts, p)
+	}
+
+	// Create a pipeline with complex type
+	product := Product{ID: 1, Name: "Widget", Price: 9.99}
+	computation := F.Pipe2(
+		Of[context.Context](product),
+		ChainFirstConsumer[context.Context](consumer),
+		Map[context.Context](func(p Product) Product {
+			p.Price = p.Price * 1.1 // Apply 10% markup
+			return p
+		}),
+	)
+
+	// Execute the computation
+	res := computation(context.Background())()
+
+	// Verify consumer saw original product
+	assert.Len(t, consumedProducts, 1)
+	assert.Equal(t, 9.99, consumedProducts[0].Price)
+
+	// Verify final result has updated price
+	assert.True(t, result.IsRight(res))
+	if result.IsRight(res) {
+		finalProduct := result.GetOrElse(func(error) Product { return Product{} })(res)
+		assert.InDelta(t, 10.989, finalProduct.Price, 0.001)
+	}
+}

v2/readerioresult/reader.go

@@ -25,10 +25,11 @@ import (
 	"github.com/IBM/fp-go/v2/readerio"
 	RIOE "github.com/IBM/fp-go/v2/readerioeither"
 	"github.com/IBM/fp-go/v2/readeroption"
+	"github.com/IBM/fp-go/v2/result"
 )
 
 //go:inline
-func FromReaderOption[R, A any](onNone func() error) Kleisli[R, ReaderOption[R, A], A] {
+func FromReaderOption[R, A any](onNone Lazy[error]) Kleisli[R, ReaderOption[R, A], A] {
 	return RIOE.FromReaderOption[R, A](onNone)
 }
 
@@ -113,7 +114,7 @@ func MonadTap[R, A, B any](fa ReaderIOResult[R, A], f Kleisli[R, A, B]) ReaderIO
 // The Either is automatically lifted into the ReaderIOResult context.
 //
 //go:inline
-func MonadChainEitherK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]) ReaderIOResult[R, B] {
+func MonadChainEitherK[R, A, B any](ma ReaderIOResult[R, A], f result.Kleisli[A, B]) ReaderIOResult[R, B] {
 	return RIOE.MonadChainEitherK(ma, f)
 }
 
@@ -121,7 +122,7 @@ func MonadChainEitherK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]
 // The Either is automatically lifted into the ReaderIOResult context.
 //
 //go:inline
-func MonadChainResultK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]) ReaderIOResult[R, B] {
+func MonadChainResultK[R, A, B any](ma ReaderIOResult[R, A], f result.Kleisli[A, B]) ReaderIOResult[R, B] {
 	return RIOE.MonadChainEitherK(ma, f)
 }
 
@@ -129,7 +130,7 @@ func MonadChainResultK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]
 // This is the curried version of MonadChainEitherK.
 //
 //go:inline
-func ChainEitherK[R, A, B any](f func(A) Result[B]) Operator[R, A, B] {
+func ChainEitherK[R, A, B any](f result.Kleisli[A, B]) Operator[R, A, B] {
 	return RIOE.ChainEitherK[R](f)
 }
 
@@ -137,7 +138,7 @@ func ChainEitherK[R, A, B any](f func(A) Result[B]) Operator[R, A, B] {
 // This is the curried version of MonadChainEitherK.
 //
 //go:inline
-func ChainResultK[R, A, B any](f func(A) Result[B]) Operator[R, A, B] {
+func ChainResultK[R, A, B any](f result.Kleisli[A, B]) Operator[R, A, B] {
 	return RIOE.ChainEitherK[R](f)
 }
 
@@ -145,12 +146,12 @@ func ChainResultK[R, A, B any](f func(A) Result[B]) Operator[R, A, B] {
 // Useful for validation or side effects that return Either.
 //
 //go:inline
-func MonadChainFirstEitherK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]) ReaderIOResult[R, A] {
+func MonadChainFirstEitherK[R, A, B any](ma ReaderIOResult[R, A], f result.Kleisli[A, B]) ReaderIOResult[R, A] {
 	return RIOE.MonadChainFirstEitherK(ma, f)
 }
 
 //go:inline
-func MonadTapEitherK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]) ReaderIOResult[R, A] {
+func MonadTapEitherK[R, A, B any](ma ReaderIOResult[R, A], f result.Kleisli[A, B]) ReaderIOResult[R, A] {
 	return RIOE.MonadTapEitherK(ma, f)
 }
 
@@ -158,12 +159,12 @@ func MonadTapEitherK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B])
 // This is the curried version of MonadChainFirstEitherK.
 //
 //go:inline
-func ChainFirstEitherK[R, A, B any](f func(A) Result[B]) Operator[R, A, A] {
+func ChainFirstEitherK[R, A, B any](f result.Kleisli[A, B]) Operator[R, A, A] {
 	return RIOE.ChainFirstEitherK[R](f)
 }
 
 //go:inline
-func TapEitherK[R, A, B any](f func(A) Result[B]) Operator[R, A, A] {
+func TapEitherK[R, A, B any](f result.Kleisli[A, B]) Operator[R, A, A] {
 	return RIOE.TapEitherK[R](f)
 }
 
@@ -171,12 +172,12 @@ func TapEitherK[R, A, B any](f func(A) Result[B]) Operator[R, A, A] {
 // Useful for validation or side effects that return Either.
 //
 //go:inline
-func MonadChainFirstResultK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]) ReaderIOResult[R, A] {
+func MonadChainFirstResultK[R, A, B any](ma ReaderIOResult[R, A], f result.Kleisli[A, B]) ReaderIOResult[R, A] {
 	return RIOE.MonadChainFirstEitherK(ma, f)
 }
 
 //go:inline
-func MonadTapResultK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B]) ReaderIOResult[R, A] {
+func MonadTapResultK[R, A, B any](ma ReaderIOResult[R, A], f result.Kleisli[A, B]) ReaderIOResult[R, A] {
 	return RIOE.MonadTapEitherK(ma, f)
 }
 
@@ -184,12 +185,12 @@ func MonadTapResultK[R, A, B any](ma ReaderIOResult[R, A], f func(A) Result[B])
 // This is the curried version of MonadChainFirstEitherK.
 //
 //go:inline
-func ChainFirstResultK[R, A, B any](f func(A) Result[B]) Operator[R, A, A] {
+func ChainFirstResultK[R, A, B any](f result.Kleisli[A, B]) Operator[R, A, A] {
 	return RIOE.ChainFirstEitherK[R](f)
 }
 
 //go:inline
-func TapResultK[R, A, B any](f func(A) Result[B]) Operator[R, A, A] {
+func TapResultK[R, A, B any](f result.Kleisli[A, B]) Operator[R, A, A] {
 	return RIOE.TapEitherK[R](f)
 }
 
@@ -230,17 +231,17 @@ func TapReaderK[R, A, B any](f reader.Kleisli[R, A, B]) Operator[R, A, A] {
 }
 
 //go:inline
-func ChainReaderOptionK[R, A, B any](onNone func() error) func(readeroption.Kleisli[R, A, B]) Operator[R, A, B] {
+func ChainReaderOptionK[R, A, B any](onNone Lazy[error]) func(readeroption.Kleisli[R, A, B]) Operator[R, A, B] {
 	return RIOE.ChainReaderOptionK[R, A, B](onNone)
 }
 
 //go:inline
-func ChainFirstReaderOptionK[R, A, B any](onNone func() error) func(readeroption.Kleisli[R, A, B]) Operator[R, A, A] {
+func ChainFirstReaderOptionK[R, A, B any](onNone Lazy[error]) func(readeroption.Kleisli[R, A, B]) Operator[R, A, A] {
 	return RIOE.ChainFirstReaderOptionK[R, A, B](onNone)
 }
 
 //go:inline
-func TapReaderOptionK[R, A, B any](onNone func() error) func(readeroption.Kleisli[R, A, B]) Operator[R, A, A] {
+func TapReaderOptionK[R, A, B any](onNone Lazy[error]) func(readeroption.Kleisli[R, A, B]) Operator[R, A, A] {
 	return RIOE.TapReaderOptionK[R, A, B](onNone)
 }
 
@@ -421,7 +422,7 @@ func TapIOK[R, A, B any](f func(A) IO[B]) Operator[R, A, A] {
 // If the Option is None, the provided error function is called to produce the error value.
 //
 //go:inline
-func ChainOptionK[R, A, B any](onNone func() error) func(func(A) Option[B]) Operator[R, A, B] {
+func ChainOptionK[R, A, B any](onNone Lazy[error]) func(func(A) Option[B]) Operator[R, A, B] {
 	return RIOE.ChainOptionK[R, A, B](onNone)
 }
 
@@ -619,7 +620,7 @@ func Asks[R, A any](r Reader[R, A]) ReaderIOResult[R, A] {
 // If the Option is None, the provided function is called to produce the error.
 //
 //go:inline
-func FromOption[R, A any](onNone func() error) Kleisli[R, Option[A], A] {
+func FromOption[R, A any](onNone Lazy[error]) Kleisli[R, Option[A], A] {
 	return RIOE.FromOption[R, A](onNone)
 }
 

v2/readerioresult/types.go

@@ -19,6 +19,7 @@ import (
 	"github.com/IBM/fp-go/v2/consumer"
 	"github.com/IBM/fp-go/v2/either"
 	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioeither"
 	"github.com/IBM/fp-go/v2/ioresult"
@@ -122,4 +123,6 @@ type (
 
 	// Predicate represents a function that tests a value of type A and returns a boolean.
 	Predicate[A any] = predicate.Predicate[A]
+
+	Void = function.Void
 )

v2/result/either.go

@@ -475,6 +475,8 @@ func Alt[A any](that Lazy[Result[A]]) Operator[A, A] {
 // If the Result is Left, it applies the provided function to the error value,
 // which returns a new Result that replaces the original.
 //
+// Note: OrElse is identical to [ChainLeft] - both provide the same functionality for error recovery.
+//
 // This is useful for error recovery, fallback logic, or chaining alternative computations.
 //
 // Example:
@@ -656,3 +658,118 @@ func InstanceOf[A any](a any) Result[A] {
 	}
 	return Left[A](fmt.Errorf("expected %T, got %T", res, a))
 }
+
+// MonadChainLeft sequences a computation on the Left (error) channel.
+// If the Result is Left, applies the function to transform or recover from the error.
+// If the Result is Right, returns the Right value unchanged.
+//
+// This is the dual of [MonadChain] - while Chain operates on Right values,
+// ChainLeft operates on Left (error) values. It's particularly useful for:
+//   - Error recovery: converting specific errors into successful values
+//   - Error transformation: changing error types or adding context
+//   - Fallback logic: providing alternative computations when errors occur
+//
+// Note: MonadChainLeft is identical to [OrElse] - both provide the same functionality for error recovery.
+//
+// The function parameter receives the error value and must return a new Result[A].
+// This allows you to:
+//   - Recover by returning Right[error](value)
+//   - Transform the error by returning Left[A](newError)
+//   - Implement conditional error handling based on error content
+//
+// Example - Error recovery:
+//
+//	result := result.MonadChainLeft(
+//	    result.Left[int](errors.New("not found")),
+//	    func(err error) result.Result[int] {
+//	        if err.Error() == "not found" {
+//	            return result.Right(0) // recover with default value
+//	        }
+//	        return result.Left[int](err) // propagate other errors
+//	    },
+//	) // Right(0)
+//
+// Example - Error type transformation:
+//
+//	result := result.MonadChainLeft(
+//	    result.Left[string](errors.New("database error")),
+//	    func(err error) result.Result[string] {
+//	        return result.Left[string](fmt.Errorf("wrapped: %w", err))
+//	    },
+//	) // Left(wrapped error)
+//
+// Example - Right values pass through:
+//
+//	result := result.MonadChainLeft(
+//	    result.Right(42),
+//	    func(err error) result.Result[int] {
+//	        return result.Right(0) // never called
+//	    },
+//	) // Right(42) - unchanged
+//
+//go:inline
+func MonadChainLeft[A any](fa Result[A], f Kleisli[error, A]) Result[A] {
+	return either.MonadChainLeft(fa, f)
+}
+
+// ChainLeft is the curried version of [MonadChainLeft].
+// Returns a function that transforms Left (error) values while preserving Right values.
+//
+// Note: ChainLeft is identical to [OrElse] - both provide the same functionality for error recovery.
+//
+// This curried form is particularly useful in functional pipelines and for creating
+// reusable error handlers that can be composed with other operations.
+//
+// The returned function can be used with [F.Pipe1], [F.Pipe2], etc., to build
+// complex error handling pipelines in a point-free style.
+//
+// Example - Creating reusable error handlers:
+//
+//	// Handler that recovers from "not found" errors
+//	recoverNotFound := result.ChainLeft(func(err error) result.Result[int] {
+//	    if err.Error() == "not found" {
+//	        return result.Right(0)
+//	    }
+//	    return result.Left[int](err)
+//	})
+//
+//	result1 := recoverNotFound(result.Left[int](errors.New("not found"))) // Right(0)
+//	result2 := recoverNotFound(result.Right(42)) // Right(42)
+//
+// Example - Using in pipelines:
+//
+//	result := F.Pipe2(
+//	    result.Left[int](errors.New("timeout")),
+//	    result.ChainLeft(func(err error) result.Result[int] {
+//	        if err.Error() == "timeout" {
+//	            return result.Right(999) // fallback value
+//	        }
+//	        return result.Left[int](err)
+//	    }),
+//	    result.Map(func(n int) string {
+//	        return fmt.Sprintf("Value: %d", n)
+//	    }),
+//	) // Right("Value: 999")
+//
+// Example - Composing multiple error handlers:
+//
+//	// First handler: convert error to string
+//	toStringError := result.ChainLeft(func(err error) result.Result[int] {
+//	    return result.Left[int](errors.New(err.Error()))
+//	})
+//
+//	// Second handler: add prefix
+//	addPrefix := result.ChainLeft(func(err error) result.Result[int] {
+//	    return result.Left[int](fmt.Errorf("Error: %w", err))
+//	})
+//
+//	result := F.Pipe2(
+//	    result.Left[int](errors.New("failed")),
+//	    toStringError,
+//	    addPrefix,
+//	) // Left(Error: failed)
+//
+//go:inline
+func ChainLeft[A any](f Kleisli[error, A]) Operator[A, A] {
+	return either.ChainLeft(f)
+}

v2/result/either_test.go

@@ -356,3 +356,296 @@ func TestInstanceOf(t *testing.T) {
 		assert.Equal(t, 2, v["b"])
 	})
 }
+
+// TestMonadChainLeft tests the MonadChainLeft function with various scenarios
+func TestMonadChainLeft(t *testing.T) {
+	t.Run("Left value is transformed by function", func(t *testing.T) {
+		// Transform error to success
+		result := MonadChainLeft(
+			Left[int](errors.New("not found")),
+			func(err error) Result[int] {
+				if err.Error() == "not found" {
+					return Right(0) // default value
+				}
+				return Left[int](err)
+			},
+		)
+		assert.Equal(t, Of(0), result)
+	})
+
+	t.Run("Left value error is transformed", func(t *testing.T) {
+		// Transform error with additional context
+		result := MonadChainLeft(
+			Left[int](errors.New("database error")),
+			func(err error) Result[int] {
+				return Left[int](fmt.Errorf("wrapped: %w", err))
+			},
+		)
+		assert.True(t, IsLeft(result))
+		_, err := UnwrapError(result)
+		assert.Contains(t, err.Error(), "wrapped:")
+		assert.Contains(t, err.Error(), "database error")
+	})
+
+	t.Run("Right value passes through unchanged", func(t *testing.T) {
+		// Right value should not be affected
+		result := MonadChainLeft(
+			Right(42),
+			func(err error) Result[int] {
+				return Left[int](errors.New("should not be called"))
+			},
+		)
+		assert.Equal(t, Of(42), result)
+	})
+
+	t.Run("Chain multiple error transformations", func(t *testing.T) {
+		// First transformation
+		step1 := MonadChainLeft(
+			Left[int](errors.New("error1")),
+			func(err error) Result[int] {
+				return Left[int](errors.New("error2"))
+			},
+		)
+		// Second transformation
+		step2 := MonadChainLeft(
+			step1,
+			func(err error) Result[int] {
+				return Left[int](fmt.Errorf("final: %s", err.Error()))
+			},
+		)
+		assert.True(t, IsLeft(step2))
+		_, err := UnwrapError(step2)
+		assert.Equal(t, "final: error2", err.Error())
+	})
+
+	t.Run("Error recovery with fallback", func(t *testing.T) {
+		// Recover from specific errors
+		result := MonadChainLeft(
+			Left[int](errors.New("timeout")),
+			func(err error) Result[int] {
+				if err.Error() == "timeout" {
+					return Right(999) // fallback value
+				}
+				return Left[int](err)
+			},
+		)
+		assert.Equal(t, Of(999), result)
+	})
+
+	t.Run("Conditional error handling", func(t *testing.T) {
+		// Handle different error types differently
+		handleError := func(err error) Result[string] {
+			switch err.Error() {
+			case "not found":
+				return Right("default")
+			case "timeout":
+				return Right("retry")
+			default:
+				return Left[string](err)
+			}
+		}
+
+		result1 := MonadChainLeft(Left[string](errors.New("not found")), handleError)
+		assert.Equal(t, Of("default"), result1)
+
+		result2 := MonadChainLeft(Left[string](errors.New("timeout")), handleError)
+		assert.Equal(t, Of("retry"), result2)
+
+		result3 := MonadChainLeft(Left[string](errors.New("other")), handleError)
+		assert.True(t, IsLeft(result3))
+	})
+
+	t.Run("Type preservation", func(t *testing.T) {
+		// Ensure type is preserved through transformation
+		result := MonadChainLeft(
+			Left[string](errors.New("error")),
+			func(err error) Result[string] {
+				return Right("recovered")
+			},
+		)
+		assert.Equal(t, Of("recovered"), result)
+	})
+}
+
+// TestChainLeft tests the curried ChainLeft function
+func TestChainLeft(t *testing.T) {
+	t.Run("Curried function transforms Left value", func(t *testing.T) {
+		// Create a reusable error handler
+		handleNotFound := ChainLeft(func(err error) Result[int] {
+			if err.Error() == "not found" {
+				return Right(0)
+			}
+			return Left[int](err)
+		})
+
+		result := handleNotFound(Left[int](errors.New("not found")))
+		assert.Equal(t, Of(0), result)
+	})
+
+	t.Run("Curried function with Right value", func(t *testing.T) {
+		handler := ChainLeft(func(err error) Result[int] {
+			return Left[int](errors.New("should not be called"))
+		})
+
+		result := handler(Right(42))
+		assert.Equal(t, Of(42), result)
+	})
+
+	t.Run("Use in pipeline with Pipe", func(t *testing.T) {
+		// Create error transformer
+		wrapError := ChainLeft(func(err error) Result[string] {
+			return Left[string](fmt.Errorf("Error: %w", err))
+		})
+
+		result := F.Pipe1(
+			Left[string](errors.New("failed")),
+			wrapError,
+		)
+		assert.True(t, IsLeft(result))
+		_, err := UnwrapError(result)
+		assert.Contains(t, err.Error(), "Error:")
+		assert.Contains(t, err.Error(), "failed")
+	})
+
+	t.Run("Compose multiple ChainLeft operations", func(t *testing.T) {
+		// First handler: convert error to string representation
+		handler1 := ChainLeft(func(err error) Result[int] {
+			return Left[int](errors.New(err.Error()))
+		})
+
+		// Second handler: add prefix to error
+		handler2 := ChainLeft(func(err error) Result[int] {
+			return Left[int](fmt.Errorf("Handled: %w", err))
+		})
+
+		result := F.Pipe2(
+			Left[int](errors.New("original")),
+			handler1,
+			handler2,
+		)
+		assert.True(t, IsLeft(result))
+		_, err := UnwrapError(result)
+		assert.Contains(t, err.Error(), "Handled:")
+		assert.Contains(t, err.Error(), "original")
+	})
+
+	t.Run("Error recovery in pipeline", func(t *testing.T) {
+		// Handler that recovers from specific errors
+		recoverFromTimeout := ChainLeft(func(err error) Result[int] {
+			if err.Error() == "timeout" {
+				return Right(0) // recovered value
+			}
+			return Left[int](err) // propagate other errors
+		})
+
+		// Test with timeout error
+		result1 := F.Pipe1(
+			Left[int](errors.New("timeout")),
+			recoverFromTimeout,
+		)
+		assert.Equal(t, Of(0), result1)
+
+		// Test with other error
+		result2 := F.Pipe1(
+			Left[int](errors.New("other error")),
+			recoverFromTimeout,
+		)
+		assert.True(t, IsLeft(result2))
+	})
+
+	t.Run("ChainLeft with Map combination", func(t *testing.T) {
+		// Combine ChainLeft with Map to handle both channels
+		errorHandler := ChainLeft(func(err error) Result[int] {
+			return Left[int](fmt.Errorf("Error: %w", err))
+		})
+
+		valueMapper := Map(func(n int) string {
+			return fmt.Sprintf("Value: %d", n)
+		})
+
+		// Test with Left
+		result1 := F.Pipe2(
+			Left[int](errors.New("fail")),
+			errorHandler,
+			valueMapper,
+		)
+		assert.True(t, IsLeft(result1))
+
+		// Test with Right
+		result2 := F.Pipe2(
+			Right(42),
+			errorHandler,
+			valueMapper,
+		)
+		assert.Equal(t, Of("Value: 42"), result2)
+	})
+
+	t.Run("Reusable error handlers", func(t *testing.T) {
+		// Create a library of reusable error handlers
+		recoverNotFound := ChainLeft(func(err error) Result[string] {
+			if err.Error() == "not found" {
+				return Right("default")
+			}
+			return Left[string](err)
+		})
+
+		recoverTimeout := ChainLeft(func(err error) Result[string] {
+			if err.Error() == "timeout" {
+				return Right("retry")
+			}
+			return Left[string](err)
+		})
+
+		// Apply handlers in sequence
+		result := F.Pipe2(
+			Left[string](errors.New("not found")),
+			recoverNotFound,
+			recoverTimeout,
+		)
+		assert.Equal(t, Of("default"), result)
+	})
+
+	t.Run("Error transformation pipeline", func(t *testing.T) {
+		// Build a pipeline that transforms errors step by step
+		addContext := ChainLeft(func(err error) Result[int] {
+			return Left[int](fmt.Errorf("context: %w", err))
+		})
+
+		addTimestamp := ChainLeft(func(err error) Result[int] {
+			return Left[int](fmt.Errorf("[2024-01-01] %w", err))
+		})
+
+		result := F.Pipe2(
+			Left[int](errors.New("base error")),
+			addContext,
+			addTimestamp,
+		)
+		assert.True(t, IsLeft(result))
+		_, err := UnwrapError(result)
+		assert.Contains(t, err.Error(), "[2024-01-01]")
+		assert.Contains(t, err.Error(), "context:")
+		assert.Contains(t, err.Error(), "base error")
+	})
+
+	t.Run("Conditional recovery based on error content", func(t *testing.T) {
+		// Recover from errors matching specific patterns
+		smartRecover := ChainLeft(func(err error) Result[int] {
+			msg := err.Error()
+			if msg == "not found" {
+				return Right(0)
+			}
+			if msg == "timeout" {
+				return Right(-1)
+			}
+			if msg == "unauthorized" {
+				return Right(-2)
+			}
+			return Left[int](err)
+		})
+
+		assert.Equal(t, Of(0), smartRecover(Left[int](errors.New("not found"))))
+		assert.Equal(t, Of(-1), smartRecover(Left[int](errors.New("timeout"))))
+		assert.Equal(t, Of(-2), smartRecover(Left[int](errors.New("unauthorized"))))
+		assert.True(t, IsLeft(smartRecover(Left[int](errors.New("unknown")))))
+	})
+}

v2/result/filterable.go

@@ -0,0 +1,262 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package result provides filterable operations for Result types.
+//
+// This package implements the Fantasy Land Filterable specification:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// Since Result[A] is an alias for Either[error, A], these functions are
+// thin wrappers around the corresponding either package functions, specialized
+// for the common case where the error type is Go's built-in error interface.
+package result
+
+import (
+	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/option"
+)
+
+// Partition separates a [Result] value into a [Pair] based on a predicate function.
+// It returns a function that takes a Result and produces a Pair of Result values,
+// where the first element contains values that fail the predicate and the second
+// contains values that pass the predicate.
+//
+// This function implements the Filterable specification's partition operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is an error (Left), both elements of the resulting Pair will be the same error
+//   - If the input is Ok (Right) and the predicate returns true, the result is (Err(empty), Ok(value))
+//   - If the input is Ok (Right) and the predicate returns false, the result is (Ok(value), Err(empty))
+//
+// Parameters:
+//   - p: A predicate function that tests values of type A
+//   - empty: The default error to use when creating error Results for partitioning
+//
+// Returns:
+//
+//	A function that takes a Result[A] and returns a Pair where:
+//	  - First element: Result values that fail the predicate (or original error)
+//	  - Second element: Result values that pass the predicate (or original error)
+//
+// Example:
+//
+//	import (
+//	    R "github.com/IBM/fp-go/v2/result"
+//	    N "github.com/IBM/fp-go/v2/number"
+//	    P "github.com/IBM/fp-go/v2/pair"
+//	    "errors"
+//	)
+//
+//	// Partition positive and non-positive numbers
+//	isPositive := N.MoreThan(0)
+//	partition := R.Partition(isPositive, errors.New("not positive"))
+//
+//	// Ok value that passes predicate
+//	result1 := partition(R.Of(5))
+//	// result1 = Pair(Err("not positive"), Ok(5))
+//
+//	// Ok value that fails predicate
+//	result2 := partition(R.Of(-3))
+//	// result2 = Pair(Ok(-3), Err("not positive"))
+//
+//	// Error passes through unchanged in both positions
+//	result3 := partition(R.Error[int](errors.New("original error")))
+//	// result3 = Pair(Err("original error"), Err("original error"))
+//
+//go:inline
+func Partition[A any](p Predicate[A], empty error) func(Result[A]) Pair[Result[A], Result[A]] {
+	return either.Partition(p, empty)
+}
+
+// Filter creates a filtering operation for [Result] values based on a predicate function.
+// It returns a function that takes a Result and produces a Result, where Ok values
+// that fail the predicate are converted to error Results with the provided error.
+//
+// This function implements the Filterable specification's filter operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is an error, it passes through unchanged
+//   - If the input is Ok and the predicate returns true, the Ok value passes through unchanged
+//   - If the input is Ok and the predicate returns false, it's converted to Err(empty)
+//
+// Parameters:
+//   - p: A predicate function that tests values of type A
+//   - empty: The default error to use when filtering out Ok values that fail the predicate
+//
+// Returns:
+//
+//	An Operator function that takes a Result[A] and returns a Result[A] where:
+//	  - Error values pass through unchanged
+//	  - Ok values that pass the predicate remain as Ok
+//	  - Ok values that fail the predicate become Err(empty)
+//
+// Example:
+//
+//	import (
+//	    R "github.com/IBM/fp-go/v2/result"
+//	    N "github.com/IBM/fp-go/v2/number"
+//	    "errors"
+//	)
+//
+//	// Filter to keep only positive numbers
+//	isPositive := N.MoreThan(0)
+//	filterPositive := R.Filter(isPositive, errors.New("not positive"))
+//
+//	// Ok value that passes predicate - remains Ok
+//	result1 := filterPositive(R.Of(5))
+//	// result1 = Ok(5)
+//
+//	// Ok value that fails predicate - becomes Err
+//	result2 := filterPositive(R.Of(-3))
+//	// result2 = Err("not positive")
+//
+//	// Error passes through unchanged
+//	result3 := filterPositive(R.Error[int](errors.New("original error")))
+//	// result3 = Err("original error")
+//
+//	// Chaining filters
+//	isEven := func(n int) bool { return n%2 == 0 }
+//	filterEven := R.Filter(isEven, errors.New("not even"))
+//
+//	result4 := filterEven(filterPositive(R.Of(4)))
+//	// result4 = Ok(4) - passes both filters
+//
+//	result5 := filterEven(filterPositive(R.Of(3)))
+//	// result5 = Err("not even") - passes first, fails second
+//
+//go:inline
+func Filter[A any](p Predicate[A], empty error) Operator[A, A] {
+	return either.Filter(p, empty)
+}
+
+// FilterMap combines filtering and mapping operations for [Result] values using an [Option]-returning function.
+// It returns a function that takes a Result[A] and produces a Result[B], where Ok values
+// are transformed by applying the function f. If f returns Some(B), the result is Ok(B). If f returns
+// None, the result is Err(empty).
+//
+// This function implements the Filterable specification's filterMap operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is an error, it passes through with its error value preserved
+//   - If the input is Ok and f returns Some(B), the result is Ok(B)
+//   - If the input is Ok and f returns None, the result is Err(empty)
+//
+// Parameters:
+//   - f: An Option Kleisli function that transforms values of type A to Option[B]
+//   - empty: The default error to use when f returns None
+//
+// Returns:
+//
+//	An Operator function that takes a Result[A] and returns a Result[B] where:
+//	  - Error values pass through with error preserved
+//	  - Ok values are transformed by f: Some(B) becomes Ok(B), None becomes Err(empty)
+//
+// Example:
+//
+//	import (
+//	    R "github.com/IBM/fp-go/v2/result"
+//	    O "github.com/IBM/fp-go/v2/option"
+//	    "errors"
+//	    "strconv"
+//	)
+//
+//	// Parse string to int, filtering out invalid values
+//	parseInt := func(s string) O.Option[int] {
+//	    if n, err := strconv.Atoi(s); err == nil {
+//	        return O.Some(n)
+//	    }
+//	    return O.None[int]()
+//	}
+//	filterMapInt := R.FilterMap(parseInt, errors.New("invalid number"))
+//
+//	// Valid number string - transforms to Ok(int)
+//	result1 := filterMapInt(R.Of("42"))
+//	// result1 = Ok(42)
+//
+//	// Invalid number string - becomes Err
+//	result2 := filterMapInt(R.Of("abc"))
+//	// result2 = Err("invalid number")
+//
+//	// Error passes through with error preserved
+//	result3 := filterMapInt(R.Error[string](errors.New("original error")))
+//	// result3 = Err("original error")
+//
+//go:inline
+func FilterMap[A, B any](f option.Kleisli[A, B], empty error) Operator[A, B] {
+	return either.FilterMap(f, empty)
+}
+
+// PartitionMap separates and transforms a [Result] value into a [Pair] of Result values using a mapping function.
+// It returns a function that takes a Result[A] and produces a Pair of Result values, where the mapping
+// function f transforms the Ok value into Either[B, C]. The result is partitioned based on whether f
+// produces a Left or Right value.
+//
+// This function implements the Filterable specification's partitionMap operation:
+// https://github.com/fantasyland/fantasy-land#filterable
+//
+// The behavior is as follows:
+//   - If the input is an error, both elements of the resulting Pair will be errors with the original error
+//   - If the input is Ok and f returns Left(B), the result is (Ok(B), Err(empty))
+//   - If the input is Ok and f returns Right(C), the result is (Err(empty), Ok(C))
+//
+// Parameters:
+//   - f: A Kleisli function that transforms values of type A to Either[B, C]
+//   - empty: The default error to use when creating error Results for partitioning
+//
+// Returns:
+//
+//	A function that takes a Result[A] and returns a Pair[Result[B], Result[C]] where:
+//	  - If input is error: (Err(original_error), Err(original_error))
+//	  - If f returns Left(B): (Ok(B), Err(empty))
+//	  - If f returns Right(C): (Err(empty), Ok(C))
+//
+// Example:
+//
+//	import (
+//	    R "github.com/IBM/fp-go/v2/result"
+//	    E "github.com/IBM/fp-go/v2/either"
+//	    P "github.com/IBM/fp-go/v2/pair"
+//	    "errors"
+//	    "strconv"
+//	)
+//
+//	// Classify and transform numbers: negative -> error message, positive -> squared value
+//	classifyNumber := func(n int) E.Either[string, int] {
+//	    if n < 0 {
+//	        return E.Left[int]("negative: " + strconv.Itoa(n))
+//	    }
+//	    return E.Right[string](n * n)
+//	}
+//	partitionMap := R.PartitionMap(classifyNumber, errors.New("not classified"))
+//
+//	// Positive number - goes to right side as squared value
+//	result1 := partitionMap(R.Of(5))
+//	// result1 = Pair(Err("not classified"), Ok(25))
+//
+//	// Negative number - goes to left side with error message
+//	result2 := partitionMap(R.Of(-3))
+//	// result2 = Pair(Ok("negative: -3"), Err("not classified"))
+//
+//	// Original error - appears in both positions
+//	result3 := partitionMap(R.Error[int](errors.New("original error")))
+//	// result3 = Pair(Err("original error"), Err("original error"))
+//
+//go:inline
+func PartitionMap[A, B, C any](f either.Kleisli[B, A, C], empty error) func(Result[A]) Pair[Result[B], Result[C]] {
+	return either.PartitionMap(f, empty)
+}

v2/result/filterable_test.go

@@ -0,0 +1,689 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package result
+
+import (
+	"errors"
+	"math"
+	"strconv"
+	"testing"
+
+	E "github.com/IBM/fp-go/v2/either"
+	N "github.com/IBM/fp-go/v2/number"
+	O "github.com/IBM/fp-go/v2/option"
+	P "github.com/IBM/fp-go/v2/pair"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPartition(t *testing.T) {
+	t.Run("Ok value that passes predicate", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		partition := Partition(isPositive, errors.New("not positive"))
+		input := Of(5)
+
+		// Act
+		result := partition(input)
+		left, right := P.Unpack(result)
+
+		// Assert
+		assert.True(t, IsLeft(left), "left should be error")
+		assert.True(t, IsRight(right), "right should be Ok")
+
+		rightVal, _ := Unwrap(right)
+		assert.Equal(t, 5, rightVal)
+	})
+
+	t.Run("Ok value that fails predicate", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		partition := Partition(isPositive, errors.New("not positive"))
+		input := Of(-3)
+
+		// Act
+		result := partition(input)
+		left, right := P.Unpack(result)
+
+		// Assert
+		assert.True(t, IsRight(left), "left should be Ok (failed predicate)")
+		assert.True(t, IsLeft(right), "right should be error")
+
+		leftVal, _ := Unwrap(left)
+		assert.Equal(t, -3, leftVal)
+	})
+
+	t.Run("Ok value at boundary (zero)", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		partition := Partition(isPositive, errors.New("not positive"))
+		input := Of(0)
+
+		// Act
+		result := partition(input)
+		left, right := P.Unpack(result)
+
+		// Assert
+		assert.True(t, IsRight(left), "left should be Ok (zero fails predicate)")
+		assert.True(t, IsLeft(right), "right should be error")
+
+		leftVal, _ := Unwrap(left)
+		assert.Equal(t, 0, leftVal)
+	})
+
+	t.Run("Error passes through unchanged", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		partition := Partition(isPositive, errors.New("not positive"))
+		originalError := errors.New("original error")
+		input := Left[int](originalError)
+
+		// Act
+		result := partition(input)
+		left, right := P.Unpack(result)
+
+		// Assert
+		assert.True(t, IsLeft(left), "left should be error")
+		assert.True(t, IsLeft(right), "right should be error")
+
+		_, leftErr := Unwrap(left)
+		_, rightErr := Unwrap(right)
+
+		assert.Equal(t, originalError, leftErr)
+		assert.Equal(t, originalError, rightErr)
+	})
+
+	t.Run("String predicate - even length strings", func(t *testing.T) {
+		// Arrange
+		isEvenLength := func(s string) bool { return len(s)%2 == 0 }
+		partition := Partition(isEvenLength, errors.New("odd length"))
+
+		// Act & Assert - passes predicate
+		result1 := partition(Of("test"))
+		left1, right1 := P.Unpack(result1)
+		assert.True(t, IsLeft(left1))
+		assert.True(t, IsRight(right1))
+		rightVal1, _ := Unwrap(right1)
+		assert.Equal(t, "test", rightVal1)
+
+		// Act & Assert - fails predicate
+		result2 := partition(Of("hello"))
+		left2, right2 := P.Unpack(result2)
+		assert.True(t, IsRight(left2))
+		assert.True(t, IsLeft(right2))
+		leftVal2, _ := Unwrap(left2)
+		assert.Equal(t, "hello", leftVal2)
+	})
+
+	t.Run("Complex type predicate - struct field check", func(t *testing.T) {
+		// Arrange
+		type Person struct {
+			Name string
+			Age  int
+		}
+		isAdult := func(p Person) bool { return p.Age >= 18 }
+		partition := Partition(isAdult, errors.New("minor"))
+
+		// Act & Assert - adult passes
+		adult := Person{Name: "Alice", Age: 25}
+		result1 := partition(Of(adult))
+		left1, right1 := P.Unpack(result1)
+		assert.True(t, IsLeft(left1))
+		assert.True(t, IsRight(right1))
+		rightVal1, _ := Unwrap(right1)
+		assert.Equal(t, adult, rightVal1)
+
+		// Act & Assert - minor fails
+		minor := Person{Name: "Bob", Age: 15}
+		result2 := partition(Of(minor))
+		left2, right2 := P.Unpack(result2)
+		assert.True(t, IsRight(left2))
+		assert.True(t, IsLeft(right2))
+		leftVal2, _ := Unwrap(left2)
+		assert.Equal(t, minor, leftVal2)
+	})
+}
+
+func TestFilter(t *testing.T) {
+	t.Run("Ok value that passes predicate", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filter := Filter(isPositive, errors.New("not positive"))
+		input := Of(5)
+
+		// Act
+		result := filter(input)
+
+		// Assert
+		assert.True(t, IsRight(result), "result should be Ok")
+		val, _ := Unwrap(result)
+		assert.Equal(t, 5, val)
+	})
+
+	t.Run("Ok value that fails predicate", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filter := Filter(isPositive, errors.New("not positive"))
+		input := Of(-3)
+
+		// Act
+		result := filter(input)
+
+		// Assert
+		assert.True(t, IsLeft(result), "result should be error")
+		_, err := Unwrap(result)
+		assert.Equal(t, "not positive", err.Error())
+	})
+
+	t.Run("Ok value at boundary (zero)", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filter := Filter(isPositive, errors.New("not positive"))
+		input := Of(0)
+
+		// Act
+		result := filter(input)
+
+		// Assert
+		assert.True(t, IsLeft(result), "zero should fail predicate")
+		_, err := Unwrap(result)
+		assert.Equal(t, "not positive", err.Error())
+	})
+
+	t.Run("Error passes through unchanged", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filter := Filter(isPositive, errors.New("not positive"))
+		originalError := errors.New("original error")
+		input := Left[int](originalError)
+
+		// Act
+		result := filter(input)
+
+		// Assert
+		assert.True(t, IsLeft(result), "result should be error")
+		_, err := Unwrap(result)
+		assert.Equal(t, originalError, err)
+	})
+
+	t.Run("Chaining multiple filters", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		isEven := func(n int) bool { return n%2 == 0 }
+		filterPositive := Filter(isPositive, errors.New("not positive"))
+		filterEven := Filter(isEven, errors.New("not even"))
+
+		// Act & Assert - passes both filters
+		result1 := filterEven(filterPositive(Of(4)))
+		assert.True(t, IsRight(result1))
+		val1, _ := Unwrap(result1)
+		assert.Equal(t, 4, val1)
+
+		// Act & Assert - passes first, fails second
+		result2 := filterEven(filterPositive(Of(3)))
+		assert.True(t, IsLeft(result2))
+		_, err2 := Unwrap(result2)
+		assert.Equal(t, "not even", err2.Error())
+
+		// Act & Assert - fails first filter
+		result3 := filterEven(filterPositive(Of(-2)))
+		assert.True(t, IsLeft(result3))
+		_, err3 := Unwrap(result3)
+		assert.Equal(t, "not positive", err3.Error())
+
+		// Act & Assert - error passes through both
+		originalErr := errors.New("original")
+		result4 := filterEven(filterPositive(Left[int](originalErr)))
+		assert.True(t, IsLeft(result4))
+		_, err4 := Unwrap(result4)
+		assert.Equal(t, originalErr, err4)
+	})
+
+	t.Run("Filter preserves error", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filter := Filter(isPositive, errors.New("default error"))
+
+		// Act - error with different message
+		originalError := errors.New("server error")
+		result := filter(Left[int](originalError))
+
+		// Assert - original error preserved
+		assert.True(t, IsLeft(result))
+		_, err := Unwrap(result)
+		assert.Equal(t, originalError, err)
+	})
+}
+
+func TestFilterMap(t *testing.T) {
+	t.Run("Ok value with Some result", func(t *testing.T) {
+		// Arrange
+		parseInt := func(s string) O.Option[int] {
+			if n, err := strconv.Atoi(s); err == nil {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		filterMap := FilterMap(parseInt, errors.New("invalid number"))
+		input := Of("42")
+
+		// Act
+		result := filterMap(input)
+
+		// Assert
+		assert.True(t, IsRight(result), "result should be Ok")
+		val, _ := Unwrap(result)
+		assert.Equal(t, 42, val)
+	})
+
+	t.Run("Ok value with None result", func(t *testing.T) {
+		// Arrange
+		parseInt := func(s string) O.Option[int] {
+			if n, err := strconv.Atoi(s); err == nil {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		filterMap := FilterMap(parseInt, errors.New("invalid number"))
+		input := Of("abc")
+
+		// Act
+		result := filterMap(input)
+
+		// Assert
+		assert.True(t, IsLeft(result), "result should be error")
+		_, err := Unwrap(result)
+		assert.Equal(t, "invalid number", err.Error())
+	})
+
+	t.Run("Error passes through", func(t *testing.T) {
+		// Arrange
+		parseInt := func(s string) O.Option[int] {
+			if n, err := strconv.Atoi(s); err == nil {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		filterMap := FilterMap(parseInt, errors.New("invalid number"))
+		originalError := errors.New("original error")
+		input := Left[string](originalError)
+
+		// Act
+		result := filterMap(input)
+
+		// Assert
+		assert.True(t, IsLeft(result), "result should be error")
+		_, err := Unwrap(result)
+		assert.Equal(t, originalError, err)
+	})
+
+	t.Run("Extract optional field from struct", func(t *testing.T) {
+		// Arrange
+		type Person struct {
+			Name  string
+			Email O.Option[string]
+		}
+		extractEmail := func(p Person) O.Option[string] { return p.Email }
+		filterMap := FilterMap(extractEmail, errors.New("no email"))
+
+		// Act & Assert - has email
+		result1 := filterMap(Of(Person{Name: "Alice", Email: O.Some("alice@example.com")}))
+		assert.True(t, IsRight(result1))
+		val1, _ := Unwrap(result1)
+		assert.Equal(t, "alice@example.com", val1)
+
+		// Act & Assert - no email
+		result2 := filterMap(Of(Person{Name: "Bob", Email: O.None[string]()}))
+		assert.True(t, IsLeft(result2))
+		_, err2 := Unwrap(result2)
+		assert.Equal(t, "no email", err2.Error())
+	})
+
+	t.Run("Transform and filter numbers", func(t *testing.T) {
+		// Arrange
+		sqrtIfPositive := func(n int) O.Option[float64] {
+			if n >= 0 {
+				return O.Some(math.Sqrt(float64(n)))
+			}
+			return O.None[float64]()
+		}
+		filterMap := FilterMap(sqrtIfPositive, errors.New("negative number"))
+
+		// Act & Assert - positive number
+		result1 := filterMap(Of(16))
+		assert.True(t, IsRight(result1))
+		val1, _ := Unwrap(result1)
+		assert.Equal(t, 4.0, val1)
+
+		// Act & Assert - negative number
+		result2 := filterMap(Of(-4))
+		assert.True(t, IsLeft(result2))
+		_, err2 := Unwrap(result2)
+		assert.Equal(t, "negative number", err2.Error())
+
+		// Act & Assert - zero
+		result3 := filterMap(Of(0))
+		assert.True(t, IsRight(result3))
+		val3, _ := Unwrap(result3)
+		assert.Equal(t, 0.0, val3)
+	})
+
+	t.Run("Chain multiple FilterMap operations", func(t *testing.T) {
+		// Arrange
+		parseInt := func(s string) O.Option[int] {
+			if n, err := strconv.Atoi(s); err == nil {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		doubleIfEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * 2)
+			}
+			return O.None[int]()
+		}
+		filterMap1 := FilterMap(parseInt, errors.New("invalid number"))
+		filterMap2 := FilterMap(doubleIfEven, errors.New("not even"))
+
+		// Act & Assert - valid even number
+		result1 := filterMap2(filterMap1(Of("4")))
+		assert.True(t, IsRight(result1))
+		val1, _ := Unwrap(result1)
+		assert.Equal(t, 8, val1)
+
+		// Act & Assert - valid odd number
+		result2 := filterMap2(filterMap1(Of("3")))
+		assert.True(t, IsLeft(result2))
+		_, err2 := Unwrap(result2)
+		assert.Equal(t, "not even", err2.Error())
+
+		// Act & Assert - invalid number
+		result3 := filterMap2(filterMap1(Of("abc")))
+		assert.True(t, IsLeft(result3))
+		_, err3 := Unwrap(result3)
+		assert.Equal(t, "invalid number", err3.Error())
+	})
+}
+
+func TestPartitionMap(t *testing.T) {
+	t.Run("Ok value that maps to Left", func(t *testing.T) {
+		// Arrange
+		classifyNumber := func(n int) E.Either[string, int] {
+			if n < 0 {
+				return E.Left[int]("negative: " + strconv.Itoa(n))
+			}
+			return E.Right[string](n * n)
+		}
+		partitionMap := PartitionMap(classifyNumber, errors.New("not classified"))
+		input := Of(-3)
+
+		// Act
+		result := partitionMap(input)
+		left, right := P.Unpack(result)
+
+		// Assert
+		assert.True(t, IsRight(left), "left should be Ok (contains error from f)")
+		assert.True(t, IsLeft(right), "right should be error")
+
+		leftVal, _ := Unwrap(left)
+		assert.Equal(t, "negative: -3", leftVal)
+	})
+
+	t.Run("Ok value that maps to Right", func(t *testing.T) {
+		// Arrange
+		classifyNumber := func(n int) E.Either[string, int] {
+			if n < 0 {
+				return E.Left[int]("negative: " + strconv.Itoa(n))
+			}
+			return E.Right[string](n * n)
+		}
+		partitionMap := PartitionMap(classifyNumber, errors.New("not classified"))
+		input := Of(5)
+
+		// Act
+		result := partitionMap(input)
+		left, right := P.Unpack(result)
+
+		// Assert
+		assert.True(t, IsLeft(left), "left should be error")
+		assert.True(t, IsRight(right), "right should be Ok (contains value from f)")
+
+		rightVal, _ := Unwrap(right)
+		assert.Equal(t, 25, rightVal)
+	})
+
+	t.Run("Error passes through to both sides", func(t *testing.T) {
+		// Arrange
+		classifyNumber := func(n int) E.Either[string, int] {
+			if n < 0 {
+				return E.Left[int]("negative")
+			}
+			return E.Right[string](n * n)
+		}
+		partitionMap := PartitionMap(classifyNumber, errors.New("not classified"))
+		originalError := errors.New("original error")
+		input := Left[int](originalError)
+
+		// Act
+		result := partitionMap(input)
+		left, right := P.Unpack(result)
+
+		// Assert
+		assert.True(t, IsLeft(left), "left should be error")
+		assert.True(t, IsLeft(right), "right should be error")
+
+		_, leftErr := Unwrap(left)
+		_, rightErr := Unwrap(right)
+
+		assert.Equal(t, originalError, leftErr)
+		assert.Equal(t, originalError, rightErr)
+	})
+
+	t.Run("Validate and transform user input", func(t *testing.T) {
+		// Arrange
+		type ValidationError struct {
+			Field   string
+			Message string
+		}
+		type User struct {
+			Name string
+			Age  int
+		}
+
+		validateUser := func(input map[string]string) E.Either[ValidationError, User] {
+			name, hasName := input["name"]
+			ageStr, hasAge := input["age"]
+			if !hasName {
+				return E.Left[User](ValidationError{"name", "missing"})
+			}
+			if !hasAge {
+				return E.Left[User](ValidationError{"age", "missing"})
+			}
+			age, err := strconv.Atoi(ageStr)
+			if err != nil {
+				return E.Left[User](ValidationError{"age", "invalid"})
+			}
+			return E.Right[ValidationError](User{name, age})
+		}
+		partitionMap := PartitionMap(validateUser, errors.New("not processed"))
+
+		// Act & Assert - valid input
+		validInput := map[string]string{"name": "Alice", "age": "30"}
+		result1 := partitionMap(Of(validInput))
+		left1, right1 := P.Unpack(result1)
+		assert.True(t, IsLeft(left1))
+		assert.True(t, IsRight(right1))
+		rightVal1, _ := Unwrap(right1)
+		assert.Equal(t, User{"Alice", 30}, rightVal1)
+
+		// Act & Assert - invalid input (missing age)
+		invalidInput := map[string]string{"name": "Bob"}
+		result2 := partitionMap(Of(invalidInput))
+		left2, right2 := P.Unpack(result2)
+		assert.True(t, IsRight(left2))
+		assert.True(t, IsLeft(right2))
+		leftVal2, _ := Unwrap(left2)
+		assert.Equal(t, ValidationError{"age", "missing"}, leftVal2)
+	})
+
+	t.Run("Classify strings by length", func(t *testing.T) {
+		// Arrange
+		classifyString := func(s string) E.Either[string, int] {
+			if len(s) < 5 {
+				return E.Left[int]("too short: " + s)
+			}
+			return E.Right[string](len(s))
+		}
+		partitionMap := PartitionMap(classifyString, errors.New("not classified"))
+
+		// Act & Assert - short string
+		result1 := partitionMap(Of("hi"))
+		left1, right1 := P.Unpack(result1)
+		assert.True(t, IsRight(left1))
+		assert.True(t, IsLeft(right1))
+		leftVal1, _ := Unwrap(left1)
+		assert.Equal(t, "too short: hi", leftVal1)
+
+		// Act & Assert - long string
+		result2 := partitionMap(Of("hello world"))
+		left2, right2 := P.Unpack(result2)
+		assert.True(t, IsLeft(left2))
+		assert.True(t, IsRight(right2))
+		rightVal2, _ := Unwrap(right2)
+		assert.Equal(t, 11, rightVal2)
+	})
+}
+
+// Benchmark tests
+func BenchmarkPartition(b *testing.B) {
+	isPositive := N.MoreThan(0)
+	partition := Partition(isPositive, errors.New("not positive"))
+	input := Of(42)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = partition(input)
+	}
+}
+
+func BenchmarkPartitionError(b *testing.B) {
+	isPositive := N.MoreThan(0)
+	partition := Partition(isPositive, errors.New("not positive"))
+	input := Left[int](errors.New("error"))
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = partition(input)
+	}
+}
+
+func BenchmarkFilter(b *testing.B) {
+	isPositive := N.MoreThan(0)
+	filter := Filter(isPositive, errors.New("not positive"))
+	input := Of(42)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = filter(input)
+	}
+}
+
+func BenchmarkFilterError(b *testing.B) {
+	isPositive := N.MoreThan(0)
+	filter := Filter(isPositive, errors.New("not positive"))
+	input := Left[int](errors.New("error"))
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = filter(input)
+	}
+}
+
+func BenchmarkFilterChained(b *testing.B) {
+	isPositive := N.MoreThan(0)
+	isEven := func(n int) bool { return n%2 == 0 }
+	filterPositive := Filter(isPositive, errors.New("not positive"))
+	filterEven := Filter(isEven, errors.New("not even"))
+	input := Of(42)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = filterEven(filterPositive(input))
+	}
+}
+
+func BenchmarkFilterMap(b *testing.B) {
+	parseInt := func(s string) O.Option[int] {
+		if n, err := strconv.Atoi(s); err == nil {
+			return O.Some(n)
+		}
+		return O.None[int]()
+	}
+	filterMap := FilterMap(parseInt, errors.New("invalid"))
+	input := Of("42")
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = filterMap(input)
+	}
+}
+
+func BenchmarkFilterMapError(b *testing.B) {
+	parseInt := func(s string) O.Option[int] {
+		if n, err := strconv.Atoi(s); err == nil {
+			return O.Some(n)
+		}
+		return O.None[int]()
+	}
+	filterMap := FilterMap(parseInt, errors.New("invalid"))
+	input := Left[string](errors.New("error"))
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = filterMap(input)
+	}
+}
+
+func BenchmarkPartitionMap(b *testing.B) {
+	classify := func(n int) E.Either[string, int] {
+		if n < 0 {
+			return E.Left[int]("negative")
+		}
+		return E.Right[string](n * n)
+	}
+	partitionMap := PartitionMap(classify, errors.New("not classified"))
+	input := Of(42)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = partitionMap(input)
+	}
+}
+
+func BenchmarkPartitionMapError(b *testing.B) {
+	classify := func(n int) E.Either[string, int] {
+		if n < 0 {
+			return E.Left[int]("negative")
+		}
+		return E.Right[string](n * n)
+	}
+	partitionMap := PartitionMap(classify, errors.New("not classified"))
+	input := Left[int](errors.New("error"))
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = partitionMap(input)
+	}
+}

v2/result/types.go

@@ -22,6 +22,7 @@ import (
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/lens"
 	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/predicate"
 	"github.com/IBM/fp-go/v2/reader"
 )
@@ -61,4 +62,6 @@ type (
 	// Predicate represents a function that tests a value of type A and returns a boolean.
 	// It's commonly used for filtering and conditional operations.
 	Predicate[A any] = predicate.Predicate[A]
+
+	Pair[L, R any] = pair.Pair[L, R]
 )