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/.bob/mcp.json

@@ -1 +0,0 @@
-{"mcpServers":{}}

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/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/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/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 +126,60 @@ 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,
@@ -57,6 +187,147 @@ func ChainLeft[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
 	)
 }
 
+// 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)
 }
@@ -138,3 +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

@@ -17,11 +17,60 @@ 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
@@ -219,4 +268,79 @@ type (
 	//	    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

@@ -18,11 +18,10 @@ package codec
 import (
 	"fmt"
 
-	"github.com/IBM/fp-go/v2/array"
 	"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"
-	"github.com/IBM/fp-go/v2/optics/codec/validation"
 )
 
 // encodeEither creates an encoder for Either[A, B] values.
@@ -151,28 +150,20 @@ func validateEither[A, B, O, I any](
 	rightItem Type[B, O, I],
 ) Validate[I, either.Either[A, B]] {
 
-	return func(i I) Decode[Context, either.Either[A, B]] {
-		valRight := rightItem.Validate(i)
-		valLeft := leftItem.Validate(i)
+	valRight := F.Pipe1(
+		rightItem.Validate,
+		validate.Map[I, B](either.Right[A]),
+	)
 
-		return func(ctx Context) Validation[either.Either[A, B]] {
+	valLeft := F.Pipe1(
+		leftItem.Validate,
+		validate.Map[I, A](either.Left[B]),
+	)
 
-			resRight := valRight(ctx)
-
-			return either.Fold(
-				func(rightErrors validate.Errors) Validation[either.Either[A, B]] {
-					resLeft := valLeft(ctx)
-					return either.Fold(
-						func(leftErrors validate.Errors) Validation[either.Either[A, B]] {
-							return validation.Failures[either.Either[A, B]](array.Concat(leftErrors)(rightErrors))
-						},
-						F.Flow2(either.Left[B, A], validation.Of),
-					)(resLeft)
-				},
-				F.Flow2(either.Right[A, B], validation.Of),
-			)(resRight)
-		}
-	}
+	return F.Pipe1(
+		valRight,
+		validate.Alt(lazy.Of(valLeft)),
+	)
 }
 
 // Either creates a codec for Either[A, B] values.
@@ -265,12 +256,9 @@ func Either[A, B, O, I any](
 	leftItem Type[A, O, I],
 	rightItem Type[B, O, I],
 ) Type[either.Either[A, B], O, I] {
-	name := fmt.Sprintf("Either[%s, %s]", leftItem.Name(), rightItem.Name())
-	isEither := Is[either.Either[A, B]]()
-
 	return MakeType(
-		name,
-		isEither,
+		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

@@ -342,6 +342,27 @@ func TestEitherErrorAccumulation(t *testing.T) {
 
 		require.NotNil(t, errors)
 		// Should have errors from both string and int validation attempts
-		assert.NotEmpty(t, errors)
+		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/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

@@ -17,6 +17,7 @@ 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"
@@ -271,4 +272,6 @@ type (
 	//   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"
@@ -429,6 +430,145 @@ func ChainLeft[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
 	)
 }
 
+// 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"
@@ -628,3 +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/validation/monad.go

@@ -474,3 +474,168 @@ func Applicative[A, B any]() applicative.Applicative[A, B, Validation[A], Valida
 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

@@ -131,17 +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)
 		})
 	})
 
@@ -156,11 +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) {
@@ -169,11 +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) {
@@ -184,11 +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)
 		})
 	})
 }
@@ -235,21 +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) {
@@ -258,17 +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)
 		})
 	})
 }
@@ -322,11 +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) {
@@ -334,10 +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

@@ -18,6 +18,7 @@ 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"
@@ -257,4 +258,6 @@ type (
 	//	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/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/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/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]
 )