fix: add support for go 1.26

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

.github/workflows/build.yml

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

v2/array/array.go

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

v2/array/array_test.go

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

v2/array/generic/array.go

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

v2/array/generic/zip.go

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

v2/array/zip.go

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

v2/array/zip_test.go

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

v2/di/erasure/injector.go

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

v2/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

@@ -20,6 +20,7 @@ import (
 
 	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"
 )
@@ -270,3 +271,210 @@ func MonadAlt[A, O, I any](first Type[A, O, I], second Lazy[Type[A, O, I]]) Type
 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

@@ -561,3 +561,361 @@ func TestAltErrorMessages(t *testing.T) {
 		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/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]
@@ -99,9 +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[A, B, O, I any]  = Reader[A, Type[B, O, I]]
+	// 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/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]
 )