doc: add fp-go/v2 LLM reference guide (#160)

Multi-layered documentation system for Claude Code sessions:
- fp-go-claude.md: compact CLAUDE.md snippet (260 lines)
- fp-go-cookbook.md: 20 migration recipes (1198 lines)
- fp-go-core-patterns.md: core types and composition (1598 lines)
- fp-go-mastery.md: advanced FP techniques (1437 lines)
- fp-go-full-reference.md: complete API inventory, 61 packages (6080 lines)

All type signatures extracted from actual source code.

Signed-off-by: franchb <hello@franchb.com>

v2/AGENTS.md

@@ -2,14 +2,30 @@
 
 This document provides guidelines for AI agents working on the fp-go/v2 project.
 
+## Table of Contents
+
+- [Documentation Standards](#documentation-standards)
+  - [Go Doc Comments](#go-doc-comments)
+  - [File Headers](#file-headers)
+- [Testing Standards](#testing-standards)
+  - [Test Structure](#test-structure)
+  - [Test Coverage](#test-coverage)
+  - [Example Test Pattern](#example-test-pattern)
+- [Code Style](#code-style)
+  - [Functional Patterns](#functional-patterns)
+  - [Error Handling](#error-handling)
+- [Checklist for New Code](#checklist-for-new-code)
+
 ## Documentation Standards
 
 ### Go Doc Comments
 
 1. **Use Standard Go Doc Format**
    - Do NOT use markdown-style links like `[text](url)`
+   - Do NOT use markdown-style headers like `# Section` or `## Subsection`
    - Use simple type references: `ReaderResult`, `Validate[I, A]`, `validation.Success`
    - Go's documentation system will automatically create links
+   - Use plain text with blank lines to separate sections
 
 2. **Structure**
    ```go
@@ -17,24 +33,20 @@ This document provides guidelines for AI agents working on the fp-go/v2 project.
    //
    // Longer description explaining the purpose and behavior.
    //
-   // # Type Parameters
-   //
+   // Type Parameters:
    //   - T: Description of type parameter
    //
-   // # Parameters
-   //
+   // Parameters:
    //   - param: Description of parameter
    //
-   // # Returns
-   //
+   // Returns:
    //   - ReturnType: Description of return value
    //
-   // # Example Usage
+   // Example:
    //
    //   code example here
    //
-   // # See Also
-   //
+   // See Also:
    //   - RelatedFunction: Brief description
    func FunctionName[T any](param T) ReturnType {
    ```
@@ -43,6 +55,7 @@ This document provides guidelines for AI agents working on the fp-go/v2 project.
    - Use idiomatic Go patterns
    - Prefer `result.Eitherize1(strconv.Atoi)` over manual error handling
    - Show realistic, runnable examples
+   - Indent code examples with spaces (not tabs) for proper godoc rendering
 
 ### File Headers
 
@@ -102,6 +115,50 @@ Always include the Apache 2.0 license header:
    - Use `result.Of` for success values
    - Use `result.Left` for error values
 
+4. **Folding Either/Result Values in Tests**
+   - Use `F.Pipe1(result, Fold(onLeft, onRight))` — avoid the `_ = Fold(...)(result)` discard pattern
+   - Use `slices.Collect[T]` instead of a manual `for n := range seq { collected = append(...) }` loop
+   - Use `t.Fatal` in the unexpected branch to combine the `IsLeft`/`IsRight` check with value extraction:
+     ```go
+     // Good: single fold combines assertion and extraction
+     collected := F.Pipe1(result, Fold(
+         func(e error) []int { t.Fatal(e); return nil },
+         slices.Collect[int],
+     ))
+
+     // Avoid: separate IsRight check + manual loop
+     assert.True(t, IsRight(result))
+     var collected []int
+     _ = MonadFold(result,
+         func(e error) []int { return nil },
+         func(seq iter.Seq[int]) []int {
+             for n := range seq { collected = append(collected, n) }
+             return collected
+         },
+     )
+     ```
+   - Use `F.Identity[error]` as the Left branch when extracting an error value:
+     ```go
+     err := F.Pipe1(result, Fold(
+         F.Identity[error],
+         func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+     ))
+     ```
+   - Extract repeated fold patterns as local helper closures within the test function:
+     ```go
+     collectInts := func(r Result[iter.Seq[int]]) []int {
+         return F.Pipe1(r, Fold(
+             func(e error) []int { t.Fatal(e); return nil },
+             slices.Collect[int],
+         ))
+     }
+     ```
+
+5. **Other Test Style Details**
+   - Use `for i := range 10` instead of `for i := 0; i < 10; i++`
+   - Chain curried calls directly: `TraverseSeq(parse)(input)` — no need for an intermediate `traverseFn` variable
+   - Use direct slice literals (`[]string{"a", "b"}`) rather than `A.From("a", "b")` in tests
+
 ### Test Coverage
 
 Include tests for:
@@ -168,56 +225,6 @@ func TestFromReaderResult_Success(t *testing.T) {
    - Check error context is preserved
    - Test error accumulation when applicable
 
-## Common Patterns
-
-### Converting Error-Based Functions
-
-```go
-// Good: Use Eitherize1
-parseIntRR := result.Eitherize1(strconv.Atoi)
-
-// Avoid: Manual error handling
-parseIntRR := func(input string) result.Result[int] {
-    val, err := strconv.Atoi(input)
-    if err != nil {
-        return result.Left[int](err)
-    }
-    return result.Of(val)
-}
-```
-
-### Testing Validation Results
-
-```go
-// Good: Direct comparison
-assert.Equal(t, validation.Success(42), result)
-
-// Avoid: Verbose extraction (unless you need to verify specific fields)
-assert.True(t, either.IsRight(result))
-value := either.MonadFold(result,
-    func(Errors) int { return 0 },
-    F.Identity[int],
-)
-assert.Equal(t, 42, value)
-```
-
-### Documentation Examples
-
-```go
-// Good: Concise and idiomatic
-//  parseIntRR := result.Eitherize1(strconv.Atoi)
-//  validator := FromReaderResult[string, int](parseIntRR)
-
-// Avoid: Verbose manual patterns
-//  parseIntRR := func(input string) result.Result[int] {
-//      val, err := strconv.Atoi(input)
-//      if err != nil {
-//          return result.Left[int](err)
-//      }
-//      return result.Of(val)
-//  }
-```
-
 ## Checklist for New Code
 
 - [ ] Apache 2.0 license header included

v2/array/array_test.go

@@ -198,11 +198,228 @@ func TestFilterMap(t *testing.T) {
 }
 
 func TestFoldMap(t *testing.T) {
-	src := From("a", "b", "c")
+	t.Run("FoldMap with 0 items", func(t *testing.T) {
+		empty := []int{}
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(empty)
+		assert.Equal(t, 0, result, "FoldMap should return monoid empty for 0 items")
+	})
 
-	fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+	t.Run("FoldMap with 1 item", func(t *testing.T) {
+		single := From(5)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(single)
+		assert.Equal(t, 10, result, "FoldMap should map and return single item")
+	})
 
-	assert.Equal(t, "ABC", fold(src))
+	t.Run("FoldMap with 2 items", func(t *testing.T) {
+		two := From(3, 4)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(two)
+		assert.Equal(t, 14, result, "FoldMap should map and fold 2 items: (3*2) + (4*2) = 14")
+	})
+
+	t.Run("FoldMap with many items", func(t *testing.T) {
+		many := From(1, 2, 3, 4, 5)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMap[int](sumMonoid)(N.Mul(2))
+		result := foldMap(many)
+		assert.Equal(t, 30, result, "FoldMap should map and fold many items: (1*2) + (2*2) + (3*2) + (4*2) + (5*2) = 30")
+	})
+
+	t.Run("FoldMap with string concatenation - 0 items", func(t *testing.T) {
+		empty := []string{}
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(empty)
+		assert.Equal(t, "", result, "FoldMap should return empty string for 0 items")
+	})
+
+	t.Run("FoldMap with string concatenation - 1 item", func(t *testing.T) {
+		single := From("a")
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(single)
+		assert.Equal(t, "A", result, "FoldMap should map single string")
+	})
+
+	t.Run("FoldMap with string concatenation - 2 items", func(t *testing.T) {
+		two := From("a", "b")
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(two)
+		assert.Equal(t, "AB", result, "FoldMap should map and concatenate 2 strings")
+	})
+
+	t.Run("FoldMap with string concatenation - many items", func(t *testing.T) {
+		many := From("a", "b", "c", "d", "e")
+		fold := FoldMap[string](S.Monoid)(strings.ToUpper)
+		result := fold(many)
+		assert.Equal(t, "ABCDE", result, "FoldMap should map and concatenate many strings")
+	})
+}
+
+func TestFold(t *testing.T) {
+	t.Run("Fold with 0 items", func(t *testing.T) {
+		empty := []int{}
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(empty)
+		assert.Equal(t, 0, result, "Fold should return monoid empty for 0 items")
+	})
+
+	t.Run("Fold with 1 item", func(t *testing.T) {
+		single := From(42)
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(single)
+		assert.Equal(t, 42, result, "Fold should return single item")
+	})
+
+	t.Run("Fold with 2 items", func(t *testing.T) {
+		two := From(10, 20)
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(two)
+		assert.Equal(t, 30, result, "Fold should combine 2 items: 10 + 20 = 30")
+	})
+
+	t.Run("Fold with many items", func(t *testing.T) {
+		many := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		sumMonoid := N.MonoidSum[int]()
+		fold := Fold[int](sumMonoid)
+		result := fold(many)
+		assert.Equal(t, 55, result, "Fold should combine many items: 1+2+3+4+5+6+7+8+9+10 = 55")
+	})
+
+	t.Run("Fold with string concatenation - 0 items", func(t *testing.T) {
+		empty := []string{}
+		fold := Fold[string](S.Monoid)
+		result := fold(empty)
+		assert.Equal(t, "", result, "Fold should return empty string for 0 items")
+	})
+
+	t.Run("Fold with string concatenation - 1 item", func(t *testing.T) {
+		single := From("hello")
+		fold := Fold[string](S.Monoid)
+		result := fold(single)
+		assert.Equal(t, "hello", result, "Fold should return single string")
+	})
+
+	t.Run("Fold with string concatenation - 2 items", func(t *testing.T) {
+		two := From("hello", "world")
+		fold := Fold[string](S.Monoid)
+		result := fold(two)
+		assert.Equal(t, "helloworld", result, "Fold should concatenate 2 strings")
+	})
+
+	t.Run("Fold with string concatenation - many items", func(t *testing.T) {
+		many := From("a", "b", "c", "d", "e", "f")
+		fold := Fold[string](S.Monoid)
+		result := fold(many)
+		assert.Equal(t, "abcdef", result, "Fold should concatenate many strings")
+	})
+
+	t.Run("Fold with product monoid - 0 items", func(t *testing.T) {
+		empty := []int{}
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(empty)
+		assert.Equal(t, 1, result, "Fold should return monoid empty (1) for product with 0 items")
+	})
+
+	t.Run("Fold with product monoid - 1 item", func(t *testing.T) {
+		single := From(7)
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(single)
+		assert.Equal(t, 7, result, "Fold should return single item for product")
+	})
+
+	t.Run("Fold with product monoid - 2 items", func(t *testing.T) {
+		two := From(3, 4)
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(two)
+		assert.Equal(t, 12, result, "Fold should multiply 2 items: 3 * 4 = 12")
+	})
+
+	t.Run("Fold with product monoid - many items", func(t *testing.T) {
+		many := From(2, 3, 4, 5)
+		productMonoid := N.MonoidProduct[int]()
+		fold := Fold[int](productMonoid)
+		result := fold(many)
+		assert.Equal(t, 120, result, "Fold should multiply many items: 2*3*4*5 = 120")
+	})
+}
+func TestFoldMapWithIndex(t *testing.T) {
+	t.Run("FoldMapWithIndex with 0 items", func(t *testing.T) {
+		empty := []int{}
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i + x })
+		result := foldMap(empty)
+		assert.Equal(t, 0, result, "FoldMapWithIndex should return monoid empty for 0 items")
+	})
+
+	t.Run("FoldMapWithIndex with 1 item", func(t *testing.T) {
+		single := From(10)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i + x })
+		result := foldMap(single)
+		assert.Equal(t, 10, result, "FoldMapWithIndex should map with index: 0 + 10 = 10")
+	})
+
+	t.Run("FoldMapWithIndex with 2 items", func(t *testing.T) {
+		two := From(10, 20)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i + x })
+		result := foldMap(two)
+		assert.Equal(t, 31, result, "FoldMapWithIndex should map with indices: (0+10) + (1+20) = 31")
+	})
+
+	t.Run("FoldMapWithIndex with many items", func(t *testing.T) {
+		many := From(5, 10, 15, 20)
+		sumMonoid := N.MonoidSum[int]()
+		foldMap := FoldMapWithIndex[int](sumMonoid)(func(i, x int) int { return i * x })
+		result := foldMap(many)
+		assert.Equal(t, 100, result, "FoldMapWithIndex should map with indices: (0*5) + (1*10) + (2*15) + (3*20) = 100")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - 0 items", func(t *testing.T) {
+		empty := []string{}
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("%d:%s", i, s)
+		})
+		result := foldMap(empty)
+		assert.Equal(t, "", result, "FoldMapWithIndex should return empty string for 0 items")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - 1 item", func(t *testing.T) {
+		single := From("a")
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("%d:%s", i, s)
+		})
+		result := foldMap(single)
+		assert.Equal(t, "0:a", result, "FoldMapWithIndex should format single item with index")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - 2 items", func(t *testing.T) {
+		two := From("a", "b")
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("%d:%s,", i, s)
+		})
+		result := foldMap(two)
+		assert.Equal(t, "0:a,1:b,", result, "FoldMapWithIndex should format 2 items with indices")
+	})
+
+	t.Run("FoldMapWithIndex with string concatenation - many items", func(t *testing.T) {
+		many := From("a", "b", "c", "d")
+		foldMap := FoldMapWithIndex[string](S.Monoid)(func(i int, s string) string {
+			return fmt.Sprintf("[%d]%s", i, s)
+		})
+		result := foldMap(many)
+		assert.Equal(t, "[0]a[1]b[2]c[3]d", result, "FoldMapWithIndex should format many items with indices")
+	})
 }
 
 func ExampleFoldMap() {
@@ -767,6 +984,25 @@ func TestExtendUseCases(t *testing.T) {
 
 // TestConcat tests the Concat function
 func TestConcat(t *testing.T) {
+	t.Run("Semantic: Concat(b)(a) produces [a... b...]", func(t *testing.T) {
+		a := []int{1, 2, 3}
+		b := []int{4, 5, 6}
+
+		// Concat(b)(a) should produce [a... b...]
+		result := Concat(b)(a)
+		expected := []int{1, 2, 3, 4, 5, 6}
+
+		assert.Equal(t, expected, result, "Concat(b)(a) should produce [a... b...]")
+
+		// Verify order: a's elements come first, then b's elements
+		assert.Equal(t, a[0], result[0], "First element should be from a")
+		assert.Equal(t, a[1], result[1], "Second element should be from a")
+		assert.Equal(t, a[2], result[2], "Third element should be from a")
+		assert.Equal(t, b[0], result[3], "Fourth element should be from b")
+		assert.Equal(t, b[1], result[4], "Fifth element should be from b")
+		assert.Equal(t, b[2], result[5], "Sixth element should be from b")
+	})
+
 	t.Run("Concat two non-empty arrays", func(t *testing.T) {
 		base := []int{1, 2, 3}
 		toAppend := []int{4, 5, 6}
@@ -870,6 +1106,54 @@ func TestConcat(t *testing.T) {
 		expected := []int{1, 2, 3}
 		assert.Equal(t, expected, result)
 	})
+	t.Run("Explicit append semantic demonstration", func(t *testing.T) {
+		// Given a base array
+		base := []string{"A", "B", "C"}
+
+		// And a suffix to append
+		suffix := []string{"D", "E", "F"}
+
+		// When we apply Concat(suffix) to base
+		appendSuffix := Concat(suffix)
+		result := appendSuffix(base)
+
+		// Then the result should be base followed by suffix
+		expected := []string{"A", "B", "C", "D", "E", "F"}
+		assert.Equal(t, expected, result)
+
+		// And the base should be unchanged
+		assert.Equal(t, []string{"A", "B", "C"}, base)
+
+		// And the suffix should be unchanged
+		assert.Equal(t, []string{"D", "E", "F"}, suffix)
+	})
+
+	t.Run("Append semantic with different types", func(t *testing.T) {
+		// Integers
+		intResult := Concat([]int{4, 5})([]int{1, 2, 3})
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, intResult)
+
+		// Strings
+		strResult := Concat([]string{"world"})([]string{"hello"})
+		assert.Equal(t, []string{"hello", "world"}, strResult)
+
+		// Floats
+		floatResult := Concat([]float64{3.3, 4.4})([]float64{1.1, 2.2})
+		assert.Equal(t, []float64{1.1, 2.2, 3.3, 4.4}, floatResult)
+	})
+
+	t.Run("Append semantic in pipeline", func(t *testing.T) {
+		// Start with [1, 2, 3]
+		// Append [4, 5] to get [1, 2, 3, 4, 5]
+		// Append [6, 7] to get [1, 2, 3, 4, 5, 6, 7]
+		result := F.Pipe2(
+			[]int{1, 2, 3},
+			Concat([]int{4, 5}),
+			Concat([]int{6, 7}),
+		)
+		expected := []int{1, 2, 3, 4, 5, 6, 7}
+		assert.Equal(t, expected, result)
+	})
 }
 
 // TestConcatComposition tests Concat with other array operations

v2/array/generic/array.go

@@ -323,34 +323,49 @@ func Clone[AS ~[]A, A any](f func(A) A) func(as AS) AS {
 }
 
 func FoldMap[AS ~[]A, A, B any](m M.Monoid[B]) func(func(A) B) func(AS) B {
-	empty := m.Empty()
 	concat := m.Concat
 	return func(f func(A) B) func(AS) B {
 		return func(as AS) B {
-			return array.Reduce(as, func(cur B, a A) B {
-				return concat(cur, f(a))
-			}, empty)
+			switch len(as) {
+			case 0:
+				return m.Empty()
+			case 1:
+				return f(as[0])
+			case 2:
+				return concat(f(as[0]), f(as[1]))
+			default:
+				return array.Reduce(as[1:], func(cur B, a A) B {
+					return concat(cur, f(a))
+				}, f(as[0]))
+			}
 		}
 	}
 }
 
 func FoldMapWithIndex[AS ~[]A, A, B any](m M.Monoid[B]) func(func(int, A) B) func(AS) B {
-	empty := m.Empty()
 	concat := m.Concat
 	return func(f func(int, A) B) func(AS) B {
 		return func(as AS) B {
 			return array.ReduceWithIndex(as, func(idx int, cur B, a A) B {
 				return concat(cur, f(idx, a))
-			}, empty)
+			}, m.Empty())
 		}
 	}
 }
 
 func Fold[AS ~[]A, A any](m M.Monoid[A]) func(AS) A {
-	empty := m.Empty()
 	concat := m.Concat
 	return func(as AS) A {
-		return array.Reduce(as, concat, empty)
+		switch len(as) {
+		case 0:
+			return m.Empty()
+		case 1:
+			return as[0]
+		case 2:
+			return concat(as[0], as[1])
+		default:
+			return array.Reduce(as[1:], concat, as[0])
+		}
 	}
 }
 
@@ -474,3 +489,16 @@ func Extend[GA ~[]A, GB ~[]B, A, B any](f func(GA) B) func(GA) GB {
 		return MakeBy[GB](len(as), func(i int) B { return f(as[i:]) })
 	}
 }
+
+func UpdateAt[GT ~[]T, T any](i int, v T) func(GT) O.Option[GT] {
+	none := O.None[GT]()
+	if i < 0 {
+		return F.Constant1[GT](none)
+	}
+	return func(g GT) O.Option[GT] {
+		if i >= len(g) {
+			return none
+		}
+		return O.Of(array.UnsafeUpdateAt(g, i, v))
+	}
+}

v2/array/sequence.go

@@ -25,7 +25,7 @@ func MonadSequence[HKTA, HKTRA any](
 	fof func(HKTA) HKTRA,
 	m M.Monoid[HKTRA],
 	ma []HKTA) HKTRA {
-	return array.MonadSequence(fof, m.Empty(), m.Concat, ma)
+	return array.MonadSequence(fof, m.Empty, m.Concat, ma)
 }
 
 // Sequence takes an array where elements are HKT<A> (higher kinded type) and,
@@ -67,7 +67,7 @@ func Sequence[HKTA, HKTRA any](
 	fof func(HKTA) HKTRA,
 	m M.Monoid[HKTRA],
 ) func([]HKTA) HKTRA {
-	return array.Sequence[[]HKTA](fof, m.Empty(), m.Concat)
+	return array.Sequence[[]HKTA](fof, m.Empty, m.Concat)
 }
 
 // ArrayOption returns a function to convert a sequence of options into an option of a sequence.

v2/constraints/constraints.go

@@ -13,28 +13,218 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+/*
+Package constraints defines a set of useful type constraints for generic programming in Go.
+
+# Overview
+
+This package provides type constraints that can be used with Go generics to restrict
+type parameters to specific categories of types. These constraints are similar to those
+in Go's standard constraints package but are defined here for consistency within the
+fp-go project.
+
+# Type Constraints
+
+Ordered - Types that support comparison operators:
+
+	type Ordered interface {
+	    Integer | Float | ~string
+	}
+
+Used for types that can be compared using <, <=, >, >= operators.
+
+Integer - All integer types (signed and unsigned):
+
+	type Integer interface {
+	    Signed | Unsigned
+	}
+
+Signed - Signed integer types:
+
+	type Signed interface {
+	    ~int | ~int8 | ~int16 | ~int32 | ~int64
+	}
+
+Unsigned - Unsigned integer types:
+
+	type Unsigned interface {
+	    ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
+	}
+
+Float - Floating-point types:
+
+	type Float interface {
+	    ~float32 | ~float64
+	}
+
+Complex - Complex number types:
+
+	type Complex interface {
+	    ~complex64 | ~complex128
+	}
+
+# Usage Examples
+
+Using Ordered constraint for comparison:
+
+	import C "github.com/IBM/fp-go/v2/constraints"
+
+	func Min[T C.Ordered](a, b T) T {
+	    if a < b {
+	        return a
+	    }
+	    return b
+	}
+
+	result := Min(5, 3)        // 3
+	result := Min(3.14, 2.71)  // 2.71
+	result := Min("apple", "banana")  // "apple"
+
+Using Integer constraint:
+
+	func Abs[T C.Integer](n T) T {
+	    if n < 0 {
+	        return -n
+	    }
+	    return n
+	}
+
+	result := Abs(-42)  // 42
+	result := Abs(uint(10))  // 10
+
+Using Float constraint:
+
+	func Average[T C.Float](a, b T) T {
+	    return (a + b) / 2
+	}
+
+	result := Average(3.14, 2.86)  // 3.0
+
+Using Complex constraint:
+
+	func Magnitude[T C.Complex](c T) float64 {
+	    r, i := real(c), imag(c)
+	    return math.Sqrt(r*r + i*i)
+	}
+
+	c := complex(3, 4)
+	result := Magnitude(c)  // 5.0
+
+# Combining Constraints
+
+Constraints can be combined to create more specific type restrictions:
+
+	type Number interface {
+	    C.Integer | C.Float | C.Complex
+	}
+
+	func Add[T Number](a, b T) T {
+	    return a + b
+	}
+
+# Tilde Operator
+
+The ~ operator in type constraints means "underlying type". For example, ~int
+matches not only int but also any type whose underlying type is int:
+
+	type MyInt int
+
+	func Double[T C.Integer](n T) T {
+	    return n * 2
+	}
+
+	var x MyInt = 5
+	result := Double(x)  // Works because MyInt's underlying type is int
+
+# Related Packages
+
+  - number: Provides algebraic structures and utilities for numeric types
+  - ord: Provides ordering operations using these constraints
+  - eq: Provides equality operations for comparable types
+*/
 package constraints
 
+// Ordered is a constraint that permits any ordered type: any type that supports
+// the operators < <= >= >. Ordered types include integers, floats, and strings.
+//
+// This constraint is commonly used for comparison operations, sorting, and
+// finding minimum/maximum values.
+//
+// Example:
+//
+//	func Max[T Ordered](a, b T) T {
+//	    if a > b {
+//	        return a
+//	    }
+//	    return b
+//	}
 type Ordered interface {
 	Integer | Float | ~string
 }
 
+// Signed is a constraint that permits any signed integer type.
+// This includes int, int8, int16, int32, and int64, as well as any
+// types whose underlying type is one of these.
+//
+// Example:
+//
+//	func Negate[T Signed](n T) T {
+//	    return -n
+//	}
 type Signed interface {
 	~int | ~int8 | ~int16 | ~int32 | ~int64
 }
 
+// Unsigned is a constraint that permits any unsigned integer type.
+// This includes uint, uint8, uint16, uint32, uint64, and uintptr, as well
+// as any types whose underlying type is one of these.
+//
+// Example:
+//
+//	func IsEven[T Unsigned](n T) bool {
+//	    return n%2 == 0
+//	}
 type Unsigned interface {
 	~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr
 }
 
+// Integer is a constraint that permits any integer type, both signed and unsigned.
+// This is a union of the Signed and Unsigned constraints.
+//
+// Example:
+//
+//	func Abs[T Integer](n T) T {
+//	    if n < 0 {
+//	        return -n
+//	    }
+//	    return n
+//	}
 type Integer interface {
 	Signed | Unsigned
 }
 
+// Float is a constraint that permits any floating-point type.
+// This includes float32 and float64, as well as any types whose
+// underlying type is one of these.
+//
+// Example:
+//
+//	func Round[T Float](f T) T {
+//	    return T(math.Round(float64(f)))
+//	}
 type Float interface {
 	~float32 | ~float64
 }
 
+// Complex is a constraint that permits any complex numeric type.
+// This includes complex64 and complex128, as well as any types whose
+// underlying type is one of these.
+//
+// Example:
+//
+//	func Conjugate[T Complex](c T) T {
+//	    return complex(real(c), -imag(c))
+//	}
 type Complex interface {
 	~complex64 | ~complex128
 }

v2/context/readerioresult/filter.go

@@ -18,6 +18,10 @@ package readerioresult
 import (
 	"context"
 
+	"github.com/IBM/fp-go/v2/array"
+	"github.com/IBM/fp-go/v2/internal/witherable"
+	"github.com/IBM/fp-go/v2/iterator/iter"
+	"github.com/IBM/fp-go/v2/option"
 	RIOR "github.com/IBM/fp-go/v2/readerioresult"
 )
 
@@ -49,3 +53,43 @@ import (
 func FilterOrElse[A any](pred Predicate[A], onFalse func(A) error) Operator[A, A] {
 	return RIOR.FilterOrElse[context.Context](pred, onFalse)
 }
+
+//go:inline
+func Filter[HKTA, A any](
+	filter func(Predicate[A]) Endomorphism[HKTA],
+) func(Predicate[A]) Operator[HKTA, HKTA] {
+	return witherable.Filter(
+		Map,
+		filter,
+	)
+}
+
+//go:inline
+func FilterArray[A any](p Predicate[A]) Operator[[]A, []A] {
+	return Filter(array.Filter[A])(p)
+}
+
+//go:inline
+func FilterIter[A any](p Predicate[A]) Operator[Seq[A], Seq[A]] {
+	return Filter(iter.Filter[A])(p)
+}
+
+//go:inline
+func FilterMap[HKTA, HKTB, A, B any](
+	filter func(option.Kleisli[A, B]) Reader[HKTA, HKTB],
+) func(option.Kleisli[A, B]) Operator[HKTA, HKTB] {
+	return witherable.FilterMap(
+		Map,
+		filter,
+	)
+}
+
+//go:inline
+func FilterMapArray[A, B any](p option.Kleisli[A, B]) Operator[[]A, []B] {
+	return FilterMap(array.FilterMap[A, B])(p)
+}
+
+//go:inline
+func FilterMapIter[A, B any](p option.Kleisli[A, B]) Operator[Seq[A], Seq[B]] {
+	return FilterMap(iter.FilterMap[A, B])(p)
+}

v2/context/readerioresult/type.go

@@ -17,6 +17,7 @@ package readerioresult
 
 import (
 	"context"
+	"iter"
 
 	"github.com/IBM/fp-go/v2/consumer"
 	"github.com/IBM/fp-go/v2/context/ioresult"
@@ -220,4 +221,10 @@ type (
 	// The first element is the CancelFunc that should be called to release resources.
 	// The second element is the new Context that was created.
 	ContextCancel = Pair[context.CancelFunc, context.Context]
+
+	// Seq is an iterator over sequences of individual values.
+	// When called as seq(yield), seq calls yield(v) for each value v in the sequence,
+	// stopping early if yield returns false.
+	// See the [iter] package documentation for more details.
+	Seq[A any] = iter.Seq[A]
 )

v2/context/readerreaderioresult/filter.go

@@ -0,0 +1,48 @@
+package readerreaderioresult
+
+import (
+	"github.com/IBM/fp-go/v2/array"
+	"github.com/IBM/fp-go/v2/internal/witherable"
+	"github.com/IBM/fp-go/v2/iterator/iter"
+	"github.com/IBM/fp-go/v2/option"
+)
+
+//go:inline
+func Filter[C, HKTA, A any](
+	filter func(Predicate[A]) Endomorphism[HKTA],
+) func(Predicate[A]) Operator[C, HKTA, HKTA] {
+	return witherable.Filter(
+		Map[C],
+		filter,
+	)
+}
+
+//go:inline
+func FilterArray[C, A any](p Predicate[A]) Operator[C, []A, []A] {
+	return Filter[C](array.Filter[A])(p)
+}
+
+//go:inline
+func FilterIter[C, A any](p Predicate[A]) Operator[C, Seq[A], Seq[A]] {
+	return Filter[C](iter.Filter[A])(p)
+}
+
+//go:inline
+func FilterMap[C, HKTA, HKTB, A, B any](
+	filter func(option.Kleisli[A, B]) Reader[HKTA, HKTB],
+) func(option.Kleisli[A, B]) Operator[C, HKTA, HKTB] {
+	return witherable.FilterMap(
+		Map[C],
+		filter,
+	)
+}
+
+//go:inline
+func FilterMapArray[C, A, B any](p option.Kleisli[A, B]) Operator[C, []A, []B] {
+	return FilterMap[C](array.FilterMap[A, B])(p)
+}
+
+//go:inline
+func FilterMapIter[C, A, B any](p option.Kleisli[A, B]) Operator[C, Seq[A], Seq[B]] {
+	return FilterMap[C](iter.FilterMap[A, B])(p)
+}

v2/context/readerreaderioresult/reader.go

@@ -834,7 +834,7 @@ func Flap[R, B, A any](a A) Operator[R, func(A) B, B] {
 // This is the monadic version that takes the computation as the first parameter.
 //
 //go:inline
-func MonadMapLeft[R, A any](fa ReaderReaderIOResult[R, A], f Endmorphism[error]) ReaderReaderIOResult[R, A] {
+func MonadMapLeft[R, A any](fa ReaderReaderIOResult[R, A], f Endomorphism[error]) ReaderReaderIOResult[R, A] {
 	return RRIOE.MonadMapLeft(fa, f)
 }
 
@@ -843,7 +843,7 @@ func MonadMapLeft[R, A any](fa ReaderReaderIOResult[R, A], f Endmorphism[error])
 // This is the curried version that returns an operator.
 //
 //go:inline
-func MapLeft[R, A any](f Endmorphism[error]) Operator[R, A, A] {
+func MapLeft[R, A any](f Endomorphism[error]) Operator[R, A, A] {
 	return RRIOE.MapLeft[R, context.Context, A](f)
 }
 

v2/context/readerreaderioresult/types.go

@@ -24,6 +24,7 @@ import (
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioeither"
 	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/iterator/iter"
 	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/optics/lens"
 	"github.com/IBM/fp-go/v2/optics/traversal/result"
@@ -146,9 +147,15 @@ type (
 	// It's an alias for predicate.Predicate[A].
 	Predicate[A any] = predicate.Predicate[A]
 
-	// Endmorphism represents a function from type A to type A.
+	// Endomorphism represents a function from type A to type A.
 	// It's an alias for endomorphism.Endomorphism[A].
-	Endmorphism[A any] = endomorphism.Endomorphism[A]
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	// Seq is an iterator over sequences of individual values.
+	// When called as seq(yield), seq calls yield(v) for each value v in the sequence,
+	// stopping early if yield returns false.
+	// See the [iter] package documentation for more details.
+	Seq[A any] = iter.Seq[A]
 
 	Void = function.Void
 )

v2/docs/fp-go-guide/fp-go-claude.md

@@ -0,0 +1,260 @@
+# fp-go/v2 Reference for Claude Code
+
+fp-go/v2 (`github.com/IBM/fp-go/v2`) is a typed functional programming library for Go 1.24+. It provides Option, Either/Result, IO, and Effect monads with data-last, curried APIs designed for pipeline composition via `Pipe` and `Flow`. The library follows Haskell/fp-ts conventions adapted for Go generics with explicit arity-numbered functions (e.g., `Pipe3`, `Flow2`). Two module families exist: **standard** (struct-based monads, full FP toolkit) and **idiomatic** (Go-native `(value, error)` tuples, zero-alloc, 2-32x faster).
+
+## Import Conventions
+
+| Alias | Package |
+|-------|---------|
+| `F`   | `github.com/IBM/fp-go/v2/function` |
+| `O`   | `github.com/IBM/fp-go/v2/option` |
+| `E`   | `github.com/IBM/fp-go/v2/either` |
+| `R`   | `github.com/IBM/fp-go/v2/result` |
+| `A`   | `github.com/IBM/fp-go/v2/array` |
+| `IO`  | `github.com/IBM/fp-go/v2/io` |
+| `IOR` | `github.com/IBM/fp-go/v2/ioresult` |
+| `IOE` | `github.com/IBM/fp-go/v2/ioeither` |
+| `RIO` | `github.com/IBM/fp-go/v2/context/readerioresult` |
+| `EFF` | `github.com/IBM/fp-go/v2/effect` |
+| `P`   | `github.com/IBM/fp-go/v2/pair` |
+| `T`   | `github.com/IBM/fp-go/v2/tuple` |
+| `N`   | `github.com/IBM/fp-go/v2/number` |
+| `S`   | `github.com/IBM/fp-go/v2/string` |
+| `B`   | `github.com/IBM/fp-go/v2/boolean` |
+| `L`   | `github.com/IBM/fp-go/v2/optics/lens` |
+| `PR`  | `github.com/IBM/fp-go/v2/optics/prism` |
+
+**Idiomatic variants** (tuple-based, zero-alloc):
+
+| Alias | Package |
+|-------|---------|
+| `IR`  | `github.com/IBM/fp-go/v2/idiomatic/result` |
+| `IO_` | `github.com/IBM/fp-go/v2/idiomatic/option` |
+| `IIR` | `github.com/IBM/fp-go/v2/idiomatic/ioresult` |
+| `IRR` | `github.com/IBM/fp-go/v2/idiomatic/context/readerresult` |
+| `IRO` | `github.com/IBM/fp-go/v2/idiomatic/readerioresult` |
+
+## Monad Selection
+
+- **Pure value** -- use the value directly, no wrapper needed
+- **May be absent** -- `Option[A]` (struct-based) or `(A, bool)` (idiomatic)
+- **Can fail with `error`** -- `Result[A]` = `Either[error, A]`
+  - Need custom error type E -- use `Either[E, A]` instead
+- **Lazy + can fail** -- `IOResult[A]` = `func() Either[error, A]`
+  - Idiomatic: `func() (A, error)`
+- **Needs `context.Context` + lazy + can fail** -- `ReaderIOResult[A]` via `context/readerioresult`
+  - Type: `func(context.Context) func() Either[error, A]`
+  - Idiomatic: `func(context.Context) (A, error)` via `idiomatic/context/readerresult`
+- **Typed DI + context + lazy + can fail** -- `Effect[C, A]` via `effect` package
+  - Type: `func(C) func(context.Context) func() Either[error, A]`
+  - C is your dependency/config struct; context.Context is handled internally
+- **Performance-critical** -- prefer `idiomatic/` variants throughout
+
+## Standard vs Idiomatic
+
+| Aspect | Standard | Idiomatic |
+|--------|----------|-----------|
+| Representation | `Either[error, A]` struct | `(A, error)` tuple |
+| Performance | Baseline | 2-32x faster, zero allocs |
+| Custom error types | `Either[E, A]` for any E | error only |
+| Do-notation | Full support | Full support |
+| FP toolkit | Complete | Complete |
+| Go interop | Requires `Unwrap`/`Eitherize` | Native `(val, err)` |
+
+**Rule of thumb**: Use idiomatic for production code and hot paths. Use standard when you need custom error types (`Either[E, A]`) or when composing with packages that use the standard types.
+
+## Core Types
+
+```go
+// function package
+type Void = struct{}
+var VOID Void = struct{}{}
+
+// option
+type Option[A any] struct { /* Some/None */ }
+
+// either
+type Either[E, A any] struct { /* Left/Right */ }
+
+// result (specialized Either)
+type Result[A any] = Either[error, A]
+
+// io
+type IO[A any] = func() A
+
+// ioresult
+type IOResult[A any] = IO[Result[A]]  // = func() Either[error, A]
+
+// context/readerioresult
+type ReaderIOResult[A any] = func(context.Context) func() Either[error, A]
+
+// effect
+type Effect[C, A any] = func(C) func(context.Context) func() Either[error, A]
+type Kleisli[C, A, B any] = func(A) Effect[C, B]
+
+// idiomatic equivalents
+type IOResult[A any] = func() (A, error)
+type ReaderResult[A any] = func(context.Context) (A, error)
+```
+
+## Key Rules
+
+1. **Data-last**: Configuration/behavior params come first, data comes last. This enables partial application and pipeline composition.
+
+2. **Type parameter ordering**: Non-inferrable type params come first. Example: `Ap[B, E, A]` -- B cannot be inferred, so it leads. `Map[A, B]` -- both usually inferred.
+
+3. **Composition direction**:
+   - `F.Flow1/2/3/.../N` -- left-to-right (use this for pipelines)
+   - `Compose` -- right-to-left (mathematical convention; avoid in pipelines)
+
+4. **Pipe vs Flow**:
+   - `F.Pipe3(value, f1, f2, f3)` -- apply data to a pipeline immediately
+   - `F.Flow3(f1, f2, f3)` -- create a reusable pipeline (returns a function)
+
+5. **Arity-numbered functions**: `Pipe1` through `Pipe20`, `Flow1` through `Flow20`. Choose the number matching your operation count.
+
+6. **Naming conventions**:
+   - `Chain` = flatMap/bind (`A -> F[B]`, flattens)
+   - `Map` = fmap (`A -> B`, lifts into context)
+   - `Ap` = applicative apply (apply wrapped function to wrapped value)
+   - `ChainFirst` / `Tap` = execute for side effects, keep original value
+   - `ChainEitherK` = lift pure `func(A) Either[E, B]` into monadic chain
+   - `Of` = pure/return (lift value into monad)
+   - `Fold` = catamorphism (handle both cases)
+   - `Left` / `Right` = Either constructors
+   - `Some` / `None` = Option constructors
+
+7. **Prefer `result` over `either`** unless you need a custom error type E. `Result[A]` = `Either[error, A]`.
+
+8. **Wrapping Go functions**:
+   - `result.Eitherize1(fn)` wraps `func(X) (Y, error)` into `func(X) Result[Y]`
+   - `result.Eitherize2(fn)` wraps `func(X, Y) (Z, error)` into `func(X, Y) Result[Z]`
+   - Variants up to `Eitherize15`
+
+9. **Use `function.Void` / `function.VOID`** instead of `struct{}` / `struct{}{}`.
+
+10. **Go 1.24+ required** (generic type aliases).
+
+## Common Patterns
+
+### Pipeline with Pipe
+```go
+result := F.Pipe3(
+    inputValue,
+    R.Map(transform),
+    R.Chain(validate),
+    R.Fold(onError, onSuccess),
+)
+```
+
+### Reusable pipeline with Flow
+```go
+pipeline := F.Flow3(
+    R.Map(normalize),
+    R.Chain(validate),
+    R.Map(format),
+)
+output := pipeline(R.Of(input))
+```
+
+### Wrapping Go error functions
+```go
+safeParseInt := R.Eitherize1(strconv.Atoi)
+// safeParseInt: func(string) Result[int]
+result := safeParseInt("42") // Right(42)
+```
+
+### Effect with DI
+```go
+type Deps struct { DB *sql.DB }
+
+fetchUser := EFF.Eitherize(func(deps Deps, ctx context.Context) (*User, error) {
+    return deps.DB.QueryRowContext(ctx, "SELECT ...").Scan(...)
+})
+// fetchUser: Effect[Deps, *User]
+
+// Execute:
+val, err := EFF.RunSync(EFF.Provide[*User](myDeps)(fetchUser))(ctx)
+```
+
+### Effect composition
+```go
+pipeline := F.Pipe1(
+    fetchUser,
+    EFF.Map[Deps](func(u *User) string { return u.Name }),
+)
+```
+
+### Do-notation (building up state)
+```go
+type State struct { X int; Y string }
+
+result := F.Pipe3(
+    R.Do(State{}),
+    R.Bind(
+        func(x int) func(State) State {
+            return func(s State) State { s.X = x; return s }
+        },
+        func(s State) Result[int] { return R.Of(42) },
+    ),
+    R.Let(
+        func(y string) func(State) State {
+            return func(s State) State { s.Y = y; return s }
+        },
+        func(s State) string { return fmt.Sprintf("val=%d", s.X) },
+    ),
+)
+```
+
+### Optics (Lens)
+```go
+type Person struct { Name string; Age int }
+
+nameLens := L.MakeLens(
+    func(p Person) string { return p.Name },
+    func(p Person, name string) Person { p.Name = name; return p },
+)
+
+name := nameLens.Get(person)              // get
+updated := nameLens.Set("Bob")(person)    // set (returns new Person)
+modified := L.Modify(strings.ToUpper)(nameLens)(person) // modify
+```
+
+### Option handling
+```go
+result := F.Pipe3(
+    O.Some(42),
+    O.Map(func(x int) int { return x * 2 }),
+    O.GetOrElse(F.Constant(0)),
+)
+```
+
+### Idiomatic IOResult
+```go
+readFile := func() ([]byte, error) { return os.ReadFile("config.json") }
+// This IS an idiomatic IOResult[[]byte] -- just a func() ([]byte, error)
+
+parsed := IIR.Map(parseConfig)(readFile)
+config, err := parsed()
+```
+
+### ReaderIOResult (context-dependent IO)
+```go
+// Eitherize1 wraps func(context.Context, T0) (R, error) -> func(T0) ReaderIOResult[R]
+fetchURL := RIO.Eitherize1(func(ctx context.Context, url string) ([]byte, error) {
+    req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
+    resp, err := http.DefaultClient.Do(req)
+    if err != nil { return nil, err }
+    defer resp.Body.Close()
+    return iolib.ReadAll(resp.Body)
+})
+// fetchURL: func(string) ReaderIOResult[[]byte]
+result := fetchURL("https://example.com")(ctx)() // execute
+```
+
+## Deeper Documentation
+
+- `fp-go-cookbook.md` -- migration recipes and "how do I X in fp-go?"
+- `fp-go-core-patterns.md` -- core types, operations, and composition details
+- `fp-go-mastery.md` -- advanced FP techniques, architecture, and Effect system
+- `fp-go-full-reference.md` -- complete API inventory across all packages

v2/effect/dependencies.go

@@ -354,3 +354,20 @@ func LocalEffectK[A, C1, C2 any](f Kleisli[C2, C2, C1]) func(Effect[C1, A]) Effe
 func LocalReaderK[A, C1, C2 any](f reader.Kleisli[C2, C1]) func(Effect[C1, A]) Effect[C2, A] {
 	return readerreaderioresult.LocalReaderK[A](f)
 }
+
+// Ask returns an Effect that produces the context C as its success value.
+// This is the fundamental operation of the reader/environment monad,
+// allowing effects to access their own context.
+//
+// # Type Parameters
+//
+//   - C: The context type (also the produced value type)
+//
+// # Returns
+//
+//   - Effect[C, C]: An effect that succeeds with its own context value
+//
+//go:inline
+func Ask[C any]() Effect[C, C] {
+	return readerreaderioresult.Ask[C]()
+}

v2/effect/dependencies_test.go

@@ -19,8 +19,6 @@ import (
 	"context"
 	"fmt"
 	"testing"
-	"time"
-
 	"github.com/IBM/fp-go/v2/context/reader"
 	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
 	"github.com/stretchr/testify/assert"
@@ -922,45 +920,77 @@ func TestLocalReaderK(t *testing.T) {
 		assert.NoError(t, err)
 		assert.Equal(t, "result", result)
 	})
+}
 
-	t.Run("runtime context deadline awareness", func(t *testing.T) {
-		type Config struct {
-			HasDeadline bool
-		}
-
-		// Reader that checks runtime context for deadline
-		checkContext := func(path string) reader.Reader[Config] {
-			return func(ctx context.Context) Config {
-				_, hasDeadline := ctx.Deadline()
-				return Config{HasDeadline: hasDeadline}
-			}
-		}
-
-		// Effect that uses the config
-		configEffect := Chain(func(cfg Config) Effect[Config, string] {
-			return Of[Config](fmt.Sprintf("Has deadline: %v", cfg.HasDeadline))
-		})(readerreaderioresult.Ask[Config]())
-
-		transform := LocalReaderK[string](checkContext)
-		pathEffect := transform(configEffect)
-
-		// Without deadline
-		ioResult := Provide[string]("config.json")(pathEffect)
-		readerResult := RunSync(ioResult)
-		result, err := readerResult(context.Background())
+func TestAsk(t *testing.T) {
+	t.Run("returns context as value", func(t *testing.T) {
+		ctx := "my-context"
+		result, err := runEffect(Ask[string](), ctx)
 
 		assert.NoError(t, err)
-		assert.Equal(t, "Has deadline: false", result)
+		assert.Equal(t, ctx, result)
+	})
 
-		// With deadline
-		ctxWithDeadline, cancel := context.WithTimeout(context.Background(), time.Second)
-		defer cancel()
+	t.Run("works with struct context", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
 
-		ioResult2 := Provide[string]("config.json")(pathEffect)
-		readerResult2 := RunSync(ioResult2)
-		result2, err2 := readerResult2(ctxWithDeadline)
+		cfg := Config{Host: "localhost", Port: 8080}
+		result, err := runEffect(Ask[Config](), cfg)
 
+		assert.NoError(t, err)
+		assert.Equal(t, cfg, result)
+	})
+
+	t.Run("can be chained with Map to extract a field", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
+
+		hostEffect := Map[Config](func(cfg Config) string {
+			return cfg.Host
+		})(Ask[Config]())
+
+		result, err := runEffect(hostEffect, Config{Host: "example.com", Port: 443})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "example.com", result)
+	})
+
+	t.Run("can be chained with Chain to produce a derived effect", func(t *testing.T) {
+		type Config struct {
+			APIKey string
+		}
+
+		derived := Chain(func(cfg Config) Effect[Config, string] {
+			if cfg.APIKey == "" {
+				return Fail[Config, string](assert.AnError)
+			}
+			return Of[Config]("authenticated: " + cfg.APIKey)
+		})(Ask[Config]())
+
+		// Valid key
+		result, err := runEffect(derived, Config{APIKey: "secret"})
+		assert.NoError(t, err)
+		assert.Equal(t, "authenticated: secret", result)
+
+		// Empty key
+		_, err = runEffect(derived, Config{APIKey: ""})
+		assert.Error(t, err)
+		assert.Equal(t, assert.AnError, err)
+	})
+
+	t.Run("is idempotent - multiple calls return same context", func(t *testing.T) {
+		ctx := TestContext{Value: "shared"}
+
+		r1, err1 := runEffect(Ask[TestContext](), ctx)
+		r2, err2 := runEffect(Ask[TestContext](), ctx)
+
+		assert.NoError(t, err1)
 		assert.NoError(t, err2)
-		assert.Equal(t, "Has deadline: true", result2)
+		assert.Equal(t, r1, r2)
 	})
 }

v2/effect/effect.go

@@ -204,6 +204,102 @@ func ChainFirst[C, A, B any](f Kleisli[C, A, B]) Operator[C, A, A] {
 	return readerreaderioresult.ChainFirst(f)
 }
 
+// ChainFirstThunkK chains an effect with a function that returns a Thunk,
+// but discards the result and returns the original value.
+// This is useful for performing side effects (like logging or IO operations) that don't
+// need the effect's context, without changing the value flowing through the computation.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The value type (preserved)
+//   - B: The type produced by the Thunk (discarded)
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns Thunk[B] for side effects
+//
+// # Returns
+//
+//   - Operator[C, A, A]: A function that executes the Thunk but preserves the original value
+//
+// # Example
+//
+//	logToFile := func(n int) readerioresult.ReaderIOResult[any] {
+//	    return func(ctx context.Context) io.IO[result.Result[any]] {
+//	        return func() result.Result[any] {
+//	            // Perform IO operation that doesn't need effect context
+//	            fmt.Printf("Logging: %d\n", n)
+//	            return result.Of[any](nil)
+//	        }
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	logged := effect.ChainFirstThunkK[MyContext](logToFile)(eff)
+//	// Prints "Logging: 42" but still produces 42
+//
+// # See Also
+//
+//   - ChainThunkK: Chains with a Thunk and uses its result
+//   - TapThunkK: Alias for ChainFirstThunkK
+//   - ChainFirstIOK: Similar but for IO operations
+//
+//go:inline
+func ChainFirstThunkK[C, A, B any](f thunk.Kleisli[A, B]) Operator[C, A, A] {
+	return fromreader.ChainFirstReaderK(
+		ChainFirst[C, A, B],
+		FromThunk[C, B],
+		f,
+	)
+}
+
+// TapThunkK is an alias for ChainFirstThunkK.
+// It chains an effect with a function that returns a Thunk for side effects,
+// but preserves the original value. This is useful for logging, debugging, or
+// performing IO operations that don't need the effect's context.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The value type (preserved)
+//   - B: The type produced by the Thunk (discarded)
+//
+// # Parameters
+//
+//   - f: A function that takes A and returns Thunk[B] for side effects
+//
+// # Returns
+//
+//   - Operator[C, A, A]: A function that executes the Thunk but preserves the original value
+//
+// # Example
+//
+//	performSideEffect := func(n int) readerioresult.ReaderIOResult[any] {
+//	    return func(ctx context.Context) io.IO[result.Result[any]] {
+//	        return func() result.Result[any] {
+//	            // Perform context-independent IO operation
+//	            log.Printf("Processing value: %d", n)
+//	            return result.Of[any](nil)
+//	        }
+//	    }
+//	}
+//
+//	eff := effect.Of[MyContext](42)
+//	tapped := effect.TapThunkK[MyContext](performSideEffect)(eff)
+//	// Logs "Processing value: 42" but still produces 42
+//
+// # See Also
+//
+//   - ChainFirstThunkK: The underlying implementation
+//   - TapIOK: Similar but for IO operations
+//   - Tap: Similar but for full effects
+//
+//go:inline
+func TapThunkK[C, A, B any](f thunk.Kleisli[A, B]) Operator[C, A, A] {
+	return ChainFirstThunkK[C](f)
+}
+
 // ChainIOK chains an effect with a function that returns an IO action.
 // This is useful for integrating IO-based computations (synchronous side effects)
 // into effect chains. The IO action is automatically lifted into the Effect context.
@@ -612,3 +708,52 @@ func ChainReaderIOK[C, A, B any](f readerio.Kleisli[C, A, B]) Operator[C, A, B]
 func Read[A, C any](c C) func(Effect[C, A]) Thunk[A] {
 	return readerreaderioresult.Read[A](c)
 }
+
+// Asks creates an Effect that projects a value from the context using a Reader function.
+// This is useful for extracting specific fields or computing derived values from the context.
+// It's essentially a lifted version of the Reader pattern into the Effect context.
+//
+// # Type Parameters
+//
+//   - C: The context type
+//   - A: The type of the projected value
+//
+// # Parameters
+//
+//   - r: A Reader function that extracts or computes a value from the context
+//
+// # Returns
+//
+//   - Effect[C, A]: An effect that succeeds with the projected value
+//
+// # Example
+//
+//	type Config struct {
+//		Host string
+//		Port int
+//	}
+//
+//	// Extract a specific field
+//	getHost := effect.Asks[Config](func(cfg Config) string {
+//		return cfg.Host
+//	})
+//
+//	// Compute a derived value
+//	getURL := effect.Asks[Config](func(cfg Config) string {
+//		return fmt.Sprintf("http://%s:%d", cfg.Host, cfg.Port)
+//	})
+//
+//	result, err := runEffect(getHost, Config{Host: "localhost", Port: 8080})
+//	// result == "localhost", err == nil
+//
+// # See Also
+//
+// See Also:
+//
+//   - Ask: Returns the entire context as the value
+//   - Map: Transforms the value after extraction
+//
+//go:inline
+func Asks[C, A any](r Reader[C, A]) Effect[C, A] {
+	return readerreaderioresult.Asks(r)
+}

v2/effect/effect_test.go

@@ -677,3 +677,992 @@ func TestChainThunkK_Integration(t *testing.T) {
 		assert.Equal(t, result.Of("Value: 100"), outcome)
 	})
 }
+
+func TestChainFirstThunkK_Success(t *testing.T) {
+	t.Run("executes thunk but preserves original value", func(t *testing.T) {
+		sideEffectExecuted := false
+
+		sideEffect := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					sideEffectExecuted = true
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](42),
+			ChainFirstThunkK[TestConfig](sideEffect),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(42), outcome)
+		assert.True(t, sideEffectExecuted)
+	})
+
+	t.Run("chains multiple side effects", func(t *testing.T) {
+		log := []string{}
+
+		logValue := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("log: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe2(
+			Of[TestConfig](10),
+			ChainFirstThunkK[TestConfig](logValue),
+			ChainFirstThunkK[TestConfig](logValue),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(10), outcome)
+		assert.Equal(t, 2, len(log))
+		assert.Equal(t, "log: 10", log[0])
+		assert.Equal(t, "log: 10", log[1])
+	})
+
+	t.Run("side effect can access runtime context", func(t *testing.T) {
+		var capturedCtx context.Context
+
+		captureContext := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					capturedCtx = ctx
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		ctx := context.Background()
+		computation := F.Pipe1(
+			Of[TestConfig](42),
+			ChainFirstThunkK[TestConfig](captureContext),
+		)
+		outcome := computation(testConfig)(ctx)()
+
+		assert.Equal(t, result.Of(42), outcome)
+		assert.Equal(t, ctx, capturedCtx)
+	})
+
+	t.Run("side effect result is discarded", func(t *testing.T) {
+		returnDifferentValue := func(n int) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) io.IO[result.Result[string]] {
+				return func() result.Result[string] {
+					return result.Of("different value")
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](42),
+			ChainFirstThunkK[TestConfig](returnDifferentValue),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(42), outcome)
+	})
+}
+
+func TestChainFirstThunkK_Failure(t *testing.T) {
+	t.Run("propagates error from previous effect", func(t *testing.T) {
+		testErr := fmt.Errorf("previous error")
+		sideEffectExecuted := false
+
+		sideEffect := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					sideEffectExecuted = true
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			ChainFirstThunkK[TestConfig](sideEffect),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Left[int](testErr), outcome)
+		assert.False(t, sideEffectExecuted)
+	})
+
+	t.Run("propagates error from thunk side effect", func(t *testing.T) {
+		testErr := fmt.Errorf("side effect error")
+
+		failingSideEffect := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					return result.Left[any](testErr)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](42),
+			ChainFirstThunkK[TestConfig](failingSideEffect),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+
+	t.Run("stops execution on first error", func(t *testing.T) {
+		testErr := fmt.Errorf("first error")
+		secondEffectExecuted := false
+
+		failingEffect := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					return result.Left[any](testErr)
+				}
+			}
+		}
+
+		secondEffect := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					secondEffectExecuted = true
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe2(
+			Of[TestConfig](42),
+			ChainFirstThunkK[TestConfig](failingEffect),
+			ChainFirstThunkK[TestConfig](secondEffect),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Left[int](testErr), outcome)
+		assert.False(t, secondEffectExecuted)
+	})
+}
+
+func TestChainFirstThunkK_EdgeCases(t *testing.T) {
+	t.Run("handles zero value", func(t *testing.T) {
+		callCount := 0
+
+		countCalls := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					callCount++
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](0),
+			ChainFirstThunkK[TestConfig](countCalls),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(0), outcome)
+		assert.Equal(t, 1, callCount)
+	})
+
+	t.Run("handles empty string", func(t *testing.T) {
+		var capturedValue string
+
+		captureValue := func(s string) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					capturedValue = s
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](""),
+			ChainFirstThunkK[TestConfig](captureValue),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(""), outcome)
+		assert.Equal(t, "", capturedValue)
+	})
+
+	t.Run("handles nil pointer", func(t *testing.T) {
+		var capturedPtr *int
+
+		capturePtr := func(ptr *int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					capturedPtr = ptr
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig]((*int)(nil)),
+			ChainFirstThunkK[TestConfig](capturePtr),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of((*int)(nil)), outcome)
+		assert.Nil(t, capturedPtr)
+	})
+}
+
+func TestChainFirstThunkK_Integration(t *testing.T) {
+	t.Run("composes with Map and Chain", func(t *testing.T) {
+		log := []string{}
+
+		logValue := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("value: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe3(
+			Of[TestConfig](5),
+			Map[TestConfig](func(x int) int { return x * 2 }),
+			ChainFirstThunkK[TestConfig](logValue),
+			Map[TestConfig](func(x int) int { return x + 3 }),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(13), outcome) // (5 * 2) + 3
+		assert.Equal(t, 1, len(log))
+		assert.Equal(t, "value: 10", log[0])
+	})
+
+	t.Run("composes with ChainThunkK", func(t *testing.T) {
+		log := []string{}
+
+		logSideEffect := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("side-effect: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		transformValue := func(n int) readerioresult.ReaderIOResult[string] {
+			return func(ctx context.Context) io.IO[result.Result[string]] {
+				return func() result.Result[string] {
+					log = append(log, fmt.Sprintf("transform: %d", n))
+					return result.Of(fmt.Sprintf("Result: %d", n))
+				}
+			}
+		}
+
+		computation := F.Pipe2(
+			Of[TestConfig](42),
+			ChainFirstThunkK[TestConfig](logSideEffect),
+			ChainThunkK[TestConfig](transformValue),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of("Result: 42"), outcome)
+		assert.Equal(t, 2, len(log))
+		assert.Equal(t, "side-effect: 42", log[0])
+		assert.Equal(t, "transform: 42", log[1])
+	})
+
+	t.Run("composes with ChainReaderK and ChainReaderIOK", func(t *testing.T) {
+		log := []string{}
+
+		addMultiplier := func(n int) reader.Reader[TestConfig, int] {
+			return func(cfg TestConfig) int {
+				return n + cfg.Multiplier
+			}
+		}
+
+		logReaderIO := func(n int) readerio.ReaderIO[TestConfig, int] {
+			return func(cfg TestConfig) io.IO[int] {
+				return func() int {
+					log = append(log, fmt.Sprintf("reader-io: %d", n))
+					return n * 2
+				}
+			}
+		}
+
+		logThunk := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("thunk: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe3(
+			Of[TestConfig](5),
+			ChainReaderK(addMultiplier),
+			ChainReaderIOK(logReaderIO),
+			ChainFirstThunkK[TestConfig](logThunk),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(16), outcome) // (5 + 3) * 2
+		assert.Equal(t, 2, len(log))
+		assert.Equal(t, "reader-io: 8", log[0])
+		assert.Equal(t, "thunk: 16", log[1])
+	})
+}
+
+func TestTapThunkK_Success(t *testing.T) {
+	t.Run("is alias for ChainFirstThunkK", func(t *testing.T) {
+		log := []string{}
+
+		logValue := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("tapped: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](42),
+			TapThunkK[TestConfig](logValue),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(42), outcome)
+		assert.Equal(t, 1, len(log))
+		assert.Equal(t, "tapped: 42", log[0])
+	})
+
+	t.Run("useful for logging without changing value", func(t *testing.T) {
+		log := []string{}
+
+		logStep := func(step string) func(int) readerioresult.ReaderIOResult[any] {
+			return func(n int) readerioresult.ReaderIOResult[any] {
+				return func(ctx context.Context) io.IO[result.Result[any]] {
+					return func() result.Result[any] {
+						log = append(log, fmt.Sprintf("%s: %d", step, n))
+						return result.Of[any](nil)
+					}
+				}
+			}
+		}
+
+		computation := F.Pipe4(
+			Of[TestConfig](10),
+			TapThunkK[TestConfig](logStep("start")),
+			Map[TestConfig](func(x int) int { return x * 2 }),
+			TapThunkK[TestConfig](logStep("after-map")),
+			Map[TestConfig](func(x int) int { return x + 5 }),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(25), outcome) // (10 * 2) + 5
+		assert.Equal(t, 2, len(log))
+		assert.Equal(t, "start: 10", log[0])
+		assert.Equal(t, "after-map: 20", log[1])
+	})
+
+	t.Run("can perform IO operations", func(t *testing.T) {
+		var ioExecuted bool
+
+		performIO := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					// Simulate IO operation
+					ioExecuted = true
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](42),
+			TapThunkK[TestConfig](performIO),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(42), outcome)
+		assert.True(t, ioExecuted)
+	})
+}
+
+func TestTapThunkK_Failure(t *testing.T) {
+	t.Run("propagates error from previous effect", func(t *testing.T) {
+		testErr := fmt.Errorf("previous error")
+		tapExecuted := false
+
+		tapValue := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					tapExecuted = true
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Fail[TestConfig, int](testErr),
+			TapThunkK[TestConfig](tapValue),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Left[int](testErr), outcome)
+		assert.False(t, tapExecuted)
+	})
+
+	t.Run("propagates error from tap operation", func(t *testing.T) {
+		testErr := fmt.Errorf("tap error")
+
+		failingTap := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					return result.Left[any](testErr)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			Of[TestConfig](42),
+			TapThunkK[TestConfig](failingTap),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Left[int](testErr), outcome)
+	})
+}
+
+func TestTapThunkK_EdgeCases(t *testing.T) {
+	t.Run("handles multiple taps in sequence", func(t *testing.T) {
+		log := []string{}
+
+		tap1 := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, "tap1")
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		tap2 := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, "tap2")
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		tap3 := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, "tap3")
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe3(
+			Of[TestConfig](42),
+			TapThunkK[TestConfig](tap1),
+			TapThunkK[TestConfig](tap2),
+			TapThunkK[TestConfig](tap3),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(42), outcome)
+		assert.Equal(t, []string{"tap1", "tap2", "tap3"}, log)
+	})
+}
+
+func TestTapThunkK_Integration(t *testing.T) {
+	t.Run("real-world logging scenario", func(t *testing.T) {
+		log := []string{}
+
+		logStart := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("Starting computation with: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		logIntermediate := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("Intermediate result: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		logFinal := func(s string) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("Final result: %s", s))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe5(
+			Of[TestConfig](10),
+			TapThunkK[TestConfig](logStart),
+			Map[TestConfig](func(x int) int { return x * 3 }),
+			TapThunkK[TestConfig](logIntermediate),
+			Map[TestConfig](func(x int) string { return fmt.Sprintf("Value: %d", x) }),
+			TapThunkK[TestConfig](logFinal),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of("Value: 30"), outcome)
+		assert.Equal(t, 3, len(log))
+		assert.Equal(t, "Starting computation with: 10", log[0])
+		assert.Equal(t, "Intermediate result: 30", log[1])
+		assert.Equal(t, "Final result: Value: 30", log[2])
+	})
+
+	t.Run("composes with FromThunk", func(t *testing.T) {
+		log := []string{}
+
+		thunk := func(ctx context.Context) io.IO[result.Result[int]] {
+			return func() result.Result[int] {
+				return result.Of(100)
+			}
+		}
+
+		logValue := func(n int) readerioresult.ReaderIOResult[any] {
+			return func(ctx context.Context) io.IO[result.Result[any]] {
+				return func() result.Result[any] {
+					log = append(log, fmt.Sprintf("value: %d", n))
+					return result.Of[any](nil)
+				}
+			}
+		}
+
+		computation := F.Pipe1(
+			FromThunk[TestConfig](thunk),
+			TapThunkK[TestConfig](logValue),
+		)
+		outcome := computation(testConfig)(context.Background())()
+
+		assert.Equal(t, result.Of(100), outcome)
+		assert.Equal(t, 1, len(log))
+		assert.Equal(t, "value: 100", log[0])
+	})
+}
+
+func TestAsks_Success(t *testing.T) {
+	t.Run("extracts a field from context", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
+
+		getHost := Asks(func(cfg Config) string {
+			return cfg.Host
+		})
+
+		result, err := runEffect(getHost, Config{Host: "localhost", Port: 8080})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "localhost", result)
+	})
+
+	t.Run("extracts multiple fields and computes derived value", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
+
+		getURL := Asks(func(cfg Config) string {
+			return fmt.Sprintf("http://%s:%d", cfg.Host, cfg.Port)
+		})
+
+		result, err := runEffect(getURL, Config{Host: "example.com", Port: 443})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "http://example.com:443", result)
+	})
+
+	t.Run("extracts numeric field", func(t *testing.T) {
+		getPort := Asks(func(cfg TestConfig) int {
+			return cfg.Multiplier
+		})
+
+		result, err := runEffect(getPort, testConfig)
+
+		assert.NoError(t, err)
+		assert.Equal(t, 3, result)
+	})
+
+	t.Run("computes value from context", func(t *testing.T) {
+		type Config struct {
+			Width  int
+			Height int
+		}
+
+		getArea := Asks(func(cfg Config) int {
+			return cfg.Width * cfg.Height
+		})
+
+		result, err := runEffect(getArea, Config{Width: 10, Height: 20})
+
+		assert.NoError(t, err)
+		assert.Equal(t, 200, result)
+	})
+
+	t.Run("transforms string field", func(t *testing.T) {
+		getUpperPrefix := Asks(func(cfg TestConfig) string {
+			return fmt.Sprintf("[%s]", cfg.Prefix)
+		})
+
+		result, err := runEffect(getUpperPrefix, testConfig)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "[LOG]", result)
+	})
+}
+
+func TestAsks_EdgeCases(t *testing.T) {
+	t.Run("handles zero values", func(t *testing.T) {
+		type Config struct {
+			Value int
+		}
+
+		getValue := Asks(func(cfg Config) int {
+			return cfg.Value
+		})
+
+		result, err := runEffect(getValue, Config{Value: 0})
+
+		assert.NoError(t, err)
+		assert.Equal(t, 0, result)
+	})
+
+	t.Run("handles empty string", func(t *testing.T) {
+		type Config struct {
+			Name string
+		}
+
+		getName := Asks(func(cfg Config) string {
+			return cfg.Name
+		})
+
+		result, err := runEffect(getName, Config{Name: ""})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "", result)
+	})
+
+	t.Run("handles nil pointer fields", func(t *testing.T) {
+		type Config struct {
+			Data *string
+		}
+
+		hasData := Asks(func(cfg Config) bool {
+			return cfg.Data != nil
+		})
+
+		result, err := runEffect(hasData, Config{Data: nil})
+
+		assert.NoError(t, err)
+		assert.False(t, result)
+	})
+
+	t.Run("handles complex nested structures", func(t *testing.T) {
+		type Database struct {
+			Host string
+			Port int
+		}
+		type Config struct {
+			DB Database
+		}
+
+		getDBHost := Asks(func(cfg Config) string {
+			return cfg.DB.Host
+		})
+
+		result, err := runEffect(getDBHost, Config{
+			DB: Database{Host: "db.example.com", Port: 5432},
+		})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "db.example.com", result)
+	})
+}
+
+func TestAsks_Integration(t *testing.T) {
+	t.Run("composes with Map", func(t *testing.T) {
+		type Config struct {
+			Value int
+		}
+
+		computation := F.Pipe1(
+			Asks(func(cfg Config) int {
+				return cfg.Value
+			}),
+			Map[Config](func(x int) int { return x * 2 }),
+		)
+
+		result, err := runEffect(computation, Config{Value: 21})
+
+		assert.NoError(t, err)
+		assert.Equal(t, 42, result)
+	})
+
+	t.Run("composes with Chain", func(t *testing.T) {
+		type Config struct {
+			Multiplier int
+		}
+
+		computation := F.Pipe1(
+			Asks(func(cfg Config) int {
+				return cfg.Multiplier
+			}),
+			Chain(func(mult int) Effect[Config, int] {
+				return Of[Config](mult * 10)
+			}),
+		)
+
+		result, err := runEffect(computation, Config{Multiplier: 5})
+
+		assert.NoError(t, err)
+		assert.Equal(t, 50, result)
+	})
+
+	t.Run("composes with ChainReaderK", func(t *testing.T) {
+		computation := F.Pipe1(
+			Asks(func(cfg TestConfig) int {
+				return cfg.Multiplier
+			}),
+			ChainReaderK(func(mult int) reader.Reader[TestConfig, int] {
+				return func(cfg TestConfig) int {
+					return mult + len(cfg.Prefix)
+				}
+			}),
+		)
+
+		result, err := runEffect(computation, testConfig)
+
+		assert.NoError(t, err)
+		assert.Equal(t, 6, result) // 3 + len("LOG")
+	})
+
+	t.Run("composes with ChainReaderIOK", func(t *testing.T) {
+		log := []string{}
+
+		computation := F.Pipe1(
+			Asks(func(cfg TestConfig) string {
+				return cfg.Prefix
+			}),
+			ChainReaderIOK(func(prefix string) readerio.ReaderIO[TestConfig, string] {
+				return func(cfg TestConfig) io.IO[string] {
+					return func() string {
+						log = append(log, "executed")
+						return fmt.Sprintf("%s:%d", prefix, cfg.Multiplier)
+					}
+				}
+			}),
+		)
+
+		result, err := runEffect(computation, testConfig)
+
+		assert.NoError(t, err)
+		assert.Equal(t, "LOG:3", result)
+		assert.Equal(t, 1, len(log))
+	})
+
+	t.Run("multiple Asks in sequence", func(t *testing.T) {
+		type Config struct {
+			First  string
+			Second string
+		}
+
+		computation := F.Pipe2(
+			Asks(func(cfg Config) string {
+				return cfg.First
+			}),
+			Chain(func(_ string) Effect[Config, string] {
+				return Asks(func(cfg Config) string {
+					return cfg.Second
+				})
+			}),
+			Map[Config](func(s string) string {
+				return "Result: " + s
+			}),
+		)
+
+		result, err := runEffect(computation, Config{First: "A", Second: "B"})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "Result: B", result)
+	})
+
+	t.Run("Asks combined with Ask", func(t *testing.T) {
+		type Config struct {
+			Value int
+		}
+
+		computation := F.Pipe1(
+			Ask[Config](),
+			Chain(func(cfg Config) Effect[Config, int] {
+				return Asks(func(c Config) int {
+					return c.Value * 2
+				})
+			}),
+		)
+
+		result, err := runEffect(computation, Config{Value: 15})
+
+		assert.NoError(t, err)
+		assert.Equal(t, 30, result)
+	})
+}
+
+func TestAsks_Comparison(t *testing.T) {
+	t.Run("Asks vs Ask with Map", func(t *testing.T) {
+		type Config struct {
+			Port int
+		}
+
+		// Using Asks
+		asksVersion := Asks(func(cfg Config) int {
+			return cfg.Port
+		})
+
+		// Using Ask + Map
+		askMapVersion := F.Pipe1(
+			Ask[Config](),
+			Map[Config](func(cfg Config) int {
+				return cfg.Port
+			}),
+		)
+
+		cfg := Config{Port: 8080}
+
+		result1, err1 := runEffect(asksVersion, cfg)
+		result2, err2 := runEffect(askMapVersion, cfg)
+
+		assert.NoError(t, err1)
+		assert.NoError(t, err2)
+		assert.Equal(t, result1, result2)
+		assert.Equal(t, 8080, result1)
+	})
+
+	t.Run("Asks is more concise than Ask + Map", func(t *testing.T) {
+		type Config struct {
+			Host string
+			Port int
+		}
+
+		// Asks is more direct for field extraction
+		getHost := Asks(func(cfg Config) string {
+			return cfg.Host
+		})
+
+		result, err := runEffect(getHost, Config{Host: "api.example.com", Port: 443})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "api.example.com", result)
+	})
+}
+
+func TestAsks_RealWorldScenarios(t *testing.T) {
+	t.Run("extract database connection string", func(t *testing.T) {
+		type DatabaseConfig struct {
+			Host     string
+			Port     int
+			Database string
+			User     string
+		}
+
+		getConnectionString := Asks(func(cfg DatabaseConfig) string {
+			return fmt.Sprintf("postgres://%s@%s:%d/%s",
+				cfg.User, cfg.Host, cfg.Port, cfg.Database)
+		})
+
+		result, err := runEffect(getConnectionString, DatabaseConfig{
+			Host:     "localhost",
+			Port:     5432,
+			Database: "myapp",
+			User:     "admin",
+		})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "postgres://admin@localhost:5432/myapp", result)
+	})
+
+	t.Run("compute API endpoint from config", func(t *testing.T) {
+		type APIConfig struct {
+			Protocol string
+			Host     string
+			Port     int
+			BasePath string
+		}
+
+		getEndpoint := Asks(func(cfg APIConfig) string {
+			return fmt.Sprintf("%s://%s:%d%s",
+				cfg.Protocol, cfg.Host, cfg.Port, cfg.BasePath)
+		})
+
+		result, err := runEffect(getEndpoint, APIConfig{
+			Protocol: "https",
+			Host:     "api.example.com",
+			Port:     443,
+			BasePath: "/v1",
+		})
+
+		assert.NoError(t, err)
+		assert.Equal(t, "https://api.example.com:443/v1", result)
+	})
+
+	t.Run("validate configuration", func(t *testing.T) {
+		type Config struct {
+			Timeout    int
+			MaxRetries int
+		}
+
+		isValid := Asks(func(cfg Config) bool {
+			return cfg.Timeout > 0 && cfg.MaxRetries >= 0
+		})
+
+		// Valid config
+		result1, err1 := runEffect(isValid, Config{Timeout: 30, MaxRetries: 3})
+		assert.NoError(t, err1)
+		assert.True(t, result1)
+
+		// Invalid config
+		result2, err2 := runEffect(isValid, Config{Timeout: 0, MaxRetries: 3})
+		assert.NoError(t, err2)
+		assert.False(t, result2)
+	})
+
+	t.Run("extract feature flags", func(t *testing.T) {
+		type FeatureFlags struct {
+			EnableNewUI     bool
+			EnableBetaAPI   bool
+			EnableAnalytics bool
+		}
+
+		hasNewUI := Asks[FeatureFlags](func(flags FeatureFlags) bool {
+			return flags.EnableNewUI
+		})
+
+		result, err := runEffect(hasNewUI, FeatureFlags{
+			EnableNewUI:     true,
+			EnableBetaAPI:   false,
+			EnableAnalytics: true,
+		})
+
+		assert.NoError(t, err)
+		assert.True(t, result)
+	})
+}

v2/effect/filter.go

@@ -0,0 +1,296 @@
+// 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 effect
+
+import (
+	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
+	"github.com/IBM/fp-go/v2/option"
+)
+
+// Filter lifts a filtering operation on a higher-kinded type into an Effect operator.
+// This is a generic function that works with any filterable data structure by taking
+// a filter function and returning an operator that can be used in effect chains.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - HKTA: The higher-kinded type being filtered (e.g., []A, Seq[A])
+//   - A: The element type being filtered
+//
+// # Parameters
+//
+//   - filter: A function that takes a predicate and returns an endomorphism on HKTA
+//
+// # Returns
+//
+//   - func(Predicate[A]) Operator[C, HKTA, HKTA]: A function that takes a predicate
+//     and returns an operator that filters effects containing HKTA values
+//
+// # Example Usage
+//
+//	import A "github.com/IBM/fp-go/v2/array"
+//
+//	// Create a custom filter operator for arrays
+//	filterOp := Filter[MyContext](A.Filter[int])
+//	isEven := func(n int) bool { return n%2 == 0 }
+//
+//	pipeline := F.Pipe2(
+//	    Succeed[MyContext]([]int{1, 2, 3, 4, 5}),
+//	    filterOp(isEven),
+//	    Map[MyContext](func(arr []int) int { return len(arr) }),
+//	)
+//	// Result: Effect that produces 2 (count of even numbers)
+//
+// # See Also
+//
+//   - FilterArray: Specialized version for array filtering
+//   - FilterIter: Specialized version for iterator filtering
+//   - FilterMap: For filtering and mapping simultaneously
+//
+//go:inline
+func Filter[C, HKTA, A any](
+	filter func(Predicate[A]) Endomorphism[HKTA],
+) func(Predicate[A]) Operator[C, HKTA, HKTA] {
+	return readerreaderioresult.Filter[C](filter)
+}
+
+// FilterArray creates an operator that filters array elements within an Effect based on a predicate.
+// Elements that satisfy the predicate are kept, while others are removed.
+// This is a specialized version of Filter for arrays.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The element type in the array
+//
+// # Parameters
+//
+//   - p: A predicate function that tests each element
+//
+// # Returns
+//
+//   - Operator[C, []A, []A]: An operator that filters array elements in an effect
+//
+// # Example Usage
+//
+//	isPositive := func(n int) bool { return n > 0 }
+//	filterPositive := FilterArray[MyContext](isPositive)
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext]([]int{-2, -1, 0, 1, 2, 3}),
+//	    filterPositive,
+//	)
+//	// Result: Effect that produces []int{1, 2, 3}
+//
+// # See Also
+//
+//   - Filter: Generic version for any filterable type
+//   - FilterIter: For filtering iterators
+//   - FilterMapArray: For filtering and mapping arrays simultaneously
+//
+//go:inline
+func FilterArray[C, A any](p Predicate[A]) Operator[C, []A, []A] {
+	return readerreaderioresult.FilterArray[C](p)
+}
+
+// FilterIter creates an operator that filters iterator elements within an Effect based on a predicate.
+// Elements that satisfy the predicate are kept in the resulting iterator, while others are removed.
+// This is a specialized version of Filter for iterators (Seq).
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The element type in the iterator
+//
+// # Parameters
+//
+//   - p: A predicate function that tests each element
+//
+// # Returns
+//
+//   - Operator[C, Seq[A], Seq[A]]: An operator that filters iterator elements in an effect
+//
+// # Example Usage
+//
+//	isEven := func(n int) bool { return n%2 == 0 }
+//	filterEven := FilterIter[MyContext](isEven)
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext](slices.Values([]int{1, 2, 3, 4, 5, 6})),
+//	    filterEven,
+//	)
+//	// Result: Effect that produces an iterator over [2, 4, 6]
+//
+// # See Also
+//
+//   - Filter: Generic version for any filterable type
+//   - FilterArray: For filtering arrays
+//   - FilterMapIter: For filtering and mapping iterators simultaneously
+//
+//go:inline
+func FilterIter[C, A any](p Predicate[A]) Operator[C, Seq[A], Seq[A]] {
+	return readerreaderioresult.FilterIter[C](p)
+}
+
+// FilterMap lifts a filter-map operation on a higher-kinded type into an Effect operator.
+// This combines filtering and mapping in a single operation: elements are transformed
+// using a function that returns Option, and only Some values are kept in the result.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - HKTA: The input higher-kinded type (e.g., []A, Seq[A])
+//   - HKTB: The output higher-kinded type (e.g., []B, Seq[B])
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - filter: A function that takes an option.Kleisli and returns a transformation from HKTA to HKTB
+//
+// # Returns
+//
+//   - func(option.Kleisli[A, B]) Operator[C, HKTA, HKTB]: A function that takes a Kleisli arrow
+//     and returns an operator that filter-maps effects
+//
+// # Example Usage
+//
+//	import A "github.com/IBM/fp-go/v2/array"
+//	import O "github.com/IBM/fp-go/v2/option"
+//
+//	// Parse and filter positive integers
+//	parsePositive := func(s string) O.Option[int] {
+//	    var n int
+//	    if _, err := fmt.Sscanf(s, "%d", &n); err == nil && n > 0 {
+//	        return O.Some(n)
+//	    }
+//	    return O.None[int]()
+//	}
+//
+//	filterMapOp := FilterMap[MyContext](A.FilterMap[string, int])
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext]([]string{"1", "-2", "3", "invalid", "5"}),
+//	    filterMapOp(parsePositive),
+//	)
+//	// Result: Effect that produces []int{1, 3, 5}
+//
+// # See Also
+//
+//   - FilterMapArray: Specialized version for arrays
+//   - FilterMapIter: Specialized version for iterators
+//   - Filter: For filtering without transformation
+//
+//go:inline
+func FilterMap[C, HKTA, HKTB, A, B any](
+	filter func(option.Kleisli[A, B]) Reader[HKTA, HKTB],
+) func(option.Kleisli[A, B]) Operator[C, HKTA, HKTB] {
+	return readerreaderioresult.FilterMap[C](filter)
+}
+
+// FilterMapArray creates an operator that filters and maps array elements within an Effect.
+// Each element is transformed using a function that returns Option[B]. Elements that
+// produce Some(b) are kept in the result array, while None values are filtered out.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - p: A Kleisli arrow from A to Option[B] that transforms and filters elements
+//
+// # Returns
+//
+//   - Operator[C, []A, []B]: An operator that filter-maps array elements in an effect
+//
+// # Example Usage
+//
+//	import O "github.com/IBM/fp-go/v2/option"
+//
+//	// Double even numbers, filter out odd numbers
+//	doubleEven := func(n int) O.Option[int] {
+//	    if n%2 == 0 {
+//	        return O.Some(n * 2)
+//	    }
+//	    return O.None[int]()
+//	}
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext]([]int{1, 2, 3, 4, 5}),
+//	    FilterMapArray[MyContext](doubleEven),
+//	)
+//	// Result: Effect that produces []int{4, 8}
+//
+// # See Also
+//
+//   - FilterMap: Generic version for any filterable type
+//   - FilterMapIter: For filter-mapping iterators
+//   - FilterArray: For filtering without transformation
+//
+//go:inline
+func FilterMapArray[C, A, B any](p option.Kleisli[A, B]) Operator[C, []A, []B] {
+	return readerreaderioresult.FilterMapArray[C](p)
+}
+
+// FilterMapIter creates an operator that filters and maps iterator elements within an Effect.
+// Each element is transformed using a function that returns Option[B]. Elements that
+// produce Some(b) are kept in the resulting iterator, while None values are filtered out.
+//
+// # Type Parameters
+//
+//   - C: The context type required by the effect
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - p: A Kleisli arrow from A to Option[B] that transforms and filters elements
+//
+// # Returns
+//
+//   - Operator[C, Seq[A], Seq[B]]: An operator that filter-maps iterator elements in an effect
+//
+// # Example Usage
+//
+//	import O "github.com/IBM/fp-go/v2/option"
+//
+//	// Parse strings to integers, keeping only valid ones
+//	parseInt := func(s string) O.Option[int] {
+//	    var n int
+//	    if _, err := fmt.Sscanf(s, "%d", &n); err == nil {
+//	        return O.Some(n)
+//	    }
+//	    return O.None[int]()
+//	}
+//
+//	pipeline := F.Pipe1(
+//	    Succeed[MyContext](slices.Values([]string{"1", "2", "invalid", "3"})),
+//	    FilterMapIter[MyContext](parseInt),
+//	)
+//	// Result: Effect that produces an iterator over [1, 2, 3]
+//
+// # See Also
+//
+//   - FilterMap: Generic version for any filterable type
+//   - FilterMapArray: For filter-mapping arrays
+//   - FilterIter: For filtering without transformation
+//
+//go:inline
+func FilterMapIter[C, A, B any](p option.Kleisli[A, B]) Operator[C, Seq[A], Seq[B]] {
+	return readerreaderioresult.FilterMapIter[C](p)
+}

v2/effect/filter_test.go

@@ -0,0 +1,653 @@
+// 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 effect
+
+import (
+	"errors"
+	"fmt"
+	"slices"
+	"testing"
+
+	A "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
+	N "github.com/IBM/fp-go/v2/number"
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+type FilterTestConfig struct {
+	MaxValue int
+	MinValue int
+}
+
+// Helper to collect iterator results from an effect
+func collectSeqEffect[C, A any](eff Effect[C, Seq[A]], cfg C) []A {
+	result, err := runEffect(eff, cfg)
+	if err != nil {
+		return nil
+	}
+	return slices.Collect(result)
+}
+
+func TestFilterArray_Success(t *testing.T) {
+	t.Run("filters array keeping matching elements", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		input := Succeed[FilterTestConfig]([]int{1, -2, 3, -4, 5})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{1, 3, 5}, result)
+	})
+
+	t.Run("returns empty array when no elements match", func(t *testing.T) {
+		// Arrange
+		isNegative := N.LessThan(0)
+		filterOp := FilterArray[FilterTestConfig](isNegative)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("returns all elements when all match", func(t *testing.T) {
+		// Arrange
+		alwaysTrue := func(n int) bool { return true }
+		filterOp := FilterArray[FilterTestConfig](alwaysTrue)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+}
+
+func TestFilterIter_Success(t *testing.T) {
+	t.Run("filters iterator keeping matching elements", func(t *testing.T) {
+		// Arrange
+		isEven := func(n int) bool { return n%2 == 0 }
+		filterOp := FilterIter[FilterTestConfig](isEven)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3, 4, 5, 6}))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, []int{2, 4, 6}, collected)
+	})
+
+	t.Run("returns empty iterator when no elements match", func(t *testing.T) {
+		// Arrange
+		isNegative := N.LessThan(0)
+		filterOp := FilterIter[FilterTestConfig](isNegative)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3}))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Empty(t, collected)
+	})
+}
+
+func TestFilterArray_WithContext(t *testing.T) {
+	t.Run("uses context for filtering", func(t *testing.T) {
+		// Arrange
+		cfg := FilterTestConfig{MaxValue: 100, MinValue: 0}
+		inRange := func(n int) bool { return n >= cfg.MinValue && n <= cfg.MaxValue }
+		filterOp := FilterArray[FilterTestConfig](inRange)
+		input := Succeed[FilterTestConfig]([]int{-10, 50, 150, 75})
+
+		// Act
+		result, err := runEffect(filterOp(input), cfg)
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{50, 75}, result)
+	})
+}
+
+func TestFilterArray_EdgeCases(t *testing.T) {
+	t.Run("handles empty array", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		input := Succeed[FilterTestConfig]([]int{})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("preserves error from input", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		inputErr := errors.New("input error")
+		input := Fail[FilterTestConfig, []int](inputErr)
+
+		// Act
+		_, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, inputErr, err)
+	})
+}
+
+func TestFilterIter_EdgeCases(t *testing.T) {
+	t.Run("handles empty iterator", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterIter[FilterTestConfig](isPositive)
+		input := Succeed[FilterTestConfig](slices.Values([]int{}))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Empty(t, collected)
+	})
+
+	t.Run("preserves error from input", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterIter[FilterTestConfig](isPositive)
+		inputErr := errors.New("input error")
+		input := Fail[FilterTestConfig, Seq[int]](inputErr)
+
+		// Act
+		_, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, inputErr, err)
+	})
+}
+
+func TestFilter_GenericFilter(t *testing.T) {
+	t.Run("works with custom filter function", func(t *testing.T) {
+		// Arrange
+		customFilter := func(p Predicate[int]) Endomorphism[[]int] {
+			return A.Filter(p)
+		}
+		filterOp := Filter[FilterTestConfig](customFilter)
+		isEven := func(n int) bool { return n%2 == 0 }
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3, 4, 5})
+
+		// Act
+		result, err := runEffect(filterOp(isEven)(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4}, result)
+	})
+}
+
+func TestFilterMapArray_Success(t *testing.T) {
+	t.Run("filters and maps array elements", func(t *testing.T) {
+		// Arrange
+		parsePositive := func(n int) O.Option[string] {
+			if n > 0 {
+				return O.Some(fmt.Sprintf("positive:%d", n))
+			}
+			return O.None[string]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](parsePositive)
+		input := Succeed[FilterTestConfig]([]int{-1, 2, -3, 4, 5})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"positive:2", "positive:4", "positive:5"}, result)
+	})
+
+	t.Run("returns empty when no elements match", func(t *testing.T) {
+		// Arrange
+		neverMatch := func(n int) O.Option[int] {
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](neverMatch)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("maps all elements when all match", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		input := Succeed[FilterTestConfig]([]int{1, 2, 3})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4, 6}, result)
+	})
+}
+
+func TestFilterMapIter_Success(t *testing.T) {
+	t.Run("filters and maps iterator elements", func(t *testing.T) {
+		// Arrange
+		doubleEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * 2)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapIter[FilterTestConfig](doubleEven)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3, 4, 5}))
+
+		// Act
+		collected := collectSeqEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, []int{4, 8}, collected)
+	})
+}
+
+func TestFilterMapArray_TypeConversion(t *testing.T) {
+	t.Run("converts int to string", func(t *testing.T) {
+		// Arrange
+		intToString := func(n int) O.Option[string] {
+			if n > 0 {
+				return O.Some(fmt.Sprintf("%d", n))
+			}
+			return O.None[string]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](intToString)
+		input := Succeed[FilterTestConfig]([]int{-1, 2, -3, 4})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"2", "4"}, result)
+	})
+
+	t.Run("converts string to int", func(t *testing.T) {
+		// Arrange
+		parseEven := func(s string) O.Option[int] {
+			var n int
+			if _, err := fmt.Sscanf(s, "%d", &n); err == nil && n%2 == 0 {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](parseEven)
+		input := Succeed[FilterTestConfig]([]string{"1", "2", "3", "4", "invalid"})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4}, result)
+	})
+}
+
+func TestFilterMapArray_EdgeCases(t *testing.T) {
+	t.Run("handles empty array", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		input := Succeed[FilterTestConfig]([]int{})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("preserves error from input", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		inputErr := errors.New("input error")
+		input := Fail[FilterTestConfig, []int](inputErr)
+
+		// Act
+		_, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, inputErr, err)
+	})
+}
+
+func TestFilterMapIter_EdgeCases(t *testing.T) {
+	t.Run("handles empty iterator", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapIter[FilterTestConfig](double)
+		input := Succeed[FilterTestConfig](slices.Values([]int{}))
+
+		// Act
+		collected := collectSeqEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Empty(t, collected)
+	})
+}
+
+func TestFilterMap_GenericFilterMap(t *testing.T) {
+	t.Run("works with custom filterMap function", func(t *testing.T) {
+		// Arrange
+		customFilterMap := func(f O.Kleisli[int, string]) Reader[[]int, []string] {
+			return A.FilterMap(f)
+		}
+		filterMapOp := FilterMap[FilterTestConfig](customFilterMap)
+		intToString := func(n int) O.Option[string] {
+			if n > 0 {
+				return O.Some(fmt.Sprintf("%d", n))
+			}
+			return O.None[string]()
+		}
+		input := Succeed[FilterTestConfig]([]int{-1, 2, -3, 4})
+
+		// Act
+		result, err := runEffect(filterMapOp(intToString)(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"2", "4"}, result)
+	})
+}
+
+func TestFilter_Composition(t *testing.T) {
+	t.Run("chains multiple filters", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		isEven := func(n int) bool { return n%2 == 0 }
+		filterPositive := FilterArray[FilterTestConfig](isPositive)
+		filterEven := FilterArray[FilterTestConfig](isEven)
+		input := Succeed[FilterTestConfig]([]int{-2, -1, 0, 1, 2, 3, 4, 5, 6})
+
+		// Act
+		result, err := runEffect(filterEven(filterPositive(input)), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{2, 4, 6}, result)
+	})
+
+	t.Run("chains filter and filterMap", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		doubleEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * 2)
+			}
+			return O.None[int]()
+		}
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		filterMapOp := FilterMapArray[FilterTestConfig](doubleEven)
+		input := Succeed[FilterTestConfig]([]int{-2, 1, 2, 3, 4, 5})
+
+		// Act
+		result, err := runEffect(filterMapOp(filterOp(input)), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{4, 8}, result)
+	})
+}
+
+func TestFilter_WithComplexTypes(t *testing.T) {
+	type User struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("filters structs", func(t *testing.T) {
+		// Arrange
+		isAdult := func(u User) bool { return u.Age >= 18 }
+		filterOp := FilterArray[FilterTestConfig](isAdult)
+		users := []User{
+			{Name: "Alice", Age: 25},
+			{Name: "Bob", Age: 16},
+			{Name: "Charlie", Age: 30},
+		}
+		input := Succeed[FilterTestConfig](users)
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		expected := []User{
+			{Name: "Alice", Age: 25},
+			{Name: "Charlie", Age: 30},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("filterMaps structs to different type", func(t *testing.T) {
+		// Arrange
+		extractAdultName := func(u User) O.Option[string] {
+			if u.Age >= 18 {
+				return O.Some(u.Name)
+			}
+			return O.None[string]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](extractAdultName)
+		users := []User{
+			{Name: "Alice", Age: 25},
+			{Name: "Bob", Age: 16},
+			{Name: "Charlie", Age: 30},
+		}
+		input := Succeed[FilterTestConfig](users)
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []string{"Alice", "Charlie"}, result)
+	})
+}
+
+func TestFilter_BoundaryConditions(t *testing.T) {
+	t.Run("filters with boundary predicate", func(t *testing.T) {
+		// Arrange
+		inRange := func(n int) bool { return n >= 0 && n <= 100 }
+		filterOp := FilterArray[FilterTestConfig](inRange)
+		input := Succeed[FilterTestConfig]([]int{-1, 0, 50, 100, 101})
+
+		// Act
+		result, err := runEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{0, 50, 100}, result)
+	})
+
+	t.Run("filterMap with boundary conditions", func(t *testing.T) {
+		// Arrange
+		clampToRange := func(n int) O.Option[int] {
+			if n >= 0 && n <= 100 {
+				return O.Some(n)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](clampToRange)
+		input := Succeed[FilterTestConfig]([]int{-1, 0, 50, 100, 101})
+
+		// Act
+		result, err := runEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		assert.Equal(t, []int{0, 50, 100}, result)
+	})
+}
+
+func TestFilter_WithIterators(t *testing.T) {
+	t.Run("filters large iterator efficiently", func(t *testing.T) {
+		// Arrange
+		isEven := func(n int) bool { return n%2 == 0 }
+		filterOp := FilterIter[FilterTestConfig](isEven)
+
+		// Create iterator for range 0-99
+		makeSeq := func(yield func(int) bool) {
+			for i := range 100 {
+				if !yield(i) {
+					return
+				}
+			}
+		}
+		input := Succeed[FilterTestConfig](Seq[int](makeSeq))
+
+		// Act
+		collected := collectSeqEffect(filterOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, 50, len(collected))
+		assert.Equal(t, 0, collected[0])
+		assert.Equal(t, 98, collected[49])
+	})
+
+	t.Run("filterMap with iterator", func(t *testing.T) {
+		// Arrange
+		squareEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * n)
+			}
+			return O.None[int]()
+		}
+		filterMapOp := FilterMapIter[FilterTestConfig](squareEven)
+		input := Succeed[FilterTestConfig](slices.Values([]int{1, 2, 3, 4, 5}))
+
+		// Act
+		collected := collectSeqEffect(filterMapOp(input), FilterTestConfig{})
+
+		// Assert
+		assert.Equal(t, []int{4, 16}, collected)
+	})
+}
+
+func TestFilter_ErrorPropagation(t *testing.T) {
+	t.Run("filter propagates Left through chain", func(t *testing.T) {
+		// Arrange
+		isPositive := N.MoreThan(0)
+		filterOp := FilterArray[FilterTestConfig](isPositive)
+		originalErr := errors.New("original error")
+
+		// Create an effect that fails
+		failedEffect := F.Pipe1(
+			Succeed[FilterTestConfig]([]int{1, 2, 3}),
+			Chain(func([]int) Effect[FilterTestConfig, []int] {
+				return Fail[FilterTestConfig, []int](originalErr)
+			}),
+		)
+
+		// Act
+		_, err := runEffect(filterOp(failedEffect), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, originalErr, err)
+	})
+
+	t.Run("filterMap propagates Left through chain", func(t *testing.T) {
+		// Arrange
+		double := func(n int) O.Option[int] {
+			return O.Some(n * 2)
+		}
+		filterMapOp := FilterMapArray[FilterTestConfig](double)
+		originalErr := errors.New("original error")
+
+		// Create an effect that fails
+		failedEffect := F.Pipe1(
+			Succeed[FilterTestConfig]([]int{1, 2, 3}),
+			Chain(func([]int) Effect[FilterTestConfig, []int] {
+				return Fail[FilterTestConfig, []int](originalErr)
+			}),
+		)
+
+		// Act
+		_, err := runEffect(filterMapOp(failedEffect), FilterTestConfig{})
+
+		// Assert
+		assert.Error(t, err)
+		assert.Equal(t, originalErr, err)
+	})
+}
+
+func TestFilter_Integration(t *testing.T) {
+	t.Run("complex filtering pipeline", func(t *testing.T) {
+		// Arrange: Filter positive numbers, then double evens, then filter > 5
+		isPositive := N.MoreThan(0)
+		doubleEven := func(n int) O.Option[int] {
+			if n%2 == 0 {
+				return O.Some(n * 2)
+			}
+			return O.None[int]()
+		}
+		isGreaterThan5 := N.MoreThan(5)
+
+		pipeline := F.Pipe3(
+			Succeed[FilterTestConfig]([]int{-2, -1, 0, 1, 2, 3, 4, 5, 6}),
+			FilterArray[FilterTestConfig](isPositive),
+			FilterMapArray[FilterTestConfig](doubleEven),
+			FilterArray[FilterTestConfig](isGreaterThan5),
+		)
+
+		// Act
+		result, err := runEffect(pipeline, FilterTestConfig{})
+
+		// Assert
+		assert.NoError(t, err)
+		// Positive: [1,2,3,4,5,6] -> DoubleEven: [4,8,12] -> >5: [8,12]
+		assert.Equal(t, []int{8, 12}, result)
+	})
+}
+
+

v2/effect/profunctor.go

@@ -0,0 +1,86 @@
+// 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 effect
+
+import (
+	F "github.com/IBM/fp-go/v2/function"
+)
+
+// Promap is the profunctor map operation that transforms both the input and output of an Effect.
+// It applies f to the input context (contravariantly) and g to the output value (covariantly).
+//
+// See: https://github.com/fantasyland/fantasy-land?tab=readme-ov-file#profunctor
+//
+// This operation allows you to:
+//   - Modify the context before passing it to the Effect (via f)
+//   - Transform the success value after the computation completes (via g)
+//
+// Promap is particularly useful for adapting effects to work with different context types
+// while simultaneously transforming their output values.
+//
+// # Type Parameters
+//
+//   - E: The original context type expected by the Effect
+//   - A: The original success type produced by the Effect
+//   - D: The new input context type
+//   - B: The new output success type
+//
+// # Parameters
+//
+//   - f: Function to transform the input context from D to E (contravariant)
+//   - g: Function to transform the output success value from A to B (covariant)
+//
+// # Returns
+//
+//   - A Kleisli arrow that takes an Effect[E, A] and returns a function from D to B
+//
+// # Example Usage
+//
+//	type AppConfig struct {
+//	    DatabaseURL string
+//	    APIKey      string
+//	}
+//
+//	type DBConfig struct {
+//	    URL string
+//	}
+//
+//	// Effect that uses DBConfig and returns an int
+//	getUserCount := func(cfg DBConfig) effect.Effect[context.Context, int] {
+//	    return effect.Succeed[context.Context](42)
+//	}
+//
+//	// Transform AppConfig to DBConfig
+//	extractDBConfig := func(app AppConfig) DBConfig {
+//	    return DBConfig{URL: app.DatabaseURL}
+//	}
+//
+//	// Transform int to string
+//	formatCount := func(count int) string {
+//	    return fmt.Sprintf("Users: %d", count)
+//	}
+//
+//	// Adapt the effect to work with AppConfig and return string
+//	adapted := effect.Promap(extractDBConfig, formatCount)(getUserCount)
+//	result := adapted(AppConfig{DatabaseURL: "localhost:5432", APIKey: "secret"})
+//
+//go:inline
+func Promap[E, A, D, B any](f Reader[D, E], g Reader[A, B]) Kleisli[D, Effect[E, A], B] {
+	return F.Flow2(
+		Local[A](f),
+		Map[D](g),
+	)
+}

v2/effect/profunctor_test.go

@@ -0,0 +1,373 @@
+// 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 effect
+
+import (
+	"context"
+	"fmt"
+	"strconv"
+	"testing"
+
+	R "github.com/IBM/fp-go/v2/result"
+	"github.com/stretchr/testify/assert"
+)
+
+// Test types for profunctor tests
+type AppConfig struct {
+	DatabaseURL string
+	APIKey      string
+	Port        int
+}
+
+type DBConfig struct {
+	URL string
+}
+
+type ServerConfig struct {
+	Host string
+	Port int
+}
+
+// TestPromapBasic tests basic Promap functionality
+func TestPromapBasic(t *testing.T) {
+	t.Run("transform both context and output", func(t *testing.T) {
+		// Effect that uses DBConfig and returns an int
+		getUserCount := Succeed[DBConfig](42)
+
+		// Transform AppConfig to DBConfig
+		extractDBConfig := func(app AppConfig) DBConfig {
+			return DBConfig{URL: app.DatabaseURL}
+		}
+
+		// Transform int to string
+		formatCount := func(count int) string {
+			return fmt.Sprintf("Users: %d", count)
+		}
+
+		// Adapt the effect to work with AppConfig and return string
+		adapted := Promap(extractDBConfig, formatCount)(getUserCount)
+		result := adapted(AppConfig{
+			DatabaseURL: "localhost:5432",
+			APIKey:      "secret",
+			Port:        8080,
+		})(context.Background())()
+
+		assert.Equal(t, R.Of("Users: 42"), result)
+	})
+
+	t.Run("identity transformations", func(t *testing.T) {
+		// Effect that returns a value
+		getValue := Succeed[DBConfig](100)
+
+		// Identity transformations
+		identity := func(x DBConfig) DBConfig { return x }
+		identityInt := func(x int) int { return x }
+
+		// Apply identity transformations
+		adapted := Promap(identity, identityInt)(getValue)
+		result := adapted(DBConfig{URL: "localhost"})(context.Background())()
+
+		assert.Equal(t, R.Of(100), result)
+	})
+}
+
+// TestPromapComposition tests that Promap composes correctly
+func TestPromapComposition(t *testing.T) {
+	t.Run("compose multiple transformations", func(t *testing.T) {
+		// Effect that uses ServerConfig and returns the port
+		getPort := Map[ServerConfig](func(cfg ServerConfig) int {
+			return cfg.Port
+		})(Ask[ServerConfig]())
+
+		// First transformation: AppConfig -> ServerConfig
+		extractServerConfig := func(app AppConfig) ServerConfig {
+			return ServerConfig{Host: "localhost", Port: app.Port}
+		}
+
+		// Second transformation: int -> string
+		formatPort := func(port int) string {
+			return fmt.Sprintf(":%d", port)
+		}
+
+		// Apply transformations
+		adapted := Promap(extractServerConfig, formatPort)(getPort)
+		result := adapted(AppConfig{
+			DatabaseURL: "db.example.com",
+			APIKey:      "key123",
+			Port:        9000,
+		})(context.Background())()
+
+		assert.Equal(t, R.Of(":9000"), result)
+	})
+}
+
+// TestPromapWithErrors tests Promap with effects that can fail
+func TestPromapWithErrors(t *testing.T) {
+	t.Run("propagates errors correctly", func(t *testing.T) {
+		// Effect that fails
+		failingEffect := Fail[DBConfig, int](fmt.Errorf("database connection failed"))
+
+		// Transformations
+		extractDBConfig := func(app AppConfig) DBConfig {
+			return DBConfig{URL: app.DatabaseURL}
+		}
+		formatCount := func(count int) string {
+			return fmt.Sprintf("Count: %d", count)
+		}
+
+		// Apply transformations
+		adapted := Promap(extractDBConfig, formatCount)(failingEffect)
+		result := adapted(AppConfig{DatabaseURL: "localhost"})(context.Background())()
+
+		assert.True(t, R.IsLeft(result))
+		err := R.MonadFold(result,
+			func(e error) error { return e },
+			func(string) error { return nil },
+		)
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "database connection failed")
+	})
+
+	t.Run("output transformation not applied on error", func(t *testing.T) {
+		callCount := 0
+
+		// Effect that fails
+		failingEffect := Fail[DBConfig, int](fmt.Errorf("error"))
+
+		// Transformation that counts calls
+		countingTransform := func(x int) string {
+			callCount++
+			return strconv.Itoa(x)
+		}
+
+		// Apply transformations
+		adapted := Promap(
+			func(app AppConfig) DBConfig { return DBConfig{URL: app.DatabaseURL} },
+			countingTransform,
+		)(failingEffect)
+		result := adapted(AppConfig{DatabaseURL: "localhost"})(context.Background())()
+
+		assert.True(t, R.IsLeft(result))
+		assert.Equal(t, 0, callCount, "output transformation should not be called on error")
+	})
+}
+
+// TestPromapWithComplexTypes tests Promap with more complex type transformations
+func TestPromapWithComplexTypes(t *testing.T) {
+	t.Run("transform struct to different struct", func(t *testing.T) {
+		type User struct {
+			ID   int
+			Name string
+		}
+
+		type UserDTO struct {
+			UserID   int
+			FullName string
+		}
+
+		// Effect that uses User and returns a string
+		getUserInfo := Map[User](func(user User) string {
+			return fmt.Sprintf("User %s (ID: %d)", user.Name, user.ID)
+		})(Ask[User]())
+
+		// Transform UserDTO to User
+		dtoToUser := func(dto UserDTO) User {
+			return User{ID: dto.UserID, Name: dto.FullName}
+		}
+
+		// Transform string to uppercase
+		toUpper := func(s string) string {
+			return fmt.Sprintf("INFO: %s", s)
+		}
+
+		// Apply transformations
+		adapted := Promap(dtoToUser, toUpper)(getUserInfo)
+		result := adapted(UserDTO{UserID: 123, FullName: "Alice"})(context.Background())()
+
+		assert.Equal(t, R.Of("INFO: User Alice (ID: 123)"), result)
+	})
+}
+
+// TestPromapChaining tests chaining multiple Promap operations
+func TestPromapChaining(t *testing.T) {
+	t.Run("chain multiple Promap operations", func(t *testing.T) {
+		// Base effect that doubles the input
+		baseEffect := Map[int](func(x int) int {
+			return x * 2
+		})(Ask[int]())
+
+		// First Promap: string -> int, int -> string
+		step1 := Promap(
+			func(s string) int {
+				n, _ := strconv.Atoi(s)
+				return n
+			},
+			strconv.Itoa,
+		)(baseEffect)
+
+		// Second Promap: float64 -> string, string -> float64
+		step2 := Promap(
+			func(f float64) string {
+				return fmt.Sprintf("%.0f", f)
+			},
+			func(s string) float64 {
+				f, _ := strconv.ParseFloat(s, 64)
+				return f
+			},
+		)(step1)
+
+		result := step2(21.0)(context.Background())()
+
+		assert.Equal(t, R.Of(42.0), result)
+	})
+}
+
+// TestPromapEdgeCases tests edge cases
+func TestPromapEdgeCases(t *testing.T) {
+	t.Run("zero values", func(t *testing.T) {
+		effect := Map[int](func(x int) int {
+			return x
+		})(Ask[int]())
+
+		adapted := Promap(
+			func(s string) int { return 0 },
+			func(x int) string { return "" },
+		)(effect)
+
+		result := adapted("anything")(context.Background())()
+
+		assert.Equal(t, R.Of(""), result)
+	})
+
+	t.Run("nil context handling", func(t *testing.T) {
+		effect := Succeed[int]("success")
+
+		adapted := Promap(
+			func(s string) int { return 42 },
+			func(s string) string { return s + "!" },
+		)(effect)
+
+		// Using background context instead of nil
+		result := adapted("test")(context.Background())()
+
+		assert.Equal(t, R.Of("success!"), result)
+	})
+}
+
+// TestPromapIntegration tests integration with other effect operations
+func TestPromapIntegration(t *testing.T) {
+	t.Run("Promap with Map", func(t *testing.T) {
+		// Base effect that adds 10
+		baseEffect := Map[int](func(x int) int {
+			return x + 10
+		})(Ask[int]())
+
+		// Apply Promap
+		promapped := Promap(
+			func(s string) int {
+				n, _ := strconv.Atoi(s)
+				return n
+			},
+			func(x int) int { return x * 2 },
+		)(baseEffect)
+
+		// Apply Map on top
+		mapped := Map[string](func(x int) string {
+			return fmt.Sprintf("Result: %d", x)
+		})(promapped)
+
+		result := mapped("5")(context.Background())()
+
+		assert.Equal(t, R.Of("Result: 30"), result)
+	})
+
+	t.Run("Promap with Chain", func(t *testing.T) {
+		// Base effect
+		baseEffect := Ask[int]()
+
+		// Apply Promap
+		promapped := Promap(
+			func(s string) int {
+				n, _ := strconv.Atoi(s)
+				return n
+			},
+			func(x int) int { return x * 2 },
+		)(baseEffect)
+
+		// Chain with another effect
+		chained := Chain(func(x int) Effect[string, string] {
+			return Succeed[string](fmt.Sprintf("Value: %d", x))
+		})(promapped)
+
+		result := chained("10")(context.Background())()
+
+		assert.Equal(t, R.Of("Value: 20"), result)
+	})
+}
+
+// BenchmarkPromap benchmarks the Promap operation
+func BenchmarkPromap(b *testing.B) {
+	effect := Map[int](func(x int) int {
+		return x * 2
+	})(Ask[int]())
+
+	adapted := Promap(
+		func(s string) int {
+			n, _ := strconv.Atoi(s)
+			return n
+		},
+		strconv.Itoa,
+	)(effect)
+
+	ctx := context.Background()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = adapted("42")(ctx)()
+	}
+}
+
+// BenchmarkPromapChained benchmarks chained Promap operations
+func BenchmarkPromapChained(b *testing.B) {
+	baseEffect := Map[int](func(x int) int {
+		return x * 2
+	})(Ask[int]())
+
+	step1 := Promap(
+		func(s string) int {
+			n, _ := strconv.Atoi(s)
+			return n
+		},
+		strconv.Itoa,
+	)(baseEffect)
+
+	step2 := Promap(
+		func(f float64) string {
+			return fmt.Sprintf("%.0f", f)
+		},
+		func(s string) float64 {
+			f, _ := strconv.ParseFloat(s, 64)
+			return f
+		},
+	)(step1)
+
+	ctx := context.Background()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = step2(21.0)(ctx)()
+	}
+}

v2/effect/types.go

@@ -19,9 +19,11 @@ import (
 	"github.com/IBM/fp-go/v2/context/readerioresult"
 	"github.com/IBM/fp-go/v2/context/readerreaderioresult"
 	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/io"
 	"github.com/IBM/fp-go/v2/ioeither"
 	"github.com/IBM/fp-go/v2/ioresult"
+	"github.com/IBM/fp-go/v2/iterator/iter"
 	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/lens"
@@ -89,4 +91,14 @@ type (
 	// Operator represents a function that transforms Effect[C, A] to Effect[C, B].
 	// It's used for lifting operations over effects.
 	Operator[C, A, B any] = readerreaderioresult.Operator[C, A, B]
+
+	// Endomorphism represents a function from type A to type A.
+	// It's an alias for endomorphism.Endomorphism[A].
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	// Seq is an iterator over sequences of individual values.
+	// When called as seq(yield), seq calls yield(v) for each value v in the sequence,
+	// stopping early if yield returns false.
+	// See the [iter] package documentation for more details.
+	Seq[A any] = iter.Seq[A]
 )

v2/either/array.go

@@ -16,6 +16,9 @@
 package either
 
 import (
+	"iter"
+	"slices"
+
 	F "github.com/IBM/fp-go/v2/function"
 	RA "github.com/IBM/fp-go/v2/internal/array"
 )
@@ -178,3 +181,92 @@ func CompactArrayG[A1 ~[]Either[E, A], A2 ~[]A, E, A any](fa A1) A2 {
 func CompactArray[E, A any](fa []Either[E, A]) []A {
 	return CompactArrayG[[]Either[E, A], []A](fa)
 }
+
+// TraverseSeq transforms an iterator by applying a function that returns an Either to each element.
+// If any element produces a Left, the entire result is that Left (short-circuits).
+// Otherwise, returns Right containing an iterator of all Right values.
+//
+// The function eagerly evaluates all elements in the input iterator to detect any Left values,
+// then returns an iterator over the collected Right values. This is necessary because Either
+// represents computations that can fail, and we need to know if any element failed before
+// producing the result iterator.
+//
+// # Type Parameters
+//
+//   - E: The error type for Left values
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - f: A function that transforms each element into an Either
+//
+// # Returns
+//
+//   - A function that takes an iterator of A and returns Either containing an iterator of B
+//
+// # Example Usage
+//
+//	parse := func(s string) either.Either[error, int] {
+//	    v, err := strconv.Atoi(s)
+//	    return either.FromError(v, err)
+//	}
+//	input := slices.Values([]string{"1", "2", "3"})
+//	result := either.TraverseSeq(parse)(input)
+//	// result is Right(iterator over [1, 2, 3])
+//
+// # See Also
+//
+//   - TraverseArray: For slice-based traversal
+//   - SequenceSeq: For sequencing iterators of Either values
+func TraverseSeq[E, A, B any](f Kleisli[E, A, B]) Kleisli[E, iter.Seq[A], iter.Seq[B]] {
+	return func(ga iter.Seq[A]) Either[E, iter.Seq[B]] {
+		var bs []B
+		for a := range ga {
+			b := f(a)
+			if b.isLeft {
+				return Left[iter.Seq[B]](b.l)
+			}
+			bs = append(bs, b.r)
+		}
+		return Of[E](slices.Values(bs))
+	}
+}
+
+// SequenceSeq converts an iterator of Either into an Either of iterator.
+// If any element is Left, returns that Left (short-circuits).
+// Otherwise, returns Right containing an iterator of all the Right values.
+//
+// This function eagerly evaluates all Either values in the input iterator to detect
+// any Left values, then returns an iterator over the collected Right values.
+//
+// # Type Parameters
+//
+//   - E: The error type for Left values
+//   - A: The value type for Right values
+//
+// # Parameters
+//
+//   - ma: An iterator of Either values
+//
+// # Returns
+//
+//   - Either containing an iterator of Right values, or the first Left encountered
+//
+// # Example Usage
+//
+//	eithers := slices.Values([]either.Either[error, int]{
+//	    either.Right[error](1),
+//	    either.Right[error](2),
+//	    either.Right[error](3),
+//	})
+//	result := either.SequenceSeq(eithers)
+//	// result is Right(iterator over [1, 2, 3])
+//
+// # See Also
+//
+//   - SequenceArray: For slice-based sequencing
+//   - TraverseSeq: For transforming and sequencing in one step
+func SequenceSeq[E, A any](ma iter.Seq[Either[E, A]]) Either[E, iter.Seq[A]] {
+	return TraverseSeq(F.Identity[Either[E, A]])(ma)
+}

v2/either/array_test.go

@@ -1,27 +1,28 @@
 package either
 
 import (
+	"errors"
 	"fmt"
+	"iter"
+	"slices"
+	"strconv"
 	"testing"
 
-	A "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
 	TST "github.com/IBM/fp-go/v2/internal/testing"
 	"github.com/stretchr/testify/assert"
 )
 
 func TestCompactArray(t *testing.T) {
-	ar := A.From(
+	ar := []Either[string, string]{
 		Of[string]("ok"),
 		Left[string]("err"),
 		Of[string]("ok"),
-	)
-
-	res := CompactArray(ar)
-	assert.Equal(t, 2, len(res))
+	}
+	assert.Equal(t, 2, len(CompactArray(ar)))
 }
 
 func TestSequenceArray(t *testing.T) {
-
 	s := TST.SequenceArrayTest(
 		FromStrictEquals[error, bool](),
 		Pointed[error, string](),
@@ -29,14 +30,12 @@ func TestSequenceArray(t *testing.T) {
 		Functor[error, []string, bool](),
 		SequenceArray[error, string],
 	)
-
-	for i := 0; i < 10; i++ {
+	for i := range 10 {
 		t.Run(fmt.Sprintf("TestSequenceArray %d", i), s(i))
 	}
 }
 
 func TestSequenceArrayError(t *testing.T) {
-
 	s := TST.SequenceArrayErrorTest(
 		FromStrictEquals[error, bool](),
 		Left[string, error],
@@ -46,6 +45,243 @@ func TestSequenceArrayError(t *testing.T) {
 		Functor[error, []string, bool](),
 		SequenceArray[error, string],
 	)
-	// run across four bits
 	s(4)(t)
 }
+
+func TestTraverseSeq_Success(t *testing.T) {
+	parse := func(s string) Either[error, int] {
+		v, err := strconv.Atoi(s)
+		return TryCatchError(v, err)
+	}
+
+	collectInts := func(result Either[error, iter.Seq[int]]) []int {
+		return F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+	}
+
+	t.Run("transforms all elements successfully", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"1", "2", "3"}))
+		assert.Equal(t, []int{1, 2, 3}, collectInts(result))
+	})
+
+	t.Run("works with empty iterator", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{}))
+		assert.Empty(t, collectInts(result))
+	})
+
+	t.Run("works with single element", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"42"}))
+		assert.Equal(t, []int{42}, collectInts(result))
+	})
+
+	t.Run("preserves order of elements", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"10", "20", "30", "40", "50"}))
+		assert.Equal(t, []int{10, 20, 30, 40, 50}, collectInts(result))
+	})
+}
+
+func TestTraverseSeq_Failure(t *testing.T) {
+	parse := func(s string) Either[error, int] {
+		v, err := strconv.Atoi(s)
+		return TryCatchError(v, err)
+	}
+
+	extractErr := func(result Either[error, iter.Seq[int]]) error {
+		return F.Pipe1(result, Fold(
+			F.Identity[error],
+			func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+		))
+	}
+
+	t.Run("short-circuits on first Left", func(t *testing.T) {
+		err := extractErr(TraverseSeq(parse)(slices.Values([]string{"1", "invalid", "3"})))
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "invalid syntax")
+	})
+
+	t.Run("returns first error when multiple failures exist", func(t *testing.T) {
+		err := extractErr(TraverseSeq(parse)(slices.Values([]string{"1", "bad1", "bad2"})))
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "bad1")
+	})
+
+	t.Run("handles custom error types", func(t *testing.T) {
+		customErr := errors.New("custom validation error")
+		validate := func(n int) Either[error, int] {
+			if n == 2 {
+				return Left[int](customErr)
+			}
+			return Right[error](n * 10)
+		}
+		err := extractErr(TraverseSeq(validate)(slices.Values([]int{1, 2, 3})))
+		assert.Equal(t, customErr, err)
+	})
+}
+
+func TestTraverseSeq_EdgeCases(t *testing.T) {
+	t.Run("handles complex transformations", func(t *testing.T) {
+		type User struct {
+			ID   int
+			Name string
+		}
+
+		transform := func(id int) Either[error, User] {
+			return Right[error](User{ID: id, Name: fmt.Sprintf("User%d", id)})
+		}
+
+		result := TraverseSeq(transform)(slices.Values([]int{1, 2, 3}))
+		collected := F.Pipe1(result, Fold(
+			func(e error) []User { t.Fatal(e); return nil },
+			slices.Collect[User],
+		))
+
+		assert.Equal(t, []User{
+			{ID: 1, Name: "User1"},
+			{ID: 2, Name: "User2"},
+			{ID: 3, Name: "User3"},
+		}, collected)
+	})
+
+	t.Run("works with identity transformation", func(t *testing.T) {
+		input := slices.Values([]Either[error, int]{
+			Right[error](1),
+			Right[error](2),
+			Right[error](3),
+		})
+
+		result := TraverseSeq(F.Identity[Either[error, int]])(input)
+		collected := F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+
+		assert.Equal(t, []int{1, 2, 3}, collected)
+	})
+}
+
+func TestSequenceSeq_Success(t *testing.T) {
+	collectInts := func(result Either[error, iter.Seq[int]]) []int {
+		return F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+	}
+
+	t.Run("sequences multiple Right values", func(t *testing.T) {
+		input := slices.Values([]Either[error, int]{Right[error](1), Right[error](2), Right[error](3)})
+		assert.Equal(t, []int{1, 2, 3}, collectInts(SequenceSeq(input)))
+	})
+
+	t.Run("works with empty iterator", func(t *testing.T) {
+		input := slices.Values([]Either[error, string]{})
+		result := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []string { t.Fatal(e); return nil },
+			slices.Collect[string],
+		))
+		assert.Empty(t, result)
+	})
+
+	t.Run("works with single Right value", func(t *testing.T) {
+		input := slices.Values([]Either[error, string]{Right[error]("hello")})
+		result := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []string { t.Fatal(e); return nil },
+			slices.Collect[string],
+		))
+		assert.Equal(t, []string{"hello"}, result)
+	})
+
+	t.Run("preserves order of results", func(t *testing.T) {
+		input := slices.Values([]Either[error, int]{
+			Right[error](5), Right[error](4), Right[error](3), Right[error](2), Right[error](1),
+		})
+		assert.Equal(t, []int{5, 4, 3, 2, 1}, collectInts(SequenceSeq(input)))
+	})
+
+	t.Run("works with complex types", func(t *testing.T) {
+		type Item struct {
+			Value int
+			Label string
+		}
+
+		input := slices.Values([]Either[error, Item]{
+			Right[error](Item{Value: 1, Label: "first"}),
+			Right[error](Item{Value: 2, Label: "second"}),
+			Right[error](Item{Value: 3, Label: "third"}),
+		})
+
+		collected := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []Item { t.Fatal(e); return nil },
+			slices.Collect[Item],
+		))
+
+		assert.Equal(t, []Item{
+			{Value: 1, Label: "first"},
+			{Value: 2, Label: "second"},
+			{Value: 3, Label: "third"},
+		}, collected)
+	})
+}
+
+func TestSequenceSeq_Failure(t *testing.T) {
+	extractErr := func(result Either[error, iter.Seq[int]]) error {
+		return F.Pipe1(result, Fold(
+			F.Identity[error],
+			func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+		))
+	}
+
+	t.Run("short-circuits on first Left", func(t *testing.T) {
+		testErr := errors.New("test error")
+		input := slices.Values([]Either[error, int]{Right[error](1), Left[int](testErr), Right[error](3)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("returns first error when multiple Left values exist", func(t *testing.T) {
+		err1 := errors.New("error 1")
+		err2 := errors.New("error 2")
+		input := slices.Values([]Either[error, int]{Right[error](1), Left[int](err1), Left[int](err2)})
+		assert.Equal(t, err1, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("handles Left at the beginning", func(t *testing.T) {
+		testErr := errors.New("first error")
+		input := slices.Values([]Either[error, int]{Left[int](testErr), Right[error](2), Right[error](3)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("handles Left at the end", func(t *testing.T) {
+		testErr := errors.New("last error")
+		input := slices.Values([]Either[error, int]{Right[error](1), Right[error](2), Left[int](testErr)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+}
+
+func TestSequenceSeq_Integration(t *testing.T) {
+	t.Run("integrates with TraverseSeq", func(t *testing.T) {
+		parse := func(s string) Either[error, int] {
+			v, err := strconv.Atoi(s)
+			return TryCatchError(v, err)
+		}
+		result := TraverseSeq(parse)(slices.Values([]string{"1", "2", "3"}))
+		assert.True(t, IsRight(result))
+	})
+
+	t.Run("SequenceSeq is equivalent to TraverseSeq with Identity", func(t *testing.T) {
+		mkInput := func() []Either[error, int] {
+			return []Either[error, int]{Right[error](10), Right[error](20), Right[error](30)}
+		}
+
+		collected1 := F.Pipe1(SequenceSeq(slices.Values(mkInput())), Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+		collected2 := F.Pipe1(TraverseSeq(F.Identity[Either[error, int]])(slices.Values(mkInput())), Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+
+		assert.Equal(t, collected1, collected2)
+	})
+}

v2/function/gen.go

@@ -236,6 +236,7 @@ func Pipe4[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, T
 // The final return value is the result of the last function application
 //go:inline
 func Flow4[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, T0, T1, T2, T3, T4 any](f1 F1, f2 F2, f3 F3, f4 F4) func(T0) T4 {
+	//go:inline
 	return func(t0 T0) T4 {
 		return Pipe4(t0, f1, f2, f3, f4)
 	}
@@ -302,6 +303,7 @@ func Pipe5[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F
 // The final return value is the result of the last function application
 //go:inline
 func Flow5[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, T0, T1, T2, T3, T4, T5 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5) func(T0) T5 {
+	//go:inline
 	return func(t0 T0) T5 {
 		return Pipe5(t0, f1, f2, f3, f4, f5)
 	}
@@ -370,6 +372,7 @@ func Pipe6[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F
 // The final return value is the result of the last function application
 //go:inline
 func Flow6[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, T0, T1, T2, T3, T4, T5, T6 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6) func(T0) T6 {
+	//go:inline
 	return func(t0 T0) T6 {
 		return Pipe6(t0, f1, f2, f3, f4, f5, f6)
 	}
@@ -440,6 +443,7 @@ func Pipe7[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F
 // The final return value is the result of the last function application
 //go:inline
 func Flow7[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, T0, T1, T2, T3, T4, T5, T6, T7 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7) func(T0) T7 {
+	//go:inline
 	return func(t0 T0) T7 {
 		return Pipe7(t0, f1, f2, f3, f4, f5, f6, f7)
 	}
@@ -512,6 +516,7 @@ func Pipe8[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F
 // The final return value is the result of the last function application
 //go:inline
 func Flow8[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, T0, T1, T2, T3, T4, T5, T6, T7, T8 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8) func(T0) T8 {
+	//go:inline
 	return func(t0 T0) T8 {
 		return Pipe8(t0, f1, f2, f3, f4, f5, f6, f7, f8)
 	}
@@ -586,6 +591,7 @@ func Pipe9[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F
 // The final return value is the result of the last function application
 //go:inline
 func Flow9[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9) func(T0) T9 {
+	//go:inline
 	return func(t0 T0) T9 {
 		return Pipe9(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9)
 	}
@@ -662,6 +668,7 @@ func Pipe10[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow10[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10) func(T0) T10 {
+	//go:inline
 	return func(t0 T0) T10 {
 		return Pipe10(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10)
 	}
@@ -740,6 +747,7 @@ func Pipe11[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow11[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11) func(T0) T11 {
+	//go:inline
 	return func(t0 T0) T11 {
 		return Pipe11(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11)
 	}
@@ -820,6 +828,7 @@ func Pipe12[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow12[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12) func(T0) T12 {
+	//go:inline
 	return func(t0 T0) T12 {
 		return Pipe12(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12)
 	}
@@ -902,6 +911,7 @@ func Pipe13[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow13[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13) func(T0) T13 {
+	//go:inline
 	return func(t0 T0) T13 {
 		return Pipe13(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13)
 	}
@@ -986,6 +996,7 @@ func Pipe14[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow14[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, F14 ~func(T13) T14, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13, f14 F14) func(T0) T14 {
+	//go:inline
 	return func(t0 T0) T14 {
 		return Pipe14(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14)
 	}
@@ -1072,6 +1083,7 @@ func Pipe15[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow15[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, F14 ~func(T13) T14, F15 ~func(T14) T15, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13, f14 F14, f15 F15) func(T0) T15 {
+	//go:inline
 	return func(t0 T0) T15 {
 		return Pipe15(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15)
 	}
@@ -1160,6 +1172,7 @@ func Pipe16[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow16[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, F14 ~func(T13) T14, F15 ~func(T14) T15, F16 ~func(T15) T16, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13, f14 F14, f15 F15, f16 F16) func(T0) T16 {
+	//go:inline
 	return func(t0 T0) T16 {
 		return Pipe16(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15, f16)
 	}
@@ -1250,6 +1263,7 @@ func Pipe17[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow17[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, F14 ~func(T13) T14, F15 ~func(T14) T15, F16 ~func(T15) T16, F17 ~func(T16) T17, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13, f14 F14, f15 F15, f16 F16, f17 F17) func(T0) T17 {
+	//go:inline
 	return func(t0 T0) T17 {
 		return Pipe17(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15, f16, f17)
 	}
@@ -1342,6 +1356,7 @@ func Pipe18[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow18[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, F14 ~func(T13) T14, F15 ~func(T14) T15, F16 ~func(T15) T16, F17 ~func(T16) T17, F18 ~func(T17) T18, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13, f14 F14, f15 F15, f16 F16, f17 F17, f18 F18) func(T0) T18 {
+	//go:inline
 	return func(t0 T0) T18 {
 		return Pipe18(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15, f16, f17, f18)
 	}
@@ -1436,6 +1451,7 @@ func Pipe19[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow19[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, F14 ~func(T13) T14, F15 ~func(T14) T15, F16 ~func(T15) T16, F17 ~func(T16) T17, F18 ~func(T17) T18, F19 ~func(T18) T19, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13, f14 F14, f15 F15, f16 F16, f17 F17, f18 F18, f19 F19) func(T0) T19 {
+	//go:inline
 	return func(t0 T0) T19 {
 		return Pipe19(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15, f16, f17, f18, f19)
 	}
@@ -1532,6 +1548,7 @@ func Pipe20[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4,
 // The final return value is the result of the last function application
 //go:inline
 func Flow20[F1 ~func(T0) T1, F2 ~func(T1) T2, F3 ~func(T2) T3, F4 ~func(T3) T4, F5 ~func(T4) T5, F6 ~func(T5) T6, F7 ~func(T6) T7, F8 ~func(T7) T8, F9 ~func(T8) T9, F10 ~func(T9) T10, F11 ~func(T10) T11, F12 ~func(T11) T12, F13 ~func(T12) T13, F14 ~func(T13) T14, F15 ~func(T14) T15, F16 ~func(T15) T16, F17 ~func(T16) T17, F18 ~func(T17) T18, F19 ~func(T18) T19, F20 ~func(T19) T20, T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20 any](f1 F1, f2 F2, f3 F3, f4 F4, f5 F5, f6 F6, f7 F7, f8 F8, f9 F9, f10 F10, f11 F11, f12 F12, f13 F13, f14 F14, f15 F15, f16 F16, f17 F17, f18 F18, f19 F19, f20 F20) func(T0) T20 {
+	//go:inline
 	return func(t0 T0) T20 {
 		return Pipe20(t0, f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15, f16, f17, f18, f19, f20)
 	}

v2/go.mod

@@ -4,7 +4,7 @@ go 1.24
 
 require (
 	github.com/stretchr/testify v1.11.1
-	github.com/urfave/cli/v3 v3.7.0
+	github.com/urfave/cli/v3 v3.8.0
 )
 
 require (

v2/go.sum

@@ -4,10 +4,8 @@ github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZb
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
-github.com/urfave/cli/v3 v3.6.2 h1:lQuqiPrZ1cIz8hz+HcrG0TNZFxU70dPZ3Yl+pSrH9A8=
-github.com/urfave/cli/v3 v3.6.2/go.mod h1:ysVLtOEmg2tOy6PknnYVhDoouyC/6N42TMeoMzskhso=
-github.com/urfave/cli/v3 v3.7.0 h1:AGSnbUyjtLiM+WJUb4dzXKldl/gL+F8OwmRDtVr6g2U=
-github.com/urfave/cli/v3 v3.7.0/go.mod h1:ysVLtOEmg2tOy6PknnYVhDoouyC/6N42TMeoMzskhso=
+github.com/urfave/cli/v3 v3.8.0 h1:XqKPrm0q4P0q5JpoclYoCAv0/MIvH/jZ2umzuf8pNTI=
+github.com/urfave/cli/v3 v3.8.0/go.mod h1:ysVLtOEmg2tOy6PknnYVhDoouyC/6N42TMeoMzskhso=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=

v2/internal/array/array.go

@@ -15,6 +15,8 @@
 
 package array
 
+import "slices"
+
 func Of[GA ~[]A, A any](a A) GA {
 	return GA{a}
 }
@@ -197,3 +199,9 @@ func Reverse[GT ~[]T, T any](as GT) GT {
 	}
 	return ras
 }
+
+func UnsafeUpdateAt[GT ~[]T, T any](as GT, i int, v T) GT {
+	c := slices.Clone(as)
+	c[i] = v
+	return c
+}

v2/internal/array/traverse.go

@@ -46,7 +46,7 @@ import (
 //   - Multiple elements: recursively divides and conquers
 func MonadSequenceSegment[HKTB, HKTRB any](
 	fof func(HKTB) HKTRB,
-	empty HKTRB,
+	empty func() HKTRB,
 	concat func(HKTRB, HKTRB) HKTRB,
 	fbs []HKTB,
 	start, end int,
@@ -54,7 +54,7 @@ func MonadSequenceSegment[HKTB, HKTRB any](
 
 	switch end - start {
 	case 0:
-		return empty
+		return empty()
 	case 1:
 		return fof(fbs[start])
 	default:
@@ -254,7 +254,7 @@ HKTAB = HKT<func(A)B>
 */
 func MonadSequence[GA ~[]HKTA, HKTA, HKTRA any](
 	fof func(HKTA) HKTRA,
-	empty HKTRA,
+	empty func() HKTRA,
 	concat func(HKTRA, HKTRA) HKTRA,
 
 	ta GA) HKTRA {
@@ -263,7 +263,7 @@ func MonadSequence[GA ~[]HKTA, HKTA, HKTRA any](
 
 func Sequence[GA ~[]HKTA, HKTA, HKTRA any](
 	fof func(HKTA) HKTRA,
-	empty HKTRA,
+	empty func() HKTRA,
 	concat func(HKTRA, HKTRA) HKTRA,
 ) func(GA) HKTRA {
 

v2/internal/filterable/types.go

@@ -0,0 +1,15 @@
+package filterable
+
+import (
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
+)
+
+type (
+	Option[A any]       = option.Option[A]
+	Separated[A, B any] = pair.Pair[A, B]
+
+	FilterType[A, HKTA any] = func(func(A) bool) func(HKTA) HKTA
+
+	FilterMapType[A, B, HKTA, HKTB any] = func(func(A) Option[B]) func(HKTA) HKTB
+)

v2/internal/iter/traverse.go

@@ -73,7 +73,7 @@ func MonadTraverse[GA ~func(yield func(A) bool), GB ~func(yield func(B) bool), A
 
 	fof := F.Bind2nd(fmap_b, Of[GB])
 
-	empty := fof_gb(Empty[GB]())
+	empty := F.Nullary2(Empty[GB], fof_gb)
 
 	cb := F.Curry2(Concat[GB])
 	concat_gb := F.Bind2nd(fmap_gb, cb)
@@ -180,7 +180,7 @@ func MonadSequence[GA ~func(yield func(HKTA) bool), HKTA, HKTRA any](
 
 	// convert to an array
 	hktb := ToArray[GA, []HKTA](ta)
-	return INTA.MonadSequenceSegment(fof, m.Empty(), m.Concat, hktb, 0, len(hktb))
+	return INTA.MonadSequenceSegment(fof, m.Empty, m.Concat, hktb, 0, len(hktb))
 }
 
 // MonadTraverseWithIndex traverses an iterator sequence with index tracking, applying an effectful
@@ -223,7 +223,7 @@ func MonadTraverseWithIndex[GA ~func(yield func(A) bool), A, HKTB, HKTRB any](
 
 	// convert to an array
 	hktb := MonadMapToArrayWithIndex[GA, []HKTB](ta, f)
-	return INTA.MonadSequenceSegment(fof, m.Empty(), m.Concat, hktb, 0, len(hktb))
+	return INTA.MonadSequenceSegment(fof, m.Empty, m.Concat, hktb, 0, len(hktb))
 }
 
 // Sequence is the curried version of MonadSequence, returning a function that sequences an iterator of effects.

v2/internal/witherable/filter.go

@@ -0,0 +1,27 @@
+package witherable
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/filterable"
+	"github.com/IBM/fp-go/v2/internal/functor"
+)
+
+func Filter[A, HKT_G_A, HKT_F_HKT_G_A any](
+	fmap functor.MapType[HKT_G_A, HKT_G_A, HKT_F_HKT_G_A, HKT_F_HKT_G_A],
+	ffilter filterable.FilterType[A, HKT_G_A],
+) func(func(A) bool) func(HKT_F_HKT_G_A) HKT_F_HKT_G_A {
+	return function.Flow2(
+		ffilter,
+		fmap,
+	)
+}
+
+func FilterMap[A, B, HKT_G_A, HKT_G_B, HKT_F_HKT_G_A, HKT_F_HKT_G_B any](
+	fmap functor.MapType[HKT_G_A, HKT_G_B, HKT_F_HKT_G_A, HKT_F_HKT_G_B],
+	ffilter filterable.FilterMapType[A, B, HKT_G_A, HKT_G_B],
+) func(func(A) Option[B]) func(HKT_F_HKT_G_A) HKT_F_HKT_G_B {
+	return function.Flow2(
+		ffilter,
+		fmap,
+	)
+}

v2/internal/witherable/partition.go

@@ -0,0 +1 @@
+package witherable

v2/internal/witherable/type.go

@@ -0,0 +1,7 @@
+package witherable
+
+import "github.com/IBM/fp-go/v2/option"
+
+type (
+	Option[A any] = option.Option[A]
+)

v2/iterator/iter/async.go

@@ -0,0 +1,254 @@
+// 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 (
+	A "github.com/IBM/fp-go/v2/array"
+)
+
+// AsyncBuf converts a synchronous sequence into an asynchronous buffered sequence.
+// It spawns a goroutine to consume the input sequence and sends values through
+// a buffered channel, allowing concurrent production and consumption of elements.
+//
+// The function provides backpressure control through the buffer size and properly
+// handles early termination when the consumer stops iterating. This is useful for
+// decoupling producers and consumers, enabling pipeline parallelism, or when you
+// need to process sequences concurrently.
+//
+// # Type Parameters
+//
+//   - T: The type of elements in the sequence
+//
+// # Parameters
+//
+//   - input: The source sequence to be consumed asynchronously
+//   - bufSize: The buffer size for the channel. Negative values are treated as 0 (unbuffered).
+//     A larger buffer allows more elements to be produced ahead of consumption,
+//     but uses more memory. A buffer of 0 creates an unbuffered channel requiring
+//     synchronization between producer and consumer.
+//
+// # Returns
+//
+//   - Seq[T]: A new sequence that yields elements from the input sequence asynchronously
+//
+// # Behavior
+//
+//   - Spawns a goroutine that consumes the input sequence
+//   - Elements are sent through a buffered channel to the output sequence
+//   - Properly handles early termination: if the consumer stops iterating (yield returns false),
+//     the producer goroutine is signaled to stop via a done channel
+//   - Both the producer goroutine and the done channel are properly cleaned up
+//   - The channel is closed when the input sequence is exhausted or early termination occurs
+//
+// # Example Usage
+//
+//	// Create an async sequence with a buffer of 10
+//	seq := From(1, 2, 3, 4, 5)
+//	async := AsyncBuf(seq, 10)
+//
+//	// Elements are produced concurrently
+//	for v := range async {
+//	    fmt.Println(v) // Prints: 1, 2, 3, 4, 5
+//	}
+//
+// # Example with Early Termination
+//
+//	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+//	async := AsyncBuf(seq, 5)
+//
+//	// Stop after 3 elements - producer goroutine will be properly cleaned up
+//	count := 0
+//	for v := range async {
+//	    fmt.Println(v)
+//	    count++
+//	    if count >= 3 {
+//	        break
+//	    }
+//	}
+//
+// # Example with Unbuffered Channel
+//
+//	// bufSize of 0 creates an unbuffered channel
+//	seq := From(1, 2, 3)
+//	async := AsyncBuf(seq, 0)
+//
+//	// Producer and consumer are synchronized
+//	for v := range async {
+//	    fmt.Println(v)
+//	}
+//
+// # See Also
+//
+//   - From: Creates a sequence from values
+//   - Map: Transforms sequence elements
+//   - Filter: Filters sequence elements
+func AsyncBuf[T any](input Seq[T], bufSize int) Seq[T] {
+	return MergeBuf(A.Of(input), bufSize)
+}
+
+// Async converts a synchronous sequence into an asynchronous sequence using a default buffer size.
+// This is a convenience wrapper around AsyncBuf that uses a default buffer size of 8.
+//
+// Type Parameters:
+//   - T: The type of elements in the sequence
+//
+// Parameters:
+//   - input: The source sequence to be consumed asynchronously
+//
+// Returns:
+//   - Seq[T]: A new sequence that yields elements from the input sequence asynchronously
+//
+// Behavior:
+//   - Uses a default buffer size of 8 for the internal channel
+//   - Spawns a goroutine that consumes the input sequence
+//   - Elements are sent through a buffered channel to the output sequence
+//   - Properly handles early termination with goroutine cleanup
+//   - The channel is closed when the input sequence is exhausted
+//
+// Example:
+//
+//	seq := From(1, 2, 3, 4, 5)
+//	async := Async(seq)
+//
+//	// Elements are produced concurrently
+//	for v := range async {
+//	    fmt.Println(v) // Prints: 1, 2, 3, 4, 5
+//	}
+//
+// See Also:
+//   - AsyncBuf: Async with custom buffer size
+//   - Async2: Asynchronous sequence for key-value sequences
+//   - Merge: Merges multiple sequences concurrently
+func Async[T any](input Seq[T]) Seq[T] {
+	return AsyncBuf(input, defaultBufferSize)
+}
+
+// Async2Buf converts a synchronous key-value sequence into an asynchronous buffered sequence.
+// It spawns a goroutine to consume the input sequence and sends key-value pairs through
+// a buffered channel, allowing concurrent production and consumption of elements.
+//
+// This function is the Seq2 variant of Async, providing the same asynchronous behavior
+// for key-value sequences. It internally converts the Seq2 to a sequence of Pairs,
+// applies Async, and converts back to Seq2.
+//
+// # Type Parameters
+//
+//   - K: The type of keys in the sequence
+//   - V: The type of values in the sequence
+//
+// # Parameters
+//
+//   - input: The source key-value sequence to be consumed asynchronously
+//   - bufSize: The buffer size for the channel. Negative values are treated as 0 (unbuffered).
+//     A larger buffer allows more elements to be produced ahead of consumption,
+//     but uses more memory. A buffer of 0 creates an unbuffered channel requiring
+//     synchronization between producer and consumer.
+//
+// # Returns
+//
+//   - Seq2[K, V]: A new key-value sequence that yields elements from the input sequence asynchronously
+//
+// # Behavior
+//
+//   - Spawns a goroutine that consumes the input key-value sequence
+//   - Key-value pairs are sent through a buffered channel to the output sequence
+//   - Properly handles early termination: if the consumer stops iterating (yield returns false),
+//     the producer goroutine is signaled to stop via a done channel
+//   - Both the producer goroutine and the done channel are properly cleaned up
+//   - The channel is closed when the input sequence is exhausted or early termination occurs
+//
+// # Example Usage
+//
+//	// Create an async key-value sequence with a buffer of 10
+//	seq := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+//	async := Async2Buf(seq, 10)
+//
+//	// Elements are produced concurrently
+//	for k, v := range async {
+//	    fmt.Printf("%d: %s\n", k, v)
+//	}
+//	// Output:
+//	// 1: a
+//	// 2: b
+//	// 3: c
+//
+// # Example with Early Termination
+//
+//	seq := MonadZip(From(1, 2, 3, 4, 5), From("a", "b", "c", "d", "e"))
+//	async := Async2Buf(seq, 5)
+//
+//	// Stop after 2 pairs - producer goroutine will be properly cleaned up
+//	count := 0
+//	for k, v := range async {
+//	    fmt.Printf("%d: %s\n", k, v)
+//	    count++
+//	    if count >= 2 {
+//	        break
+//	    }
+//	}
+//
+// # See Also
+//
+//   - Async: Asynchronous sequence for single-value sequences
+//   - ToSeqPair: Converts Seq2 to Seq of Pairs
+//   - FromSeqPair: Converts Seq of Pairs to Seq2
+//   - MonadZip: Creates key-value sequences from two sequences
+func Async2Buf[K, V any](input Seq2[K, V], bufSize int) Seq2[K, V] {
+	return FromSeqPair(AsyncBuf(ToSeqPair(input), bufSize))
+}
+
+// Async2 converts a synchronous key-value sequence into an asynchronous sequence using a default buffer size.
+// This is a convenience wrapper around Async2Buf that uses a default buffer size of 8.
+// It's the Seq2 variant of Async, providing the same asynchronous behavior for key-value sequences.
+//
+// Type Parameters:
+//   - K: The type of keys in the sequence
+//   - V: The type of values in the sequence
+//
+// Parameters:
+//   - input: The source key-value sequence to be consumed asynchronously
+//
+// Returns:
+//   - Seq2[K, V]: A new key-value sequence that yields elements from the input sequence asynchronously
+//
+// Behavior:
+//   - Uses a default buffer size of 8 for the internal channel
+//   - Spawns a goroutine that consumes the input key-value sequence
+//   - Key-value pairs are sent through a buffered channel to the output sequence
+//   - Properly handles early termination with goroutine cleanup
+//   - The channel is closed when the input sequence is exhausted
+//
+// Example:
+//
+//	seq := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+//	async := Async2(seq)
+//
+//	// Elements are produced concurrently
+//	for k, v := range async {
+//	    fmt.Printf("%d: %s\n", k, v)
+//	}
+//	// Output:
+//	// 1: a
+//	// 2: b
+//	// 3: c
+//
+// See Also:
+//   - Async2Buf: Async2 with custom buffer size
+//   - Async: Asynchronous sequence for single-value sequences
+//   - MonadZip: Creates key-value sequences from two sequences
+func Async2[K, V any](input Seq2[K, V]) Seq2[K, V] {
+	return Async2Buf(input, defaultBufferSize)
+}

v2/iterator/iter/async_test.go

@@ -0,0 +1,903 @@
+// 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"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	N "github.com/IBM/fp-go/v2/number"
+	"github.com/IBM/fp-go/v2/pair"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestAsync_Success tests basic Async functionality
+func TestAsync_Success(t *testing.T) {
+	t.Run("converts sequence to async with buffer", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async := AsyncBuf(seq, 10)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("preserves element order", func(t *testing.T) {
+		seq := From("a", "b", "c", "d", "e")
+		async := AsyncBuf(seq, 5)
+		result := toSlice(async)
+		assert.Equal(t, []string{"a", "b", "c", "d", "e"}, result)
+	})
+
+	t.Run("works with single element", func(t *testing.T) {
+		seq := From(42)
+		async := AsyncBuf(seq, 1)
+		result := toSlice(async)
+		assert.Equal(t, []int{42}, result)
+	})
+
+	t.Run("works with large sequence", func(t *testing.T) {
+		data := make([]int, 100)
+		for i := range data {
+			data[i] = i
+		}
+		seq := From(data...)
+		async := AsyncBuf(seq, 20)
+		result := toSlice(async)
+		assert.Equal(t, data, result)
+	})
+}
+
+// TestAsync_BufferSizes tests different buffer sizes
+func TestAsync_BufferSizes(t *testing.T) {
+	t.Run("unbuffered channel (bufSize 0)", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		async := AsyncBuf(seq, 0)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+
+	t.Run("small buffer", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async := AsyncBuf(seq, 2)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("large buffer", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async := AsyncBuf(seq, 100)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("negative buffer size treated as 0", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		async := AsyncBuf(seq, -5)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+
+	t.Run("buffer size equals sequence length", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async := AsyncBuf(seq, 5)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("buffer size larger than sequence", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		async := AsyncBuf(seq, 10)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+}
+
+// TestAsync_Empty tests Async with empty sequences
+func TestAsync_Empty(t *testing.T) {
+	t.Run("empty integer sequence", func(t *testing.T) {
+		seq := Empty[int]()
+		async := AsyncBuf(seq, 5)
+		result := toSlice(async)
+		assert.Empty(t, result)
+	})
+
+	t.Run("empty string sequence", func(t *testing.T) {
+		seq := Empty[string]()
+		async := AsyncBuf(seq, 10)
+		result := toSlice(async)
+		assert.Empty(t, result)
+	})
+
+	t.Run("empty with zero buffer", func(t *testing.T) {
+		seq := Empty[int]()
+		async := AsyncBuf(seq, 0)
+		result := toSlice(async)
+		assert.Empty(t, result)
+	})
+}
+
+// TestAsync_EarlyTermination tests that Async properly handles early termination
+func TestAsync_EarlyTermination(t *testing.T) {
+	t.Run("stops producer when consumer breaks", func(t *testing.T) {
+		var producerCount atomic.Int32
+
+		// Create a sequence that tracks how many elements were produced
+		seq := func(yield func(int) bool) {
+			for i := range 100 {
+				producerCount.Add(1)
+				if !yield(i) {
+					return
+				}
+			}
+		}
+
+		async := AsyncBuf(seq, 10)
+
+		// Consume only 5 elements
+		count := 0
+		for range async {
+			count++
+			if count >= 5 {
+				break
+			}
+		}
+
+		// Give goroutine time to clean up
+		time.Sleep(10 * time.Millisecond)
+
+		// Producer should have stopped shortly after consumer stopped
+		// It may produce a few extra due to buffering, but not all 100
+		produced := producerCount.Load()
+		assert.LessOrEqual(t, produced, int32(20), "producer should stop after consumer breaks")
+		assert.GreaterOrEqual(t, produced, int32(5), "producer should produce at least what was consumed")
+	})
+
+	t.Run("handles yield returning false", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		async := AsyncBuf(seq, 5)
+
+		collected := []int{}
+		for v := range async {
+			collected = append(collected, v)
+			if v == 3 {
+				break
+			}
+		}
+
+		assert.Equal(t, []int{1, 2, 3}, collected)
+	})
+
+	t.Run("early termination with unbuffered channel", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async := AsyncBuf(seq, 0)
+
+		collected := []int{}
+		for v := range async {
+			collected = append(collected, v)
+			if v == 2 {
+				break
+			}
+		}
+
+		assert.Equal(t, []int{1, 2}, collected)
+	})
+}
+
+// TestAsync_WithComplexTypes tests Async with complex data types
+func TestAsync_WithComplexTypes(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("works with structs", func(t *testing.T) {
+		seq := From(
+			Person{"Alice", 30},
+			Person{"Bob", 25},
+			Person{"Charlie", 35},
+		)
+		async := AsyncBuf(seq, 5)
+		result := toSlice(async)
+		expected := []Person{
+			{"Alice", 30},
+			{"Bob", 25},
+			{"Charlie", 35},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("works with pointers", func(t *testing.T) {
+		p1 := &Person{"Alice", 30}
+		p2 := &Person{"Bob", 25}
+		p3 := &Person{"Charlie", 35}
+		seq := From(p1, p2, p3)
+		async := AsyncBuf(seq, 3)
+		result := toSlice(async)
+		assert.Equal(t, []*Person{p1, p2, p3}, result)
+	})
+
+	t.Run("works with slices", func(t *testing.T) {
+		seq := From([]int{1, 2}, []int{3, 4}, []int{5, 6})
+		async := AsyncBuf(seq, 2)
+		result := toSlice(async)
+		expected := [][]int{{1, 2}, {3, 4}, {5, 6}}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("works with maps", func(t *testing.T) {
+		m1 := map[string]int{"a": 1}
+		m2 := map[string]int{"b": 2}
+		m3 := map[string]int{"c": 3}
+		seq := From(m1, m2, m3)
+		async := AsyncBuf(seq, 3)
+		result := toSlice(async)
+		assert.Equal(t, []map[string]int{m1, m2, m3}, result)
+	})
+}
+
+// TestAsync_WithChainedOperations tests Async with other sequence operations
+func TestAsync_WithChainedOperations(t *testing.T) {
+	t.Run("async after map", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		mapped := MonadMap(seq, N.Mul(2))
+		async := AsyncBuf(mapped, 5)
+		result := toSlice(async)
+		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)
+	})
+
+	t.Run("map after async", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async := AsyncBuf(seq, 5)
+		mapped := MonadMap(async, N.Mul(2))
+		result := toSlice(mapped)
+		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)
+	})
+
+	t.Run("async after filter", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		filtered := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+		async := AsyncBuf(filtered, 5)
+		result := toSlice(async)
+		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)
+	})
+
+	t.Run("filter after async", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		async := AsyncBuf(seq, 5)
+		filtered := MonadFilter(async, func(x int) bool { return x%2 == 0 })
+		result := toSlice(filtered)
+		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)
+	})
+
+	t.Run("async after chain", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		chained := MonadChain(seq, func(x int) Seq[int] {
+			return From(x, x*10)
+		})
+		async := AsyncBuf(chained, 10)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 10, 2, 20, 3, 30}, result)
+	})
+
+	t.Run("multiple async operations", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async1 := AsyncBuf(seq, 3)
+		async2 := AsyncBuf(async1, 2)
+		result := toSlice(async2)
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+}
+
+// TestAsync_Concurrency tests concurrent behavior
+func TestAsync_Concurrency(t *testing.T) {
+	t.Run("allows concurrent production and consumption", func(t *testing.T) {
+		// Create a slow producer
+		seq := func(yield func(int) bool) {
+			for i := range 5 {
+				time.Sleep(5 * time.Millisecond)
+				if !yield(i) {
+					return
+				}
+			}
+		}
+
+		async := AsyncBuf(seq, 10)
+
+		result := toSlice(async)
+
+		// Verify all elements are produced correctly
+		assert.Equal(t, []int{0, 1, 2, 3, 4}, result)
+	})
+
+	t.Run("handles concurrent consumption safely", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		async := AsyncBuf(seq, 5)
+
+		// Consume with some processing time
+		var sum atomic.Int32
+		for v := range async {
+			sum.Add(int32(v))
+			time.Sleep(1 * time.Millisecond)
+		}
+
+		assert.Equal(t, int32(55), sum.Load())
+	})
+}
+
+// TestAsync_EdgeCases tests edge cases
+func TestAsync_EdgeCases(t *testing.T) {
+	t.Run("very large buffer size", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		async := AsyncBuf(seq, 1000000)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+
+	t.Run("buffer size of 1", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		async := AsyncBuf(seq, 1)
+		result := toSlice(async)
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("works with replicate", func(t *testing.T) {
+		seq := Replicate(5, 42)
+		async := AsyncBuf(seq, 3)
+		result := toSlice(async)
+		assert.Equal(t, []int{42, 42, 42, 42, 42}, result)
+	})
+
+	t.Run("works with makeBy", func(t *testing.T) {
+		seq := MakeBy(5, func(i int) int { return i * i })
+		async := AsyncBuf(seq, 3)
+		result := toSlice(async)
+		assert.Equal(t, []int{0, 1, 4, 9, 16}, result)
+	})
+}
+
+// Benchmark tests
+func BenchmarkAsync(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		async := AsyncBuf(seq, 5)
+		for range async {
+		}
+	}
+}
+
+func BenchmarkAsync_LargeSequence(b *testing.B) {
+	data := make([]int, 1000)
+	for i := range data {
+		data[i] = i
+	}
+	seq := From(data...)
+	b.ResetTimer()
+	for range b.N {
+		async := AsyncBuf(seq, 100)
+		for range async {
+		}
+	}
+}
+
+func BenchmarkAsync_SmallBuffer(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		async := AsyncBuf(seq, 1)
+		for range async {
+		}
+	}
+}
+
+func BenchmarkAsync_LargeBuffer(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		async := AsyncBuf(seq, 100)
+		for range async {
+		}
+	}
+}
+
+func BenchmarkAsync_Unbuffered(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		async := AsyncBuf(seq, 0)
+		for range async {
+		}
+	}
+}
+
+func BenchmarkAsync_WithMap(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		async := AsyncBuf(seq, 5)
+		mapped := MonadMap(async, N.Mul(2))
+		for range mapped {
+		}
+	}
+}
+
+func BenchmarkAsync_WithFilter(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		async := AsyncBuf(seq, 5)
+		filtered := MonadFilter(async, func(x int) bool { return x%2 == 0 })
+		for range filtered {
+		}
+	}
+}
+
+// Example tests for documentation
+func ExampleAsync() {
+	seq := From(1, 2, 3, 4, 5)
+	async := AsyncBuf(seq, 10)
+
+	for v := range async {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 1 2 3 4 5
+}
+
+func ExampleAsync_unbuffered() {
+	seq := From(1, 2, 3)
+	async := AsyncBuf(seq, 0)
+
+	for v := range async {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 1 2 3
+}
+
+func ExampleAsync_earlyTermination() {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	async := AsyncBuf(seq, 5)
+
+	count := 0
+	for v := range async {
+		fmt.Printf("%d ", v)
+		count++
+		if count >= 3 {
+			break
+		}
+	}
+	// Output: 1 2 3
+}
+
+func ExampleAsync_withMap() {
+	seq := From(1, 2, 3, 4, 5)
+	async := AsyncBuf(seq, 5)
+	doubled := MonadMap(async, N.Mul(2))
+
+	for v := range doubled {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 2 4 6 8 10
+}
+
+func ExampleAsync_withFilter() {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	async := AsyncBuf(seq, 5)
+	evens := MonadFilter(async, func(x int) bool { return x%2 == 0 })
+
+	for v := range evens {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 2 4 6 8 10
+}
+
+// TestAsync2_Success tests basic Async2 functionality
+func TestAsync2_Success(t *testing.T) {
+	t.Run("converts Seq2 to async with buffer", func(t *testing.T) {
+		seq := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+		async := Async2Buf(seq, 10)
+		result := toMap(async)
+		expected := map[int]string{1: "a", 2: "b", 3: "c"}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("preserves key-value pairs order", func(t *testing.T) {
+		seq := MonadZip(From("x", "y", "z"), From(10, 20, 30))
+		async := Async2Buf(seq, 5)
+
+		keys := []string{}
+		values := []int{}
+		for k, v := range async {
+			keys = append(keys, k)
+			values = append(values, v)
+		}
+
+		assert.Equal(t, []string{"x", "y", "z"}, keys)
+		assert.Equal(t, []int{10, 20, 30}, values)
+	})
+
+	t.Run("works with single pair", func(t *testing.T) {
+		seq := Of2("key", 42)
+		async := Async2Buf(seq, 1)
+		result := toMap(async)
+		assert.Equal(t, map[string]int{"key": 42}, result)
+	})
+
+	t.Run("works with large Seq2", func(t *testing.T) {
+		keys := make([]int, 100)
+		values := make([]string, 100)
+		for i := range keys {
+			keys[i] = i
+			values[i] = fmt.Sprintf("val%d", i)
+		}
+		seq := MonadZip(From(keys...), From(values...))
+		async := Async2Buf(seq, 20)
+		result := toMap(async)
+		assert.Equal(t, 100, len(result))
+		for i := range 100 {
+			assert.Equal(t, fmt.Sprintf("val%d", i), result[i])
+		}
+	})
+}
+
+// TestAsync2_BufferSizes tests different buffer sizes
+func TestAsync2_BufferSizes(t *testing.T) {
+	t.Run("unbuffered channel (bufSize 0)", func(t *testing.T) {
+		seq := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+		async := Async2Buf(seq, 0)
+		result := toMap(async)
+		expected := map[int]string{1: "a", 2: "b", 3: "c"}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("negative buffer size treated as 0", func(t *testing.T) {
+		seq := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+		async := Async2Buf(seq, -5)
+		result := toMap(async)
+		expected := map[int]string{1: "a", 2: "b", 3: "c"}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("large buffer", func(t *testing.T) {
+		seq := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+		async := Async2Buf(seq, 100)
+		result := toMap(async)
+		expected := map[int]string{1: "a", 2: "b", 3: "c"}
+		assert.Equal(t, expected, result)
+	})
+}
+
+// TestAsync2_Empty tests Async2 with empty sequences
+func TestAsync2_Empty(t *testing.T) {
+	t.Run("empty Seq2", func(t *testing.T) {
+		seq := MonadZip(Empty[int](), Empty[string]())
+		async := Async2Buf(seq, 5)
+		result := toMap(async)
+		assert.Empty(t, result)
+	})
+}
+
+// TestAsync2_EarlyTermination tests that Async2 properly handles early termination
+func TestAsync2_EarlyTermination(t *testing.T) {
+	t.Run("stops producer when consumer breaks", func(t *testing.T) {
+		seq := MonadZip(From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), From("a", "b", "c", "d", "e", "f", "g", "h", "i", "j"))
+		async := Async2Buf(seq, 5)
+
+		count := 0
+		for range async {
+			count++
+			if count >= 3 {
+				break
+			}
+		}
+
+		assert.Equal(t, 3, count)
+	})
+}
+
+// TestAsync2_WithChainedOperations tests Async2 with other operations
+func TestAsync2_WithChainedOperations(t *testing.T) {
+	t.Run("async2 after map", func(t *testing.T) {
+		seq := MonadZip(From(1, 2, 3), From(10, 20, 30))
+		mapped := MonadMapWithKey(seq, func(k, v int) int { return k + v })
+		async := Async2Buf(mapped, 5)
+		result := toMap(async)
+		expected := map[int]int{1: 11, 2: 22, 3: 33}
+		assert.Equal(t, expected, result)
+	})
+}
+
+// TestToSeqPair_Success tests basic ToSeqPair functionality
+func TestToSeqPair_Success(t *testing.T) {
+	t.Run("converts Seq2 to Seq of Pairs", func(t *testing.T) {
+		seq2 := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+		pairs := ToSeqPair(seq2)
+		result := toSlice(pairs)
+
+		assert.Equal(t, 3, len(result))
+		assert.Equal(t, 1, pair.Head(result[0]))
+		assert.Equal(t, "a", pair.Tail(result[0]))
+		assert.Equal(t, 2, pair.Head(result[1]))
+		assert.Equal(t, "b", pair.Tail(result[1]))
+		assert.Equal(t, 3, pair.Head(result[2]))
+		assert.Equal(t, "c", pair.Tail(result[2]))
+	})
+
+	t.Run("preserves order", func(t *testing.T) {
+		seq2 := MonadZip(From("x", "y", "z"), From(10, 20, 30))
+		pairs := ToSeqPair(seq2)
+		result := toSlice(pairs)
+
+		assert.Equal(t, 3, len(result))
+		for i, p := range result {
+			expectedKey := string(rune('x' + i))
+			expectedVal := (i + 1) * 10
+			assert.Equal(t, expectedKey, pair.Head(p))
+			assert.Equal(t, expectedVal, pair.Tail(p))
+		}
+	})
+
+	t.Run("works with single pair", func(t *testing.T) {
+		seq2 := Of2("key", 42)
+		pairs := ToSeqPair(seq2)
+		result := toSlice(pairs)
+
+		assert.Equal(t, 1, len(result))
+		assert.Equal(t, "key", pair.Head(result[0]))
+		assert.Equal(t, 42, pair.Tail(result[0]))
+	})
+}
+
+// TestToSeqPair_Empty tests ToSeqPair with empty sequences
+func TestToSeqPair_Empty(t *testing.T) {
+	t.Run("empty Seq2 produces empty Seq", func(t *testing.T) {
+		seq2 := MonadZip(Empty[int](), Empty[string]())
+		pairs := ToSeqPair(seq2)
+		result := toSlice(pairs)
+		assert.Empty(t, result)
+	})
+}
+
+// TestToSeqPair_WithComplexTypes tests ToSeqPair with complex types
+func TestToSeqPair_WithComplexTypes(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("works with struct values", func(t *testing.T) {
+		seq2 := MonadZip(
+			From(1, 2, 3),
+			From(Person{"Alice", 30}, Person{"Bob", 25}, Person{"Charlie", 35}),
+		)
+		pairs := ToSeqPair(seq2)
+		result := toSlice(pairs)
+
+		assert.Equal(t, 3, len(result))
+		assert.Equal(t, 1, pair.Head(result[0]))
+		assert.Equal(t, Person{"Alice", 30}, pair.Tail(result[0]))
+	})
+}
+
+// TestFromSeqPair_Success tests basic FromSeqPair functionality
+func TestFromSeqPair_Success(t *testing.T) {
+	t.Run("converts Seq of Pairs to Seq2", func(t *testing.T) {
+		pairs := From(
+			pair.MakePair(1, "a"),
+			pair.MakePair(2, "b"),
+			pair.MakePair(3, "c"),
+		)
+		seq2 := FromSeqPair(pairs)
+		result := toMap(seq2)
+
+		expected := map[int]string{1: "a", 2: "b", 3: "c"}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("preserves order", func(t *testing.T) {
+		pairs := From(
+			pair.MakePair("x", 10),
+			pair.MakePair("y", 20),
+			pair.MakePair("z", 30),
+		)
+		seq2 := FromSeqPair(pairs)
+
+		keys := []string{}
+		values := []int{}
+		for k, v := range seq2 {
+			keys = append(keys, k)
+			values = append(values, v)
+		}
+
+		assert.Equal(t, []string{"x", "y", "z"}, keys)
+		assert.Equal(t, []int{10, 20, 30}, values)
+	})
+
+	t.Run("works with single pair", func(t *testing.T) {
+		pairs := From(pair.MakePair("key", 42))
+		seq2 := FromSeqPair(pairs)
+		result := toMap(seq2)
+
+		assert.Equal(t, map[string]int{"key": 42}, result)
+	})
+}
+
+// TestFromSeqPair_Empty tests FromSeqPair with empty sequences
+func TestFromSeqPair_Empty(t *testing.T) {
+	t.Run("empty Seq produces empty Seq2", func(t *testing.T) {
+		pairs := Empty[Pair[int, string]]()
+		seq2 := FromSeqPair(pairs)
+		result := toMap(seq2)
+		assert.Empty(t, result)
+	})
+}
+
+// TestFromSeqPair_WithComplexTypes tests FromSeqPair with complex types
+func TestFromSeqPair_WithComplexTypes(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("works with struct values", func(t *testing.T) {
+		pairs := From(
+			pair.MakePair(1, Person{"Alice", 30}),
+			pair.MakePair(2, Person{"Bob", 25}),
+			pair.MakePair(3, Person{"Charlie", 35}),
+		)
+		seq2 := FromSeqPair(pairs)
+		result := toMap(seq2)
+
+		expected := map[int]Person{
+			1: {"Alice", 30},
+			2: {"Bob", 25},
+			3: {"Charlie", 35},
+		}
+		assert.Equal(t, expected, result)
+	})
+}
+
+// TestRoundTrip tests that ToSeqPair and FromSeqPair are inverses
+func TestRoundTrip(t *testing.T) {
+	t.Run("ToSeqPair then FromSeqPair", func(t *testing.T) {
+		original := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+		pairs := ToSeqPair(original)
+		restored := FromSeqPair(pairs)
+		result := toMap(restored)
+
+		expected := map[int]string{1: "a", 2: "b", 3: "c"}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("FromSeqPair then ToSeqPair", func(t *testing.T) {
+		original := From(
+			pair.MakePair(1, "a"),
+			pair.MakePair(2, "b"),
+			pair.MakePair(3, "c"),
+		)
+		seq2 := FromSeqPair(original)
+		restored := ToSeqPair(seq2)
+		result := toSlice(restored)
+
+		assert.Equal(t, 3, len(result))
+		assert.Equal(t, 1, pair.Head(result[0]))
+		assert.Equal(t, "a", pair.Tail(result[0]))
+	})
+}
+
+// Benchmark tests for Async2
+func BenchmarkAsync2(b *testing.B) {
+	seq := MonadZip(From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), From("a", "b", "c", "d", "e", "f", "g", "h", "i", "j"))
+	b.ResetTimer()
+	for range b.N {
+		async := Async2Buf(seq, 5)
+		for range async {
+		}
+	}
+}
+
+func BenchmarkAsync2_LargeSequence(b *testing.B) {
+	keys := make([]int, 1000)
+	values := make([]string, 1000)
+	for i := range keys {
+		keys[i] = i
+		values[i] = fmt.Sprintf("val%d", i)
+	}
+	seq := MonadZip(From(keys...), From(values...))
+	b.ResetTimer()
+	for range b.N {
+		async := Async2Buf(seq, 100)
+		for range async {
+		}
+	}
+}
+
+// Benchmark tests for FromSeqPair
+func BenchmarkFromSeqPair(b *testing.B) {
+	pairs := From(
+		pair.MakePair(1, "a"),
+		pair.MakePair(2, "b"),
+		pair.MakePair(3, "c"),
+		pair.MakePair(4, "d"),
+		pair.MakePair(5, "e"),
+	)
+	b.ResetTimer()
+	for range b.N {
+		seq2 := FromSeqPair(pairs)
+		for range seq2 {
+		}
+	}
+}
+
+func BenchmarkRoundTrip(b *testing.B) {
+	seq := MonadZip(From(1, 2, 3, 4, 5), From("a", "b", "c", "d", "e"))
+	b.ResetTimer()
+	for range b.N {
+		pairs := ToSeqPair(seq)
+		restored := FromSeqPair(pairs)
+		for range restored {
+		}
+	}
+}
+
+// Example tests for Async2
+func ExampleAsync2() {
+	seq := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+	async := Async2Buf(seq, 10)
+
+	for k, v := range async {
+		fmt.Printf("%d: %s\n", k, v)
+	}
+	// Output:
+	// 1: a
+	// 2: b
+	// 3: c
+}
+
+func ExampleAsync2_earlyTermination() {
+	seq := MonadZip(From(1, 2, 3, 4, 5), From("a", "b", "c", "d", "e"))
+	async := Async2Buf(seq, 5)
+
+	count := 0
+	for k, v := range async {
+		fmt.Printf("%d: %s\n", k, v)
+		count++
+		if count >= 2 {
+			break
+		}
+	}
+	// Output:
+	// 1: a
+	// 2: b
+}
+
+// Example tests for FromSeqPair
+func ExampleFromSeqPair() {
+	pairs := From(
+		pair.MakePair(1, "a"),
+		pair.MakePair(2, "b"),
+		pair.MakePair(3, "c"),
+	)
+	seq2 := FromSeqPair(pairs)
+
+	for k, v := range seq2 {
+		fmt.Printf("%d: %s\n", k, v)
+	}
+	// Output:
+	// 1: a
+	// 2: b
+	// 3: c
+}

v2/iterator/iter/compress.go

@@ -34,6 +34,13 @@ import (
 //  3. Filtering to keep only pairs where the boolean (tail) is true
 //  4. Extracting the original values (head) from the filtered pairs
 //
+// Marble Diagram:
+//
+//	Data:      --1--2--3--4--5-->
+//	Selectors: --T--F--T--F--T-->
+//	Compress
+//	Output:    --1-----3-----5-->
+//
 // RxJS Equivalent: Similar to combining [zip] with [filter] - https://rxjs.dev/api/operators/zip
 //
 // Type Parameters:

v2/iterator/iter/cycle.go

@@ -21,6 +21,12 @@ package iter
 // all elements repeatedly. When the end of the input sequence is reached, it starts over
 // from the beginning, continuing this pattern forever.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3|
+//	Cycle
+//	Output: --1--2--3--1--2--3--1--2--3--> (infinite)
+//
 // RxJS Equivalent: [repeat] - https://rxjs.dev/api/operators/repeat
 //
 // WARNING: This creates an INFINITE sequence for non-empty inputs. It must be used with

v2/iterator/iter/first.go

@@ -23,6 +23,16 @@ import "github.com/IBM/fp-go/v2/option"
 // contains at least one element, it returns Some(element). If the iterator is empty,
 // it returns None. The function consumes only the first element of the iterator.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5-->
+//	First
+//	Output: --Some(1)|
+//
+//	Input:  --|
+//	First
+//	Output: --None|
+//
 // RxJS Equivalent: [first] - https://rxjs.dev/api/operators/first
 //
 // Type Parameters:

v2/iterator/iter/iter.go

@@ -82,6 +82,12 @@ func Of2[K, A any](k K, a A) Seq2[K, A] {
 // MonadMap transforms each element in a sequence using the provided function.
 // This is the monadic version that takes the sequence as the first parameter.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3-->
+//	Map(x => x * 2)
+//	Output: --2--4--6-->
+//
 // RxJS Equivalent: [map] - https://rxjs.dev/api/operators/map
 //
 // Example:
@@ -186,6 +192,12 @@ func MapWithKey[K, A, B any](f func(K, A) B) Operator2[K, A, B] {
 
 // MonadFilter returns a sequence containing only elements that satisfy the predicate.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5-->
+//	Filter(x => x % 2 == 0)
+//	Output: -----2-----4----->
+//
 // RxJS Equivalent: [filter] - https://rxjs.dev/api/operators/filter
 //
 // Example:
@@ -293,6 +305,12 @@ func FilterWithKey[K, A any](pred func(K, A) bool) Operator2[K, A, A] {
 // MonadFilterMap applies a function that returns an Option to each element,
 // keeping only the Some values and unwrapping them.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5-->
+//	FilterMap(x => x % 2 == 0 ? Some(x * 10) : None)
+//	Output: -----20----40---->
+//
 // Example:
 //
 //	seq := From(1, 2, 3, 4, 5)
@@ -430,6 +448,12 @@ func FilterMapWithKey[K, A, B any](f func(K, A) Option[B]) Operator2[K, A, B] {
 // MonadChain applies a function that returns a sequence to each element and flattens the results.
 // This is the monadic bind operation (flatMap).
 //
+// Marble Diagram:
+//
+//	Input:  --1-----2-----3---->
+//	Chain(x => [x, x*10])
+//	Output: --1-10--2-20--3-30->
+//
 // RxJS Equivalent: [mergeMap/flatMap] - https://rxjs.dev/api/operators/mergeMap
 //
 // Example:
@@ -471,8 +495,34 @@ func FlatMap[A, B any](f func(A) Seq[B]) Operator[A, B] {
 	return Chain(f)
 }
 
+// ConcatMap is an alias for Chain that emphasizes sequential concatenation.
+// It maps each element to a sequence and concatenates the results in order.
+//
+// Unlike concurrent operations, ConcatMap preserves the order of elements:
+// it fully processes each input element (yielding all elements from f(a))
+// before moving to the next input element.
+//
+// Example:
+//
+//	seq := From(1, 2, 3)
+//	result := ConcatMap(func(x int) Seq[int] {
+//	    return From(x, x*10)
+//	})(seq)
+//	// yields: 1, 10, 2, 20, 3, 30 (order preserved)
+//
+//go:inline
+func ConcatMap[A, B any](f func(A) Seq[B]) Operator[A, B] {
+	return Chain(f)
+}
+
 // Flatten flattens a sequence of sequences into a single sequence.
 //
+// Marble Diagram:
+//
+//	Input:  --[1,2]--[3,4]--[5]-->
+//	Flatten
+//	Output: --1-2----3-4----5---->
+//
 // RxJS Equivalent: [mergeAll] - https://rxjs.dev/api/operators/mergeAll
 //
 // Example:
@@ -486,9 +536,22 @@ func Flatten[A any](mma Seq[Seq[A]]) Seq[A] {
 	return MonadChain(mma, F.Identity[Seq[A]])
 }
 
+//go:inline
+func ConcatAll[A any](mma Seq[Seq[A]]) Seq[A] {
+	return Flatten(mma)
+}
+
 // MonadAp applies a sequence of functions to a sequence of values.
 // This is the applicative apply operation.
 //
+// Marble Diagram:
+//
+//	Functions: --(*2)---(+10)-->
+//	Values:    --5------3------>
+//	Ap
+//	Output:    --10-6---15-13-->
+//	           (each function applied to each value)
+//
 // Example:
 //
 //	fns := From(N.Mul(2), N.Add(10))
@@ -577,6 +640,13 @@ func Replicate[A any](n int, a A) Seq[A] {
 // MonadReduce reduces a sequence to a single value by applying a function to each element
 // and an accumulator, starting with an initial value.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5--|
+//	Reduce((acc, x) => acc + x, 0)
+//	Output: ------------------15|
+//	        (emits final result only)
+//
 // RxJS Equivalent: [reduce] - https://rxjs.dev/api/operators/reduce
 //
 // Example:
@@ -811,6 +881,13 @@ func FoldMapWithKey[K, A, B any](m M.Monoid[B]) func(func(K, A) B) func(Seq2[K,
 // MonadFlap applies a fixed value to a sequence of functions.
 // This is the dual of MonadAp.
 //
+// Marble Diagram:
+//
+//	Functions: --(*2)---(+10)-->
+//	Value:     5 (fixed)
+//	Flap
+//	Output:    --10-----15----->
+//
 // Example:
 //
 //	fns := From(N.Mul(2), N.Add(10))
@@ -832,6 +909,12 @@ func Flap[B, A any](a A) Operator[func(A) B, B] {
 
 // Prepend returns a function that adds an element to the beginning of a sequence.
 //
+// Marble Diagram:
+//
+//	Input:  -----2--3--4-->
+//	Prepend(1)
+//	Output: --1--2--3--4-->
+//
 // RxJS Equivalent: [startWith] - https://rxjs.dev/api/operators/startWith
 //
 // Example:
@@ -847,6 +930,12 @@ func Prepend[A any](head A) Operator[A, A] {
 
 // Append returns a function that adds an element to the end of a sequence.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3-----|
+//	Append(4)
+//	Output: --1--2--3--4--|
+//
 // RxJS Equivalent: [endWith] - https://rxjs.dev/api/operators/endWith
 //
 // Example:
@@ -863,6 +952,14 @@ func Append[A any](tail A) Operator[A, A] {
 // MonadZip combines two sequences into a sequence of pairs.
 // The resulting sequence stops when either input sequence is exhausted.
 //
+// Marble Diagram:
+//
+//	SeqA:   --1--2--3---->
+//	SeqB:   --a--b------->
+//	Zip
+//	Output: --(1,a)-(2,b)|
+//	        (stops when shorter sequence ends)
+//
 // RxJS Equivalent: [zip] - https://rxjs.dev/api/operators/zip
 //
 // Example:
@@ -1002,3 +1099,138 @@ func ToSeqPair[A, B any](as Seq2[A, B]) Seq[Pair[A, B]] {
 		}
 	}
 }
+
+// FromSeqPair converts a sequence of Pairs into a key-value sequence.
+//
+// This function transforms a Seq[Pair[A, B]] (which yields Pair objects when iterated)
+// into a Seq2[A, B] (which yields key-value pairs as separate arguments). This is the
+// inverse operation of ToSeqPair and is useful when you need to convert from working
+// with pairs as first-class values back to the key-value iteration pattern.
+//
+// # Type Parameters
+//
+//   - A: The type of the first element (key) in each pair
+//   - B: The type of the second element (value) in each pair
+//
+// # Parameters
+//
+//   - as: A Seq that yields Pair objects
+//
+// # Returns
+//
+//   - Seq2[A, B]: A key-value sequence that yields the unpacked pairs
+//
+// # Example Usage
+//
+//	// Create a sequence of pairs
+//	pairs := From(
+//	    pair.MakePair("a", 1),
+//	    pair.MakePair("b", 2),
+//	    pair.MakePair("c", 3),
+//	)
+//	seq2 := FromSeqPair(pairs)
+//
+//	// Iterate as key-value pairs
+//	for k, v := range seq2 {
+//	    fmt.Printf("%s: %d\n", k, v)
+//	}
+//	// Output:
+//	// a: 1
+//	// b: 2
+//	// c: 3
+//
+// # Example with Map
+//
+//	pairs := From(
+//	    pair.MakePair(1, 10),
+//	    pair.MakePair(2, 20),
+//	    pair.MakePair(3, 30),
+//	)
+//	seq2 := FromSeqPair(pairs)
+//
+//	// Use with Seq2 operations
+//	mapped := MonadMapWithKey(seq2, func(k, v int) int {
+//	    return k + v
+//	})
+//	// yields: 11, 22, 33
+//
+// # Example - Round-trip conversion
+//
+//	original := MonadZip(From(1, 2, 3), From("a", "b", "c"))
+//	pairs := ToSeqPair(original)
+//	restored := FromSeqPair(pairs)
+//	// restored is equivalent to original
+//
+// # See Also
+//
+//   - ToSeqPair: Converts Seq2 to Seq of Pairs (inverse operation)
+//   - MonadZip: Creates key-value sequences from two sequences
+//   - pair.MakePair: Creates a Pair from two values
+//   - pair.Unpack: Unpacks a Pair into two values
+func FromSeqPair[A, B any](as Seq[Pair[A, B]]) Seq2[A, B] {
+	return func(yield func(A, B) bool) {
+		for p := range as {
+			if !yield(pair.Unpack(p)) {
+				return
+			}
+		}
+	}
+}
+
+// Skip returns an operator that skips the first n elements of a sequence.
+//
+// This function creates a transformation that discards the first n elements from
+// the source sequence and yields all remaining elements. If n is less than or equal
+// to 0, all elements are yielded. If n is greater than or equal to the sequence length,
+// an empty sequence is returned.
+//
+// The operation is lazy and only consumes elements from the source sequence as needed.
+// The first n elements are consumed and discarded, then subsequent elements are yielded.
+//
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5--6--7--8-->
+//	Skip(3)
+//	Output: -----------4--5--6--7--8-->
+//
+// RxJS Equivalent: [skip] - https://rxjs.dev/api/operators/skip
+//
+// Type Parameters:
+//   - U: The type of elements in the sequence
+//
+// Parameters:
+//   - count: The number of elements to skip from the beginning of the sequence
+//
+// Returns:
+//   - An Operator that transforms a Seq[U] by skipping the first count elements
+//
+// Example - Skip first 3 elements:
+//
+//	seq := From(1, 2, 3, 4, 5)
+//	result := Skip[int](3)(seq)
+//	// yields: 4, 5
+//
+// Example - Skip more than available:
+//
+//	seq := From(1, 2)
+//	result := Skip[int](5)(seq)
+//	// yields: nothing (empty sequence)
+//
+// Example - Skip zero or negative:
+//
+//	seq := From(1, 2, 3)
+//	result := Skip[int](0)(seq)
+//	// yields: 1, 2, 3 (all elements)
+//
+// Example - Chaining with other operations:
+//
+//	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+//	result := F.Pipe2(
+//	    seq,
+//	    Skip[int](3),
+//	    MonadFilter(seq, func(x int) bool { return x%2 == 0 }),
+//	)
+//	// yields: 4, 6, 8, 10 (skip first 3, then filter evens)
+func Skip[U any](count int) Operator[U, U] {
+	return FilterWithIndex(func(idx int, _ U) bool { return idx >= count })
+}

v2/iterator/iter/iter_test.go

@@ -612,3 +612,440 @@ func TestMapToArrayIdentity(t *testing.T) {
 	result := mapper(seq)
 	assert.Equal(t, []string{"a", "b", "c"}, result)
 }
+
+// TestSkip tests basic Skip functionality
+func TestSkip(t *testing.T) {
+	t.Run("skips first n elements from sequence", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(Skip[int](3)(seq))
+		assert.Equal(t, []int{4, 5}, result)
+	})
+
+	t.Run("skips first element", func(t *testing.T) {
+		seq := From(10, 20, 30)
+		result := toSlice(Skip[int](1)(seq))
+		assert.Equal(t, []int{20, 30}, result)
+	})
+
+	t.Run("skips all elements when n equals length", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		result := toSlice(Skip[int](3)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skips all elements when n exceeds length", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		result := toSlice(Skip[int](10)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skips from string sequence", func(t *testing.T) {
+		seq := From("a", "b", "c", "d", "e")
+		result := toSlice(Skip[string](2)(seq))
+		assert.Equal(t, []string{"c", "d", "e"}, result)
+	})
+
+	t.Run("skips from single element sequence", func(t *testing.T) {
+		seq := From(42)
+		result := toSlice(Skip[int](1)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skips from large sequence", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		result := toSlice(Skip[int](7)(seq))
+		assert.Equal(t, []int{8, 9, 10}, result)
+	})
+}
+
+// TestSkipZeroOrNegative tests Skip with zero or negative values
+func TestSkipZeroOrNegative(t *testing.T) {
+	t.Run("returns all elements when n is zero", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(Skip[int](0)(seq))
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("returns all elements when n is negative", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(Skip[int](-1)(seq))
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("returns all elements when n is large negative", func(t *testing.T) {
+		seq := From("a", "b", "c")
+		result := toSlice(Skip[string](-100)(seq))
+		assert.Equal(t, []string{"a", "b", "c"}, result)
+	})
+}
+
+// TestSkipEmpty tests Skip with empty sequences
+func TestSkipEmpty(t *testing.T) {
+	t.Run("returns empty from empty integer sequence", func(t *testing.T) {
+		seq := Empty[int]()
+		result := toSlice(Skip[int](5)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("returns empty from empty string sequence", func(t *testing.T) {
+		seq := Empty[string]()
+		result := toSlice(Skip[string](3)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("returns empty when skipping zero from empty", func(t *testing.T) {
+		seq := Empty[int]()
+		result := toSlice(Skip[int](0)(seq))
+		assert.Empty(t, result)
+	})
+}
+
+// TestSkipWithComplexTypes tests Skip with complex data types
+func TestSkipWithComplexTypes(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("skips structs", func(t *testing.T) {
+		seq := From(
+			Person{"Alice", 30},
+			Person{"Bob", 25},
+			Person{"Charlie", 35},
+			Person{"David", 28},
+		)
+		result := toSlice(Skip[Person](2)(seq))
+		expected := []Person{
+			{"Charlie", 35},
+			{"David", 28},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("skips pointers", func(t *testing.T) {
+		p1 := &Person{"Alice", 30}
+		p2 := &Person{"Bob", 25}
+		p3 := &Person{"Charlie", 35}
+		seq := From(p1, p2, p3)
+		result := toSlice(Skip[*Person](1)(seq))
+		assert.Equal(t, []*Person{p2, p3}, result)
+	})
+
+	t.Run("skips slices", func(t *testing.T) {
+		seq := From([]int{1, 2}, []int{3, 4}, []int{5, 6}, []int{7, 8})
+		result := toSlice(Skip[[]int](2)(seq))
+		expected := [][]int{{5, 6}, {7, 8}}
+		assert.Equal(t, expected, result)
+	})
+}
+
+// TestSkipWithChainedOperations tests Skip with other sequence operations
+func TestSkipWithChainedOperations(t *testing.T) {
+	t.Run("skip after map", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		mapped := MonadMap(seq, N.Mul(2))
+		result := toSlice(Skip[int](2)(mapped))
+		assert.Equal(t, []int{6, 8, 10}, result)
+	})
+
+	t.Run("skip after filter", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		filtered := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+		result := toSlice(Skip[int](2)(filtered))
+		assert.Equal(t, []int{6, 8, 10}, result)
+	})
+
+	t.Run("map after skip", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		skipped := Skip[int](2)(seq)
+		result := toSlice(MonadMap(skipped, N.Mul(10)))
+		assert.Equal(t, []int{30, 40, 50}, result)
+	})
+
+	t.Run("filter after skip", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8)
+		skipped := Skip[int](2)(seq)
+		result := toSlice(MonadFilter(skipped, func(x int) bool { return x%2 == 0 }))
+		assert.Equal(t, []int{4, 6, 8}, result)
+	})
+
+	t.Run("skip after chain", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		chained := MonadChain(seq, func(x int) Seq[int] {
+			return From(x, x*10)
+		})
+		result := toSlice(Skip[int](3)(chained))
+		assert.Equal(t, []int{20, 3, 30}, result)
+	})
+
+	t.Run("multiple skips", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		skipped1 := Skip[int](2)(seq)
+		skipped2 := Skip[int](3)(skipped1)
+		result := toSlice(skipped2)
+		assert.Equal(t, []int{6, 7, 8, 9, 10}, result)
+	})
+
+	t.Run("skip and take", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		skipped := Skip[int](3)(seq)
+		taken := Take[int](3)(skipped)
+		result := toSlice(taken)
+		assert.Equal(t, []int{4, 5, 6}, result)
+	})
+
+	t.Run("take and skip", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		taken := Take[int](7)(seq)
+		skipped := Skip[int](2)(taken)
+		result := toSlice(skipped)
+		assert.Equal(t, []int{3, 4, 5, 6, 7}, result)
+	})
+}
+
+// TestSkipWithReplicate tests Skip with Replicate
+func TestSkipWithReplicate(t *testing.T) {
+	t.Run("skips from replicated sequence", func(t *testing.T) {
+		seq := Replicate(10, 42)
+		result := toSlice(Skip[int](7)(seq))
+		assert.Equal(t, []int{42, 42, 42}, result)
+	})
+
+	t.Run("skips all from short replicate", func(t *testing.T) {
+		seq := Replicate(2, "hello")
+		result := toSlice(Skip[string](5)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skips zero from replicate", func(t *testing.T) {
+		seq := Replicate(3, 100)
+		result := toSlice(Skip[int](0)(seq))
+		assert.Equal(t, []int{100, 100, 100}, result)
+	})
+}
+
+// TestSkipWithMakeBy tests Skip with MakeBy
+func TestSkipWithMakeBy(t *testing.T) {
+	t.Run("skips from generated sequence", func(t *testing.T) {
+		seq := MakeBy(10, func(i int) int { return i * i })
+		result := toSlice(Skip[int](5)(seq))
+		assert.Equal(t, []int{25, 36, 49, 64, 81}, result)
+	})
+
+	t.Run("skips more than generated", func(t *testing.T) {
+		seq := MakeBy(3, func(i int) int { return i + 1 })
+		result := toSlice(Skip[int](10)(seq))
+		assert.Empty(t, result)
+	})
+}
+
+// TestSkipWithPrependAppend tests Skip with Prepend and Append
+func TestSkipWithPrependAppend(t *testing.T) {
+	t.Run("skip from prepended sequence", func(t *testing.T) {
+		seq := From(2, 3, 4, 5)
+		prepended := Prepend(1)(seq)
+		result := toSlice(Skip[int](2)(prepended))
+		assert.Equal(t, []int{3, 4, 5}, result)
+	})
+
+	t.Run("skip from appended sequence", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		appended := Append(4)(seq)
+		result := toSlice(Skip[int](2)(appended))
+		assert.Equal(t, []int{3, 4}, result)
+	})
+
+	t.Run("skip includes appended element", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		appended := Append(4)(seq)
+		result := toSlice(Skip[int](3)(appended))
+		assert.Equal(t, []int{4}, result)
+	})
+}
+
+// TestSkipWithFlatten tests Skip with Flatten
+func TestSkipWithFlatten(t *testing.T) {
+	t.Run("skips from flattened sequence", func(t *testing.T) {
+		nested := From(From(1, 2), From(3, 4), From(5, 6))
+		flattened := Flatten(nested)
+		result := toSlice(Skip[int](3)(flattened))
+		assert.Equal(t, []int{4, 5, 6}, result)
+	})
+
+	t.Run("skips from flattened with empty inner sequences", func(t *testing.T) {
+		nested := From(From(1, 2), Empty[int](), From(3, 4))
+		flattened := Flatten(nested)
+		result := toSlice(Skip[int](2)(flattened))
+		assert.Equal(t, []int{3, 4}, result)
+	})
+}
+
+// TestSkipDoesNotConsumeSkippedElements tests that Skip is efficient
+func TestSkipDoesNotConsumeSkippedElements(t *testing.T) {
+	t.Run("processes all elements including skipped", func(t *testing.T) {
+		callCount := 0
+		seq := MonadMap(From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), func(x int) int {
+			callCount++
+			return x * 2
+		})
+
+		skipped := Skip[int](7)(seq)
+
+		result := []int{}
+		for v := range skipped {
+			result = append(result, v)
+		}
+
+		assert.Equal(t, []int{16, 18, 20}, result)
+		// Skip still needs to iterate through skipped elements to count them
+		assert.Equal(t, 10, callCount, "should process all elements")
+	})
+}
+
+// TestSkipEdgeCases tests edge cases
+func TestSkipEdgeCases(t *testing.T) {
+	t.Run("skip 0 from single element", func(t *testing.T) {
+		seq := From(42)
+		result := toSlice(Skip[int](0)(seq))
+		assert.Equal(t, []int{42}, result)
+	})
+
+	t.Run("skip 1 from single element", func(t *testing.T) {
+		seq := From(42)
+		result := toSlice(Skip[int](1)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skip large number from small sequence", func(t *testing.T) {
+		seq := From(1, 2)
+		result := toSlice(Skip[int](1000000)(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skip with very large n", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		result := toSlice(Skip[int](int(^uint(0) >> 1))(seq)) // max int
+		assert.Empty(t, result)
+	})
+
+	t.Run("skip all but one", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(Skip[int](4)(seq))
+		assert.Equal(t, []int{5}, result)
+	})
+}
+
+// Benchmark tests for Skip
+func BenchmarkSkip(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		skipped := Skip[int](5)(seq)
+		for range skipped {
+		}
+	}
+}
+
+func BenchmarkSkipLargeSequence(b *testing.B) {
+	data := make([]int, 1000)
+	for i := range data {
+		data[i] = i
+	}
+	seq := From(data...)
+	b.ResetTimer()
+	for range b.N {
+		skipped := Skip[int](900)(seq)
+		for range skipped {
+		}
+	}
+}
+
+func BenchmarkSkipWithMap(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		mapped := MonadMap(seq, N.Mul(2))
+		skipped := Skip[int](5)(mapped)
+		for range skipped {
+		}
+	}
+}
+
+func BenchmarkSkipWithFilter(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		filtered := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+		skipped := Skip[int](2)(filtered)
+		for range skipped {
+		}
+	}
+}
+
+// Example tests for documentation
+func ExampleSkip() {
+	seq := From(1, 2, 3, 4, 5)
+	skipped := Skip[int](3)(seq)
+
+	for v := range skipped {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 4 5
+}
+
+func ExampleSkip_moreThanAvailable() {
+	seq := From(1, 2, 3)
+	skipped := Skip[int](10)(seq)
+
+	count := 0
+	for range skipped {
+		count++
+	}
+	fmt.Printf("Count: %d\n", count)
+	// Output: Count: 0
+}
+
+func ExampleSkip_zero() {
+	seq := From(1, 2, 3, 4, 5)
+	skipped := Skip[int](0)(seq)
+
+	for v := range skipped {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 1 2 3 4 5
+}
+
+func ExampleSkip_withFilter() {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	evens := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+	skipped := Skip[int](2)(evens)
+
+	for v := range skipped {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 6 8 10
+}
+
+func ExampleSkip_withMap() {
+	seq := From(1, 2, 3, 4, 5)
+	doubled := MonadMap(seq, N.Mul(2))
+	skipped := Skip[int](2)(doubled)
+
+	for v := range skipped {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 6 8 10
+}
+
+func ExampleSkip_chained() {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	result := F.Pipe3(
+		seq,
+		Skip[int](3),
+		Filter(func(x int) bool { return x%2 == 0 }),
+		toSlice[int],
+	)
+
+	fmt.Println(result)
+	// Output: [4 6 8 10]
+}

v2/iterator/iter/last.go

@@ -10,6 +10,16 @@ import (
 // sequence. If the iterator contains at least one element, it returns Some(element).
 // If the iterator is empty, it returns None.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5--|
+//	Last
+//	Output: -----------------Some(5)|
+//
+//	Input:  --|
+//	Last
+//	Output: --None|
+//
 // RxJS Equivalent: [last] - https://rxjs.dev/api/operators/last
 //
 // Type Parameters:

v2/iterator/iter/mergeall.go

@@ -0,0 +1,563 @@
+// 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 (
+	"slices"
+	"sync"
+
+	A "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
+	M "github.com/IBM/fp-go/v2/monoid"
+	N "github.com/IBM/fp-go/v2/number"
+)
+
+const (
+	defaultBufferSize = 8
+)
+
+// MergeBuf merges multiple sequences concurrently into a single sequence.
+// It spawns a goroutine for each input sequence and merges their elements through
+// a buffered channel, allowing concurrent production from all sources. The output
+// order is non-deterministic and depends on the timing of concurrent producers.
+//
+// This function is useful for combining results from multiple concurrent operations,
+// processing data from multiple sources in parallel, or implementing fan-in patterns
+// where multiple producers feed into a single consumer.
+//
+// Type Parameters:
+//   - T: The type of elements in the sequences
+//
+// Parameters:
+//   - iterables: A slice of sequences to merge. If empty, returns an empty sequence.
+//   - bufSize: The buffer size for the internal channel. Negative values are treated as 0 (unbuffered).
+//     A larger buffer allows more elements to be produced ahead of consumption,
+//     reducing contention between producers but using more memory.
+//     A buffer of 0 creates an unbuffered channel requiring synchronization.
+//
+// Returns:
+//   - Seq[T]: A new sequence that yields elements from all input sequences in non-deterministic order
+//
+// Behavior:
+//   - Spawns one goroutine per input sequence to produce elements concurrently
+//   - Elements from different sequences are interleaved non-deterministically
+//   - Properly handles early termination: if the consumer stops iterating (yield returns false),
+//     all producer goroutines are signaled to stop and cleaned up
+//   - The output channel is closed when all input sequences are exhausted
+//   - No goroutines leak even with early termination
+//   - Thread-safe: multiple producers can safely send to the shared channel
+//
+// Example Usage:
+//
+//	// MergeBuf three sequences concurrently
+//	seq1 := From(1, 2, 3)
+//	seq2 := From(4, 5, 6)
+//	seq3 := From(7, 8, 9)
+//	merged := MergeBuf([]Seq[int]{seq1, seq2, seq3}, 10)
+//
+//	// Elements appear in non-deterministic order
+//	for v := range merged {
+//	    fmt.Println(v) // May print: 1, 4, 7, 2, 5, 8, 3, 6, 9 (order varies)
+//	}
+//
+// Example with Early Termination:
+//
+//	seq1 := From(1, 2, 3, 4, 5)
+//	seq2 := From(6, 7, 8, 9, 10)
+//	merged := MergeBuf([]Seq[int]{seq1, seq2}, 5)
+//
+//	// Stop after 3 elements - all producer goroutines will be properly cleaned up
+//	count := 0
+//	for v := range merged {
+//	    fmt.Println(v)
+//	    count++
+//	    if count >= 3 {
+//	        break
+//	    }
+//	}
+//
+// Example with Unbuffered Channel:
+//
+//	// bufSize of 0 creates an unbuffered channel
+//	seq1 := From(1, 2, 3)
+//	seq2 := From(4, 5, 6)
+//	merged := MergeBuf([]Seq[int]{seq1, seq2}, 0)
+//
+//	// Producers and consumer are synchronized
+//	for v := range merged {
+//	    fmt.Println(v)
+//	}
+//
+// See Also:
+//   - Async: Converts a single sequence to asynchronous
+//   - From: Creates a sequence from values
+//   - MonadChain: Sequentially chains sequences (deterministic order)
+func MergeBuf[T any](iterables []Seq[T], bufSize int) Seq[T] {
+	return F.Pipe2(
+		iterables,
+		slices.Values,
+		MergeAll[T](bufSize),
+	)
+}
+
+// Merge merges multiple sequences concurrently into a single sequence using a default buffer size.
+// This is a convenience wrapper around MergeBuf that uses a default buffer size of 8.
+//
+// Type Parameters:
+//   - T: The type of elements in the sequences
+//
+// Parameters:
+//   - iterables: A slice of sequences to merge. If empty, returns an empty sequence.
+//
+// Returns:
+//   - Seq[T]: A new sequence that yields elements from all input sequences in non-deterministic order
+//
+// Behavior:
+//   - Uses a default buffer size of 8 for the internal channel
+//   - Spawns one goroutine per input sequence to produce elements concurrently
+//   - Elements from different sequences are interleaved non-deterministically
+//   - Properly handles early termination with goroutine cleanup
+//   - Thread-safe: multiple producers can safely send to the shared channel
+//
+// Example:
+//
+//	seq1 := From(1, 2, 3)
+//	seq2 := From(4, 5, 6)
+//	seq3 := From(7, 8, 9)
+//	merged := Merge([]Seq[int]{seq1, seq2, seq3})
+//
+//	// Elements appear in non-deterministic order
+//	for v := range merged {
+//	    fmt.Println(v) // May print: 1, 4, 7, 2, 5, 8, 3, 6, 9 (order varies)
+//	}
+//
+// See Also:
+//   - MergeBuf: Merge with custom buffer size
+//   - MergeAll: Merges a sequence of sequences
+//   - Async: Converts a single sequence to asynchronous
+func Merge[T any](iterables []Seq[T]) Seq[T] {
+	return MergeBuf(iterables, defaultBufferSize)
+}
+
+// MergeMonoid creates a Monoid for merging sequences concurrently.
+// The monoid combines two sequences by merging them concurrently with the specified
+// buffer size, and uses an empty sequence as the identity element.
+//
+// A Monoid is an algebraic structure with an associative binary operation (concat)
+// and an identity element (empty). For sequences, the concat operation merges two
+// sequences concurrently, and the identity is an empty sequence.
+//
+// This is useful for functional composition patterns where you need to combine
+// multiple sequences using monoid operations like Reduce, FoldMap, or when working
+// with monadic operations that require a monoid instance.
+//
+// Marble Diagram (Concurrent Merging):
+//
+//	Seq1:   --1--2--3--|
+//	Seq2:   --4--5--6--|
+//	Merge:  --1-4-2-5-3-6--|
+//	        (non-deterministic order)
+//
+// Marble Diagram (vs ConcatMonoid):
+//
+//	MergeMonoid (concurrent):
+//	  Seq1:   --1--2--3--|
+//	  Seq2:   --4--5--6--|
+//	  Result: --1-4-2-5-3-6--|
+//	          (elements interleaved)
+//
+//	ConcatMonoid (sequential):
+//	  Seq1:   --1--2--3--|
+//	  Seq2:              --4--5--6--|
+//	  Result: --1--2--3--4--5--6--|
+//	          (deterministic order)
+//
+// Type Parameters:
+//   - T: The type of elements in the sequences
+//
+// Parameters:
+//   - bufSize: The buffer size for the internal channel used during merging.
+//     This buffer size will be used for all merge operations performed by the monoid.
+//     Negative values are treated as 0 (unbuffered).
+//
+// Returns:
+//   - Monoid[Seq[T]]: A monoid instance with:
+//   - Concat: Merges two sequences concurrently using Merge
+//   - Empty: Returns an empty sequence
+//
+// Properties:
+//   - Identity: concat(empty, x) = concat(x, empty) = x
+//   - Associativity: concat(concat(a, b), c) = concat(a, concat(b, c))
+//     Note: Due to concurrent execution, element order may vary between equivalent expressions
+//
+// Example Usage:
+//
+//	// Create a monoid for merging integer sequences
+//	monoid := MergeMonoid[int](10)
+//
+//	// Use with Reduce to merge multiple sequences
+//	sequences := []Seq[int]{
+//	    From(1, 2, 3),
+//	    From(4, 5, 6),
+//	    From(7, 8, 9),
+//	}
+//	merged := MonadReduce(From(sequences...), monoid.Concat, monoid.Empty)
+//	// merged contains all elements from all sequences (order non-deterministic)
+//
+// Example with Empty Identity:
+//
+//	monoid := MergeMonoid[int](5)
+//	seq := From(1, 2, 3)
+//
+//	// Merging with empty is identity
+//	result1 := monoid.Concat(monoid.Empty, seq)  // same as seq
+//	result2 := monoid.Concat(seq, monoid.Empty)  // same as seq
+//
+// Example with FoldMap:
+//
+//	// Convert each number to a sequence and merge all results
+//	monoid := MergeMonoid[int](10)
+//	numbers := From(1, 2, 3)
+//	result := MonadFoldMap(numbers, func(n int) Seq[int] {
+//	    return From(n, n*10, n*100)
+//	}, monoid)
+//	// result contains: 1, 10, 100, 2, 20, 200, 3, 30, 300 (order varies)
+//
+// See Also:
+//   - Merge: The underlying merge function
+//   - MergeAll: Merges multiple sequences at once
+//   - Empty: Creates an empty sequence
+func MergeMonoid[T any](bufSize int) M.Monoid[Seq[T]] {
+	return M.MakeMonoid(
+		func(l, r Seq[T]) Seq[T] {
+			return MergeBuf(A.From(l, r), bufSize)
+		},
+		Empty[T](),
+	)
+}
+
+// MergeAll creates an operator that flattens and merges a sequence of sequences concurrently.
+// It takes a sequence of sequences (Seq[Seq[T]]) and produces a single flat sequence (Seq[T])
+// by spawning a goroutine for each inner sequence as it arrives, merging all their elements
+// through a buffered channel. This enables dynamic concurrent processing where inner sequences
+// can be produced and consumed concurrently.
+//
+// Unlike Merge which takes a pre-defined slice of sequences, MergeAll processes sequences
+// dynamically as they are produced by the outer sequence. This makes it ideal for scenarios
+// where the number of sequences isn't known upfront or where sequences are generated on-the-fly.
+//
+// Type Parameters:
+//   - T: The type of elements in the inner sequences
+//
+// Parameters:
+//   - bufSize: The buffer size for the internal channel. Negative values are treated as 0 (unbuffered).
+//     A larger buffer allows more elements to be produced ahead of consumption,
+//     reducing contention between producers but using more memory.
+//
+// Returns:
+//   - Operator[Seq[T], T]: A function that takes a sequence of sequences and returns a flat sequence
+//
+// Behavior:
+//   - Spawns one goroutine for the outer sequence to iterate and spawn inner producers
+//   - Spawns one goroutine per inner sequence as it arrives from the outer sequence
+//   - Elements from different inner sequences are interleaved non-deterministically
+//   - Properly handles early termination: if the consumer stops iterating, all goroutines are cleaned up
+//   - The output channel is closed when both the outer sequence and all inner sequences are exhausted
+//   - No goroutines leak even with early termination
+//   - Thread-safe: multiple producers can safely send to the shared channel
+//
+// Example Usage:
+//
+//	// Create a sequence of sequences dynamically
+//	outer := From(
+//	    From(1, 2, 3),
+//	    From(4, 5, 6),
+//	    From(7, 8, 9),
+//	)
+//	mergeAll := MergeAll[int](10)
+//	merged := mergeAll(outer)
+//
+//	// Elements appear in non-deterministic order
+//	for v := range merged {
+//	    fmt.Println(v) // May print: 1, 4, 7, 2, 5, 8, 3, 6, 9 (order varies)
+//	}
+//
+// Example with Dynamic Generation:
+//
+//	// Generate sequences on-the-fly
+//	outer := Map(func(n int) Seq[int] {
+//	    return From(n, n*10, n*100)
+//	})(From(1, 2, 3))
+//	mergeAll := MergeAll[int](10)
+//	merged := mergeAll(outer)
+//
+//	// Yields: 1, 10, 100, 2, 20, 200, 3, 30, 300 (order varies)
+//	for v := range merged {
+//	    fmt.Println(v)
+//	}
+//
+// Example with Early Termination:
+//
+//	outer := From(
+//	    From(1, 2, 3, 4, 5),
+//	    From(6, 7, 8, 9, 10),
+//	    From(11, 12, 13, 14, 15),
+//	)
+//	mergeAll := MergeAll[int](5)
+//	merged := mergeAll(outer)
+//
+//	// Stop after 5 elements - all goroutines will be properly cleaned up
+//	count := 0
+//	for v := range merged {
+//	    fmt.Println(v)
+//	    count++
+//	    if count >= 5 {
+//	        break
+//	    }
+//	}
+//
+// Example with Chain:
+//
+//	// Use with Chain to flatten nested sequences
+//	numbers := From(1, 2, 3)
+//	result := Chain(func(n int) Seq[int] {
+//	    return From(n, n*10)
+//	})(numbers)
+//	// This is equivalent to: MergeAll[int](0)(Map(...)(numbers))
+//
+// See Also:
+//   - Merge: Merges a pre-defined slice of sequences
+//   - Chain: Sequentially flattens sequences (deterministic order)
+//   - Flatten: Flattens nested sequences sequentially
+//   - Async: Converts a single sequence to asynchronous
+func MergeAll[T any](bufSize int) Operator[Seq[T], T] {
+	buf := N.Max(bufSize, 0)
+
+	return func(s Seq[Seq[T]]) Seq[T] {
+
+		return func(yield func(T) bool) {
+
+			ch := make(chan T, buf)
+			done := make(chan Void)
+			var wg sync.WaitGroup
+
+			// Outer producer: iterates the outer Seq and spawns an inner
+			// goroutine for each inner Seq it emits.
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				s(func(inner Seq[T]) bool {
+					select {
+					case <-done:
+						return false
+					default:
+					}
+
+					wg.Add(1)
+					go func(seq Seq[T]) {
+						defer wg.Done()
+						seq(func(v T) bool {
+							select {
+							case ch <- v:
+								return true
+							case <-done:
+								return false
+							}
+						})
+					}(inner)
+
+					return true
+				})
+			}()
+
+			// Close ch once the outer producer and all inner producers finish.
+			go func() {
+				wg.Wait()
+				close(ch)
+			}()
+
+			// On exit, signal cancellation and drain so no producer blocks
+			// forever on `ch <- v`.
+			defer func() {
+				close(done)
+				for range ch {
+				}
+			}()
+
+			for v := range ch {
+				if !yield(v) {
+					return
+				}
+			}
+		}
+	}
+}
+
+// MergeMapBuf applies a function that returns a sequence to each element and merges the results concurrently.
+// This is the concurrent version of Chain (flatMap), where each mapped sequence is processed in parallel
+// rather than sequentially. It combines Map and MergeAll into a single operation.
+//
+// Unlike Chain which processes sequences sequentially (deterministic order), MergeMapBuf spawns a goroutine
+// for each mapped sequence and merges their elements concurrently through a buffered channel. This makes
+// it ideal for I/O-bound operations, parallel data processing, or when the order of results doesn't matter.
+//
+// Type Parameters:
+//   - A: The type of elements in the input sequence
+//   - B: The type of elements in the output sequences
+//
+// Parameters:
+//   - f: A function that transforms each input element into a sequence of output elements
+//   - bufSize: The buffer size for the internal channel. Negative values are treated as 0 (unbuffered).
+//     A larger buffer allows more elements to be produced ahead of consumption,
+//     reducing contention between producers but using more memory.
+//
+// Returns:
+//   - Operator[A, B]: A function that takes a sequence of A and returns a flat sequence of B
+//
+// Behavior:
+//   - Applies f to each element in the input sequence to produce inner sequences
+//   - Spawns one goroutine per inner sequence to produce elements concurrently
+//   - Elements from different inner sequences are interleaved non-deterministically
+//   - Properly handles early termination: if the consumer stops iterating, all goroutines are cleaned up
+//   - No goroutines leak even with early termination
+//   - Thread-safe: multiple producers can safely send to the shared channel
+//
+// Comparison with Chain:
+//   - Chain: Sequential processing, deterministic order, no concurrency overhead
+//   - MergeMapBuf: Concurrent processing, non-deterministic order, better for I/O-bound tasks
+//
+// Example Usage:
+//
+//	// Expand each number into a sequence concurrently
+//	expand := MergeMapBuf(func(n int) Seq[int] {
+//	    return From(n, n*10, n*100)
+//	}, 10)
+//	seq := From(1, 2, 3)
+//	result := expand(seq)
+//
+//	// Yields: 1, 10, 100, 2, 20, 200, 3, 30, 300 (order varies)
+//	for v := range result {
+//	    fmt.Println(v)
+//	}
+//
+// Example with I/O Operations:
+//
+//	// Fetch data concurrently for each ID
+//	fetchData := MergeMapBuf(func(id int) Seq[string] {
+//	    // Simulate I/O operation
+//	    data := fetchFromAPI(id)
+//	    return From(data...)
+//	}, 20)
+//	ids := From(1, 2, 3, 4, 5)
+//	results := fetchData(ids)
+//
+//	// All fetches happen concurrently
+//	for data := range results {
+//	    fmt.Println(data)
+//	}
+//
+// Example with Early Termination:
+//
+//	expand := MergeMapBuf(func(n int) Seq[int] {
+//	    return From(n, n*10, n*100)
+//	}, 5)
+//	seq := From(1, 2, 3, 4, 5)
+//	result := expand(seq)
+//
+//	// Stop after 5 elements - all goroutines will be properly cleaned up
+//	count := 0
+//	for v := range result {
+//	    fmt.Println(v)
+//	    count++
+//	    if count >= 5 {
+//	        break
+//	    }
+//	}
+//
+// Example with Unbuffered Channel:
+//
+//	// bufSize of 0 creates an unbuffered channel
+//	expand := MergeMapBuf(func(n int) Seq[int] {
+//	    return From(n, n*2)
+//	}, 0)
+//	seq := From(1, 2, 3)
+//	result := expand(seq)
+//
+//	// Producers and consumer are synchronized
+//	for v := range result {
+//	    fmt.Println(v)
+//	}
+//
+// See Also:
+//   - Chain: Sequential version (deterministic order)
+//   - MergeAll: Merges pre-existing sequences concurrently
+//   - Map: Transforms elements without flattening
+//   - Async: Converts a single sequence to asynchronous
+func MergeMapBuf[A, B any](f func(A) Seq[B], bufSize int) Operator[A, B] {
+	return F.Flow2(
+		Map(f),
+		MergeAll[B](bufSize),
+	)
+}
+
+// MergeMap applies a function that returns a sequence to each element and merges the results concurrently using a default buffer size.
+// This is a convenience wrapper around MergeMapBuf that uses a default buffer size of 8.
+// It's the concurrent version of Chain (flatMap), where each mapped sequence is processed in parallel.
+//
+// Type Parameters:
+//   - A: The type of elements in the input sequence
+//   - B: The type of elements in the output sequences
+//
+// Parameters:
+//   - f: A function that transforms each input element into a sequence of output elements
+//
+// Returns:
+//   - Operator[A, B]: A function that takes a sequence of A and returns a flat sequence of B
+//
+// Behavior:
+//   - Uses a default buffer size of 8 for the internal channel
+//   - Applies f to each element in the input sequence to produce inner sequences
+//   - Spawns one goroutine per inner sequence to produce elements concurrently
+//   - Elements from different inner sequences are interleaved non-deterministically
+//   - Properly handles early termination with goroutine cleanup
+//   - Thread-safe: multiple producers can safely send to the shared channel
+//
+// Comparison with Chain:
+//   - Chain: Sequential processing, deterministic order, no concurrency overhead
+//   - MergeMap: Concurrent processing, non-deterministic order, better for I/O-bound tasks
+//
+// Example:
+//
+//	// Expand each number into a sequence concurrently
+//	expand := MergeMap(func(n int) Seq[int] {
+//	    return From(n, n*10, n*100)
+//	})
+//	seq := From(1, 2, 3)
+//	result := expand(seq)
+//
+//	// Yields: 1, 10, 100, 2, 20, 200, 3, 30, 300 (order varies)
+//	for v := range result {
+//	    fmt.Println(v)
+//	}
+//
+// See Also:
+//   - MergeMapBuf: MergeMap with custom buffer size
+//   - Chain: Sequential version (deterministic order)
+//   - MergeAll: Merges pre-existing sequences concurrently
+//   - Map: Transforms elements without flattening
+func MergeMap[A, B any](f func(A) Seq[B]) Operator[A, B] {
+	return MergeMapBuf(f, defaultBufferSize)
+}

v2/iterator/iter/monoid.go

@@ -21,7 +21,13 @@ import (
 )
 
 // Monoid returns a Monoid instance for Seq[T].
-// The monoid's concat operation concatenates sequences, and the empty value is an empty sequence.
+// The monoid's concat operation concatenates sequences sequentially, and the empty value is an empty sequence.
+//
+// Marble Diagram:
+//
+//	Seq1:   --1--2--3--|
+//	Seq2:   --4--5--6--|
+//	Concat: --1--2--3--4--5--6--|
 //
 // Example:
 //
@@ -35,3 +41,130 @@ import (
 func Monoid[T any]() M.Monoid[Seq[T]] {
 	return G.Monoid[Seq[T]]()
 }
+
+// ConcatMonoid returns a Monoid instance for Seq[T] that concatenates sequences sequentially.
+// This is an alias for Monoid that makes the sequential concatenation behavior explicit.
+//
+// A Monoid is an algebraic structure with an associative binary operation (concat)
+// and an identity element (empty). For sequences, the concat operation appends one
+// sequence after another in deterministic order, and the identity is an empty sequence.
+//
+// This monoid is useful for functional composition patterns where you need to combine
+// multiple sequences sequentially using monoid operations like Reduce, FoldMap, or when
+// working with monadic operations that require a monoid instance.
+//
+// Marble Diagram (Sequential Concatenation):
+//
+//	Seq1:   --1--2--3--|
+//	Seq2:   --4--5--6--|
+//	Concat: --1--2--3--4--5--6--|
+//	        (deterministic order)
+//
+// Marble Diagram (vs MergeMonoid):
+//
+//	ConcatMonoid:
+//	  Seq1:   --1--2--3--|
+//	  Seq2:              --4--5--6--|
+//	  Result: --1--2--3--4--5--6--|
+//
+//	MergeMonoid:
+//	  Seq1:   --1--2--3--|
+//	  Seq2:   --4--5--6--|
+//	  Result: --1-4-2-5-3-6--|
+//	          (non-deterministic)
+//
+// Type Parameters:
+//   - T: The type of elements in the sequences
+//
+// Returns:
+//   - Monoid[Seq[T]]: A monoid instance with:
+//   - Concat: Appends sequences sequentially (deterministic order)
+//   - Empty: Returns an empty sequence
+//
+// Properties:
+//   - Identity: concat(empty, x) = concat(x, empty) = x
+//   - Associativity: concat(concat(a, b), c) = concat(a, concat(b, c))
+//   - Deterministic: Elements always appear in the order of the input sequences
+//
+// Comparison with MergeMonoid:
+//
+// ConcatMonoid and MergeMonoid serve different purposes:
+//
+//   - ConcatMonoid: Sequential concatenation
+//
+//   - Order: Deterministic - elements from first sequence, then second, etc.
+//
+//   - Concurrency: No concurrency - sequences are processed one after another
+//
+//   - Performance: Lower overhead, no goroutines or channels
+//
+//   - Use when: Order matters, no I/O operations, or simplicity is preferred
+//
+//   - MergeMonoid: Concurrent merging
+//
+//   - Order: Non-deterministic - elements interleaved based on timing
+//
+//   - Concurrency: Spawns goroutines for each sequence
+//
+//   - Performance: Better for I/O-bound operations, higher overhead for CPU-bound
+//
+//   - Use when: Order doesn't matter, parallel I/O, or concurrent processing needed
+//
+// Example Usage:
+//
+//	// Create a monoid for concatenating integer sequences
+//	monoid := ConcatMonoid[int]()
+//
+//	// Use with Reduce to concatenate multiple sequences
+//	sequences := []Seq[int]{
+//	    From(1, 2, 3),
+//	    From(4, 5, 6),
+//	    From(7, 8, 9),
+//	}
+//	concatenated := MonadReduce(From(sequences...), monoid.Concat, monoid.Empty)
+//	// yields: 1, 2, 3, 4, 5, 6, 7, 8, 9 (deterministic order)
+//
+// Example with Empty Identity:
+//
+//	monoid := ConcatMonoid[int]()
+//	seq := From(1, 2, 3)
+//
+//	// Concatenating with empty is identity
+//	result1 := monoid.Concat(monoid.Empty, seq)  // same as seq
+//	result2 := monoid.Concat(seq, monoid.Empty)  // same as seq
+//
+// Example with FoldMap:
+//
+//	// Convert each number to a sequence and concatenate all results
+//	monoid := ConcatMonoid[int]()
+//	numbers := From(1, 2, 3)
+//	result := MonadFoldMap(numbers, func(n int) Seq[int] {
+//	    return From(n, n*10, n*100)
+//	}, monoid)
+//	// yields: 1, 10, 100, 2, 20, 200, 3, 30, 300 (deterministic order)
+//
+// Example Comparing ConcatMonoid vs MergeMonoid:
+//
+//	seq1 := From(1, 2, 3)
+//	seq2 := From(4, 5, 6)
+//
+//	// ConcatMonoid: Sequential, deterministic
+//	concatMonoid := ConcatMonoid[int]()
+//	concat := concatMonoid.Concat(seq1, seq2)
+//	// Always yields: 1, 2, 3, 4, 5, 6
+//
+//	// MergeMonoid: Concurrent, non-deterministic
+//	mergeMonoid := MergeMonoid[int](10)
+//	merged := mergeMonoid.Concat(seq1, seq2)
+//	// May yield: 1, 4, 2, 5, 3, 6 (order varies)
+//
+// See Also:
+//   - Monoid: The base monoid function (alias)
+//   - MergeMonoid: Concurrent merging monoid
+//   - MonadChain: Sequential flattening of sequences
+//   - Empty: Creates an empty sequence
+//
+//go:inline
+func ConcatMonoid[T any]() M.Monoid[Seq[T]] {
+	return Monoid[T]()
+}

v2/iterator/iter/monoid_test.go

@@ -0,0 +1,363 @@
+// 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"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestConcatMonoid_Identity(t *testing.T) {
+	t.Run("left identity", func(t *testing.T) {
+		monoid := ConcatMonoid[int]()
+		seq := From(1, 2, 3)
+
+		result := monoid.Concat(monoid.Empty(), seq)
+		collected := slices.Collect(result)
+
+		assert.Equal(t, []int{1, 2, 3}, collected)
+	})
+
+	t.Run("right identity", func(t *testing.T) {
+		monoid := ConcatMonoid[int]()
+		seq := From(1, 2, 3)
+
+		result := monoid.Concat(seq, monoid.Empty())
+		collected := slices.Collect(result)
+
+		assert.Equal(t, []int{1, 2, 3}, collected)
+	})
+}
+
+func TestConcatMonoid_Associativity(t *testing.T) {
+	monoid := ConcatMonoid[int]()
+	seq1 := From(1, 2)
+	seq2 := From(3, 4)
+	seq3 := From(5, 6)
+
+	// (a + b) + c
+	left := monoid.Concat(monoid.Concat(seq1, seq2), seq3)
+	leftResult := slices.Collect(left)
+
+	// a + (b + c)
+	right := monoid.Concat(seq1, monoid.Concat(seq2, seq3))
+	rightResult := slices.Collect(right)
+
+	assert.Equal(t, leftResult, rightResult)
+	assert.Equal(t, []int{1, 2, 3, 4, 5, 6}, leftResult)
+}
+
+func TestConcatMonoid_DeterministicOrder(t *testing.T) {
+	t.Run("concatenates in deterministic order", func(t *testing.T) {
+		monoid := ConcatMonoid[int]()
+		seq1 := From(1, 2, 3)
+		seq2 := From(4, 5, 6)
+		seq3 := From(7, 8, 9)
+
+		result := monoid.Concat(monoid.Concat(seq1, seq2), seq3)
+		collected := slices.Collect(result)
+
+		// Order is always deterministic
+		assert.Equal(t, []int{1, 2, 3, 4, 5, 6, 7, 8, 9}, collected)
+	})
+
+	t.Run("multiple runs produce same order", func(t *testing.T) {
+		monoid := ConcatMonoid[int]()
+		seq1 := From(1, 2, 3)
+		seq2 := From(4, 5, 6)
+
+		// Run multiple times
+		results := make([][]int, 5)
+		for i := range 5 {
+			result := monoid.Concat(seq1, seq2)
+			results[i] = slices.Collect(result)
+		}
+
+		// All results should be identical
+		expected := []int{1, 2, 3, 4, 5, 6}
+		for i, result := range results {
+			assert.Equal(t, expected, result, "run %d should match", i)
+		}
+	})
+}
+
+func TestConcatMonoid_WithReduce(t *testing.T) {
+	monoid := ConcatMonoid[int]()
+	sequences := []Seq[int]{
+		From(1, 2, 3),
+		From(4, 5, 6),
+		From(7, 8, 9),
+	}
+
+	result := MonadReduce(From(sequences...), monoid.Concat, monoid.Empty())
+	collected := slices.Collect(result)
+
+	assert.Equal(t, []int{1, 2, 3, 4, 5, 6, 7, 8, 9}, collected)
+}
+
+func TestConcatMonoid_WithFoldMap(t *testing.T) {
+	monoid := ConcatMonoid[int]()
+	numbers := From(1, 2, 3)
+
+	result := MonadFoldMap(numbers, func(n int) Seq[int] {
+		return From(n, n*10, n*100)
+	}, monoid)
+	collected := slices.Collect(result)
+
+	// Deterministic order: each number's expansion in sequence
+	assert.Equal(t, []int{1, 10, 100, 2, 20, 200, 3, 30, 300}, collected)
+}
+
+func TestConcatMonoid_ComparisonWithMergeMonoid(t *testing.T) {
+	t.Run("ConcatMonoid is deterministic", func(t *testing.T) {
+		concatMonoid := ConcatMonoid[int]()
+		seq1 := From(1, 2, 3)
+		seq2 := From(4, 5, 6)
+
+		result := concatMonoid.Concat(seq1, seq2)
+		collected := slices.Collect(result)
+
+		// Always the same order
+		assert.Equal(t, []int{1, 2, 3, 4, 5, 6}, collected)
+	})
+
+	t.Run("MergeMonoid may be non-deterministic", func(t *testing.T) {
+		mergeMonoid := MergeMonoid[int](10)
+		seq1 := From(1, 2, 3)
+		seq2 := From(4, 5, 6)
+
+		result := mergeMonoid.Concat(seq1, seq2)
+		collected := slices.Collect(result)
+
+		// Contains all elements but order may vary
+		assert.ElementsMatch(t, []int{1, 2, 3, 4, 5, 6}, collected)
+		// Note: We can't assert exact order as it's non-deterministic
+	})
+}
+
+func TestConcatMonoid_EmptySequences(t *testing.T) {
+	t.Run("concatenating empty sequences", func(t *testing.T) {
+		monoid := ConcatMonoid[int]()
+		empty1 := Empty[int]()
+		empty2 := Empty[int]()
+
+		result := monoid.Concat(empty1, empty2)
+		collected := slices.Collect(result)
+
+		assert.Empty(t, collected)
+	})
+
+	t.Run("concatenating with empty in middle", func(t *testing.T) {
+		monoid := ConcatMonoid[int]()
+		seq1 := From(1, 2)
+		empty := Empty[int]()
+		seq2 := From(3, 4)
+
+		result := monoid.Concat(monoid.Concat(seq1, empty), seq2)
+		collected := slices.Collect(result)
+
+		assert.Equal(t, []int{1, 2, 3, 4}, collected)
+	})
+}
+
+func TestConcatMonoid_WithComplexTypes(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	monoid := ConcatMonoid[Person]()
+	seq1 := From(Person{"Alice", 30}, Person{"Bob", 25})
+	seq2 := From(Person{"Charlie", 35}, Person{"Diana", 28})
+
+	result := monoid.Concat(seq1, seq2)
+	collected := slices.Collect(result)
+
+	expected := []Person{
+		{"Alice", 30},
+		{"Bob", 25},
+		{"Charlie", 35},
+		{"Diana", 28},
+	}
+	assert.Equal(t, expected, collected)
+}
+
+func BenchmarkConcatMonoid_TwoSequences(b *testing.B) {
+	monoid := ConcatMonoid[int]()
+	seq1 := From(1, 2, 3, 4, 5)
+	seq2 := From(6, 7, 8, 9, 10)
+
+	b.ResetTimer()
+	for range b.N {
+		result := monoid.Concat(seq1, seq2)
+		for range result {
+		}
+	}
+}
+
+func BenchmarkConcatMonoid_Reduce(b *testing.B) {
+	monoid := ConcatMonoid[int]()
+	sequences := []Seq[int]{
+		From(1, 2, 3),
+		From(4, 5, 6),
+		From(7, 8, 9),
+		From(10, 11, 12),
+	}
+
+	b.ResetTimer()
+	for range b.N {
+		result := MonadReduce(From(sequences...), monoid.Concat, monoid.Empty())
+		for range result {
+		}
+	}
+}
+
+func BenchmarkConcatMonoid_VsMergeMonoid(b *testing.B) {
+	seq1 := From(1, 2, 3, 4, 5)
+	seq2 := From(6, 7, 8, 9, 10)
+
+	b.Run("ConcatMonoid", func(b *testing.B) {
+		monoid := ConcatMonoid[int]()
+		b.ResetTimer()
+		for range b.N {
+			result := monoid.Concat(seq1, seq2)
+			for range result {
+			}
+		}
+	})
+
+	b.Run("MergeMonoid", func(b *testing.B) {
+		monoid := MergeMonoid[int](10)
+		b.ResetTimer()
+		for range b.N {
+			result := monoid.Concat(seq1, seq2)
+			for range result {
+			}
+		}
+	})
+}
+
+func ExampleConcatMonoid() {
+	monoid := ConcatMonoid[int]()
+	seq1 := From(1, 2, 3)
+	seq2 := From(4, 5, 6)
+
+	result := monoid.Concat(seq1, seq2)
+	for v := range result {
+		fmt.Println(v)
+	}
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+	// 6
+}
+
+func ExampleConcatMonoid_identity() {
+	monoid := ConcatMonoid[int]()
+	seq := From(1, 2, 3)
+
+	// Left identity
+	result1 := monoid.Concat(monoid.Empty(), seq)
+	for v := range result1 {
+		fmt.Println(v)
+	}
+
+	// Right identity
+	result2 := monoid.Concat(seq, monoid.Empty())
+	for v := range result2 {
+		fmt.Println(v)
+	}
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 1
+	// 2
+	// 3
+}
+
+func ExampleConcatMonoid_reduce() {
+	monoid := ConcatMonoid[int]()
+	sequences := []Seq[int]{
+		From(1, 2, 3),
+		From(4, 5, 6),
+		From(7, 8, 9),
+	}
+
+	result := MonadReduce(From(sequences...), monoid.Concat, monoid.Empty())
+	for v := range result {
+		fmt.Println(v)
+	}
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+	// 6
+	// 7
+	// 8
+	// 9
+}
+
+func ExampleConcatMonoid_comparison() {
+	seq1 := From(1, 2, 3)
+	seq2 := From(4, 5, 6)
+
+	// ConcatMonoid: Sequential, deterministic
+	concatMonoid := ConcatMonoid[int]()
+	concat := concatMonoid.Concat(seq1, seq2)
+	fmt.Println("ConcatMonoid (always same order):")
+	for v := range concat {
+		fmt.Println(v)
+	}
+
+	// MergeMonoid: Concurrent, non-deterministic
+	// Note: Output order may vary in actual runs
+	mergeMonoid := MergeMonoid[int](10)
+	merged := mergeMonoid.Concat(seq1, seq2)
+	fmt.Println("\nMergeMonoid (order may vary):")
+	collected := slices.Collect(merged)
+	// Sort for consistent test output
+	slices.Sort(collected)
+	for _, v := range collected {
+		fmt.Println(v)
+	}
+	// Output:
+	// ConcatMonoid (always same order):
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+	// 6
+	//
+	// MergeMonoid (order may vary):
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+	// 6
+}
+
+// Made with Bob

v2/iterator/iter/option.go

@@ -28,6 +28,13 @@ import (
 //
 // This is the monadic form that takes the sequence as the first parameter.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5-->
+//	ChainOptionK(x => x % 2 == 0 ? Some(x * 10) : None)
+//	Output: -----20----40---->
+//	        (filters and transforms)
+//
 // RxJS Equivalent: [concatMap] combined with [filter] - https://rxjs.dev/api/operators/concatMap
 //
 // Type parameters:
@@ -72,6 +79,13 @@ func MonadChainOptionK[A, B any](as Seq[A], f option.Kleisli[A, B]) Seq[B] {
 // This is the curried version of [MonadChainOptionK], useful for function composition
 // and creating reusable transformations.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5-->
+//	ChainOptionK(x => x > 2 ? Some(x) : None)
+//	Output: --------3--4--5-->
+//	        (filters out values <= 2)
+//
 // RxJS Equivalent: [concatMap] combined with [filter] - https://rxjs.dev/api/operators/concatMap
 //
 // Type parameters:

v2/iterator/iter/scan.go

@@ -24,6 +24,13 @@ package iter
 //
 // The operation is lazy - intermediate values are computed only as they are consumed.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5-->
+//	Scan((acc, x) => acc + x, 0)
+//	Output: --1--3--6--10-15->
+//	        (running sum)
+//
 // RxJS Equivalent: [scan] - https://rxjs.dev/api/operators/scan
 //
 // Scan is useful for:

v2/iterator/iter/take.go

@@ -27,6 +27,12 @@ import F "github.com/IBM/fp-go/v2/function"
 // Once n elements have been yielded, iteration stops immediately without consuming
 // the remaining elements from the source.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--4--5--6--7--8-->
+//	Take(3)
+//	Output: --1--2--3|
+//
 // RxJS Equivalent: [take] - https://rxjs.dev/api/operators/take
 //
 // Type Parameters:
@@ -78,3 +84,158 @@ func Take[U any](n int) Operator[U, U] {
 		}
 	}
 }
+
+// TakeWhile returns an operator that emits elements from a sequence while a predicate is satisfied.
+//
+// This function creates a transformation that yields elements from the source sequence
+// as long as each element satisfies the provided predicate. Once an element fails the
+// predicate test, the sequence terminates immediately, and no further elements are
+// emitted, even if subsequent elements would satisfy the predicate.
+//
+// The operation is lazy and only consumes elements from the source sequence as needed.
+// Once the predicate returns false, iteration stops immediately without consuming
+// the remaining elements from the source.
+//
+// Marble Diagram:
+//
+//	Input:       --1--2--3--4--5--2--1-->
+//	TakeWhile(x < 4)
+//	Output:      --1--2--3|
+//	                      (stops at 4)
+//
+// RxJS Equivalent: [takeWhile] - https://rxjs.dev/api/operators/takeWhile
+//
+// Type Parameters:
+//   - U: The type of elements in the sequence
+//
+// Parameters:
+//   - p: A predicate function that tests each element. Returns true to continue, false to stop
+//
+// Returns:
+//   - An Operator that transforms a Seq[U] by taking elements while the predicate is satisfied
+//
+// Example - Take while less than threshold:
+//
+//	seq := From(1, 2, 3, 4, 5, 2, 1)
+//	result := TakeWhile(func(x int) bool { return x < 4 })(seq)
+//	// yields: 1, 2, 3 (stops at 4, doesn't continue to 2, 1)
+//
+// Example - Take while condition is met:
+//
+//	seq := From("a", "b", "c", "1", "d", "e")
+//	isLetter := func(s string) bool { return s >= "a" && s <= "z" }
+//	result := TakeWhile(isLetter)(seq)
+//	// yields: "a", "b", "c" (stops at "1")
+//
+// Example - Take all when predicate always true:
+//
+//	seq := From(2, 4, 6, 8)
+//	result := TakeWhile(func(x int) bool { return x%2 == 0 })(seq)
+//	// yields: 2, 4, 6, 8 (all elements satisfy predicate)
+//
+// Example - Take none when first element fails:
+//
+//	seq := From(5, 1, 2, 3)
+//	result := TakeWhile(func(x int) bool { return x < 5 })(seq)
+//	// yields: nothing (first element fails predicate)
+//
+// Example - Chaining with other operations:
+//
+//	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+//	result := F.Pipe2(
+//	    seq,
+//	    MonadMap(seq, func(x int) int { return x * 2 }),
+//	    TakeWhile(func(x int) bool { return x < 10 }),
+//	)
+//	// yields: 2, 4, 6, 8 (stops when doubled value reaches 10)
+func TakeWhile[U any](p Predicate[U]) Operator[U, U] {
+	return func(s Seq[U]) Seq[U] {
+		return func(yield func(U) bool) {
+			for u := range s {
+				if !p(u) || !yield(u) {
+					return
+				}
+			}
+		}
+	}
+}
+
+// SkipWhile returns an operator that skips elements from a sequence while a predicate is satisfied.
+//
+// This function creates a transformation that discards elements from the source sequence
+// as long as each element satisfies the provided predicate. Once an element fails the
+// predicate test, that element and all subsequent elements are yielded, regardless of
+// whether they satisfy the predicate.
+//
+// The operation is lazy and only consumes elements from the source sequence as needed.
+// Once the predicate returns false, all remaining elements are yielded without further
+// predicate evaluation.
+//
+// Marble Diagram:
+//
+//	Input:        --1--2--3--4--5--2--1-->
+//	SkipWhile(x < 4)
+//	Output:       -----------4--5--2--1-->
+//	                         (starts at 4, continues with all)
+//
+// RxJS Equivalent: [skipWhile] - https://rxjs.dev/api/operators/skipWhile
+//
+// Type Parameters:
+//   - U: The type of elements in the sequence
+//
+// Parameters:
+//   - p: A predicate function that tests each element. Returns true to skip, false to start yielding
+//
+// Returns:
+//   - An Operator that transforms a Seq[U] by skipping elements while the predicate is satisfied
+//
+// Example - Skip while less than threshold:
+//
+//	seq := From(1, 2, 3, 4, 5, 2, 1)
+//	result := SkipWhile(func(x int) bool { return x < 4 })(seq)
+//	// yields: 4, 5, 2, 1 (starts at 4, continues with all remaining)
+//
+// Example - Skip while condition is met:
+//
+//	seq := From("a", "b", "c", "1", "d", "e")
+//	isLetter := func(s string) bool { return s >= "a" && s <= "z" }
+//	result := SkipWhile(isLetter)(seq)
+//	// yields: "1", "d", "e" (starts at "1", continues with all remaining)
+//
+// Example - Skip none when first element fails:
+//
+//	seq := From(5, 1, 2, 3)
+//	result := SkipWhile(func(x int) bool { return x < 5 })(seq)
+//	// yields: 5, 1, 2, 3 (first element fails predicate, all yielded)
+//
+// Example - Skip all when predicate always true:
+//
+//	seq := From(2, 4, 6, 8)
+//	result := SkipWhile(func(x int) bool { return x%2 == 0 })(seq)
+//	// yields: nothing (all elements satisfy predicate)
+//
+// Example - Chaining with other operations:
+//
+//	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+//	result := F.Pipe2(
+//	    seq,
+//	    SkipWhile(func(x int) bool { return x < 5 }),
+//	    MonadMap(seq, func(x int) int { return x * 2 }),
+//	)
+//	// yields: 10, 12, 14, 16, 18, 20 (skip until 5, then double remaining)
+func SkipWhile[U any](p Predicate[U]) Operator[U, U] {
+	return func(s Seq[U]) Seq[U] {
+		return func(yield func(U) bool) {
+			skipping := true
+			for u := range s {
+				if skipping && p(u) {
+					continue
+				}
+				skipping = false
+				if !yield(u) {
+					return
+				}
+			}
+		}
+	}
+}

v2/iterator/iter/take_test.go

@@ -461,3 +461,831 @@ func ExampleTake_chained() {
 	}
 	// Output: 4 5 6 7 8
 }
+
+// TestSkipWhile tests basic SkipWhile functionality
+func TestSkipWhile(t *testing.T) {
+	t.Run("skips while predicate is true", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 2, 1)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 4 })(seq))
+		assert.Equal(t, []int{4, 5, 2, 1}, result)
+	})
+
+	t.Run("skips none when first element fails", func(t *testing.T) {
+		seq := From(5, 1, 2, 3)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 5 })(seq))
+		assert.Equal(t, []int{5, 1, 2, 3}, result)
+	})
+
+	t.Run("skips all when predicate always true", func(t *testing.T) {
+		seq := From(2, 4, 6, 8)
+		result := toSlice(SkipWhile(func(x int) bool { return x%2 == 0 })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skips from string sequence", func(t *testing.T) {
+		seq := From("a", "b", "c", "1", "d", "e")
+		isLetter := func(s string) bool { return s >= "a" && s <= "z" }
+		result := toSlice(SkipWhile(isLetter)(seq))
+		assert.Equal(t, []string{"1", "d", "e"}, result)
+	})
+
+	t.Run("continues after predicate fails", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 1, 2, 3)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 4 })(seq))
+		assert.Equal(t, []int{4, 1, 2, 3}, result)
+	})
+
+	t.Run("skips single element", func(t *testing.T) {
+		seq := From(1, 10, 2, 3)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 10 })(seq))
+		assert.Equal(t, []int{10, 2, 3}, result)
+	})
+}
+
+// TestSkipWhileEmpty tests SkipWhile with empty sequences
+func TestSkipWhileEmpty(t *testing.T) {
+	t.Run("returns empty from empty sequence", func(t *testing.T) {
+		seq := Empty[int]()
+		result := toSlice(SkipWhile(func(x int) bool { return x > 0 })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("returns empty when predicate always satisfied", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 10 })(seq))
+		assert.Empty(t, result)
+	})
+}
+
+// TestSkipWhileWithComplexTypes tests SkipWhile with complex data types
+func TestSkipWhileWithComplexTypes(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("skips structs while condition met", func(t *testing.T) {
+		seq := From(
+			Person{"Alice", 25},
+			Person{"Bob", 30},
+			Person{"Charlie", 35},
+			Person{"David", 28},
+		)
+		result := toSlice(SkipWhile(func(p Person) bool { return p.Age < 35 })(seq))
+		expected := []Person{
+			{"Charlie", 35},
+			{"David", 28},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("skips pointers while condition met", func(t *testing.T) {
+		p1 := &Person{"Alice", 25}
+		p2 := &Person{"Bob", 30}
+		p3 := &Person{"Charlie", 35}
+		p4 := &Person{"David", 28}
+		seq := From(p1, p2, p3, p4)
+		result := toSlice(SkipWhile(func(p *Person) bool { return p.Age < 35 })(seq))
+		assert.Equal(t, []*Person{p3, p4}, result)
+	})
+
+	t.Run("skips slices while condition met", func(t *testing.T) {
+		seq := From([]int{1}, []int{1, 2}, []int{1, 2, 3}, []int{1})
+		result := toSlice(SkipWhile(func(s []int) bool { return len(s) < 3 })(seq))
+		expected := [][]int{{1, 2, 3}, {1}}
+		assert.Equal(t, expected, result)
+	})
+}
+
+// TestSkipWhileWithChainedOperations tests SkipWhile with other sequence operations
+func TestSkipWhileWithChainedOperations(t *testing.T) {
+	t.Run("skipWhile after map", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		mapped := MonadMap(seq, N.Mul(2))
+		result := toSlice(SkipWhile(func(x int) bool { return x < 8 })(mapped))
+		assert.Equal(t, []int{8, 10}, result)
+	})
+
+	t.Run("skipWhile after filter", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		filtered := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+		result := toSlice(SkipWhile(func(x int) bool { return x < 6 })(filtered))
+		assert.Equal(t, []int{6, 8, 10}, result)
+	})
+
+	t.Run("map after skipWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		skipped := SkipWhile(func(x int) bool { return x < 4 })(seq)
+		result := toSlice(MonadMap(skipped, N.Mul(10)))
+		assert.Equal(t, []int{40, 50}, result)
+	})
+
+	t.Run("filter after skipWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8)
+		skipped := SkipWhile(func(x int) bool { return x < 4 })(seq)
+		result := toSlice(MonadFilter(skipped, func(x int) bool { return x%2 == 0 }))
+		assert.Equal(t, []int{4, 6, 8}, result)
+	})
+
+	t.Run("skipWhile after chain", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		chained := MonadChain(seq, func(x int) Seq[int] {
+			return From(x, x*10)
+		})
+		result := toSlice(SkipWhile(func(x int) bool { return x < 20 })(chained))
+		assert.Equal(t, []int{20, 3, 30}, result)
+	})
+
+	t.Run("skip after skipWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		skipped1 := SkipWhile(func(x int) bool { return x < 4 })(seq)
+		skipped2 := Skip[int](2)(skipped1)
+		result := toSlice(skipped2)
+		assert.Equal(t, []int{6, 7, 8, 9, 10}, result)
+	})
+
+	t.Run("skipWhile after skip", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		skipped := Skip[int](3)(seq)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 7 })(skipped))
+		assert.Equal(t, []int{7, 8, 9, 10}, result)
+	})
+
+	t.Run("takeWhile after skipWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		skipped := SkipWhile(func(x int) bool { return x < 4 })(seq)
+		taken := TakeWhile(func(x int) bool { return x < 8 })(skipped)
+		result := toSlice(taken)
+		assert.Equal(t, []int{4, 5, 6, 7}, result)
+	})
+
+	t.Run("skipWhile after takeWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		taken := TakeWhile(func(x int) bool { return x < 8 })(seq)
+		skipped := SkipWhile(func(x int) bool { return x < 4 })(taken)
+		result := toSlice(skipped)
+		assert.Equal(t, []int{4, 5, 6, 7}, result)
+	})
+}
+
+// TestSkipWhileWithReplicate tests SkipWhile with Replicate
+func TestSkipWhileWithReplicate(t *testing.T) {
+	t.Run("skips all from replicated sequence", func(t *testing.T) {
+		seq := Replicate(10, 5)
+		result := toSlice(SkipWhile(func(x int) bool { return x == 5 })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skips none when predicate fails on replicate", func(t *testing.T) {
+		seq := Replicate(5, 10)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 10 })(seq))
+		assert.Equal(t, []int{10, 10, 10, 10, 10}, result)
+	})
+}
+
+// TestSkipWhileWithMakeBy tests SkipWhile with MakeBy
+func TestSkipWhileWithMakeBy(t *testing.T) {
+	t.Run("skips from generated sequence", func(t *testing.T) {
+		seq := MakeBy(10, func(i int) int { return i * i })
+		result := toSlice(SkipWhile(func(x int) bool { return x < 25 })(seq))
+		assert.Equal(t, []int{25, 36, 49, 64, 81}, result)
+	})
+
+	t.Run("skips all from generated sequence", func(t *testing.T) {
+		seq := MakeBy(5, func(i int) int { return i + 1 })
+		result := toSlice(SkipWhile(func(x int) bool { return x < 100 })(seq))
+		assert.Empty(t, result)
+	})
+}
+
+// TestSkipWhileWithPrependAppend tests SkipWhile with Prepend and Append
+func TestSkipWhileWithPrependAppend(t *testing.T) {
+	t.Run("skipWhile from prepended sequence", func(t *testing.T) {
+		seq := From(2, 3, 4, 5)
+		prepended := Prepend(1)(seq)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 4 })(prepended))
+		assert.Equal(t, []int{4, 5}, result)
+	})
+
+	t.Run("skipWhile from appended sequence", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		appended := Append(10)(seq)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 10 })(appended))
+		assert.Equal(t, []int{10}, result)
+	})
+
+	t.Run("skipWhile includes appended element", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		appended := Append(4)(seq)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 3 })(appended))
+		assert.Equal(t, []int{3, 4}, result)
+	})
+}
+
+// TestSkipWhileWithFlatten tests SkipWhile with Flatten
+func TestSkipWhileWithFlatten(t *testing.T) {
+	t.Run("skips from flattened sequence", func(t *testing.T) {
+		nested := From(From(1, 2), From(3, 4), From(5, 6))
+		flattened := Flatten(nested)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 4 })(flattened))
+		assert.Equal(t, []int{4, 5, 6}, result)
+	})
+
+	t.Run("skips from flattened with empty inner sequences", func(t *testing.T) {
+		nested := From(From(1, 2), Empty[int](), From(3, 4))
+		flattened := Flatten(nested)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 3 })(flattened))
+		assert.Equal(t, []int{3, 4}, result)
+	})
+}
+
+// TestSkipWhileDoesNotConsumeEntireSequence tests that SkipWhile is lazy
+func TestSkipWhileDoesNotConsumeEntireSequence(t *testing.T) {
+	t.Run("only consumes needed elements", func(t *testing.T) {
+		callCount := 0
+		seq := MonadMap(From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), func(x int) int {
+			callCount++
+			return x * 2
+		})
+
+		skipped := SkipWhile(func(x int) bool { return x < 8 })(seq)
+
+		result := []int{}
+		for v := range skipped {
+			result = append(result, v)
+		}
+
+		assert.Equal(t, []int{8, 10, 12, 14, 16, 18, 20}, result)
+		// Should process all elements since we iterate through all remaining
+		assert.Equal(t, 10, callCount, "should process all elements")
+	})
+
+	t.Run("stops early when consumer stops", func(t *testing.T) {
+		callCount := 0
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		filtered := MonadFilter(seq, func(x int) bool {
+			callCount++
+			return x%2 == 0
+		})
+
+		skipped := SkipWhile(func(x int) bool { return x < 6 })(filtered)
+
+		result := []int{}
+		count := 0
+		for v := range skipped {
+			result = append(result, v)
+			count++
+			if count == 2 {
+				break
+			}
+		}
+
+		assert.Equal(t, []int{6, 8}, result)
+		// Should stop after getting 2 elements
+		assert.LessOrEqual(t, callCount, 9, "should not consume all elements")
+	})
+}
+
+// TestSkipWhileEdgeCases tests edge cases
+func TestSkipWhileEdgeCases(t *testing.T) {
+	t.Run("skipWhile with always false predicate", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(SkipWhile(func(x int) bool { return false })(seq))
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("skipWhile with always true predicate", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(SkipWhile(func(x int) bool { return true })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skipWhile from single element that passes", func(t *testing.T) {
+		seq := From(42)
+		result := toSlice(SkipWhile(func(x int) bool { return x > 0 })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("skipWhile from single element that fails", func(t *testing.T) {
+		seq := From(42)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 0 })(seq))
+		assert.Equal(t, []int{42}, result)
+	})
+
+	t.Run("skipWhile with complex predicate", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		result := toSlice(SkipWhile(func(x int) bool {
+			return x%2 == 1 || x < 5
+		})(seq))
+		assert.Equal(t, []int{6, 7, 8, 9, 10}, result)
+	})
+
+	t.Run("skipWhile yields elements that satisfy predicate after first failure", func(t *testing.T) {
+		seq := From(1, 2, 3, 10, 1, 2, 3)
+		result := toSlice(SkipWhile(func(x int) bool { return x < 10 })(seq))
+		assert.Equal(t, []int{10, 1, 2, 3}, result)
+	})
+}
+
+// Benchmark tests for SkipWhile
+func BenchmarkSkipWhile(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		skipped := SkipWhile(func(x int) bool { return x < 6 })(seq)
+		for range skipped {
+		}
+	}
+}
+
+func BenchmarkSkipWhileLargeSequence(b *testing.B) {
+	data := make([]int, 1000)
+	for i := range data {
+		data[i] = i
+	}
+	seq := From(data...)
+	b.ResetTimer()
+	for range b.N {
+		skipped := SkipWhile(func(x int) bool { return x < 100 })(seq)
+		for range skipped {
+		}
+	}
+}
+
+func BenchmarkSkipWhileWithMap(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		mapped := MonadMap(seq, N.Mul(2))
+		skipped := SkipWhile(func(x int) bool { return x < 12 })(mapped)
+		for range skipped {
+		}
+	}
+}
+
+func BenchmarkSkipWhileWithFilter(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		filtered := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+		skipped := SkipWhile(func(x int) bool { return x < 6 })(filtered)
+		for range skipped {
+		}
+	}
+}
+
+// Example tests for documentation
+func ExampleSkipWhile() {
+	seq := From(1, 2, 3, 4, 5, 2, 1)
+	skipped := SkipWhile(func(x int) bool { return x < 4 })(seq)
+
+	for v := range skipped {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 4 5 2 1
+}
+
+func ExampleSkipWhile_allSatisfy() {
+	seq := From(2, 4, 6, 8)
+	skipped := SkipWhile(func(x int) bool { return x%2 == 0 })(seq)
+
+	count := 0
+	for range skipped {
+		count++
+	}
+	fmt.Printf("Count: %d\n", count)
+	// Output: Count: 0
+}
+
+func ExampleSkipWhile_firstFails() {
+	seq := From(5, 1, 2, 3)
+	skipped := SkipWhile(func(x int) bool { return x < 5 })(seq)
+
+	for v := range skipped {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 5 1 2 3
+}
+
+func ExampleSkipWhile_withMap() {
+	seq := From(1, 2, 3, 4, 5)
+	doubled := MonadMap(seq, N.Mul(2))
+	skipped := SkipWhile(func(x int) bool { return x < 8 })(doubled)
+
+	for v := range skipped {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 8 10
+}
+
+func ExampleSkipWhile_strings() {
+	seq := From("a", "b", "c", "1", "d", "e")
+	isLetter := func(s string) bool { return s >= "a" && s <= "z" }
+	skipped := SkipWhile(isLetter)(seq)
+
+	for v := range skipped {
+		fmt.Printf("%s ", v)
+	}
+	// Output: 1 d e
+}
+
+// TestTakeWhile tests basic TakeWhile functionality
+func TestTakeWhile(t *testing.T) {
+	t.Run("takes while predicate is true", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 2, 1)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 4 })(seq))
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+
+	t.Run("takes all when predicate always true", func(t *testing.T) {
+		seq := From(2, 4, 6, 8)
+		result := toSlice(TakeWhile(func(x int) bool { return x%2 == 0 })(seq))
+		assert.Equal(t, []int{2, 4, 6, 8}, result)
+	})
+
+	t.Run("takes none when first element fails", func(t *testing.T) {
+		seq := From(5, 1, 2, 3)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 5 })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("takes from string sequence", func(t *testing.T) {
+		seq := From("a", "b", "c", "1", "d", "e")
+		isLetter := func(s string) bool { return s >= "a" && s <= "z" }
+		result := toSlice(TakeWhile(isLetter)(seq))
+		assert.Equal(t, []string{"a", "b", "c"}, result)
+	})
+
+	t.Run("takes single element", func(t *testing.T) {
+		seq := From(1, 10, 2, 3)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 10 })(seq))
+		assert.Equal(t, []int{1}, result)
+	})
+
+	t.Run("stops at first false predicate", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 1, 2, 3)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 4 })(seq))
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+}
+
+// TestTakeWhileEmpty tests TakeWhile with empty sequences
+func TestTakeWhileEmpty(t *testing.T) {
+	t.Run("returns empty from empty sequence", func(t *testing.T) {
+		seq := Empty[int]()
+		result := toSlice(TakeWhile(func(x int) bool { return x > 0 })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("returns empty when predicate never satisfied", func(t *testing.T) {
+		seq := From(10, 20, 30)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 5 })(seq))
+		assert.Empty(t, result)
+	})
+}
+
+// TestTakeWhileWithComplexTypes tests TakeWhile with complex data types
+func TestTakeWhileWithComplexTypes(t *testing.T) {
+	type Person struct {
+		Name string
+		Age  int
+	}
+
+	t.Run("takes structs while condition met", func(t *testing.T) {
+		seq := From(
+			Person{"Alice", 25},
+			Person{"Bob", 30},
+			Person{"Charlie", 35},
+			Person{"David", 28},
+		)
+		result := toSlice(TakeWhile(func(p Person) bool { return p.Age < 35 })(seq))
+		expected := []Person{
+			{"Alice", 25},
+			{"Bob", 30},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("takes pointers while condition met", func(t *testing.T) {
+		p1 := &Person{"Alice", 25}
+		p2 := &Person{"Bob", 30}
+		p3 := &Person{"Charlie", 35}
+		seq := From(p1, p2, p3)
+		result := toSlice(TakeWhile(func(p *Person) bool { return p.Age < 35 })(seq))
+		assert.Equal(t, []*Person{p1, p2}, result)
+	})
+
+	t.Run("takes slices while condition met", func(t *testing.T) {
+		seq := From([]int{1}, []int{1, 2}, []int{1, 2, 3}, []int{1})
+		result := toSlice(TakeWhile(func(s []int) bool { return len(s) < 3 })(seq))
+		expected := [][]int{{1}, {1, 2}}
+		assert.Equal(t, expected, result)
+	})
+}
+
+// TestTakeWhileWithChainedOperations tests TakeWhile with other sequence operations
+func TestTakeWhileWithChainedOperations(t *testing.T) {
+	t.Run("takeWhile after map", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		mapped := MonadMap(seq, N.Mul(2))
+		result := toSlice(TakeWhile(func(x int) bool { return x < 8 })(mapped))
+		assert.Equal(t, []int{2, 4, 6}, result)
+	})
+
+	t.Run("takeWhile after filter", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		filtered := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+		result := toSlice(TakeWhile(func(x int) bool { return x < 7 })(filtered))
+		assert.Equal(t, []int{2, 4, 6}, result)
+	})
+
+	t.Run("map after takeWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		taken := TakeWhile(func(x int) bool { return x < 4 })(seq)
+		result := toSlice(MonadMap(taken, N.Mul(10)))
+		assert.Equal(t, []int{10, 20, 30}, result)
+	})
+
+	t.Run("filter after takeWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8)
+		taken := TakeWhile(func(x int) bool { return x < 7 })(seq)
+		result := toSlice(MonadFilter(taken, func(x int) bool { return x%2 == 0 }))
+		assert.Equal(t, []int{2, 4, 6}, result)
+	})
+
+	t.Run("takeWhile after chain", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		chained := MonadChain(seq, func(x int) Seq[int] {
+			return From(x, x*10)
+		})
+		result := toSlice(TakeWhile(func(x int) bool { return x < 20 })(chained))
+		assert.Equal(t, []int{1, 10, 2}, result)
+	})
+
+	t.Run("take after takeWhile", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		taken1 := TakeWhile(func(x int) bool { return x < 8 })(seq)
+		taken2 := Take[int](3)(taken1)
+		result := toSlice(taken2)
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+
+	t.Run("takeWhile after take", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		taken := Take[int](7)(seq)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 5 })(taken))
+		assert.Equal(t, []int{1, 2, 3, 4}, result)
+	})
+}
+
+// TestTakeWhileWithReplicate tests TakeWhile with Replicate
+func TestTakeWhileWithReplicate(t *testing.T) {
+	t.Run("takes from replicated sequence", func(t *testing.T) {
+		seq := Replicate(10, 5)
+		result := toSlice(TakeWhile(func(x int) bool { return x == 5 })(seq))
+		assert.Equal(t, []int{5, 5, 5, 5, 5, 5, 5, 5, 5, 5}, result)
+	})
+
+	t.Run("takes none when predicate fails on replicate", func(t *testing.T) {
+		seq := Replicate(5, 10)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 10 })(seq))
+		assert.Empty(t, result)
+	})
+}
+
+// TestTakeWhileWithMakeBy tests TakeWhile with MakeBy
+func TestTakeWhileWithMakeBy(t *testing.T) {
+	t.Run("takes from generated sequence", func(t *testing.T) {
+		seq := MakeBy(10, func(i int) int { return i * i })
+		result := toSlice(TakeWhile(func(x int) bool { return x < 25 })(seq))
+		assert.Equal(t, []int{0, 1, 4, 9, 16}, result)
+	})
+
+	t.Run("takes all from generated sequence", func(t *testing.T) {
+		seq := MakeBy(5, func(i int) int { return i + 1 })
+		result := toSlice(TakeWhile(func(x int) bool { return x < 100 })(seq))
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+}
+
+// TestTakeWhileWithPrependAppend tests TakeWhile with Prepend and Append
+func TestTakeWhileWithPrependAppend(t *testing.T) {
+	t.Run("takeWhile from prepended sequence", func(t *testing.T) {
+		seq := From(2, 3, 4, 5)
+		prepended := Prepend(1)(seq)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 4 })(prepended))
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+
+	t.Run("takeWhile from appended sequence", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		appended := Append(10)(seq)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 10 })(appended))
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+
+	t.Run("takeWhile includes appended element", func(t *testing.T) {
+		seq := From(1, 2, 3)
+		appended := Append(4)(seq)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 5 })(appended))
+		assert.Equal(t, []int{1, 2, 3, 4}, result)
+	})
+}
+
+// TestTakeWhileWithFlatten tests TakeWhile with Flatten
+func TestTakeWhileWithFlatten(t *testing.T) {
+	t.Run("takes from flattened sequence", func(t *testing.T) {
+		nested := From(From(1, 2), From(3, 4), From(5, 6))
+		flattened := Flatten(nested)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 5 })(flattened))
+		assert.Equal(t, []int{1, 2, 3, 4}, result)
+	})
+
+	t.Run("takes from flattened with empty inner sequences", func(t *testing.T) {
+		nested := From(From(1, 2), Empty[int](), From(3, 4))
+		flattened := Flatten(nested)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 4 })(flattened))
+		assert.Equal(t, []int{1, 2, 3}, result)
+	})
+}
+
+// TestTakeWhileDoesNotConsumeEntireSequence tests that TakeWhile is lazy
+func TestTakeWhileDoesNotConsumeEntireSequence(t *testing.T) {
+	t.Run("only consumes needed elements", func(t *testing.T) {
+		callCount := 0
+		seq := MonadMap(From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10), func(x int) int {
+			callCount++
+			return x * 2
+		})
+
+		taken := TakeWhile(func(x int) bool { return x < 8 })(seq)
+
+		result := []int{}
+		for v := range taken {
+			result = append(result, v)
+		}
+
+		assert.Equal(t, []int{2, 4, 6}, result)
+		// Should stop after finding element that fails predicate
+		assert.LessOrEqual(t, callCount, 5, "should not consume significantly more than needed")
+		assert.GreaterOrEqual(t, callCount, 4, "should consume at least enough to find failure")
+	})
+
+	t.Run("stops early with filter", func(t *testing.T) {
+		callCount := 0
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		filtered := MonadFilter(seq, func(x int) bool {
+			callCount++
+			return x%2 == 0
+		})
+
+		taken := TakeWhile(func(x int) bool { return x < 7 })(filtered)
+
+		result := []int{}
+		for v := range taken {
+			result = append(result, v)
+		}
+
+		assert.Equal(t, []int{2, 4, 6}, result)
+		// Should stop after finding even number >= 7
+		assert.LessOrEqual(t, callCount, 9, "should not consume significantly more than needed")
+		assert.GreaterOrEqual(t, callCount, 7, "should consume at least enough to find 8")
+	})
+}
+
+// TestTakeWhileEdgeCases tests edge cases
+func TestTakeWhileEdgeCases(t *testing.T) {
+	t.Run("takeWhile with always false predicate", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(TakeWhile(func(x int) bool { return false })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("takeWhile with always true predicate", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5)
+		result := toSlice(TakeWhile(func(x int) bool { return true })(seq))
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+
+	t.Run("takeWhile from single element that passes", func(t *testing.T) {
+		seq := From(42)
+		result := toSlice(TakeWhile(func(x int) bool { return x > 0 })(seq))
+		assert.Equal(t, []int{42}, result)
+	})
+
+	t.Run("takeWhile from single element that fails", func(t *testing.T) {
+		seq := From(42)
+		result := toSlice(TakeWhile(func(x int) bool { return x < 0 })(seq))
+		assert.Empty(t, result)
+	})
+
+	t.Run("takeWhile with complex predicate", func(t *testing.T) {
+		seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+		result := toSlice(TakeWhile(func(x int) bool {
+			return x%2 == 1 || x < 5
+		})(seq))
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+}
+
+// Benchmark tests for TakeWhile
+func BenchmarkTakeWhile(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		taken := TakeWhile(func(x int) bool { return x < 6 })(seq)
+		for range taken {
+		}
+	}
+}
+
+func BenchmarkTakeWhileLargeSequence(b *testing.B) {
+	data := make([]int, 1000)
+	for i := range data {
+		data[i] = i
+	}
+	seq := From(data...)
+	b.ResetTimer()
+	for range b.N {
+		taken := TakeWhile(func(x int) bool { return x < 100 })(seq)
+		for range taken {
+		}
+	}
+}
+
+func BenchmarkTakeWhileWithMap(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		mapped := MonadMap(seq, N.Mul(2))
+		taken := TakeWhile(func(x int) bool { return x < 12 })(mapped)
+		for range taken {
+		}
+	}
+}
+
+func BenchmarkTakeWhileWithFilter(b *testing.B) {
+	seq := From(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+	b.ResetTimer()
+	for range b.N {
+		filtered := MonadFilter(seq, func(x int) bool { return x%2 == 0 })
+		taken := TakeWhile(func(x int) bool { return x < 7 })(filtered)
+		for range taken {
+		}
+	}
+}
+
+// Example tests for documentation
+func ExampleTakeWhile() {
+	seq := From(1, 2, 3, 4, 5, 2, 1)
+	taken := TakeWhile(func(x int) bool { return x < 4 })(seq)
+
+	for v := range taken {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 1 2 3
+}
+
+func ExampleTakeWhile_allSatisfy() {
+	seq := From(2, 4, 6, 8)
+	taken := TakeWhile(func(x int) bool { return x%2 == 0 })(seq)
+
+	for v := range taken {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 2 4 6 8
+}
+
+func ExampleTakeWhile_firstFails() {
+	seq := From(5, 1, 2, 3)
+	taken := TakeWhile(func(x int) bool { return x < 5 })(seq)
+
+	count := 0
+	for range taken {
+		count++
+	}
+	fmt.Printf("Count: %d\n", count)
+	// Output: Count: 0
+}
+
+func ExampleTakeWhile_withMap() {
+	seq := From(1, 2, 3, 4, 5)
+	doubled := MonadMap(seq, N.Mul(2))
+	taken := TakeWhile(func(x int) bool { return x < 8 })(doubled)
+
+	for v := range taken {
+		fmt.Printf("%d ", v)
+	}
+	// Output: 2 4 6
+}
+
+func ExampleTakeWhile_strings() {
+	seq := From("a", "b", "c", "1", "d", "e")
+	isLetter := func(s string) bool { return s >= "a" && s <= "z" }
+	taken := TakeWhile(isLetter)(seq)
+
+	for v := range taken {
+		fmt.Printf("%s ", v)
+	}
+	// Output: a b c
+}

v2/iterator/iter/uniq.go

@@ -32,6 +32,13 @@ import (
 // the number of unique keys encountered. The operation is lazy - elements are processed
 // and filtered as they are consumed.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--2--4--1--5-->
+//	Uniq(identity)
+//	Output: --1--2--3-----4-----5-->
+//	        (first occurrence only)
+//
 // RxJS Equivalent: [distinct] - https://rxjs.dev/api/operators/distinct
 //
 // Type Parameters:
@@ -119,6 +126,13 @@ func Uniq[A any, K comparable](f func(A) K) Operator[A, A] {
 // The operation maintains a map of seen elements internally, so memory usage grows with
 // the number of unique elements. Only the first occurrence of each unique element is kept.
 //
+// Marble Diagram:
+//
+//	Input:  --1--2--3--2--4--1--5-->
+//	StrictUniq
+//	Output: --1--2--3-----4-----5-->
+//	        (first occurrence only)
+//
 // RxJS Equivalent: [distinct] - https://rxjs.dev/api/operators/distinct
 //
 // Type Parameters:

v2/number/utils.go

@@ -115,10 +115,7 @@ func Inc[T Number](value T) T {
 //	result := Min(5, 10) // returns 5
 //	result := Min(3.14, 2.71) // returns 2.71
 func Min[A C.Ordered](a, b A) A {
-	if a < b {
-		return a
-	}
-	return b
+	return min(a, b)
 }
 
 // Max returns the maximum of two ordered values.
@@ -132,10 +129,7 @@ func Min[A C.Ordered](a, b A) A {
 //	result := Max(5, 10) // returns 10
 //	result := Max(3.14, 2.71) // returns 3.14
 func Max[A C.Ordered](a, b A) A {
-	if a > b {
-		return a
-	}
-	return b
+	return max(a, b)
 }
 
 // MoreThan is a curried comparison function that checks if a value is more than (greater than) another.

v2/optics/codec/codecs.go

@@ -522,3 +522,199 @@ func MarshalJSON[T any](
 		},
 	)
 }
+
+// FromNonZero creates a bidirectional codec for non-zero values of comparable types.
+// This codec validates that values are not equal to their zero value (e.g., 0 for int,
+// "" for string, false for bool, nil for pointers).
+//
+// The codec uses a refinement (prism) that:
+//   - Decodes: Validates that the input is not the zero value of type T
+//   - Encodes: Returns the value unchanged (identity function)
+//   - Validates: Ensures the value is non-zero/non-default
+//
+// This is useful for enforcing that required fields have meaningful values rather than
+// their default zero values, which often represent "not set" or "missing" states.
+//
+// Type Parameters:
+//   - T: A comparable type (must support == and != operators)
+//
+// Returns:
+//   - A Type[T, T, T] codec that validates non-zero values
+//
+// Example:
+//
+//	// Create a codec for non-zero integers
+//	nonZeroInt := FromNonZero[int]()
+//
+//	// Decode non-zero value succeeds
+//	result := nonZeroInt.Decode(42)
+//	// result is Right(42)
+//
+//	// Decode zero value fails
+//	result := nonZeroInt.Decode(0)
+//	// result is Left(ValidationError{...})
+//
+//	// Encode is identity
+//	encoded := nonZeroInt.Encode(42)
+//	// encoded is 42
+//
+//	// Works with strings
+//	nonEmptyStr := FromNonZero[string]()
+//	result := nonEmptyStr.Decode("hello")  // Right("hello")
+//	result = nonEmptyStr.Decode("")        // Left(ValidationError{...})
+//
+//	// Works with pointers
+//	nonNilPtr := FromNonZero[*int]()
+//	value := 42
+//	result := nonNilPtr.Decode(&value)  // Right(&value)
+//	result = nonNilPtr.Decode(nil)      // Left(ValidationError{...})
+//
+// Common use cases:
+//   - Validating required numeric fields are not zero
+//   - Ensuring string fields are not empty
+//   - Checking pointers are not nil
+//   - Validating boolean flags are explicitly set to true
+//   - Composing with other codecs for multi-stage validation
+//
+// See Also:
+//   - NonEmptyString: Specialized version for strings with clearer intent
+//   - FromRefinement: General function for creating codecs from prisms
+func FromNonZero[T comparable]() Type[T, T, T] {
+	return FromRefinement(prism.FromNonZero[T]())
+}
+
+// NonEmptyString creates a bidirectional codec for non-empty strings.
+// This codec validates that string values are not empty, providing a type-safe
+// way to work with strings that must contain at least one character.
+//
+// This is a specialized version of FromNonZero[string]() that makes the intent
+// clearer when working specifically with strings that must not be empty.
+//
+// The codec:
+//   - Decodes: Validates that the input string is not empty ("")
+//   - Encodes: Returns the string unchanged (identity function)
+//   - Validates: Ensures the string has length > 0
+//
+// Note: This codec only checks for empty strings, not whitespace-only strings.
+// A string containing only spaces, tabs, or newlines will pass validation.
+//
+// Returns:
+//   - A Type[string, string, string] codec that validates non-empty strings
+//
+// Example:
+//
+//	nonEmpty := NonEmptyString()
+//
+//	// Decode non-empty string succeeds
+//	result := nonEmpty.Decode("hello")
+//	// result is Right("hello")
+//
+//	// Decode empty string fails
+//	result := nonEmpty.Decode("")
+//	// result is Left(ValidationError{...})
+//
+//	// Whitespace-only strings pass validation
+//	result := nonEmpty.Decode("   ")
+//	// result is Right("   ")
+//
+//	// Encode is identity
+//	encoded := nonEmpty.Encode("world")
+//	// encoded is "world"
+//
+//	// Compose with other codecs for validation pipelines
+//	intFromNonEmptyString := Pipe(IntFromString())(nonEmpty)
+//	result := intFromNonEmptyString.Decode("42")   // Right(42)
+//	result = intFromNonEmptyString.Decode("")      // Left(ValidationError{...})
+//	result = intFromNonEmptyString.Decode("abc")   // Left(ValidationError{...})
+//
+// Common use cases:
+//   - Validating required string fields (usernames, names, IDs)
+//   - Ensuring configuration values are provided
+//   - Validating user input before processing
+//   - Composing with parsing codecs to validate before parsing
+//   - Building validation pipelines for string data
+//
+// See Also:
+//   - FromNonZero: General version for any comparable type
+//   - String: Basic string codec without validation
+//   - IntFromString: Codec for parsing integers from strings
+func NonEmptyString() Type[string, string, string] {
+	return F.Pipe1(
+		FromRefinement(prism.NonEmptyString()),
+		WithName[string, string, string]("NonEmptyString"),
+	)
+}
+
+// WithName creates an endomorphism that renames a codec without changing its behavior.
+// This function returns a higher-order function that takes a codec and returns a new codec
+// with the specified name, while preserving all validation, encoding, and type-checking logic.
+//
+// This is useful for:
+//   - Providing more descriptive names for composed codecs
+//   - Creating domain-specific codec names for better error messages
+//   - Documenting the purpose of complex codec pipelines
+//   - Improving debugging and logging output
+//
+// The renamed codec maintains the same:
+//   - Type checking behavior (Is function)
+//   - Validation logic (Validate function)
+//   - Encoding behavior (Encode function)
+//
+// Only the name returned by the Name() method changes.
+//
+// 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)
+//
+// Parameters:
+//   - name: The new name for the codec
+//
+// Returns:
+//   - An Endomorphism[Type[A, O, I]] that renames the codec
+//
+// Example:
+//
+//	// Create a codec with a generic name
+//	positiveInt := Pipe[int, int, string, int](
+//	    FromRefinement(prism.FromPredicate(func(n int) bool { return n > 0 })),
+//	)(IntFromString())
+//	// positiveInt.Name() returns something like "Pipe(FromRefinement(...), IntFromString)"
+//
+//	// Rename it for clarity
+//	namedCodec := WithName[int, string, string]("PositiveIntFromString")(positiveInt)
+//	// namedCodec.Name() returns "PositiveIntFromString"
+//
+//	// Use in a pipeline with F.Pipe
+//	userAgeCodec := F.Pipe1(
+//	    IntFromString(),
+//	    WithName[int, string, string]("UserAge"),
+//	)
+//
+//	// Validation errors will show the custom name
+//	result := userAgeCodec.Decode("invalid")
+//	// Error context will reference "UserAge" instead of "IntFromString"
+//
+// Common use cases:
+//   - Naming composed codecs for better error messages
+//   - Creating domain-specific codec names (e.g., "EmailAddress", "PhoneNumber")
+//   - Documenting complex validation pipelines
+//   - Improving debugging output in logs
+//   - Making codec composition more readable
+//
+// Note: This function creates a new codec instance with the same behavior but a different
+// name. The original codec is not modified.
+//
+// See Also:
+//   - MakeType: For creating codecs with custom names from scratch
+//   - Pipe: For composing codecs (which generates automatic names)
+func WithName[A, O, I any](name string) Endomorphism[Type[A, O, I]] {
+	return func(codec Type[A, O, I]) Type[A, O, I] {
+		return MakeType(
+			name,
+			codec.Is,
+			codec.Validate,
+			codec.Encode,
+		)
+	}
+}

v2/optics/codec/codecs_test.go

@@ -23,6 +23,7 @@ import (
 	"time"
 
 	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
 	"github.com/IBM/fp-go/v2/optics/prism"
 	"github.com/IBM/fp-go/v2/option"
@@ -688,6 +689,596 @@ func TestBoolFromString_Integration(t *testing.T) {
 	})
 }
 
+// ---------------------------------------------------------------------------
+// FromNonZero
+// ---------------------------------------------------------------------------
+
+func TestFromNonZero_Decode_Success(t *testing.T) {
+	t.Run("int - decodes non-zero value", func(t *testing.T) {
+		c := FromNonZero[int]()
+		result := c.Decode(42)
+		assert.Equal(t, validation.Success(42), result)
+	})
+
+	t.Run("int - decodes negative value", func(t *testing.T) {
+		c := FromNonZero[int]()
+		result := c.Decode(-5)
+		assert.Equal(t, validation.Success(-5), result)
+	})
+
+	t.Run("string - decodes non-empty string", func(t *testing.T) {
+		c := FromNonZero[string]()
+		result := c.Decode("hello")
+		assert.Equal(t, validation.Success("hello"), result)
+	})
+
+	t.Run("string - decodes whitespace string", func(t *testing.T) {
+		c := FromNonZero[string]()
+		result := c.Decode("   ")
+		assert.Equal(t, validation.Success("   "), result)
+	})
+
+	t.Run("bool - decodes true", func(t *testing.T) {
+		c := FromNonZero[bool]()
+		result := c.Decode(true)
+		assert.Equal(t, validation.Success(true), result)
+	})
+
+	t.Run("float64 - decodes non-zero value", func(t *testing.T) {
+		c := FromNonZero[float64]()
+		result := c.Decode(3.14)
+		assert.Equal(t, validation.Success(3.14), result)
+	})
+
+	t.Run("float64 - decodes negative value", func(t *testing.T) {
+		c := FromNonZero[float64]()
+		result := c.Decode(-2.5)
+		assert.Equal(t, validation.Success(-2.5), result)
+	})
+
+	t.Run("pointer - decodes non-nil pointer", func(t *testing.T) {
+		c := FromNonZero[*int]()
+		value := 42
+		result := c.Decode(&value)
+		assert.True(t, either.IsRight(result))
+		ptr := either.MonadFold(result, func(validation.Errors) *int { return nil }, func(p *int) *int { return p })
+		require.NotNil(t, ptr)
+		assert.Equal(t, 42, *ptr)
+	})
+}
+
+func TestFromNonZero_Decode_Failure(t *testing.T) {
+	t.Run("int - fails on zero", func(t *testing.T) {
+		c := FromNonZero[int]()
+		result := c.Decode(0)
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("string - fails on empty string", func(t *testing.T) {
+		c := FromNonZero[string]()
+		result := c.Decode("")
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("bool - fails on false", func(t *testing.T) {
+		c := FromNonZero[bool]()
+		result := c.Decode(false)
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("float64 - fails on zero", func(t *testing.T) {
+		c := FromNonZero[float64]()
+		result := c.Decode(0.0)
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("pointer - fails on nil", func(t *testing.T) {
+		c := FromNonZero[*int]()
+		result := c.Decode(nil)
+		assert.True(t, either.IsLeft(result))
+	})
+}
+
+func TestFromNonZero_Encode(t *testing.T) {
+	t.Run("int - encodes value unchanged", func(t *testing.T) {
+		c := FromNonZero[int]()
+		assert.Equal(t, 42, c.Encode(42))
+	})
+
+	t.Run("string - encodes value unchanged", func(t *testing.T) {
+		c := FromNonZero[string]()
+		assert.Equal(t, "hello", c.Encode("hello"))
+	})
+
+	t.Run("bool - encodes value unchanged", func(t *testing.T) {
+		c := FromNonZero[bool]()
+		assert.Equal(t, true, c.Encode(true))
+	})
+
+	t.Run("float64 - encodes value unchanged", func(t *testing.T) {
+		c := FromNonZero[float64]()
+		assert.Equal(t, 3.14, c.Encode(3.14))
+	})
+
+	t.Run("pointer - encodes value unchanged", func(t *testing.T) {
+		c := FromNonZero[*int]()
+		value := 42
+		ptr := &value
+		assert.Equal(t, ptr, c.Encode(ptr))
+	})
+
+	t.Run("round-trip: decode then encode", func(t *testing.T) {
+		c := FromNonZero[int]()
+		original := 42
+		result := c.Decode(original)
+		require.True(t, either.IsRight(result))
+		decoded := either.MonadFold(result, func(validation.Errors) int { return 0 }, func(n int) int { return n })
+		assert.Equal(t, original, c.Encode(decoded))
+	})
+}
+
+func TestFromNonZero_Name(t *testing.T) {
+	t.Run("int codec name", func(t *testing.T) {
+		c := FromNonZero[int]()
+		assert.Contains(t, c.Name(), "FromRefinement")
+		assert.Contains(t, c.Name(), "PrismFromNonZero")
+	})
+
+	t.Run("string codec name", func(t *testing.T) {
+		c := FromNonZero[string]()
+		assert.Contains(t, c.Name(), "FromRefinement")
+		assert.Contains(t, c.Name(), "PrismFromNonZero")
+	})
+}
+
+func TestFromNonZero_Integration(t *testing.T) {
+	t.Run("validates multiple non-zero integers", func(t *testing.T) {
+		c := FromNonZero[int]()
+		values := []int{1, -1, 42, -100, 999}
+		for _, v := range values {
+			result := c.Decode(v)
+			require.True(t, either.IsRight(result), "expected success for %d", v)
+			decoded := either.MonadFold(result, func(validation.Errors) int { return 0 }, func(n int) int { return n })
+			assert.Equal(t, v, decoded)
+			assert.Equal(t, v, c.Encode(decoded))
+		}
+	})
+
+	t.Run("rejects zero values", func(t *testing.T) {
+		c := FromNonZero[int]()
+		result := c.Decode(0)
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("works with custom comparable types", func(t *testing.T) {
+		type UserID string
+		c := FromNonZero[UserID]()
+
+		result := c.Decode(UserID("user123"))
+		assert.Equal(t, validation.Success(UserID("user123")), result)
+
+		result = c.Decode(UserID(""))
+		assert.True(t, either.IsLeft(result))
+	})
+}
+
+// ---------------------------------------------------------------------------
+// NonEmptyString
+// ---------------------------------------------------------------------------
+
+func TestNonEmptyString_Decode_Success(t *testing.T) {
+	t.Run("decodes non-empty string", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("hello")
+		assert.Equal(t, validation.Success("hello"), result)
+	})
+
+	t.Run("decodes single character", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("a")
+		assert.Equal(t, validation.Success("a"), result)
+	})
+
+	t.Run("decodes whitespace string", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("   ")
+		assert.Equal(t, validation.Success("   "), result)
+	})
+
+	t.Run("decodes string with newlines", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("\n\t")
+		assert.Equal(t, validation.Success("\n\t"), result)
+	})
+
+	t.Run("decodes unicode string", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("你好")
+		assert.Equal(t, validation.Success("你好"), result)
+	})
+
+	t.Run("decodes emoji string", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("🎉")
+		assert.Equal(t, validation.Success("🎉"), result)
+	})
+
+	t.Run("decodes multiline string", func(t *testing.T) {
+		c := NonEmptyString()
+		multiline := "line1\nline2\nline3"
+		result := c.Decode(multiline)
+		assert.Equal(t, validation.Success(multiline), result)
+	})
+}
+
+func TestNonEmptyString_Decode_Failure(t *testing.T) {
+	t.Run("fails on empty string", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("")
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("error contains context", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("")
+		require.True(t, either.IsLeft(result))
+		errors := either.MonadFold(result, func(e validation.Errors) validation.Errors { return e }, func(string) validation.Errors { return nil })
+		require.NotEmpty(t, errors)
+	})
+}
+
+func TestNonEmptyString_Encode(t *testing.T) {
+	t.Run("encodes string unchanged", func(t *testing.T) {
+		c := NonEmptyString()
+		assert.Equal(t, "hello", c.Encode("hello"))
+	})
+
+	t.Run("encodes unicode string unchanged", func(t *testing.T) {
+		c := NonEmptyString()
+		assert.Equal(t, "你好", c.Encode("你好"))
+	})
+
+	t.Run("encodes whitespace string unchanged", func(t *testing.T) {
+		c := NonEmptyString()
+		assert.Equal(t, "   ", c.Encode("   "))
+	})
+
+	t.Run("round-trip: decode then encode", func(t *testing.T) {
+		c := NonEmptyString()
+		original := "test string"
+		result := c.Decode(original)
+		require.True(t, either.IsRight(result))
+		decoded := either.MonadFold(result, func(validation.Errors) string { return "" }, func(s string) string { return s })
+		assert.Equal(t, original, c.Encode(decoded))
+	})
+}
+
+func TestNonEmptyString_Name(t *testing.T) {
+	c := NonEmptyString()
+	assert.Equal(t, c.Name(), "NonEmptyString")
+}
+
+func TestNonEmptyString_Integration(t *testing.T) {
+	t.Run("validates multiple non-empty strings", func(t *testing.T) {
+		c := NonEmptyString()
+		strings := []string{"a", "hello", "world", "test123", "  spaces  ", "🎉"}
+		for _, s := range strings {
+			result := c.Decode(s)
+			require.True(t, either.IsRight(result), "expected success for %q", s)
+			decoded := either.MonadFold(result, func(validation.Errors) string { return "" }, func(str string) string { return str })
+			assert.Equal(t, s, decoded)
+			assert.Equal(t, s, c.Encode(decoded))
+		}
+	})
+
+	t.Run("rejects empty string", func(t *testing.T) {
+		c := NonEmptyString()
+		result := c.Decode("")
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("compose with IntFromString", func(t *testing.T) {
+		// Create a codec that only parses non-empty strings to integers
+		nonEmptyThenInt := Pipe[string, string](IntFromString())(NonEmptyString())
+
+		// Valid non-empty string with integer
+		result := nonEmptyThenInt.Decode("42")
+		assert.Equal(t, validation.Success(42), result)
+
+		// Empty string fails at NonEmptyString stage
+		result = nonEmptyThenInt.Decode("")
+		assert.True(t, either.IsLeft(result))
+
+		// Non-empty but invalid integer fails at IntFromString stage
+		result = nonEmptyThenInt.Decode("abc")
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("use in validation pipeline", func(t *testing.T) {
+		c := NonEmptyString()
+
+		// Simulate validating user input
+		inputs := []struct {
+			value    string
+			expected bool
+		}{
+			{"john_doe", true},
+			{"", false},
+			{"a", true},
+			{"user@example.com", true},
+		}
+
+		for _, input := range inputs {
+			result := c.Decode(input.value)
+			if input.expected {
+				assert.True(t, either.IsRight(result), "expected success for %q", input.value)
+			} else {
+				assert.True(t, either.IsLeft(result), "expected failure for %q", input.value)
+			}
+		}
+	})
+}
+
+// ---------------------------------------------------------------------------
+// WithName
+// ---------------------------------------------------------------------------
+
+func TestWithName_BasicFunctionality(t *testing.T) {
+	t.Run("renames codec without changing behavior", func(t *testing.T) {
+		original := IntFromString()
+		renamed := WithName[int, string, string]("CustomIntCodec")(original)
+
+		// Name should be changed
+		assert.Equal(t, "CustomIntCodec", renamed.Name())
+		assert.NotEqual(t, original.Name(), renamed.Name())
+
+		// Behavior should be unchanged
+		result := renamed.Decode("42")
+		assert.Equal(t, validation.Success(42), result)
+
+		encoded := renamed.Encode(42)
+		assert.Equal(t, "42", encoded)
+	})
+
+	t.Run("preserves validation logic", func(t *testing.T) {
+		original := IntFromString()
+		renamed := WithName[int, string, string]("MyInt")(original)
+
+		// Valid input should succeed
+		result := renamed.Decode("123")
+		assert.True(t, either.IsRight(result))
+
+		// Invalid input should fail
+		result = renamed.Decode("not a number")
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("preserves encoding logic", func(t *testing.T) {
+		original := BoolFromString()
+		renamed := WithName[bool, string, string]("CustomBool")(original)
+
+		assert.Equal(t, "true", renamed.Encode(true))
+		assert.Equal(t, "false", renamed.Encode(false))
+	})
+}
+
+func TestWithName_WithComposedCodecs(t *testing.T) {
+	t.Run("renames composed codec", func(t *testing.T) {
+		// Create a composed codec
+		composed := Pipe[string, string](IntFromString())(NonEmptyString())
+
+		// Rename it
+		renamed := WithName[int, string, string]("NonEmptyIntString")(composed)
+
+		assert.Equal(t, "NonEmptyIntString", renamed.Name())
+
+		// Behavior should be preserved
+		result := renamed.Decode("42")
+		assert.Equal(t, validation.Success(42), result)
+
+		// Empty string should fail
+		result = renamed.Decode("")
+		assert.True(t, either.IsLeft(result))
+
+		// Non-numeric should fail
+		result = renamed.Decode("abc")
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("works in pipeline with F.Pipe", func(t *testing.T) {
+		codec := F.Pipe1(
+			IntFromString(),
+			WithName[int, string, string]("UserAge"),
+		)
+
+		assert.Equal(t, "UserAge", codec.Name())
+
+		result := codec.Decode("25")
+		assert.Equal(t, validation.Success(25), result)
+	})
+}
+
+func TestWithName_PreservesTypeChecking(t *testing.T) {
+	t.Run("preserves Is function", func(t *testing.T) {
+		original := String()
+		renamed := WithName[string, string, any]("CustomString")(original)
+
+		// Should accept string
+		result := renamed.Is("hello")
+		assert.True(t, either.IsRight(result))
+
+		// Should reject non-string
+		result = renamed.Is(42)
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("preserves complex type checking", func(t *testing.T) {
+		original := Array(Int())
+		renamed := WithName[[]int, []int, any]("IntArray")(original)
+
+		// Should accept []int
+		result := renamed.Is([]int{1, 2, 3})
+		assert.True(t, either.IsRight(result))
+
+		// Should reject []string
+		result = renamed.Is([]string{"a", "b"})
+		assert.True(t, either.IsLeft(result))
+	})
+}
+
+func TestWithName_RoundTrip(t *testing.T) {
+	t.Run("maintains round-trip property", func(t *testing.T) {
+		original := Int64FromString()
+		renamed := WithName[int64, string, string]("CustomInt64")(original)
+
+		testValues := []string{"0", "42", "-100", "9223372036854775807"}
+		for _, input := range testValues {
+			result := renamed.Decode(input)
+			require.True(t, either.IsRight(result), "expected success for %s", input)
+
+			decoded := either.MonadFold(result, func(validation.Errors) int64 { return 0 }, func(n int64) int64 { return n })
+			encoded := renamed.Encode(decoded)
+			assert.Equal(t, input, encoded)
+		}
+	})
+}
+
+func TestWithName_ErrorMessages(t *testing.T) {
+	t.Run("custom name appears in validation context", func(t *testing.T) {
+		codec := WithName[int, string, string]("PositiveInteger")(IntFromString())
+
+		result := codec.Decode("not a number")
+		require.True(t, either.IsLeft(result))
+
+		// The error context should reference the custom name
+		errors := either.MonadFold(result, func(e validation.Errors) validation.Errors { return e }, func(int) validation.Errors { return nil })
+		require.NotEmpty(t, errors)
+
+		// Check that at least one error references our custom name
+		found := false
+		for _, err := range errors {
+			if len(err.Context) > 0 {
+				for _, ctx := range err.Context {
+					if ctx.Type == "PositiveInteger" {
+						found = true
+						break
+					}
+				}
+			}
+		}
+		assert.True(t, found, "expected custom name 'PositiveInteger' in error context")
+	})
+}
+
+func TestWithName_MultipleRenames(t *testing.T) {
+	t.Run("can rename multiple times", func(t *testing.T) {
+		codec := IntFromString()
+
+		renamed1 := WithName[int, string, string]("FirstName")(codec)
+		assert.Equal(t, "FirstName", renamed1.Name())
+
+		renamed2 := WithName[int, string, string]("SecondName")(renamed1)
+		assert.Equal(t, "SecondName", renamed2.Name())
+
+		// Behavior should still work
+		result := renamed2.Decode("42")
+		assert.Equal(t, validation.Success(42), result)
+	})
+}
+
+func TestWithName_WithDifferentTypes(t *testing.T) {
+	t.Run("works with string codec", func(t *testing.T) {
+		codec := WithName[string, string, string]("Username")(NonEmptyString())
+		assert.Equal(t, "Username", codec.Name())
+
+		result := codec.Decode("john_doe")
+		assert.Equal(t, validation.Success("john_doe"), result)
+	})
+
+	t.Run("works with bool codec", func(t *testing.T) {
+		codec := WithName[bool, string, string]("IsActive")(BoolFromString())
+		assert.Equal(t, "IsActive", codec.Name())
+
+		result := codec.Decode("true")
+		assert.Equal(t, validation.Success(true), result)
+	})
+
+	t.Run("works with URL codec", func(t *testing.T) {
+		codec := WithName[*url.URL, string, string]("WebsiteURL")(URL())
+		assert.Equal(t, "WebsiteURL", codec.Name())
+
+		result := codec.Decode("https://example.com")
+		assert.True(t, either.IsRight(result))
+	})
+
+	t.Run("works with array codec", func(t *testing.T) {
+		codec := WithName[[]int, []int, any]("Numbers")(Array(Int()))
+		assert.Equal(t, "Numbers", codec.Name())
+
+		result := codec.Decode([]int{1, 2, 3})
+		assert.Equal(t, validation.Success([]int{1, 2, 3}), result)
+	})
+}
+
+func TestWithName_AsDecoderEncoder(t *testing.T) {
+	t.Run("AsDecoder returns decoder interface", func(t *testing.T) {
+		codec := WithName[int, string, string]("MyInt")(IntFromString())
+		decoder := codec.AsDecoder()
+
+		result := decoder.Decode("42")
+		assert.Equal(t, validation.Success(42), result)
+	})
+
+	t.Run("AsEncoder returns encoder interface", func(t *testing.T) {
+		codec := WithName[int, string, string]("MyInt")(IntFromString())
+		encoder := codec.AsEncoder()
+
+		encoded := encoder.Encode(42)
+		assert.Equal(t, "42", encoded)
+	})
+}
+
+func TestWithName_Integration(t *testing.T) {
+	t.Run("domain-specific codec names", func(t *testing.T) {
+		// Create domain-specific codecs with meaningful names
+		emailCodec := WithName[string, string, string]("EmailAddress")(NonEmptyString())
+		phoneCodec := WithName[string, string, string]("PhoneNumber")(NonEmptyString())
+		ageCodec := WithName[int, string, string]("Age")(IntFromString())
+
+		// Test email
+		result := emailCodec.Decode("user@example.com")
+		assert.True(t, either.IsRight(result))
+		assert.Equal(t, "EmailAddress", emailCodec.Name())
+
+		// Test phone
+		result = phoneCodec.Decode("+1234567890")
+		assert.True(t, either.IsRight(result))
+		assert.Equal(t, "PhoneNumber", phoneCodec.Name())
+
+		// Test age
+		ageResult := ageCodec.Decode("25")
+		assert.True(t, either.IsRight(ageResult))
+		assert.Equal(t, "Age", ageCodec.Name())
+	})
+
+	t.Run("naming complex validation pipelines", func(t *testing.T) {
+		// Create a complex codec and give it a clear name
+		positiveIntCodec := F.Pipe2(
+			NonEmptyString(),
+			Pipe[string, string](IntFromString()),
+			WithName[int, string, string]("PositiveIntegerFromString"),
+		)
+
+		assert.Equal(t, "PositiveIntegerFromString", positiveIntCodec.Name())
+
+		result := positiveIntCodec.Decode("42")
+		assert.True(t, either.IsRight(result))
+
+		result = positiveIntCodec.Decode("")
+		assert.True(t, either.IsLeft(result))
+	})
+}
+
 // ---------------------------------------------------------------------------
 // MarshalJSON
 // ---------------------------------------------------------------------------
@@ -773,7 +1364,7 @@ func TestIntFromString_PipeComposition(t *testing.T) {
 		func(n int) int { return n },
 		"PositiveInt",
 	)
-	positiveIntCodec := Pipe[string, string, int, int](
+	positiveIntCodec := Pipe[string, string](
 		FromRefinement(positiveIntPrism),
 	)(IntFromString())
 

v2/optics/codec/iso.go

@@ -0,0 +1,169 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/decode"
+)
+
+// FromIso creates a Type codec from an Iso (isomorphism).
+//
+// An isomorphism represents a bidirectional transformation between types I and A
+// without any loss of information. This function converts an Iso[I, A] into a
+// Type[A, I, I] codec that can validate, decode, and encode values using the
+// isomorphism's transformations.
+//
+// The resulting codec:
+//   - Decode: Uses iso.Get to transform I → A, always succeeds (no validation)
+//   - Encode: Uses iso.ReverseGet to transform A → I
+//   - Validation: Always succeeds since isomorphisms are lossless transformations
+//   - Type checking: Uses standard type checking for type A
+//
+// This is particularly useful for:
+//   - Creating codecs for newtype patterns (wrapping/unwrapping types)
+//   - Building codecs for types with lossless conversions
+//   - Composing with other codecs using Pipe or other operators
+//   - Implementing bidirectional transformations in codec pipelines
+//
+// # Type Parameters
+//
+//   - A: The target type (what we decode to and encode from)
+//   - I: The input/output type (what we decode from and encode to)
+//
+// # Parameters
+//
+//   - iso: An Iso[I, A] that defines the bidirectional transformation:
+//   - Get: I → A (converts input to target type)
+//   - ReverseGet: A → I (converts target back to input type)
+//
+// # Returns
+//
+//   - A Type[A, I, I] codec where:
+//   - Decode: I → Validation[A] - transforms using iso.Get, always succeeds
+//   - Encode: A → I - transforms using iso.ReverseGet
+//   - Is: Checks if a value is of type A
+//   - Name: Returns "FromIso[iso_string_representation]"
+//
+// # Behavior
+//
+// Decoding:
+//   - Applies iso.Get to transform the input value
+//   - Wraps the result in decode.Of (always successful validation)
+//   - No validation errors can occur since isomorphisms are lossless
+//
+// Encoding:
+//   - Applies iso.ReverseGet to transform back to the input type
+//   - Always succeeds as isomorphisms guarantee reversibility
+//
+// # Example Usage
+//
+// Creating a codec for a newtype pattern:
+//
+//	type UserId int
+//
+//	// Define an isomorphism between int and UserId
+//	userIdIso := iso.MakeIso(
+//	    func(id UserId) int { return int(id) },
+//	    func(i int) UserId { return UserId(i) },
+//	)
+//
+//	// Create a codec from the isomorphism
+//	userIdCodec := codec.FromIso[int, UserId](userIdIso)
+//
+//	// Decode: UserId → int
+//	result := userIdCodec.Decode(UserId(42))  // Success: Right(42)
+//
+//	// Encode: int → UserId
+//	encoded := userIdCodec.Encode(42)         // Returns: UserId(42)
+//
+// Using with temperature conversions:
+//
+//	type Celsius float64
+//	type Fahrenheit float64
+//
+//	celsiusToFahrenheit := iso.MakeIso(
+//	    func(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) },
+//	    func(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) },
+//	)
+//
+//	tempCodec := codec.FromIso[Fahrenheit, Celsius](celsiusToFahrenheit)
+//
+//	// Decode: Celsius → Fahrenheit
+//	result := tempCodec.Decode(Celsius(20))   // Success: Right(68°F)
+//
+//	// Encode: Fahrenheit → Celsius
+//	encoded := tempCodec.Encode(Fahrenheit(68)) // Returns: 20°C
+//
+// Composing with other codecs:
+//
+//	type Email string
+//	type ValidatedEmail struct{ value Email }
+//
+//	emailIso := iso.MakeIso(
+//	    func(ve ValidatedEmail) Email { return ve.value },
+//	    func(e Email) ValidatedEmail { return ValidatedEmail{value: e} },
+//	)
+//
+//	// Compose with string codec for validation
+//	emailCodec := F.Pipe2(
+//	    codec.String(),                           // Type[string, string, any]
+//	    codec.Pipe(codec.FromIso[Email, string](  // Add string → Email iso
+//	        iso.MakeIso(
+//	            func(s string) Email { return Email(s) },
+//	            func(e Email) string { return string(e) },
+//	        ),
+//	    )),
+//	    codec.Pipe(codec.FromIso[ValidatedEmail, Email](emailIso)), // Add Email → ValidatedEmail iso
+//	)
+//
+// # Use Cases
+//
+//   - Newtype patterns: Wrapping primitive types for type safety
+//   - Unit conversions: Temperature, distance, time, etc.
+//   - Format transformations: Between equivalent representations
+//   - Type aliasing: Creating semantic types from base types
+//   - Codec composition: Building complex codecs from simple isomorphisms
+//
+// # Notes
+//
+//   - Isomorphisms must satisfy the round-trip laws:
+//   - iso.ReverseGet(iso.Get(i)) == i
+//   - iso.Get(iso.ReverseGet(a)) == a
+//   - Validation always succeeds since isomorphisms are lossless
+//   - The codec name includes the isomorphism's string representation
+//   - Type checking is performed using the standard Is[A]() function
+//   - This codec is ideal for lossless transformations without validation logic
+//
+// # See Also
+//
+//   - iso.Iso: The isomorphism type used by this function
+//   - iso.MakeIso: Constructor for creating isomorphisms
+//   - Pipe: For composing this codec with other codecs
+//   - MakeType: For creating codecs with custom validation logic
+func FromIso[A, I any](iso Iso[I, A]) Type[A, I, I] {
+	return MakeType(
+		fmt.Sprintf("FromIso[%s]", iso),
+		Is[A](),
+		F.Flow2(
+			iso.Get,
+			decode.Of[Context],
+		),
+		iso.ReverseGet,
+	)
+}

v2/optics/codec/iso_test.go

@@ -0,0 +1,504 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	"github.com/IBM/fp-go/v2/optics/iso"
+	"github.com/stretchr/testify/assert"
+)
+
+// Test types for newtype pattern
+type UserId int
+type Email string
+type Celsius float64
+type Fahrenheit float64
+
+func TestFromIso_Success(t *testing.T) {
+	t.Run("decodes using iso.Get", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		result := codec.Decode(UserId(42))
+
+		// Assert
+		assert.Equal(t, validation.Success(42), result)
+	})
+
+	t.Run("encodes using iso.ReverseGet", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		encoded := codec.Encode(42)
+
+		// Assert
+		assert.Equal(t, UserId(42), encoded)
+	})
+
+	t.Run("round-trip preserves value", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+		original := UserId(123)
+
+		// Act
+		decoded := codec.Decode(original)
+
+		// Assert
+		assert.True(t, either.IsRight(decoded))
+		roundTrip := either.Fold[validation.Errors, int, UserId](
+			func(validation.Errors) UserId { return UserId(0) },
+			codec.Encode,
+		)(decoded)
+		assert.Equal(t, original, roundTrip)
+	})
+}
+
+func TestFromIso_StringTypes(t *testing.T) {
+	t.Run("handles string newtype", func(t *testing.T) {
+		// Arrange
+		emailIso := iso.MakeIso(
+			func(e Email) string { return string(e) },
+			func(s string) Email { return Email(s) },
+		)
+		codec := FromIso[string, Email](emailIso)
+
+		// Act
+		result := codec.Decode(Email("user@example.com"))
+
+		// Assert
+		assert.Equal(t, validation.Success("user@example.com"), result)
+	})
+
+	t.Run("encodes string newtype", func(t *testing.T) {
+		// Arrange
+		emailIso := iso.MakeIso(
+			func(e Email) string { return string(e) },
+			func(s string) Email { return Email(s) },
+		)
+		codec := FromIso[string, Email](emailIso)
+
+		// Act
+		encoded := codec.Encode("admin@example.com")
+
+		// Assert
+		assert.Equal(t, Email("admin@example.com"), encoded)
+	})
+
+	t.Run("handles empty string", func(t *testing.T) {
+		// Arrange
+		emailIso := iso.MakeIso(
+			func(e Email) string { return string(e) },
+			func(s string) Email { return Email(s) },
+		)
+		codec := FromIso[string, Email](emailIso)
+
+		// Act
+		result := codec.Decode(Email(""))
+
+		// Assert
+		assert.Equal(t, validation.Success(""), result)
+	})
+}
+
+func TestFromIso_NumericConversions(t *testing.T) {
+	t.Run("converts Celsius to Fahrenheit", func(t *testing.T) {
+		// Arrange
+		tempIso := iso.MakeIso(
+			func(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) },
+			func(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) },
+		)
+		codec := FromIso[Fahrenheit, Celsius](tempIso)
+
+		// Act
+		result := codec.Decode(Celsius(0))
+
+		// Assert
+		assert.Equal(t, validation.Success(Fahrenheit(32)), result)
+	})
+
+	t.Run("converts Fahrenheit to Celsius", func(t *testing.T) {
+		// Arrange
+		tempIso := iso.MakeIso(
+			func(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) },
+			func(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) },
+		)
+		codec := FromIso[Fahrenheit, Celsius](tempIso)
+
+		// Act
+		encoded := codec.Encode(Fahrenheit(68))
+
+		// Assert
+		assert.Equal(t, Celsius(20), encoded)
+	})
+
+	t.Run("handles negative temperatures", func(t *testing.T) {
+		// Arrange
+		tempIso := iso.MakeIso(
+			func(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) },
+			func(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) },
+		)
+		codec := FromIso[Fahrenheit, Celsius](tempIso)
+
+		// Act
+		result := codec.Decode(Celsius(-40))
+
+		// Assert
+		assert.Equal(t, validation.Success(Fahrenheit(-40)), result)
+	})
+
+	t.Run("temperature round-trip", func(t *testing.T) {
+		// Arrange
+		tempIso := iso.MakeIso(
+			func(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) },
+			func(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) },
+		)
+		codec := FromIso[Fahrenheit, Celsius](tempIso)
+		original := Celsius(25)
+
+		// Act
+		decoded := codec.Decode(original)
+
+		// Assert
+		assert.True(t, either.IsRight(decoded))
+		roundTrip := either.Fold[validation.Errors, Fahrenheit, Celsius](
+			func(validation.Errors) Celsius { return Celsius(0) },
+			codec.Encode,
+		)(decoded)
+		// Allow small floating point error
+		diff := float64(original - roundTrip)
+		if diff < 0 {
+			diff = -diff
+		}
+		assert.True(t, diff < 0.0001)
+	})
+}
+
+func TestFromIso_EdgeCases(t *testing.T) {
+	t.Run("handles zero values", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		result := codec.Decode(UserId(0))
+
+		// Assert
+		assert.Equal(t, validation.Success(0), result)
+	})
+
+	t.Run("handles negative values", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		result := codec.Decode(UserId(-1))
+
+		// Assert
+		assert.Equal(t, validation.Success(-1), result)
+	})
+
+	t.Run("handles large values", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		result := codec.Decode(UserId(999999999))
+
+		// Assert
+		assert.Equal(t, validation.Success(999999999), result)
+	})
+}
+
+func TestFromIso_TypeChecking(t *testing.T) {
+	t.Run("Is checks target type", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		isResult := codec.Is(42)
+
+		// Assert
+		assert.True(t, either.IsRight(isResult))
+	})
+
+	t.Run("Is rejects wrong type", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		isResult := codec.Is("not an int")
+
+		// Assert
+		assert.True(t, either.IsLeft(isResult))
+	})
+}
+
+func TestFromIso_Name(t *testing.T) {
+	t.Run("includes iso in name", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		name := codec.Name()
+
+		// Assert
+		assert.True(t, len(name) > 0)
+		assert.True(t, name[:7] == "FromIso")
+	})
+}
+
+func TestFromIso_Composition(t *testing.T) {
+	t.Run("composes with Pipe", func(t *testing.T) {
+		// Arrange
+		type PositiveInt int
+
+		// First iso: UserId -> int
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+
+		// Second iso: int -> PositiveInt (no validation, just type conversion)
+		positiveIso := iso.MakeIso(
+			func(i int) PositiveInt { return PositiveInt(i) },
+			func(p PositiveInt) int { return int(p) },
+		)
+
+		// Compose codecs
+		codec := F.Pipe1(
+			FromIso[int, UserId](userIdIso),
+			Pipe[UserId, UserId](FromIso[PositiveInt, int](positiveIso)),
+		)
+
+		// Act
+		result := codec.Decode(UserId(42))
+
+		// Assert
+		assert.Equal(t, validation.Of(PositiveInt(42)), result)
+	})
+
+	t.Run("composed codec encodes correctly", func(t *testing.T) {
+		// Arrange
+		type PositiveInt int
+
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+
+		positiveIso := iso.MakeIso(
+			func(i int) PositiveInt { return PositiveInt(i) },
+			func(p PositiveInt) int { return int(p) },
+		)
+
+		codec := F.Pipe1(
+			FromIso[int, UserId](userIdIso),
+			Pipe[UserId, UserId](FromIso[PositiveInt, int](positiveIso)),
+		)
+
+		// Act
+		encoded := codec.Encode(PositiveInt(42))
+
+		// Assert
+		assert.Equal(t, UserId(42), encoded)
+	})
+}
+
+func TestFromIso_Integration(t *testing.T) {
+	t.Run("works with Array codec", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		userIdCodec := FromIso[int, UserId](userIdIso)
+		arrayCodec := TranscodeArray(userIdCodec)
+
+		// Act
+		result := arrayCodec.Decode([]UserId{UserId(1), UserId(2), UserId(3)})
+
+		// Assert
+		assert.Equal(t, validation.Success([]int{1, 2, 3}), result)
+	})
+
+	t.Run("encodes array correctly", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		userIdCodec := FromIso[int, UserId](userIdIso)
+		arrayCodec := TranscodeArray(userIdCodec)
+
+		// Act
+		encoded := arrayCodec.Encode([]int{1, 2, 3})
+
+		// Assert
+		assert.Equal(t, []UserId{UserId(1), UserId(2), UserId(3)}, encoded)
+	})
+
+	t.Run("handles empty array", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		userIdCodec := FromIso[int, UserId](userIdIso)
+		arrayCodec := TranscodeArray(userIdCodec)
+
+		// Act
+		result := arrayCodec.Decode([]UserId{})
+
+		// Assert
+		assert.True(t, either.IsRight(result))
+		decoded := either.Fold[validation.Errors, []int, []int](
+			func(validation.Errors) []int { return nil },
+			func(arr []int) []int { return arr },
+		)(result)
+		assert.Equal(t, 0, len(decoded))
+	})
+}
+
+func TestFromIso_ComplexTypes(t *testing.T) {
+	t.Run("handles struct wrapping", func(t *testing.T) {
+		// Arrange
+		type Wrapper struct{ Value int }
+
+		wrapperIso := iso.MakeIso(
+			func(w Wrapper) int { return w.Value },
+			func(i int) Wrapper { return Wrapper{Value: i} },
+		)
+		codec := FromIso[int, Wrapper](wrapperIso)
+
+		// Act
+		result := codec.Decode(Wrapper{Value: 42})
+
+		// Assert
+		assert.Equal(t, validation.Success(42), result)
+	})
+
+	t.Run("encodes struct wrapping", func(t *testing.T) {
+		// Arrange
+		type Wrapper struct{ Value int }
+
+		wrapperIso := iso.MakeIso(
+			func(w Wrapper) int { return w.Value },
+			func(i int) Wrapper { return Wrapper{Value: i} },
+		)
+		codec := FromIso[int, Wrapper](wrapperIso)
+
+		// Act
+		encoded := codec.Encode(42)
+
+		// Assert
+		assert.Equal(t, Wrapper{Value: 42}, encoded)
+	})
+}
+
+func TestFromIso_AsDecoder(t *testing.T) {
+	t.Run("returns decoder interface", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		decoder := codec.AsDecoder()
+
+		// Assert
+		result := decoder.Decode(UserId(42))
+		assert.Equal(t, validation.Success(42), result)
+	})
+}
+
+func TestFromIso_AsEncoder(t *testing.T) {
+	t.Run("returns encoder interface", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		encoder := codec.AsEncoder()
+
+		// Assert
+		encoded := encoder.Encode(42)
+		assert.Equal(t, UserId(42), encoded)
+	})
+}
+
+func TestFromIso_Validate(t *testing.T) {
+	t.Run("validate method works correctly", func(t *testing.T) {
+		// Arrange
+		userIdIso := iso.MakeIso(
+			func(id UserId) int { return int(id) },
+			func(i int) UserId { return UserId(i) },
+		)
+		codec := FromIso[int, UserId](userIdIso)
+
+		// Act
+		validateFn := codec.Validate(UserId(42))
+		result := validateFn([]validation.ContextEntry{})
+
+		// Assert
+		assert.Equal(t, validation.Success(42), result)
+	})
+}

v2/optics/codec/types.go

@@ -11,6 +11,7 @@ import (
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
 	"github.com/IBM/fp-go/v2/optics/decoder"
 	"github.com/IBM/fp-go/v2/optics/encoder"
+	"github.com/IBM/fp-go/v2/optics/iso"
 	"github.com/IBM/fp-go/v2/optics/lens"
 	"github.com/IBM/fp-go/v2/optics/optional"
 	"github.com/IBM/fp-go/v2/optics/prism"
@@ -494,4 +495,7 @@ type (
 	//   - function.VOID: The single value of type Void
 	//   - Empty: Codec function that uses Void for unit types
 	Void = function.Void
+
+	// Iso represents an isomorphism - a bidirectional transformation between two types.
+	Iso[S, A any] = iso.Iso[S, A]
 )

v2/optics/iso/generic/iso.go

@@ -0,0 +1,23 @@
+package generic
+
+import (
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	I "github.com/IBM/fp-go/v2/optics/iso"
+)
+
+// AsTraversal converts a iso to a traversal
+func AsTraversal[R ~func(func(A) HKTA) func(S) HKTS, S, A, HKTS, HKTA any](
+	fmap functor.MapType[A, S, HKTA, HKTS],
+) func(I.Iso[S, A]) R {
+	return func(sa I.Iso[S, A]) R {
+		saSet := fmap(sa.ReverseGet)
+		return func(f func(A) HKTA) func(S) HKTS {
+			return F.Flow3(
+				sa.Get,
+				f,
+				saSet,
+			)
+		}
+	}
+}

v2/optics/lens/either/either.go

@@ -23,5 +23,5 @@ import (
 )
 
 func AsTraversal[E, S, A any]() func(L.Lens[S, A]) T.Traversal[E, S, A] {
-	return LG.AsTraversal[T.Traversal[E, S, A]](ET.MonadMap[E, A, S])
+	return LG.AsTraversal[T.Traversal[E, S, A]](ET.Map[E, A, S])
 }

v2/optics/lens/generic/lens.go

@@ -16,19 +16,24 @@
 package generic
 
 import (
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/functor"
 	L "github.com/IBM/fp-go/v2/optics/lens"
 )
 
 // AsTraversal converts a lens to a traversal
 func AsTraversal[R ~func(func(A) HKTA) func(S) HKTS, S, A, HKTS, HKTA any](
-	fmap func(HKTA, func(A) S) HKTS,
+	fmap functor.MapType[A, S, HKTA, HKTS],
 ) func(L.Lens[S, A]) R {
 	return func(sa L.Lens[S, A]) R {
-		return func(f func(a A) HKTA) func(S) HKTS {
+		return func(f func(A) HKTA) func(S) HKTS {
 			return func(s S) HKTS {
-				return fmap(f(sa.Get(s)), func(a A) S {
-					return sa.Set(a)(s)
-				})
+				return F.Pipe1(
+					f(sa.Get(s)),
+					fmap(func(a A) S {
+						return sa.Set(a)(s)
+					}),
+				)
 			}
 		}
 	}

v2/optics/lens/lens.go

@@ -22,6 +22,7 @@ import (
 	"github.com/IBM/fp-go/v2/endomorphism"
 	EQ "github.com/IBM/fp-go/v2/eq"
 	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/functor"
 )
 
 // setCopy wraps a setter for a pointer into a setter that first creates a copy before
@@ -909,6 +910,83 @@ func Modify[S any, FCT ~func(A) A, A any](f FCT) func(Lens[S, A]) Endomorphism[S
 	}
 }
 
+// ModifyF transforms a value through a lens using a function that returns a value in a functor context.
+//
+// This is the functorial version of Modify, allowing transformations that produce effects
+// (like Option, Either, IO, etc.) while updating the focused value. The functor's map operation
+// is used to apply the lens's setter to the transformed value, preserving the computational context.
+//
+// This function corresponds to modifyF from monocle-ts, enabling effectful updates through lenses.
+//
+// # Type Parameters
+//
+//   - S: Structure type
+//   - A: Focus type (the value being transformed)
+//   - HKTA: Higher-kinded type containing the transformed value (e.g., Option[A], Either[E, A])
+//   - HKTS: Higher-kinded type containing the updated structure (e.g., Option[S], Either[E, S])
+//
+// # Parameters
+//
+//   - fmap: A functor map operation that transforms A to S within the functor context
+//
+// # Returns
+//
+//   - A curried function that takes:
+//     1. A transformation function (A → HKTA)
+//     2. A Lens[S, A]
+//     3. A structure S
+//     And returns the updated structure in the functor context (HKTS)
+//
+// # Example Usage
+//
+//	type Person struct {
+//	    Name string
+//	    Age  int
+//	}
+//
+//	ageLens := lens.MakeLens(
+//	    func(p Person) int { return p.Age },
+//	    func(p Person, age int) Person { p.Age = age; return p },
+//	)
+//
+//	// Validate age is positive, returning Option
+//	validateAge := func(age int) option.Option[int] {
+//	    if age > 0 {
+//	        return option.Some(age)
+//	    }
+//	    return option.None[int]()
+//	}
+//
+//	// Create a modifier that validates while updating
+//	modifyAge := lens.ModifyF[Person, int](option.Functor[int, Person]().Map)
+//
+//	person := Person{Name: "Alice", Age: 30}
+//	result := modifyAge(validateAge)(ageLens)(person)
+//	// result is Some(Person{Name: "Alice", Age: 30})
+//
+//	invalidResult := modifyAge(func(age int) option.Option[int] {
+//	    return option.None[int]()
+//	})(ageLens)(person)
+//	// invalidResult is None[Person]()
+//
+// # See Also
+//
+//   - Modify: Non-functorial version for simple transformations
+//   - functor.Functor: The functor interface used for mapping
+func ModifyF[S, A, HKTA, HKTS any](
+	fmap functor.MapType[A, S, HKTA, HKTS],
+) func(func(A) HKTA) func(Lens[S, A]) func(S) HKTS {
+	return func(f func(A) HKTA) func(Lens[S, A]) func(S) HKTS {
+		return func(sa Lens[S, A]) func(S) HKTS {
+			return func(s S) HKTS {
+				return fmap(func(a A) S {
+					return sa.Set(a)(s)
+				})(f(sa.Get(s)))
+			}
+		}
+	}
+}
+
 // IMap transforms the focus type of a lens using an isomorphism.
 //
 // An isomorphism is a pair of functions (A → B, B → A) that are inverses of each other.

v2/optics/lens/lens_test.go

@@ -16,6 +16,7 @@
 package lens
 
 import (
+	"errors"
 	"testing"
 
 	EQ "github.com/IBM/fp-go/v2/eq"
@@ -937,3 +938,367 @@ func TestMakeLensWithEq_WithNilState_MultipleOperations(t *testing.T) {
 	assert.NotNil(t, street4)
 	assert.Equal(t, "", street4.name)
 }
+
+// TestModifyF_Success tests ModifyF with a simple Maybe-like functor for successful transformations
+func TestModifyF_Success(t *testing.T) {
+	// Define a simple Maybe type for testing
+	type Maybe[A any] struct {
+		value *A
+	}
+
+	some := func(a int) Maybe[int] {
+		return Maybe[int]{value: &a}
+	}
+
+	none := func() Maybe[int] {
+		return Maybe[int]{value: nil}
+	}
+
+	// Functor map for Maybe
+	maybeMap := func(f func(int) Inner) func(Maybe[int]) Maybe[Inner] {
+		return func(ma Maybe[int]) Maybe[Inner] {
+			if ma.value == nil {
+				return Maybe[Inner]{value: nil}
+			}
+			result := f(*ma.value)
+			return Maybe[Inner]{value: &result}
+		}
+	}
+
+	t.Run("transforms value with successful result", func(t *testing.T) {
+		ageLens := MakeLens(
+			func(p Inner) int { return p.Value },
+			func(p Inner, age int) Inner { p.Value = age; return p },
+		)
+
+		// Function that returns Some for positive values
+		validatePositive := func(n int) Maybe[int] {
+			if n > 0 {
+				return some(n * 2)
+			}
+			return none()
+		}
+
+		modifyAge := ModifyF[Inner, int](maybeMap)
+
+		person := Inner{Value: 5, Foo: "test"}
+		result := modifyAge(validatePositive)(ageLens)(person)
+
+		assert.NotNil(t, result.value)
+		updated := *result.value
+		assert.Equal(t, 10, updated.Value)
+		assert.Equal(t, "test", updated.Foo)
+	})
+
+	t.Run("preserves structure with identity transformation", func(t *testing.T) {
+		type MaybeStr struct {
+			value *string
+		}
+
+		someStr := func(s string) MaybeStr {
+			return MaybeStr{value: &s}
+		}
+
+		maybeStrMap := func(f func(string) Street) func(MaybeStr) struct{ value *Street } {
+			return func(ma MaybeStr) struct{ value *Street } {
+				if ma.value == nil {
+					return struct{ value *Street }{value: nil}
+				}
+				result := f(*ma.value)
+				return struct{ value *Street }{value: &result}
+			}
+		}
+
+		nameLens := MakeLens(
+			func(s Street) string { return s.name },
+			func(s Street, name string) Street { s.name = name; return s },
+		)
+
+		identity := func(s string) MaybeStr {
+			return someStr(s)
+		}
+
+		modifyName := ModifyF[Street, string](maybeStrMap)
+
+		street := Street{num: 1, name: "Main"}
+		result := modifyName(identity)(nameLens)(street)
+
+		assert.NotNil(t, result.value)
+		assert.Equal(t, street, *result.value)
+	})
+}
+
+// TestModifyF_Failure tests ModifyF with failures
+func TestModifyF_Failure(t *testing.T) {
+	type Maybe[A any] struct {
+		value *A
+	}
+
+	some := func(a int) Maybe[int] {
+		return Maybe[int]{value: &a}
+	}
+
+	none := func() Maybe[int] {
+		return Maybe[int]{value: nil}
+	}
+
+	maybeMap := func(f func(int) Inner) func(Maybe[int]) Maybe[Inner] {
+		return func(ma Maybe[int]) Maybe[Inner] {
+			if ma.value == nil {
+				return Maybe[Inner]{value: nil}
+			}
+			result := f(*ma.value)
+			return Maybe[Inner]{value: &result}
+		}
+	}
+
+	t.Run("returns None when transformation fails", func(t *testing.T) {
+		ageLens := MakeLens(
+			func(p Inner) int { return p.Value },
+			func(p Inner, age int) Inner { p.Value = age; return p },
+		)
+
+		validatePositive := func(n int) Maybe[int] {
+			if n > 0 {
+				return some(n)
+			}
+			return none()
+		}
+
+		modifyAge := ModifyF[Inner, int](maybeMap)
+
+		person := Inner{Value: -5, Foo: "test"}
+		result := modifyAge(validatePositive)(ageLens)(person)
+
+		assert.Nil(t, result.value)
+	})
+}
+
+// TestModifyF_WithResult tests ModifyF with Result/Either-like functor
+func TestModifyF_WithResult(t *testing.T) {
+	type Result[A any] struct {
+		value *A
+		err   error
+	}
+
+	ok := func(a int) Result[int] {
+		return Result[int]{value: &a, err: nil}
+	}
+
+	fail := func(e error) Result[int] {
+		return Result[int]{value: nil, err: e}
+	}
+
+	resultMap := func(f func(int) Inner) func(Result[int]) Result[Inner] {
+		return func(r Result[int]) Result[Inner] {
+			if r.err != nil {
+				return Result[Inner]{value: nil, err: r.err}
+			}
+			result := f(*r.value)
+			return Result[Inner]{value: &result, err: nil}
+		}
+	}
+
+	t.Run("returns success for valid transformation", func(t *testing.T) {
+		ageLens := MakeLens(
+			func(p Inner) int { return p.Value },
+			func(p Inner, age int) Inner { p.Value = age; return p },
+		)
+
+		validateAge := func(n int) Result[int] {
+			if n >= 0 && n <= 150 {
+				return ok(n + 1)
+			}
+			return fail(errors.New("age out of range"))
+		}
+
+		modifyAge := ModifyF[Inner, int](resultMap)
+
+		person := Inner{Value: 30, Foo: "test"}
+		result := modifyAge(validateAge)(ageLens)(person)
+
+		assert.Nil(t, result.err)
+		assert.NotNil(t, result.value)
+		assert.Equal(t, 31, result.value.Value)
+		assert.Equal(t, "test", result.value.Foo)
+	})
+
+	t.Run("returns error for failed validation", func(t *testing.T) {
+		ageLens := MakeLens(
+			func(p Inner) int { return p.Value },
+			func(p Inner, age int) Inner { p.Value = age; return p },
+		)
+
+		validateAge := func(n int) Result[int] {
+			if n >= 0 && n <= 150 {
+				return ok(n)
+			}
+			return fail(errors.New("age out of range"))
+		}
+
+		modifyAge := ModifyF[Inner, int](resultMap)
+
+		person := Inner{Value: 200, Foo: "test"}
+		result := modifyAge(validateAge)(ageLens)(person)
+
+		assert.NotNil(t, result.err)
+		assert.Equal(t, "age out of range", result.err.Error())
+		assert.Nil(t, result.value)
+	})
+}
+
+// TestModifyF_EdgeCases tests edge cases for ModifyF
+func TestModifyF_EdgeCases(t *testing.T) {
+	type Maybe[A any] struct {
+		value *A
+	}
+
+	some := func(a int) Maybe[int] {
+		return Maybe[int]{value: &a}
+	}
+
+	maybeMap := func(f func(int) Inner) func(Maybe[int]) Maybe[Inner] {
+		return func(ma Maybe[int]) Maybe[Inner] {
+			if ma.value == nil {
+				return Maybe[Inner]{value: nil}
+			}
+			result := f(*ma.value)
+			return Maybe[Inner]{value: &result}
+		}
+	}
+
+	t.Run("handles zero values", func(t *testing.T) {
+		ageLens := MakeLens(
+			func(p Inner) int { return p.Value },
+			func(p Inner, age int) Inner { p.Value = age; return p },
+		)
+
+		identity := func(n int) Maybe[int] {
+			return some(n)
+		}
+
+		modifyAge := ModifyF[Inner, int](maybeMap)
+
+		person := Inner{Value: 0, Foo: ""}
+		result := modifyAge(identity)(ageLens)(person)
+
+		assert.NotNil(t, result.value)
+		assert.Equal(t, person, *result.value)
+	})
+
+	t.Run("works with composed lenses", func(t *testing.T) {
+		innerLens := MakeLens(
+			Outer.GetInner,
+			Outer.SetInner,
+		)
+		valueLens := MakeLensRef(
+			(*Inner).GetValue,
+			(*Inner).SetValue,
+		)
+
+		composedLens := Compose[Outer](valueLens)(innerLens)
+
+		maybeMapOuter := func(f func(int) Outer) func(Maybe[int]) Maybe[Outer] {
+			return func(ma Maybe[int]) Maybe[Outer] {
+				if ma.value == nil {
+					return Maybe[Outer]{value: nil}
+				}
+				result := f(*ma.value)
+				return Maybe[Outer]{value: &result}
+			}
+		}
+
+		validatePositive := func(n int) Maybe[int] {
+			if n > 0 {
+				return some(n * 2)
+			}
+			return Maybe[int]{value: nil}
+		}
+
+		modifyValue := ModifyF[Outer, int](maybeMapOuter)
+
+		outer := Outer{inner: &Inner{Value: 5, Foo: "test"}}
+		result := modifyValue(validatePositive)(composedLens)(outer)
+
+		assert.NotNil(t, result.value)
+		assert.Equal(t, 10, result.value.inner.Value)
+		assert.Equal(t, "test", result.value.inner.Foo)
+	})
+}
+
+// TestModifyF_Integration tests integration scenarios
+func TestModifyF_Integration(t *testing.T) {
+	type Maybe[A any] struct {
+		value *A
+	}
+
+	some := func(a int) Maybe[int] {
+		return Maybe[int]{value: &a}
+	}
+
+	maybeMap := func(f func(int) Inner) func(Maybe[int]) Maybe[Inner] {
+		return func(ma Maybe[int]) Maybe[Inner] {
+			if ma.value == nil {
+				return Maybe[Inner]{value: nil}
+			}
+			result := f(*ma.value)
+			return Maybe[Inner]{value: &result}
+		}
+	}
+
+	t.Run("chains multiple ModifyF operations", func(t *testing.T) {
+		ageLens := MakeLens(
+			func(p Inner) int { return p.Value },
+			func(p Inner, age int) Inner { p.Value = age; return p },
+		)
+
+		increment := func(n int) Maybe[int] {
+			return some(n + 1)
+		}
+
+		modifyAge := ModifyF[Inner, int](maybeMap)
+
+		person := Inner{Value: 5, Foo: "test"}
+
+		// Apply transformation twice
+		result1 := modifyAge(increment)(ageLens)(person)
+		assert.NotNil(t, result1.value)
+
+		result2 := modifyAge(increment)(ageLens)(*result1.value)
+		assert.NotNil(t, result2.value)
+
+		assert.Equal(t, 7, result2.value.Value)
+	})
+
+	t.Run("combines with regular Modify", func(t *testing.T) {
+		ageLens := MakeLens(
+			func(p Inner) int { return p.Value },
+			func(p Inner, age int) Inner { p.Value = age; return p },
+		)
+
+		// First use regular Modify
+		person := Inner{Value: 5, Foo: "test"}
+		modified := F.Pipe2(
+			ageLens,
+			Modify[Inner](func(n int) int { return n * 2 }),
+			func(endoFn func(Inner) Inner) Inner {
+				return endoFn(person)
+			},
+		)
+
+		assert.Equal(t, 10, modified.Value)
+
+		// Then use ModifyF with validation
+		validateRange := func(n int) Maybe[int] {
+			if n >= 0 && n <= 100 {
+				return some(n)
+			}
+			return Maybe[int]{value: nil}
+		}
+
+		modifyAge := ModifyF[Inner, int](maybeMap)
+		result := modifyAge(validateRange)(ageLens)(modified)
+
+		assert.NotNil(t, result.value)
+	})
+}

v2/optics/lens/option/option.go

@@ -60,5 +60,5 @@ import (
 //	configs := []Config{{Timeout: O.Some(30)}, {Timeout: O.None[int]()}}
 //	// Apply operations across all configs using the traversal
 func AsTraversal[S, A any]() func(Lens[S, A]) T.Traversal[S, A] {
-	return LG.AsTraversal[T.Traversal[S, A]](O.MonadMap[A, S])
+	return LG.AsTraversal[T.Traversal[S, A]](O.Map[A, S])
 }

v2/optics/lens/traversal/generic/identity/traversal.go

@@ -0,0 +1,86 @@
+// 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 identity
+
+import (
+	I "github.com/IBM/fp-go/v2/identity"
+	G "github.com/IBM/fp-go/v2/optics/lens/traversal/generic"
+)
+
+// Compose composes a lens with a traversal to create a new traversal.
+//
+// This function allows you to focus deeper into a data structure by first using
+// a lens to access a field, then using a traversal to access multiple values within
+// that field. The result is a traversal that can operate on all the nested values.
+//
+// The composition follows the pattern: Lens[S, A] → Traversal[A, B] → Traversal[S, B]
+// where the lens focuses on field A within structure S, and the traversal focuses on
+// multiple B values within A.
+//
+// Type Parameters:
+//   - S: The outer structure type
+//   - A: The intermediate field type (target of the lens)
+//   - B: The final focus type (targets of the traversal)
+//
+// Parameters:
+//   - t: A traversal that focuses on B values within A
+//
+// Returns:
+//   - A function that takes a Lens[S, A] and returns a Traversal[S, B]
+//
+// Example:
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    "github.com/IBM/fp-go/v2/optics/lens"
+//	    LT "github.com/IBM/fp-go/v2/optics/lens/traversal"
+//	    AI "github.com/IBM/fp-go/v2/optics/traversal/array/identity"
+//	)
+//
+//	type Team struct {
+//	    Name    string
+//	    Members []string
+//	}
+//
+//	// Lens to access the Members field
+//	membersLens := lens.MakeLens(
+//	    func(t Team) []string { return t.Members },
+//	    func(t Team, m []string) Team { t.Members = m; return t },
+//	)
+//
+//	// Traversal for array elements
+//	arrayTraversal := AI.FromArray[string]()
+//
+//	// Compose lens with traversal to access all member names
+//	memberTraversal := F.Pipe1(
+//	    membersLens,
+//	    LT.Compose[Team, []string, string](arrayTraversal),
+//	)
+//
+//	team := Team{Name: "Engineering", Members: []string{"Alice", "Bob"}}
+//	// Uppercase all member names
+//	updated := memberTraversal(strings.ToUpper)(team)
+//	// updated.Members: ["ALICE", "BOB"]
+//
+// See Also:
+//   - Lens: A functional reference to a subpart of a data structure
+//   - Traversal: A functional reference to multiple subparts
+//   - traversal.Compose: Composes two traversals
+func Compose[S, A, B any](t Traversal[A, B, A, B]) func(Lens[S, A]) Traversal[S, B, S, B] {
+	return G.Compose[S, A, B, S, A, B](
+		I.Map,
+	)(t)
+}

v2/optics/lens/traversal/generic/identity/traversal_test.go

@@ -0,0 +1,253 @@
+// 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 identity
+
+import (
+	"strings"
+	"testing"
+
+	AR "github.com/IBM/fp-go/v2/array"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/lens"
+	AI "github.com/IBM/fp-go/v2/optics/traversal/array/identity"
+	"github.com/stretchr/testify/assert"
+)
+
+type Team struct {
+	Name    string
+	Members []string
+}
+
+type Company struct {
+	Name  string
+	Teams []Team
+}
+
+func TestCompose_Success(t *testing.T) {
+	t.Run("composes lens with array traversal to modify nested values", func(t *testing.T) {
+		// Arrange
+		membersLens := lens.MakeLens(
+			func(team Team) []string { return team.Members },
+			func(team Team, members []string) Team {
+				team.Members = members
+				return team
+			},
+		)
+		arrayTraversal := AI.FromArray[string]()
+
+		memberTraversal := F.Pipe1(
+			membersLens,
+			Compose[Team](arrayTraversal),
+		)
+
+		team := Team{
+			Name:    "Engineering",
+			Members: []string{"alice", "bob", "charlie"},
+		}
+
+		// Act - uppercase all member names
+		result := memberTraversal(strings.ToUpper)(team)
+
+		// Assert
+		expected := Team{
+			Name:    "Engineering",
+			Members: []string{"ALICE", "BOB", "CHARLIE"},
+		}
+		assert.Equal(t, expected, result)
+	})
+
+	t.Run("composes lens with array traversal on empty array", func(t *testing.T) {
+		// Arrange
+		membersLens := lens.MakeLens(
+			func(team Team) []string { return team.Members },
+			func(team Team, members []string) Team {
+				team.Members = members
+				return team
+			},
+		)
+		arrayTraversal := AI.FromArray[string]()
+
+		memberTraversal := F.Pipe1(
+			membersLens,
+			Compose[Team](arrayTraversal),
+		)
+
+		team := Team{
+			Name:    "Engineering",
+			Members: []string{},
+		}
+
+		// Act
+		result := memberTraversal(strings.ToUpper)(team)
+
+		// Assert
+		assert.Equal(t, team, result)
+	})
+
+	t.Run("composes lens with array traversal to transform numbers", func(t *testing.T) {
+		// Arrange
+		type Stats struct {
+			Name   string
+			Scores []int
+		}
+
+		scoresLens := lens.MakeLens(
+			func(s Stats) []int { return s.Scores },
+			func(s Stats, scores []int) Stats {
+				s.Scores = scores
+				return s
+			},
+		)
+		arrayTraversal := AI.FromArray[int]()
+
+		scoreTraversal := F.Pipe1(
+			scoresLens,
+			Compose[Stats, []int, int](arrayTraversal),
+		)
+
+		stats := Stats{
+			Name:   "Player1",
+			Scores: []int{10, 20, 30},
+		}
+
+		// Act - double all scores
+		result := scoreTraversal(func(n int) int { return n * 2 })(stats)
+
+		// Assert
+		expected := Stats{
+			Name:   "Player1",
+			Scores: []int{20, 40, 60},
+		}
+		assert.Equal(t, expected, result)
+	})
+}
+
+func TestCompose_Integration(t *testing.T) {
+	t.Run("composes multiple lenses and traversals", func(t *testing.T) {
+		// Arrange - nested structure with Company -> Teams -> Members
+		teamsLens := lens.MakeLens(
+			func(c Company) []Team { return c.Teams },
+			func(c Company, teams []Team) Company {
+				c.Teams = teams
+				return c
+			},
+		)
+
+		// First compose: Company -> []Team -> Team
+		teamArrayTraversal := AI.FromArray[Team]()
+		companyToTeamTraversal := F.Pipe1(
+			teamsLens,
+			Compose[Company, []Team, Team](teamArrayTraversal),
+		)
+
+		// Second compose: Team -> []string -> string
+		membersLens := lens.MakeLens(
+			func(team Team) []string { return team.Members },
+			func(team Team, members []string) Team {
+				team.Members = members
+				return team
+			},
+		)
+		memberArrayTraversal := AI.FromArray[string]()
+		teamToMemberTraversal := F.Pipe1(
+			membersLens,
+			Compose[Team](memberArrayTraversal),
+		)
+
+		company := Company{
+			Name: "TechCorp",
+			Teams: []Team{
+				{Name: "Engineering", Members: []string{"alice", "bob"}},
+				{Name: "Design", Members: []string{"charlie", "diana"}},
+			},
+		}
+
+		// Act - uppercase all members in all teams
+		// First traverse to teams, then for each team traverse to members
+		result := companyToTeamTraversal(func(team Team) Team {
+			return teamToMemberTraversal(strings.ToUpper)(team)
+		})(company)
+
+		// Assert
+		expected := Company{
+			Name: "TechCorp",
+			Teams: []Team{
+				{Name: "Engineering", Members: []string{"ALICE", "BOB"}},
+				{Name: "Design", Members: []string{"CHARLIE", "DIANA"}},
+			},
+		}
+		assert.Equal(t, expected, result)
+	})
+}
+
+func TestCompose_EdgeCases(t *testing.T) {
+	t.Run("preserves structure name when modifying members", func(t *testing.T) {
+		// Arrange
+		membersLens := lens.MakeLens(
+			func(team Team) []string { return team.Members },
+			func(team Team, members []string) Team {
+				team.Members = members
+				return team
+			},
+		)
+		arrayTraversal := AI.FromArray[string]()
+
+		memberTraversal := F.Pipe1(
+			membersLens,
+			Compose[Team](arrayTraversal),
+		)
+
+		team := Team{
+			Name:    "Engineering",
+			Members: []string{"alice"},
+		}
+
+		// Act
+		result := memberTraversal(strings.ToUpper)(team)
+
+		// Assert - Name should be unchanged
+		assert.Equal(t, "Engineering", result.Name)
+		assert.Equal(t, AR.From("ALICE"), result.Members)
+	})
+
+	t.Run("handles identity transformation", func(t *testing.T) {
+		// Arrange
+		membersLens := lens.MakeLens(
+			func(team Team) []string { return team.Members },
+			func(team Team, members []string) Team {
+				team.Members = members
+				return team
+			},
+		)
+		arrayTraversal := AI.FromArray[string]()
+
+		memberTraversal := F.Pipe1(
+			membersLens,
+			Compose[Team](arrayTraversal),
+		)
+
+		team := Team{
+			Name:    "Engineering",
+			Members: []string{"alice", "bob"},
+		}
+
+		// Act - apply identity function
+		result := memberTraversal(F.Identity[string])(team)
+
+		// Assert - should be unchanged
+		assert.Equal(t, team, result)
+	})
+}

v2/optics/lens/traversal/generic/identity/types.go

@@ -0,0 +1,14 @@
+package identity
+
+import (
+	"github.com/IBM/fp-go/v2/optics/lens"
+	T "github.com/IBM/fp-go/v2/optics/traversal"
+)
+
+type (
+
+	// Lens is a functional reference to a subpart of a data structure.
+	Lens[S, A any] = lens.Lens[S, A]
+
+	Traversal[S, A, HKTS, HKTA any] = T.Traversal[S, A, HKTS, HKTA]
+)

v2/optics/lens/traversal/generic/traversal.go

@@ -0,0 +1,25 @@
+package generic
+
+import (
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	G "github.com/IBM/fp-go/v2/optics/lens/generic"
+	TG "github.com/IBM/fp-go/v2/optics/traversal/generic"
+)
+
+func Compose[S, A, B, HKTS, HKTA, HKTB any](
+	fmap functor.MapType[A, S, HKTA, HKTS],
+) func(Traversal[A, B, HKTA, HKTB]) func(Lens[S, A]) Traversal[S, B, HKTS, HKTB] {
+	lensTrav := G.AsTraversal[Traversal[S, A, HKTS, HKTA]](fmap)
+
+	return func(ab Traversal[A, B, HKTA, HKTB]) func(Lens[S, A]) Traversal[S, B, HKTS, HKTB] {
+		return F.Flow2(
+			lensTrav,
+			TG.Compose[
+				Traversal[A, B, HKTA, HKTB],
+				Traversal[S, A, HKTS, HKTA],
+				Traversal[S, B, HKTS, HKTB],
+			](ab),
+		)
+	}
+}

v2/optics/lens/traversal/generic/types.go

@@ -0,0 +1,14 @@
+package generic
+
+import (
+	"github.com/IBM/fp-go/v2/optics/lens"
+	T "github.com/IBM/fp-go/v2/optics/traversal"
+)
+
+type (
+
+	// Lens is a functional reference to a subpart of a data structure.
+	Lens[S, A any] = lens.Lens[S, A]
+
+	Traversal[S, A, HKTS, HKTA any] = T.Traversal[S, A, HKTS, HKTA]
+)

v2/optics/lenses/url.go

@@ -5,6 +5,7 @@ package lenses
 // 2026-01-27 16:08:47.5483589 +0100 CET m=+0.003380301
 
 import (
+	"net"
 	url "net/url"
 
 	__iso_option "github.com/IBM/fp-go/v2/optics/iso/option"
@@ -119,6 +120,8 @@ type URLLenses struct {
 	RawQuery    __lens.Lens[url.URL, string]
 	Fragment    __lens.Lens[url.URL, string]
 	RawFragment __lens.Lens[url.URL, string]
+	Hostname    __lens.Lens[url.URL, string]
+	Port        __lens.Lens[url.URL, string]
 	// optional fields
 	SchemeO      __lens_option.LensO[url.URL, string]
 	OpaqueO      __lens_option.LensO[url.URL, string]
@@ -131,6 +134,8 @@ type URLLenses struct {
 	RawQueryO    __lens_option.LensO[url.URL, string]
 	FragmentO    __lens_option.LensO[url.URL, string]
 	RawFragmentO __lens_option.LensO[url.URL, string]
+	HostnameO    __lens_option.LensO[url.URL, string]
+	PortO        __lens_option.LensO[url.URL, string]
 }
 
 // URLRefLenses provides lenses for accessing fields of url.URL via a reference to url.URL
@@ -147,6 +152,8 @@ type URLRefLenses struct {
 	RawQuery    __lens.Lens[*url.URL, string]
 	Fragment    __lens.Lens[*url.URL, string]
 	RawFragment __lens.Lens[*url.URL, string]
+	Hostname    __lens.Lens[*url.URL, string]
+	Port        __lens.Lens[*url.URL, string]
 	// optional fields
 	SchemeO      __lens_option.LensO[*url.URL, string]
 	OpaqueO      __lens_option.LensO[*url.URL, string]
@@ -159,6 +166,8 @@ type URLRefLenses struct {
 	RawQueryO    __lens_option.LensO[*url.URL, string]
 	FragmentO    __lens_option.LensO[*url.URL, string]
 	RawFragmentO __lens_option.LensO[*url.URL, string]
+	HostnameO    __lens_option.LensO[*url.URL, string]
+	PortO        __lens_option.LensO[*url.URL, string]
 }
 
 // MakeURLLenses creates a new URLLenses with lenses for all fields
@@ -219,6 +228,38 @@ func MakeURLLenses() URLLenses {
 		func(s url.URL, v string) url.URL { s.RawFragment = v; return s },
 		"URL.RawFragment",
 	)
+	lensHostname := __lens.MakeLensWithName(
+		func(s url.URL) string {
+			host, _, err := net.SplitHostPort(s.Host)
+			if err != nil {
+				return s.Host
+			}
+			return host
+		},
+		func(s url.URL, v string) url.URL {
+			_, port, err := net.SplitHostPort(s.Host)
+			if err != nil {
+				s.Host = v
+			} else {
+				s.Host = net.JoinHostPort(v, port)
+			}
+			return s
+		},
+		"URL.Hostname",
+	)
+	lensPort := __lens.MakeLensWithName(
+		func(s url.URL) string { return s.Port() },
+		func(s url.URL, v string) url.URL {
+			host, _, err := net.SplitHostPort(s.Host)
+			if err != nil {
+				s.Host = net.JoinHostPort(s.Host, v)
+			} else {
+				s.Host = net.JoinHostPort(host, v)
+			}
+			return s
+		},
+		"URL.Port",
+	)
 	// optional lenses
 	lensSchemeO := __lens_option.FromIso[url.URL](__iso_option.FromZero[string]())(lensScheme)
 	lensOpaqueO := __lens_option.FromIso[url.URL](__iso_option.FromZero[string]())(lensOpaque)
@@ -231,6 +272,8 @@ func MakeURLLenses() URLLenses {
 	lensRawQueryO := __lens_option.FromIso[url.URL](__iso_option.FromZero[string]())(lensRawQuery)
 	lensFragmentO := __lens_option.FromIso[url.URL](__iso_option.FromZero[string]())(lensFragment)
 	lensRawFragmentO := __lens_option.FromIso[url.URL](__iso_option.FromZero[string]())(lensRawFragment)
+	lensHostnameO := __lens_option.FromIso[url.URL](__iso_option.FromZero[string]())(lensHostname)
+	lensPortO := __lens_option.FromIso[url.URL](__iso_option.FromZero[string]())(lensPort)
 	return URLLenses{
 		// mandatory lenses
 		Scheme:      lensScheme,
@@ -244,6 +287,8 @@ func MakeURLLenses() URLLenses {
 		RawQuery:    lensRawQuery,
 		Fragment:    lensFragment,
 		RawFragment: lensRawFragment,
+		Hostname:    lensHostname,
+		Port:        lensPort,
 		// optional lenses
 		SchemeO:      lensSchemeO,
 		OpaqueO:      lensOpaqueO,
@@ -256,6 +301,8 @@ func MakeURLLenses() URLLenses {
 		RawQueryO:    lensRawQueryO,
 		FragmentO:    lensFragmentO,
 		RawFragmentO: lensRawFragmentO,
+		HostnameO:    lensHostnameO,
+		PortO:        lensPortO,
 	}
 }
 
@@ -317,6 +364,38 @@ func MakeURLRefLenses() URLRefLenses {
 		func(s *url.URL, v string) *url.URL { s.RawFragment = v; return s },
 		"(*url.URL).RawFragment",
 	)
+	lensHostname := __lens.MakeLensStrictWithName(
+		func(s *url.URL) string {
+			host, _, err := net.SplitHostPort(s.Host)
+			if err != nil {
+				return s.Host
+			}
+			return host
+		},
+		func(s *url.URL, v string) *url.URL {
+			_, port, err := net.SplitHostPort(s.Host)
+			if err != nil {
+				s.Host = v
+			} else {
+				s.Host = net.JoinHostPort(v, port)
+			}
+			return s
+		},
+		"URL.Hostname",
+	)
+	lensPort := __lens.MakeLensStrictWithName(
+		(*url.URL).Port,
+		func(s *url.URL, v string) *url.URL {
+			host, _, err := net.SplitHostPort(s.Host)
+			if err != nil {
+				s.Host = net.JoinHostPort(s.Host, v)
+			} else {
+				s.Host = net.JoinHostPort(host, v)
+			}
+			return s
+		},
+		"URL.Port",
+	)
 	// optional lenses
 	lensSchemeO := __lens_option.FromIso[*url.URL](__iso_option.FromZero[string]())(lensScheme)
 	lensOpaqueO := __lens_option.FromIso[*url.URL](__iso_option.FromZero[string]())(lensOpaque)
@@ -329,6 +408,8 @@ func MakeURLRefLenses() URLRefLenses {
 	lensRawQueryO := __lens_option.FromIso[*url.URL](__iso_option.FromZero[string]())(lensRawQuery)
 	lensFragmentO := __lens_option.FromIso[*url.URL](__iso_option.FromZero[string]())(lensFragment)
 	lensRawFragmentO := __lens_option.FromIso[*url.URL](__iso_option.FromZero[string]())(lensRawFragment)
+	lensHostnameO := __lens_option.FromIso[*url.URL](__iso_option.FromZero[string]())(lensHostname)
+	lensPortO := __lens_option.FromIso[*url.URL](__iso_option.FromZero[string]())(lensPort)
 	return URLRefLenses{
 		// mandatory lenses
 		Scheme:      lensScheme,
@@ -342,6 +423,8 @@ func MakeURLRefLenses() URLRefLenses {
 		RawQuery:    lensRawQuery,
 		Fragment:    lensFragment,
 		RawFragment: lensRawFragment,
+		Hostname:    lensHostname,
+		Port:        lensPort,
 		// optional lenses
 		SchemeO:      lensSchemeO,
 		OpaqueO:      lensOpaqueO,
@@ -354,6 +437,8 @@ func MakeURLRefLenses() URLRefLenses {
 		RawQueryO:    lensRawQueryO,
 		FragmentO:    lensFragmentO,
 		RawFragmentO: lensRawFragmentO,
+		HostnameO:    lensHostnameO,
+		PortO:        lensPortO,
 	}
 }
 

v2/optics/optional/array/array.go

@@ -0,0 +1,79 @@
+// 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 array
+
+import (
+	OP "github.com/IBM/fp-go/v2/optics/optional"
+	G "github.com/IBM/fp-go/v2/optics/optional/array/generic"
+)
+
+// At creates an Optional that focuses on the element at a specific index in an array.
+//
+// This function returns an Optional that can get and set the element at the given index.
+// If the index is out of bounds, GetOption returns None and Set operations are no-ops
+// (the array is returned unchanged). This follows the Optional laws where operations
+// on non-existent values have no effect.
+//
+// The Optional provides safe array access without panicking on invalid indices, making
+// it ideal for functional transformations where you want to modify array elements only
+// when they exist.
+//
+// Type Parameters:
+//   - A: The type of elements in the array
+//
+// Parameters:
+//   - idx: The zero-based index to focus on
+//
+// Returns:
+//   - An Optional that focuses on the element at the specified index
+//
+// Example:
+//
+//	import (
+//	    AR "github.com/IBM/fp-go/v2/array"
+//	    OP "github.com/IBM/fp-go/v2/optics/optional"
+//	    OA "github.com/IBM/fp-go/v2/optics/optional/array"
+//	)
+//
+//	numbers := []int{10, 20, 30, 40}
+//
+//	// Create an optional focusing on index 1
+//	second := OA.At[int](1)
+//
+//	// Get the element at index 1
+//	value := second.GetOption(numbers)
+//	// value: option.Some(20)
+//
+//	// Set the element at index 1
+//	updated := second.Set(25)(numbers)
+//	// updated: []int{10, 25, 30, 40}
+//
+//	// Out of bounds access returns None
+//	outOfBounds := OA.At[int](10)
+//	value = outOfBounds.GetOption(numbers)
+//	// value: option.None[int]()
+//
+//	// Out of bounds set is a no-op
+//	unchanged := outOfBounds.Set(99)(numbers)
+//	// unchanged: []int{10, 20, 30, 40} (original array)
+//
+// See Also:
+//   - AR.Lookup: Gets an element at an index, returning an Option
+//   - AR.UpdateAt: Updates an element at an index, returning an Option
+//   - OP.Optional: The Optional optic type
+func At[A any](idx int) OP.Optional[[]A, A] {
+	return G.At[[]A](idx)
+}

v2/optics/optional/array/array_test.go

@@ -0,0 +1,466 @@
+// 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 array
+
+import (
+	"testing"
+
+	EQ "github.com/IBM/fp-go/v2/eq"
+	F "github.com/IBM/fp-go/v2/function"
+	O "github.com/IBM/fp-go/v2/option"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestAt_GetOption tests the GetOption functionality
+func TestAt_GetOption(t *testing.T) {
+	t.Run("returns Some for valid index", func(t *testing.T) {
+		numbers := []int{10, 20, 30, 40}
+		optional := At[int](1)
+
+		result := optional.GetOption(numbers)
+
+		assert.Equal(t, O.Some(20), result)
+	})
+
+	t.Run("returns Some for first element", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](0)
+
+		result := optional.GetOption(numbers)
+
+		assert.Equal(t, O.Some(10), result)
+	})
+
+	t.Run("returns Some for last element", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](2)
+
+		result := optional.GetOption(numbers)
+
+		assert.Equal(t, O.Some(30), result)
+	})
+
+	t.Run("returns None for negative index", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](-1)
+
+		result := optional.GetOption(numbers)
+
+		assert.Equal(t, O.None[int](), result)
+	})
+
+	t.Run("returns None for out of bounds index", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](10)
+
+		result := optional.GetOption(numbers)
+
+		assert.Equal(t, O.None[int](), result)
+	})
+
+	t.Run("returns None for empty array", func(t *testing.T) {
+		numbers := []int{}
+		optional := At[int](0)
+
+		result := optional.GetOption(numbers)
+
+		assert.Equal(t, O.None[int](), result)
+	})
+
+	t.Run("returns None for nil array", func(t *testing.T) {
+		var numbers []int
+		optional := At[int](0)
+
+		result := optional.GetOption(numbers)
+
+		assert.Equal(t, O.None[int](), result)
+	})
+}
+
+// TestAt_Set tests the Set functionality
+func TestAt_Set(t *testing.T) {
+	t.Run("updates element at valid index", func(t *testing.T) {
+		numbers := []int{10, 20, 30, 40}
+		optional := At[int](1)
+
+		result := optional.Set(25)(numbers)
+
+		assert.Equal(t, []int{10, 25, 30, 40}, result)
+		assert.Equal(t, []int{10, 20, 30, 40}, numbers) // Original unchanged
+	})
+
+	t.Run("updates first element", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](0)
+
+		result := optional.Set(5)(numbers)
+
+		assert.Equal(t, []int{5, 20, 30}, result)
+	})
+
+	t.Run("updates last element", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](2)
+
+		result := optional.Set(35)(numbers)
+
+		assert.Equal(t, []int{10, 20, 35}, result)
+	})
+
+	t.Run("is no-op for negative index", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](-1)
+
+		result := optional.Set(99)(numbers)
+
+		assert.Equal(t, numbers, result)
+	})
+
+	t.Run("is no-op for out of bounds index", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](10)
+
+		result := optional.Set(99)(numbers)
+
+		assert.Equal(t, numbers, result)
+	})
+
+	t.Run("is no-op for empty array", func(t *testing.T) {
+		numbers := []int{}
+		optional := At[int](0)
+
+		result := optional.Set(99)(numbers)
+
+		assert.Equal(t, numbers, result)
+	})
+
+	t.Run("is no-op for nil array", func(t *testing.T) {
+		var numbers []int
+		optional := At[int](0)
+
+		result := optional.Set(99)(numbers)
+
+		assert.Equal(t, numbers, result)
+	})
+}
+
+// TestAt_OptionalLaw1_GetSetNoOp tests Optional Law 1: GetSet Law (No-op on None)
+// If GetOption(s) returns None, then Set(a)(s) must return s unchanged (no-op).
+func TestAt_OptionalLaw1_GetSetNoOp(t *testing.T) {
+	t.Run("out of bounds index - set is no-op", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](10)
+
+		// Verify GetOption returns None
+		assert.Equal(t, O.None[int](), optional.GetOption(numbers))
+
+		// Set should be a no-op
+		result := optional.Set(99)(numbers)
+		assert.Equal(t, numbers, result)
+	})
+
+	t.Run("negative index - set is no-op", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](-1)
+
+		// Verify GetOption returns None
+		assert.Equal(t, O.None[int](), optional.GetOption(numbers))
+
+		// Set should be a no-op
+		result := optional.Set(99)(numbers)
+		assert.Equal(t, numbers, result)
+	})
+
+	t.Run("empty array - set is no-op", func(t *testing.T) {
+		numbers := []int{}
+		optional := At[int](0)
+
+		// Verify GetOption returns None
+		assert.Equal(t, O.None[int](), optional.GetOption(numbers))
+
+		// Set should be a no-op
+		result := optional.Set(99)(numbers)
+		assert.Equal(t, numbers, result)
+	})
+
+	t.Run("nil array - set is no-op", func(t *testing.T) {
+		var numbers []int
+		optional := At[int](0)
+
+		// Verify GetOption returns None
+		assert.Equal(t, O.None[int](), optional.GetOption(numbers))
+
+		// Set should be a no-op
+		result := optional.Set(99)(numbers)
+		assert.Equal(t, numbers, result)
+	})
+}
+
+// TestAt_OptionalLaw2_SetGet tests Optional Law 2: SetGet Law (Get what you Set)
+// If GetOption(s) returns Some(_), then GetOption(Set(a)(s)) must return Some(a).
+func TestAt_OptionalLaw2_SetGet(t *testing.T) {
+	t.Run("set then get returns the set value", func(t *testing.T) {
+		numbers := []int{10, 20, 30, 40}
+		optional := At[int](1)
+
+		// Verify GetOption returns Some (precondition)
+		assert.True(t, O.IsSome(optional.GetOption(numbers)))
+
+		// Set a new value
+		newValue := 25
+		updated := optional.Set(newValue)(numbers)
+
+		// GetOption on updated should return Some(newValue)
+		result := optional.GetOption(updated)
+		assert.Equal(t, O.Some(newValue), result)
+	})
+
+	t.Run("set first element then get", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](0)
+
+		assert.True(t, O.IsSome(optional.GetOption(numbers)))
+
+		newValue := 5
+		updated := optional.Set(newValue)(numbers)
+
+		result := optional.GetOption(updated)
+		assert.Equal(t, O.Some(newValue), result)
+	})
+
+	t.Run("set last element then get", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](2)
+
+		assert.True(t, O.IsSome(optional.GetOption(numbers)))
+
+		newValue := 35
+		updated := optional.Set(newValue)(numbers)
+
+		result := optional.GetOption(updated)
+		assert.Equal(t, O.Some(newValue), result)
+	})
+
+	t.Run("multiple indices satisfy law", func(t *testing.T) {
+		numbers := []int{10, 20, 30, 40, 50}
+
+		for i := range 5 {
+			optional := At[int](i)
+
+			assert.True(t, O.IsSome(optional.GetOption(numbers)))
+
+			newValue := i * 100
+			updated := optional.Set(newValue)(numbers)
+
+			result := optional.GetOption(updated)
+			assert.Equal(t, O.Some(newValue), result)
+		}
+	})
+}
+
+// TestAt_OptionalLaw3_SetSet tests Optional Law 3: SetSet Law (Last Set Wins)
+// Setting twice is the same as setting once with the final value.
+// Formally: Set(b)(Set(a)(s)) = Set(b)(s)
+func TestAt_OptionalLaw3_SetSet(t *testing.T) {
+	eqSlice := EQ.FromEquals(func(a, b []int) bool {
+		if len(a) != len(b) {
+			return false
+		}
+		for i := range len(a) {
+			if a[i] != b[i] {
+				return false
+			}
+		}
+		return true
+	})
+
+	t.Run("setting twice equals setting once with final value", func(t *testing.T) {
+		numbers := []int{10, 20, 30, 40}
+		optional := At[int](1)
+
+		// Set twice: first to 25, then to 99
+		setTwice := F.Pipe2(
+			numbers,
+			optional.Set(25),
+			optional.Set(99),
+		)
+
+		// Set once with final value
+		setOnce := optional.Set(99)(numbers)
+
+		assert.True(t, eqSlice.Equals(setTwice, setOnce))
+	})
+
+	t.Run("multiple sets - last one wins", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](0)
+
+		// Set multiple times
+		result := F.Pipe4(
+			numbers,
+			optional.Set(1),
+			optional.Set(2),
+			optional.Set(3),
+			optional.Set(4),
+		)
+
+		// Should equal setting once with final value
+		expected := optional.Set(4)(numbers)
+
+		assert.True(t, eqSlice.Equals(result, expected))
+	})
+
+	t.Run("set twice on out of bounds - both no-ops", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](10)
+
+		// Set twice on out of bounds
+		setTwice := F.Pipe2(
+			numbers,
+			optional.Set(25),
+			optional.Set(99),
+		)
+
+		// Set once on out of bounds
+		setOnce := optional.Set(99)(numbers)
+
+		// Both should be no-ops, returning original
+		assert.True(t, eqSlice.Equals(setTwice, numbers))
+		assert.True(t, eqSlice.Equals(setOnce, numbers))
+		assert.True(t, eqSlice.Equals(setTwice, setOnce))
+	})
+}
+
+// TestAt_EdgeCases tests edge cases and boundary conditions
+func TestAt_EdgeCases(t *testing.T) {
+	t.Run("single element array", func(t *testing.T) {
+		numbers := []int{42}
+		optional := At[int](0)
+
+		// Get
+		assert.Equal(t, O.Some(42), optional.GetOption(numbers))
+
+		// Set
+		updated := optional.Set(99)(numbers)
+		assert.Equal(t, []int{99}, updated)
+
+		// Out of bounds
+		outOfBounds := At[int](1)
+		assert.Equal(t, O.None[int](), outOfBounds.GetOption(numbers))
+		assert.Equal(t, numbers, outOfBounds.Set(99)(numbers))
+	})
+
+	t.Run("large array", func(t *testing.T) {
+		numbers := make([]int, 1000)
+		for i := range 1000 {
+			numbers[i] = i
+		}
+
+		optional := At[int](500)
+
+		// Get
+		assert.Equal(t, O.Some(500), optional.GetOption(numbers))
+
+		// Set
+		updated := optional.Set(9999)(numbers)
+		assert.Equal(t, 9999, updated[500])
+		assert.Equal(t, 500, numbers[500]) // Original unchanged
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		// String array
+		strings := []string{"a", "b", "c"}
+		strOptional := At[string](1)
+		assert.Equal(t, O.Some("b"), strOptional.GetOption(strings))
+		assert.Equal(t, []string{"a", "x", "c"}, strOptional.Set("x")(strings))
+
+		// Bool array
+		bools := []bool{true, false, true}
+		boolOptional := At[bool](1)
+		assert.Equal(t, O.Some(false), boolOptional.GetOption(bools))
+		assert.Equal(t, []bool{true, true, true}, boolOptional.Set(true)(bools))
+	})
+
+	t.Run("preserves array capacity", func(t *testing.T) {
+		numbers := make([]int, 3, 10)
+		numbers[0], numbers[1], numbers[2] = 10, 20, 30
+
+		optional := At[int](1)
+		updated := optional.Set(25)(numbers)
+
+		assert.Equal(t, []int{10, 25, 30}, updated)
+		assert.Equal(t, 3, len(updated))
+	})
+}
+
+// TestAt_Integration tests integration scenarios
+func TestAt_Integration(t *testing.T) {
+	t.Run("multiple optionals on same array", func(t *testing.T) {
+		numbers := []int{10, 20, 30, 40}
+
+		first := At[int](0)
+		second := At[int](1)
+		third := At[int](2)
+
+		// Update multiple indices
+		result := F.Pipe3(
+			numbers,
+			first.Set(1),
+			second.Set(2),
+			third.Set(3),
+		)
+
+		assert.Equal(t, []int{1, 2, 3, 40}, result)
+		assert.Equal(t, []int{10, 20, 30, 40}, numbers) // Original unchanged
+	})
+
+	t.Run("chaining operations", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](1)
+
+		// Get, verify, set, get again
+		original := optional.GetOption(numbers)
+		assert.Equal(t, O.Some(20), original)
+
+		updated := optional.Set(25)(numbers)
+		newValue := optional.GetOption(updated)
+		assert.Equal(t, O.Some(25), newValue)
+
+		// Original still unchanged
+		assert.Equal(t, O.Some(20), optional.GetOption(numbers))
+	})
+
+	t.Run("conditional update based on current value", func(t *testing.T) {
+		numbers := []int{10, 20, 30}
+		optional := At[int](1)
+
+		// Get current value and conditionally update
+		result := F.Pipe1(
+			optional.GetOption(numbers),
+			O.Fold(
+				func() []int { return numbers },
+				func(current int) []int {
+					if current > 15 {
+						return optional.Set(current * 2)(numbers)
+					}
+					return numbers
+				},
+			),
+		)
+
+		assert.Equal(t, []int{10, 40, 30}, result)
+	})
+}

v2/optics/optional/array/generic/array.go

@@ -0,0 +1,98 @@
+// 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 generic
+
+import (
+	"fmt"
+
+	AR "github.com/IBM/fp-go/v2/array/generic"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/lazy"
+	OP "github.com/IBM/fp-go/v2/optics/optional"
+	O "github.com/IBM/fp-go/v2/option"
+)
+
+// At creates an Optional that focuses on the element at a specific index in an array.
+//
+// This function returns an Optional that can get and set the element at the given index.
+// If the index is out of bounds, GetOption returns None and Set operations are no-ops
+// (the array is returned unchanged). This follows the Optional laws where operations
+// on non-existent values have no effect.
+//
+// The Optional provides safe array access without panicking on invalid indices, making
+// it ideal for functional transformations where you want to modify array elements only
+// when they exist.
+//
+// Type Parameters:
+//   - A: The type of elements in the array
+//
+// Parameters:
+//   - idx: The zero-based index to focus on
+//
+// Returns:
+//   - An Optional that focuses on the element at the specified index
+//
+// Example:
+//
+//	import (
+//	    AR "github.com/IBM/fp-go/v2/array"
+//	    OP "github.com/IBM/fp-go/v2/optics/optional"
+//	    OA "github.com/IBM/fp-go/v2/optics/optional/array"
+//	)
+//
+//	numbers := []int{10, 20, 30, 40}
+//
+//	// Create an optional focusing on index 1
+//	second := OA.At[int](1)
+//
+//	// Get the element at index 1
+//	value := second.GetOption(numbers)
+//	// value: option.Some(20)
+//
+//	// Set the element at index 1
+//	updated := second.Set(25)(numbers)
+//	// updated: []int{10, 25, 30, 40}
+//
+//	// Out of bounds access returns None
+//	outOfBounds := OA.At[int](10)
+//	value = outOfBounds.GetOption(numbers)
+//	// value: option.None[int]()
+//
+//	// Out of bounds set is a no-op
+//	unchanged := outOfBounds.Set(99)(numbers)
+//	// unchanged: []int{10, 20, 30, 40} (original array)
+//
+// See Also:
+//   - AR.Lookup: Gets an element at an index, returning an Option
+//   - AR.UpdateAt: Updates an element at an index, returning an Option
+//   - OP.Optional: The Optional optic type
+func At[GA ~[]A, A any](idx int) OP.Optional[GA, A] {
+	lookup := AR.Lookup[GA](idx)
+	return OP.MakeOptionalCurriedWithName(
+		lookup,
+		func(a A) func(GA) GA {
+			update := AR.UpdateAt[GA](idx, a)
+			return func(as GA) GA {
+				return F.Pipe2(
+					as,
+					update,
+					O.GetOrElse(lazy.Of(as)),
+				)
+			}
+		},
+		fmt.Sprintf("At[%d]", idx),
+	)
+}

v2/optics/optional/traversal.go

@@ -0,0 +1,34 @@
+package optional
+
+import (
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	"github.com/IBM/fp-go/v2/internal/pointed"
+	"github.com/IBM/fp-go/v2/lazy"
+	O "github.com/IBM/fp-go/v2/option"
+)
+
+func AsTraversal[R ~func(func(A) HKTA) func(S) HKTS, S, A, HKTS, HKTA any](
+	fof pointed.OfType[S, HKTS],
+	fmap functor.MapType[A, S, HKTA, HKTS],
+) func(Optional[S, A]) R {
+	return func(sa Optional[S, A]) R {
+		return func(f func(A) HKTA) func(S) HKTS {
+			return func(s S) HKTS {
+				return F.Pipe2(
+					s,
+					sa.GetOption,
+					O.Fold(
+						lazy.Of(fof(s)),
+						F.Flow2(
+							f,
+							fmap(func(a A) S {
+								return sa.Set(a)(s)
+							}),
+						),
+					),
+				)
+			}
+		}
+	}
+}

v2/optics/prism/prism_test.go

@@ -310,8 +310,10 @@ func TestAsTraversal(t *testing.T) {
 		return Identity[Option[int]]{Value: s}
 	}
 
-	fmap := func(ia Identity[int], f func(int) Option[int]) Identity[Option[int]] {
-		return Identity[Option[int]]{Value: f(ia.Value)}
+	fmap := func(f func(int) Option[int]) func(Identity[int]) Identity[Option[int]] {
+		return func(ia Identity[int]) Identity[Option[int]] {
+			return Identity[Option[int]]{Value: f(ia.Value)}
+		}
 	}
 
 	type TraversalFunc func(func(int) Identity[int]) func(Option[int]) Identity[Option[int]]

v2/optics/prism/traversal.go

@@ -17,6 +17,9 @@ package prism
 
 import (
 	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	"github.com/IBM/fp-go/v2/internal/pointed"
+	"github.com/IBM/fp-go/v2/lazy"
 	O "github.com/IBM/fp-go/v2/option"
 )
 
@@ -58,24 +61,23 @@ import (
 // higher-kinded types and applicative functors. Most users will work
 // directly with prisms rather than converting them to traversals.
 func AsTraversal[R ~func(func(A) HKTA) func(S) HKTS, S, A, HKTS, HKTA any](
-	fof func(S) HKTS,
-	fmap func(HKTA, func(A) S) HKTS,
+	fof pointed.OfType[S, HKTS],
+	fmap functor.MapType[A, S, HKTA, HKTS],
 ) func(Prism[S, A]) R {
 	return func(sa Prism[S, A]) R {
-		return func(f func(a A) HKTA) func(S) HKTS {
+		return func(f func(A) HKTA) func(S) HKTS {
 			return func(s S) HKTS {
 				return F.Pipe2(
 					s,
 					sa.GetOption,
 					O.Fold(
-						// If prism doesn't match, return the original value lifted into HKTS
-						F.Nullary2(F.Constant(s), fof),
-						// If prism matches, apply f to the extracted value and map back
-						func(a A) HKTS {
-							return fmap(f(a), func(a A) S {
-								return prismModify(F.Constant1[A](a), sa, s)
-							})
-						},
+						lazy.Of(fof(s)),
+						F.Flow2(
+							f,
+							fmap(func(a A) S {
+								return Set[S](a)(sa)(s)
+							}),
+						),
 					),
 				)
 			}

v2/optics/traversal/array/const/traversal.go

@@ -23,6 +23,6 @@ import (
 )
 
 // FromArray returns a traversal from an array for the identity [Monoid]
-func FromArray[E, A any](m M.Monoid[E]) G.Traversal[[]A, A, C.Const[E, []A], C.Const[E, A]] {
+func FromArray[A, E any](m M.Monoid[E]) G.Traversal[[]A, A, C.Const[E, []A], C.Const[E, A]] {
 	return AR.FromArray[[]A](m)
 }

v2/optics/traversal/array/generic/identity/traversal.go

@@ -21,7 +21,51 @@ import (
 	G "github.com/IBM/fp-go/v2/optics/traversal/generic"
 )
 
-// FromArray returns a traversal from an array for the identity monad
+// FromArray creates a traversal for array elements using the Identity functor.
+//
+// This is a specialized version of the generic FromArray that uses the Identity
+// functor, which provides the simplest possible computational context (no context).
+// This makes it ideal for straightforward array transformations where you want to
+// modify elements directly without additional effects.
+//
+// The Identity functor means that operations are applied directly to values without
+// wrapping them in any additional structure. This results in clean, efficient
+// traversals that simply map functions over array elements.
+//
+// Type Parameters:
+//   - GA: Array type constraint (e.g., []A)
+//   - A: The element type within the array
+//
+// Returns:
+//   - A Traversal that can transform all elements in an array
+//
+// Example:
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    T "github.com/IBM/fp-go/v2/optics/traversal"
+//	    TI "github.com/IBM/fp-go/v2/optics/traversal/array/generic/identity"
+//	)
+//
+//	// Create a traversal for integer arrays
+//	arrayTraversal := TI.FromArray[[]int, int]()
+//
+//	// Compose with identity traversal
+//	traversal := F.Pipe1(
+//	    T.Id[[]int, []int](),
+//	    T.Compose[[]int, []int, []int, int](arrayTraversal),
+//	)
+//
+//	// Double all numbers in the array
+//	numbers := []int{1, 2, 3, 4, 5}
+//	doubled := traversal(func(n int) int { return n * 2 })(numbers)
+//	// doubled: []int{2, 4, 6, 8, 10}
+//
+// See Also:
+//   - AR.FromArray: Generic version with configurable functor
+//   - I.Of: Identity functor's pure/of operation
+//   - I.Map: Identity functor's map operation
+//   - I.Ap: Identity functor's applicative operation
 func FromArray[GA ~[]A, A any]() G.Traversal[GA, A, GA, A] {
 	return AR.FromArray[GA](
 		I.Of[GA],
@@ -29,3 +73,75 @@ func FromArray[GA ~[]A, A any]() G.Traversal[GA, A, GA, A] {
 		I.Ap[GA, A],
 	)
 }
+
+// At creates a function that focuses a traversal on a specific array index using the Identity functor.
+//
+// This is a specialized version of the generic At that uses the Identity functor,
+// providing the simplest computational context for array element access. It transforms
+// a traversal focusing on an array into a traversal focusing on the element at the
+// specified index.
+//
+// The Identity functor means operations are applied directly without additional wrapping,
+// making this ideal for straightforward element modifications. If the index is out of
+// bounds, the traversal focuses on zero elements (no-op).
+//
+// Type Parameters:
+//   - GA: Array type constraint (e.g., []A)
+//   - S: The source type of the outer traversal
+//   - A: The element type within the array
+//
+// Parameters:
+//   - idx: The zero-based index to focus on
+//
+// Returns:
+//   - A function that transforms a traversal on arrays into a traversal on a specific element
+//
+// Example:
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    T "github.com/IBM/fp-go/v2/optics/traversal"
+//	    TI "github.com/IBM/fp-go/v2/optics/traversal/array/generic/identity"
+//	)
+//
+//	type Person struct {
+//	    Name    string
+//	    Hobbies []string
+//	}
+//
+//	// Create a traversal focusing on hobbies
+//	hobbiesTraversal := T.Id[Person, []string]()
+//
+//	// Focus on the second hobby (index 1)
+//	secondHobby := F.Pipe1(
+//	    hobbiesTraversal,
+//	    TI.At[[]string, Person, string](1),
+//	)
+//
+//	// Modify the second hobby
+//	person := Person{Name: "Alice", Hobbies: []string{"reading", "coding", "gaming"}}
+//	updated := secondHobby(func(s string) string {
+//	    return s + "!"
+//	})(person)
+//	// updated.Hobbies: []string{"reading", "coding!", "gaming"}
+//
+//	// Out of bounds index is a no-op
+//	outOfBounds := F.Pipe1(
+//	    hobbiesTraversal,
+//	    TI.At[[]string, Person, string](10),
+//	)
+//	unchanged := outOfBounds(func(s string) string {
+//	    return s + "!"
+//	})(person)
+//	// unchanged.Hobbies: []string{"reading", "coding", "gaming"} (no change)
+//
+// See Also:
+//   - AR.At: Generic version with configurable functor
+//   - I.Of: Identity functor's pure/of operation
+//   - I.Map: Identity functor's map operation
+func At[GA ~[]A, S, A any](idx int) func(G.Traversal[S, GA, S, GA]) G.Traversal[S, A, S, A] {
+	return AR.At[GA, S, A, S](
+		I.Of[GA],
+		I.Map[A, GA],
+	)(idx)
+}

v2/optics/traversal/array/generic/traversal.go

@@ -16,19 +16,105 @@
 package generic
 
 import (
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/apply"
 	AR "github.com/IBM/fp-go/v2/internal/array"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	"github.com/IBM/fp-go/v2/internal/pointed"
+	"github.com/IBM/fp-go/v2/optics/optional"
+	OA "github.com/IBM/fp-go/v2/optics/optional/array/generic"
 	G "github.com/IBM/fp-go/v2/optics/traversal/generic"
 )
 
 // FromArray returns a traversal from an array
 func FromArray[GA ~[]A, GB ~[]B, A, B, HKTB, HKTAB, HKTRB any](
-	fof func(GB) HKTRB,
-	fmap func(func(GB) func(B) GB) func(HKTRB) HKTAB,
-	fap func(HKTB) func(HKTAB) HKTRB,
+	fof pointed.OfType[GB, HKTRB],
+	fmap functor.MapType[GB, func(B) GB, HKTRB, HKTAB],
+	fap apply.ApType[HKTB, HKTRB, HKTAB],
 ) G.Traversal[GA, A, HKTRB, HKTB] {
-	return func(f func(A) HKTB) func(s GA) HKTRB {
-		return func(s GA) HKTRB {
-			return AR.MonadTraverse(fof, fmap, fap, s, f)
-		}
+	return func(f func(A) HKTB) func(GA) HKTRB {
+		return AR.Traverse[GA](fof, fmap, fap, f)
 	}
 }
+
+// At creates a function that focuses a traversal on a specific array index.
+//
+// This function takes an index and returns a function that transforms a traversal
+// focusing on an array into a traversal focusing on the element at that index.
+// It works by:
+//  1. Creating an Optional that focuses on the array element at the given index
+//  2. Converting that Optional into a Traversal
+//  3. Composing it with the original traversal
+//
+// If the index is out of bounds, the traversal will focus on zero elements (no-op),
+// following the Optional laws where operations on non-existent values have no effect.
+//
+// This is particularly useful when you have a nested structure containing arrays
+// and want to traverse to a specific element within those arrays.
+//
+// Type Parameters:
+//   - GA: Array type constraint (e.g., []A)
+//   - S: The source type of the outer traversal
+//   - A: The element type within the array
+//   - HKTS: Higher-kinded type for S (functor/applicative context)
+//   - HKTGA: Higher-kinded type for GA (functor/applicative context)
+//   - HKTA: Higher-kinded type for A (functor/applicative context)
+//
+// Parameters:
+//   - fof: Function to lift GA into the higher-kinded type HKTGA (pure/of operation)
+//   - fmap: Function to map over HKTA and produce HKTGA (functor map operation)
+//
+// Returns:
+//   - A function that takes an index and returns a traversal transformer
+//
+// Example:
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    "github.com/IBM/fp-go/v2/identity"
+//	    T "github.com/IBM/fp-go/v2/optics/traversal"
+//	    TA "github.com/IBM/fp-go/v2/optics/traversal/array/generic"
+//	)
+//
+//	type Person struct {
+//	    Name    string
+//	    Hobbies []string
+//	}
+//
+//	// Create a traversal focusing on the hobbies array
+//	hobbiesTraversal := T.Id[Person, []string]()
+//
+//	// Focus on the first hobby (index 0)
+//	firstHobby := F.Pipe1(
+//	    hobbiesTraversal,
+//	    TA.At[[]string, Person, string](
+//	        identity.Of[[]string],
+//	        identity.Map[string, []string],
+//	    )(0),
+//	)
+//
+//	// Modify the first hobby
+//	person := Person{Name: "Alice", Hobbies: []string{"reading", "coding"}}
+//	updated := firstHobby(func(s string) string {
+//	    return s + "!"
+//	})(person)
+//	// updated.Hobbies: []string{"reading!", "coding"}
+//
+// See Also:
+//   - OA.At: Creates an Optional focusing on an array element
+//   - optional.AsTraversal: Converts an Optional to a Traversal
+//   - G.Compose: Composes two traversals
+func At[GA ~[]A, S, A, HKTS, HKTGA, HKTA any](
+	fof pointed.OfType[GA, HKTGA],
+	fmap functor.MapType[A, GA, HKTA, HKTGA],
+) func(int) func(G.Traversal[S, GA, HKTS, HKTGA]) G.Traversal[S, A, HKTS, HKTA] {
+	return F.Flow3(
+		OA.At[GA],
+		optional.AsTraversal[G.Traversal[GA, A, HKTGA, HKTA]](fof, fmap),
+		G.Compose[
+			G.Traversal[GA, A, HKTGA, HKTA],
+			G.Traversal[S, GA, HKTS, HKTGA],
+			G.Traversal[S, A, HKTS, HKTA],
+		],
+	)
+}

v2/optics/traversal/generic/traversal.go

@@ -18,7 +18,12 @@ package generic
 import (
 	AR "github.com/IBM/fp-go/v2/array/generic"
 	C "github.com/IBM/fp-go/v2/constant"
+	"github.com/IBM/fp-go/v2/endomorphism"
 	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	"github.com/IBM/fp-go/v2/internal/pointed"
+	"github.com/IBM/fp-go/v2/optics/prism"
+	"github.com/IBM/fp-go/v2/predicate"
 )
 
 type (
@@ -47,7 +52,7 @@ func FromTraversable[
 }
 
 // FoldMap maps each target to a `Monoid` and combines the result
-func FoldMap[M, S, A any](f func(A) M) func(sa Traversal[S, A, C.Const[M, S], C.Const[M, A]]) func(S) M {
+func FoldMap[S, M, A any](f func(A) M) func(sa Traversal[S, A, C.Const[M, S], C.Const[M, A]]) func(S) M {
 	return func(sa Traversal[S, A, C.Const[M, S], C.Const[M, A]]) func(S) M {
 		return F.Flow2(
 			F.Pipe1(
@@ -61,13 +66,84 @@ func FoldMap[M, S, A any](f func(A) M) func(sa Traversal[S, A, C.Const[M, S], C.
 
 // Fold maps each target to a `Monoid` and combines the result
 func Fold[S, A any](sa Traversal[S, A, C.Const[A, S], C.Const[A, A]]) func(S) A {
-	return FoldMap[A, S](F.Identity[A])(sa)
+	return FoldMap[S](F.Identity[A])(sa)
 }
 
 // GetAll gets all the targets of a traversal
 func GetAll[GA ~[]A, S, A any](s S) func(sa Traversal[S, A, C.Const[GA, S], C.Const[GA, A]]) GA {
-	fmap := FoldMap[GA, S](AR.Of[GA, A])
+	fmap := FoldMap[S](AR.Of[GA, A])
 	return func(sa Traversal[S, A, C.Const[GA, S], C.Const[GA, A]]) GA {
 		return fmap(sa)(s)
 	}
 }
+
+// Filter creates a function that filters the targets of a traversal based on a predicate.
+//
+// This function allows you to refine a traversal to only focus on values that satisfy
+// a given predicate. It works by converting the predicate into a prism, then converting
+// that prism into a traversal, and finally composing it with the original traversal.
+//
+// The filtering is selective: when modifying values through the filtered traversal,
+// only values that satisfy the predicate will be transformed. Values that don't
+// satisfy the predicate remain unchanged.
+//
+// Type Parameters:
+//   - S: The source type
+//   - A: The focus type (the values being filtered)
+//   - HKTS: Higher-kinded type for S (functor/applicative context)
+//   - HKTA: Higher-kinded type for A (functor/applicative context)
+//
+// Parameters:
+//   - fof: Function to lift A into the higher-kinded type HKTA (pure/of operation)
+//   - fmap: Function to map over HKTA (functor map operation)
+//
+// Returns:
+//   - A function that takes a predicate and returns an endomorphism on traversals
+//
+// Example:
+//
+//	import (
+//	    AR "github.com/IBM/fp-go/v2/array"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    "github.com/IBM/fp-go/v2/identity"
+//	    N "github.com/IBM/fp-go/v2/number"
+//	    AI "github.com/IBM/fp-go/v2/optics/traversal/array/identity"
+//	)
+//
+//	// Create a traversal for array elements
+//	arrayTraversal := AI.FromArray[int]()
+//	baseTraversal := F.Pipe1(
+//	    Id[[]int, []int](),
+//	    Compose[[]int, []int, []int, int](arrayTraversal),
+//	)
+//
+//	// Filter to only positive numbers
+//	isPositive := N.MoreThan(0)
+//	filteredTraversal := F.Pipe1(
+//	    baseTraversal,
+//	    Filter[[]int, int](identity.Of[int], identity.Map[int, int])(isPositive),
+//	)
+//
+//	// Double only positive numbers
+//	numbers := []int{-2, -1, 0, 1, 2, 3}
+//	result := filteredTraversal(func(n int) int { return n * 2 })(numbers)
+//	// result: [-2, -1, 0, 2, 4, 6]
+//
+// See Also:
+//   - prism.FromPredicate: Creates a prism from a predicate
+//   - prism.AsTraversal: Converts a prism to a traversal
+//   - Compose: Composes two traversals
+func Filter[
+	S, HKTS, A, HKTA any](
+	fof pointed.OfType[A, HKTA],
+	fmap functor.MapType[A, A, HKTA, HKTA],
+) func(predicate.Predicate[A]) endomorphism.Endomorphism[Traversal[S, A, HKTS, HKTA]] {
+	return F.Flow3(
+		prism.FromPredicate,
+		prism.AsTraversal[Traversal[A, A, HKTA, HKTA]](fof, fmap),
+		Compose[
+			Traversal[A, A, HKTA, HKTA],
+			Traversal[S, A, HKTS, HKTA],
+			Traversal[S, A, HKTS, HKTA]],
+	)
+}

v2/optics/traversal/traversal.go

@@ -18,46 +18,110 @@ package traversal
 import (
 	C "github.com/IBM/fp-go/v2/constant"
 	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/identity"
+	"github.com/IBM/fp-go/v2/internal/functor"
+	"github.com/IBM/fp-go/v2/internal/pointed"
 	G "github.com/IBM/fp-go/v2/optics/traversal/generic"
 )
 
 // Id is the identity constructor of a traversal
-func Id[S, A any]() G.Traversal[S, S, A, A] {
+func Id[S, A any]() Traversal[S, S, A, A] {
 	return F.Identity[func(S) A]
 }
 
 // Modify applies a transformation function to a traversal
-func Modify[S, A any](f func(A) A) func(sa G.Traversal[S, A, S, A]) func(S) S {
-	return func(sa G.Traversal[S, A, S, A]) func(S) S {
-		return sa(f)
-	}
+func Modify[S, A any](f Endomorphism[A]) func(Traversal[S, A, S, A]) Endomorphism[S] {
+	return identity.Flap[Endomorphism[S]](f)
 }
 
 // Set sets a constant value for all values of the traversal
-func Set[S, A any](a A) func(sa G.Traversal[S, A, S, A]) func(S) S {
+func Set[S, A any](a A) func(Traversal[S, A, S, A]) Endomorphism[S] {
 	return Modify[S](F.Constant1[A](a))
 }
 
 // FoldMap maps each target to a `Monoid` and combines the result
-func FoldMap[M, S, A any](f func(A) M) func(sa G.Traversal[S, A, C.Const[M, S], C.Const[M, A]]) func(S) M {
-	return G.FoldMap[M, S](f)
+func FoldMap[S, M, A any](f func(A) M) func(sa Traversal[S, A, C.Const[M, S], C.Const[M, A]]) func(S) M {
+	return G.FoldMap[S](f)
 }
 
 // Fold maps each target to a `Monoid` and combines the result
-func Fold[S, A any](sa G.Traversal[S, A, C.Const[A, S], C.Const[A, A]]) func(S) A {
+func Fold[S, A any](sa Traversal[S, A, C.Const[A, S], C.Const[A, A]]) func(S) A {
 	return G.Fold(sa)
 }
 
 // GetAll gets all the targets of a traversal
-func GetAll[S, A any](s S) func(sa G.Traversal[S, A, C.Const[[]A, S], C.Const[[]A, A]]) []A {
+func GetAll[A, S any](s S) func(sa Traversal[S, A, C.Const[[]A, S], C.Const[[]A, A]]) []A {
 	return G.GetAll[[]A](s)
 }
 
 // Compose composes two traversables
 func Compose[
-	S, A, B, HKTS, HKTA, HKTB any](ab G.Traversal[A, B, HKTA, HKTB]) func(sa G.Traversal[S, A, HKTS, HKTA]) G.Traversal[S, B, HKTS, HKTB] {
+	S, HKTS, A, B, HKTA, HKTB any](ab Traversal[A, B, HKTA, HKTB]) func(Traversal[S, A, HKTS, HKTA]) Traversal[S, B, HKTS, HKTB] {
 	return G.Compose[
-		G.Traversal[A, B, HKTA, HKTB],
-		G.Traversal[S, A, HKTS, HKTA],
-		G.Traversal[S, B, HKTS, HKTB]](ab)
+		Traversal[A, B, HKTA, HKTB],
+		Traversal[S, A, HKTS, HKTA],
+		Traversal[S, B, HKTS, HKTB]](ab)
+}
+
+// Filter creates a function that filters the targets of a traversal based on a predicate.
+//
+// This function allows you to refine a traversal to only focus on values that satisfy
+// a given predicate. It works by converting the predicate into a prism, then converting
+// that prism into a traversal, and finally composing it with the original traversal.
+//
+// The filtering is selective: when modifying values through the filtered traversal,
+// only values that satisfy the predicate will be transformed. Values that don't
+// satisfy the predicate remain unchanged.
+//
+// Type Parameters:
+//   - S: The source type
+//   - A: The focus type (the values being filtered)
+//   - HKTS: Higher-kinded type for S (functor/applicative context)
+//   - HKTA: Higher-kinded type for A (functor/applicative context)
+//
+// Parameters:
+//   - fof: Function to lift A into the higher-kinded type HKTA (pure/of operation)
+//   - fmap: Function to map over HKTA (functor map operation)
+//
+// Returns:
+//   - A function that takes a predicate and returns an endomorphism on traversals
+//
+// Example:
+//
+//	import (
+//	    AR "github.com/IBM/fp-go/v2/array"
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    "github.com/IBM/fp-go/v2/identity"
+//	    N "github.com/IBM/fp-go/v2/number"
+//	    AI "github.com/IBM/fp-go/v2/optics/traversal/array/identity"
+//	)
+//
+//	// Create a traversal for array elements
+//	arrayTraversal := AI.FromArray[int]()
+//	baseTraversal := F.Pipe1(
+//	    Id[[]int, []int](),
+//	    Compose[[]int, []int, []int, int](arrayTraversal),
+//	)
+//
+//	// Filter to only positive numbers
+//	isPositive := N.MoreThan(0)
+//	filteredTraversal := F.Pipe1(
+//	    baseTraversal,
+//	    Filter[[]int, int](identity.Of[int], identity.Map[int, int])(isPositive),
+//	)
+//
+//	// Double only positive numbers
+//	numbers := []int{-2, -1, 0, 1, 2, 3}
+//	result := filteredTraversal(func(n int) int { return n * 2 })(numbers)
+//	// result: [-2, -1, 0, 2, 4, 6]
+//
+// See Also:
+//   - prism.FromPredicate: Creates a prism from a predicate
+//   - prism.AsTraversal: Converts a prism to a traversal
+//   - Compose: Composes two traversals
+func Filter[S, HKTS, A, HKTA any](
+	fof pointed.OfType[A, HKTA],
+	fmap functor.MapType[A, A, HKTA, HKTA],
+) func(Predicate[A]) Endomorphism[Traversal[S, A, HKTS, HKTA]] {
+	return G.Filter[S, HKTS](fof, fmap)
 }

v2/optics/traversal/traversal_test.go

@@ -32,14 +32,14 @@ func TestGetAll(t *testing.T) {
 
 	as := AR.From(1, 2, 3)
 
-	tr := AT.FromArray[[]int, int](AR.Monoid[int]())
+	tr := AT.FromArray[int](AR.Monoid[int]())
 
 	sa := F.Pipe1(
 		Id[[]int, C.Const[[]int, []int]](),
-		Compose[[]int, []int, int, C.Const[[]int, []int]](tr),
+		Compose[[]int, C.Const[[]int, []int], []int, int](tr),
 	)
 
-	getall := GetAll[[]int, int](as)(sa)
+	getall := GetAll[int](as)(sa)
 
 	assert.Equal(t, AR.From(1, 2, 3), getall)
 }
@@ -54,7 +54,7 @@ func TestFold(t *testing.T) {
 
 	sa := F.Pipe1(
 		Id[[]int, C.Const[int, []int]](),
-		Compose[[]int, []int, int, C.Const[int, []int]](tr),
+		Compose[[]int, C.Const[int, []int], []int, int](tr),
 	)
 
 	folded := Fold(sa)(as)
@@ -70,10 +70,245 @@ func TestTraverse(t *testing.T) {
 
 	sa := F.Pipe1(
 		Id[[]int, []int](),
-		Compose[[]int, []int, int, []int](tr),
+		Compose[[]int, []int](tr),
 	)
 
 	res := sa(utils.Double)(as)
 
 	assert.Equal(t, AR.From(2, 4, 6), res)
 }
+
+func TestFilter_Success(t *testing.T) {
+	t.Run("filters and modifies only matching elements", func(t *testing.T) {
+		// Arrange
+		numbers := []int{-2, -1, 0, 1, 2, 3}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		// Filter to only positive numbers
+		isPositive := N.MoreThan(0)
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isPositive),
+		)
+
+		// Act - double only positive numbers
+		result := filteredTraversal(func(n int) int { return n * 2 })(numbers)
+
+		// Assert
+		assert.Equal(t, []int{-2, -1, 0, 2, 4, 6}, result)
+	})
+
+	t.Run("filters even numbers and triples them", func(t *testing.T) {
+		// Arrange
+		numbers := []int{1, 2, 3, 4, 5, 6}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		// Filter to only even numbers
+		isEven := func(n int) bool { return n%2 == 0 }
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isEven),
+		)
+
+		// Act
+		result := filteredTraversal(func(n int) int { return n * 3 })(numbers)
+
+		// Assert
+		assert.Equal(t, []int{1, 6, 3, 12, 5, 18}, result)
+	})
+
+	t.Run("filters strings by length", func(t *testing.T) {
+		// Arrange
+		words := []string{"a", "ab", "abc", "abcd", "abcde"}
+		arrayTraversal := AI.FromArray[string]()
+		baseTraversal := F.Pipe1(
+			Id[[]string, []string](),
+			Compose[[]string, []string, []string, string](arrayTraversal),
+		)
+
+		// Filter strings with length > 2
+		longerThanTwo := func(s string) bool { return len(s) > 2 }
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]string, []string, string, string](F.Identity[string], F.Identity[func(string) string])(longerThanTwo),
+		)
+
+		// Act - convert to uppercase
+		result := filteredTraversal(func(s string) string {
+			return s + "!"
+		})(words)
+
+		// Assert
+		assert.Equal(t, []string{"a", "ab", "abc!", "abcd!", "abcde!"}, result)
+	})
+}
+
+func TestFilter_EdgeCases(t *testing.T) {
+	t.Run("empty array returns empty array", func(t *testing.T) {
+		// Arrange
+		numbers := []int{}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		isPositive := N.MoreThan(0)
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isPositive),
+		)
+
+		// Act
+		result := filteredTraversal(utils.Double)(numbers)
+
+		// Assert
+		assert.Equal(t, []int{}, result)
+	})
+
+	t.Run("no elements match predicate", func(t *testing.T) {
+		// Arrange
+		numbers := []int{-5, -4, -3, -2, -1}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		isPositive := N.MoreThan(0)
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isPositive),
+		)
+
+		// Act
+		result := filteredTraversal(utils.Double)(numbers)
+
+		// Assert - all elements unchanged
+		assert.Equal(t, []int{-5, -4, -3, -2, -1}, result)
+	})
+
+	t.Run("all elements match predicate", func(t *testing.T) {
+		// Arrange
+		numbers := []int{1, 2, 3, 4, 5}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		isPositive := N.MoreThan(0)
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isPositive),
+		)
+
+		// Act
+		result := filteredTraversal(utils.Double)(numbers)
+
+		// Assert - all elements doubled
+		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)
+	})
+
+	t.Run("single element matching", func(t *testing.T) {
+		// Arrange
+		numbers := []int{42}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		isPositive := N.MoreThan(0)
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isPositive),
+		)
+
+		// Act
+		result := filteredTraversal(utils.Double)(numbers)
+
+		// Assert
+		assert.Equal(t, []int{84}, result)
+	})
+
+	t.Run("single element not matching", func(t *testing.T) {
+		// Arrange
+		numbers := []int{-42}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		isPositive := N.MoreThan(0)
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isPositive),
+		)
+
+		// Act
+		result := filteredTraversal(utils.Double)(numbers)
+
+		// Assert
+		assert.Equal(t, []int{-42}, result)
+	})
+}
+
+func TestFilter_Integration(t *testing.T) {
+	t.Run("multiple filters composed", func(t *testing.T) {
+		// Arrange
+		numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		// Filter to only even numbers, then only those > 4
+		isEven := func(n int) bool { return n%2 == 0 }
+		greaterThanFour := N.MoreThan(4)
+
+		filteredTraversal := F.Pipe2(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isEven),
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(greaterThanFour),
+		)
+
+		// Act - add 100 to matching elements
+		result := filteredTraversal(func(n int) int { return n + 100 })(numbers)
+
+		// Assert - only 6, 8, 10 should be modified
+		assert.Equal(t, []int{1, 2, 3, 4, 5, 106, 7, 108, 9, 110}, result)
+	})
+
+	t.Run("filter with identity transformation", func(t *testing.T) {
+		// Arrange
+		numbers := []int{1, 2, 3, 4, 5}
+		arrayTraversal := AI.FromArray[int]()
+		baseTraversal := F.Pipe1(
+			Id[[]int, []int](),
+			Compose[[]int, []int](arrayTraversal),
+		)
+
+		isEven := func(n int) bool { return n%2 == 0 }
+		filteredTraversal := F.Pipe1(
+			baseTraversal,
+			Filter[[]int, []int](F.Identity[int], F.Identity[func(int) int])(isEven),
+		)
+
+		// Act - identity transformation
+		result := filteredTraversal(F.Identity[int])(numbers)
+
+		// Assert - array unchanged
+		assert.Equal(t, []int{1, 2, 3, 4, 5}, result)
+	})
+}

v2/optics/traversal/types.go

@@ -0,0 +1,15 @@
+package traversal
+
+import (
+	"github.com/IBM/fp-go/v2/endomorphism"
+	G "github.com/IBM/fp-go/v2/optics/traversal/generic"
+	"github.com/IBM/fp-go/v2/predicate"
+)
+
+type (
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
+
+	Traversal[S, A, HKTS, HKTA any] = G.Traversal[S, A, HKTS, HKTA]
+
+	Predicate[A any] = predicate.Predicate[A]
+)

v2/record/generic/record.go

@@ -16,6 +16,7 @@
 package generic
 
 import (
+	"maps"
 	"sort"
 
 	F "github.com/IBM/fp-go/v2/function"
@@ -301,13 +302,8 @@ func unionLast[M ~map[K]V, K comparable, V any](left, right M) M {
 
 	result := make(M, lenLeft+lenRight)
 
-	for k, v := range left {
-		result[k] = v
-	}
-
-	for k, v := range right {
-		result[k] = v
-	}
+	maps.Copy(result, left)
+	maps.Copy(result, right)
 
 	return result
 }

v2/result/array.go

@@ -16,6 +16,8 @@
 package result
 
 import (
+	"iter"
+
 	"github.com/IBM/fp-go/v2/either"
 )
 
@@ -155,3 +157,84 @@ func CompactArrayG[A1 ~[]Result[A], A2 ~[]A, A any](fa A1) A2 {
 func CompactArray[A any](fa []Result[A]) []A {
 	return either.CompactArray(fa)
 }
+
+// TraverseSeq transforms an iterator by applying a function that returns a Result to each element.
+// If any element produces a Left, the entire result is that Left (short-circuits).
+// Otherwise, returns Right containing an iterator of all Right values.
+//
+// The function eagerly evaluates all elements in the input iterator to detect any Left values,
+// then returns an iterator over the collected Right values. This is necessary because Result
+// represents computations that can fail, and we need to know if any element failed before
+// producing the result iterator.
+//
+// # Type Parameters
+//
+//   - A: The input element type
+//   - B: The output element type
+//
+// # Parameters
+//
+//   - f: A function that transforms each element into a Result
+//
+// # Returns
+//
+//   - A function that takes an iterator of A and returns Result containing an iterator of B
+//
+// # Example Usage
+//
+//	parse := func(s string) result.Result[int] {
+//	    v, err := strconv.Atoi(s)
+//	    return result.TryCatchError(v, err)
+//	}
+//	input := slices.Values([]string{"1", "2", "3"})
+//	result := result.TraverseSeq(parse)(input)
+//	// result is Right(iterator over [1, 2, 3])
+//
+// # See Also
+//
+//   - TraverseArray: For slice-based traversal
+//   - SequenceSeq: For sequencing iterators of Result values
+//
+//go:inline
+func TraverseSeq[A, B any](f Kleisli[A, B]) Kleisli[iter.Seq[A], iter.Seq[B]] {
+	return either.TraverseSeq(f)
+}
+
+// SequenceSeq converts an iterator of Result into a Result of iterator.
+// If any element is Left, returns that Left (short-circuits).
+// Otherwise, returns Right containing an iterator of all the Right values.
+//
+// This function eagerly evaluates all Result values in the input iterator to detect
+// any Left values, then returns an iterator over the collected Right values.
+//
+// # Type Parameters
+//
+//   - A: The value type for Right values
+//
+// # Parameters
+//
+//   - ma: An iterator of Result values
+//
+// # Returns
+//
+//   - Result containing an iterator of Right values, or the first Left encountered
+//
+// # Example Usage
+//
+//	results := slices.Values([]result.Result[int]{
+//	    result.Of(1),
+//	    result.Of(2),
+//	    result.Of(3),
+//	})
+//	result := result.SequenceSeq(results)
+//	// result is Right(iterator over [1, 2, 3])
+//
+// # See Also
+//
+//   - SequenceArray: For slice-based sequencing
+//   - TraverseSeq: For transforming and sequencing in one step
+//
+//go:inline
+func SequenceSeq[A any](ma iter.Seq[Result[A]]) Result[iter.Seq[A]] {
+	return either.SequenceSeq(ma)
+}

v2/result/array_test.go

@@ -3,8 +3,12 @@ package result
 import (
 	"errors"
 	"fmt"
+	"iter"
+	"slices"
+	"strconv"
 	"testing"
 
+	F "github.com/IBM/fp-go/v2/function"
 	TST "github.com/IBM/fp-go/v2/internal/testing"
 	"github.com/stretchr/testify/assert"
 )
@@ -15,13 +19,10 @@ func TestCompactArray(t *testing.T) {
 		Left[string](errors.New("err")),
 		Of("ok"),
 	}
-
-	res := CompactArray(ar)
-	assert.Equal(t, 2, len(res))
+	assert.Equal(t, 2, len(CompactArray(ar)))
 }
 
 func TestSequenceArray(t *testing.T) {
-
 	s := TST.SequenceArrayTest(
 		FromStrictEquals[bool](),
 		Pointed[string](),
@@ -29,14 +30,12 @@ func TestSequenceArray(t *testing.T) {
 		Functor[[]string, bool](),
 		SequenceArray[string],
 	)
-
-	for i := 0; i < 10; i++ {
+	for i := range 10 {
 		t.Run(fmt.Sprintf("TestSequenceArray %d", i), s(i))
 	}
 }
 
 func TestSequenceArrayError(t *testing.T) {
-
 	s := TST.SequenceArrayErrorTest(
 		FromStrictEquals[bool](),
 		Left[string],
@@ -46,6 +45,237 @@ func TestSequenceArrayError(t *testing.T) {
 		Functor[[]string, bool](),
 		SequenceArray[string],
 	)
-	// run across four bits
 	s(4)(t)
 }
+
+func TestTraverseSeq_Success(t *testing.T) {
+	parse := func(s string) Result[int] {
+		v, err := strconv.Atoi(s)
+		return TryCatchError(v, err)
+	}
+
+	collectInts := func(result Result[iter.Seq[int]]) []int {
+		return F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+	}
+
+	t.Run("transforms all elements successfully", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"1", "2", "3"}))
+		assert.Equal(t, []int{1, 2, 3}, collectInts(result))
+	})
+
+	t.Run("works with empty iterator", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{}))
+		assert.Empty(t, collectInts(result))
+	})
+
+	t.Run("works with single element", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"42"}))
+		assert.Equal(t, []int{42}, collectInts(result))
+	})
+
+	t.Run("preserves order of elements", func(t *testing.T) {
+		result := TraverseSeq(parse)(slices.Values([]string{"10", "20", "30", "40", "50"}))
+		assert.Equal(t, []int{10, 20, 30, 40, 50}, collectInts(result))
+	})
+}
+
+func TestTraverseSeq_Failure(t *testing.T) {
+	parse := func(s string) Result[int] {
+		v, err := strconv.Atoi(s)
+		return TryCatchError(v, err)
+	}
+
+	extractErr := func(result Result[iter.Seq[int]]) error {
+		return F.Pipe1(result, Fold(
+			F.Identity[error],
+			func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+		))
+	}
+
+	t.Run("short-circuits on first Left", func(t *testing.T) {
+		err := extractErr(TraverseSeq(parse)(slices.Values([]string{"1", "invalid", "3"})))
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "invalid syntax")
+	})
+
+	t.Run("returns first error when multiple failures exist", func(t *testing.T) {
+		err := extractErr(TraverseSeq(parse)(slices.Values([]string{"1", "bad1", "bad2"})))
+		assert.Error(t, err)
+		assert.Contains(t, err.Error(), "bad1")
+	})
+
+	t.Run("handles custom error types", func(t *testing.T) {
+		customErr := errors.New("custom validation error")
+		validate := func(n int) Result[int] {
+			if n == 2 {
+				return Left[int](customErr)
+			}
+			return Of(n * 10)
+		}
+		err := extractErr(TraverseSeq(validate)(slices.Values([]int{1, 2, 3})))
+		assert.Equal(t, customErr, err)
+	})
+}
+
+func TestTraverseSeq_EdgeCases(t *testing.T) {
+	t.Run("handles complex transformations", func(t *testing.T) {
+		type User struct {
+			ID   int
+			Name string
+		}
+
+		transform := func(id int) Result[User] {
+			return Of(User{ID: id, Name: fmt.Sprintf("User%d", id)})
+		}
+
+		result := TraverseSeq(transform)(slices.Values([]int{1, 2, 3}))
+		collected := F.Pipe1(result, Fold(
+			func(e error) []User { t.Fatal(e); return nil },
+			slices.Collect[User],
+		))
+
+		assert.Equal(t, []User{
+			{ID: 1, Name: "User1"},
+			{ID: 2, Name: "User2"},
+			{ID: 3, Name: "User3"},
+		}, collected)
+	})
+
+	t.Run("works with identity transformation", func(t *testing.T) {
+		input := slices.Values([]Result[int]{Of(1), Of(2), Of(3)})
+
+		result := TraverseSeq(F.Identity[Result[int]])(input)
+		collected := F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+
+		assert.Equal(t, []int{1, 2, 3}, collected)
+	})
+}
+
+func TestSequenceSeq_Success(t *testing.T) {
+	collectInts := func(result Result[iter.Seq[int]]) []int {
+		return F.Pipe1(result, Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+	}
+
+	t.Run("sequences multiple Right values", func(t *testing.T) {
+		input := slices.Values([]Result[int]{Of(1), Of(2), Of(3)})
+		assert.Equal(t, []int{1, 2, 3}, collectInts(SequenceSeq(input)))
+	})
+
+	t.Run("works with empty iterator", func(t *testing.T) {
+		input := slices.Values([]Result[string]{})
+		result := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []string { t.Fatal(e); return nil },
+			slices.Collect[string],
+		))
+		assert.Empty(t, result)
+	})
+
+	t.Run("works with single Right value", func(t *testing.T) {
+		input := slices.Values([]Result[string]{Of("hello")})
+		result := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []string { t.Fatal(e); return nil },
+			slices.Collect[string],
+		))
+		assert.Equal(t, []string{"hello"}, result)
+	})
+
+	t.Run("preserves order of results", func(t *testing.T) {
+		input := slices.Values([]Result[int]{Of(5), Of(4), Of(3), Of(2), Of(1)})
+		assert.Equal(t, []int{5, 4, 3, 2, 1}, collectInts(SequenceSeq(input)))
+	})
+
+	t.Run("works with complex types", func(t *testing.T) {
+		type Item struct {
+			Value int
+			Label string
+		}
+
+		input := slices.Values([]Result[Item]{
+			Of(Item{Value: 1, Label: "first"}),
+			Of(Item{Value: 2, Label: "second"}),
+			Of(Item{Value: 3, Label: "third"}),
+		})
+
+		collected := F.Pipe1(SequenceSeq(input), Fold(
+			func(e error) []Item { t.Fatal(e); return nil },
+			slices.Collect[Item],
+		))
+
+		assert.Equal(t, []Item{
+			{Value: 1, Label: "first"},
+			{Value: 2, Label: "second"},
+			{Value: 3, Label: "third"},
+		}, collected)
+	})
+}
+
+func TestSequenceSeq_Failure(t *testing.T) {
+	extractErr := func(result Result[iter.Seq[int]]) error {
+		return F.Pipe1(result, Fold(
+			F.Identity[error],
+			func(_ iter.Seq[int]) error { t.Fatal("expected Left but got Right"); return nil },
+		))
+	}
+
+	t.Run("short-circuits on first Left", func(t *testing.T) {
+		testErr := errors.New("test error")
+		input := slices.Values([]Result[int]{Of(1), Left[int](testErr), Of(3)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("returns first error when multiple Left values exist", func(t *testing.T) {
+		err1 := errors.New("error 1")
+		err2 := errors.New("error 2")
+		input := slices.Values([]Result[int]{Of(1), Left[int](err1), Left[int](err2)})
+		assert.Equal(t, err1, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("handles Left at the beginning", func(t *testing.T) {
+		testErr := errors.New("first error")
+		input := slices.Values([]Result[int]{Left[int](testErr), Of(2), Of(3)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+
+	t.Run("handles Left at the end", func(t *testing.T) {
+		testErr := errors.New("last error")
+		input := slices.Values([]Result[int]{Of(1), Of(2), Left[int](testErr)})
+		assert.Equal(t, testErr, extractErr(SequenceSeq(input)))
+	})
+}
+
+func TestSequenceSeq_Integration(t *testing.T) {
+	t.Run("integrates with TraverseSeq", func(t *testing.T) {
+		parse := func(s string) Result[int] {
+			v, err := strconv.Atoi(s)
+			return TryCatchError(v, err)
+		}
+		result := TraverseSeq(parse)(slices.Values([]string{"1", "2", "3"}))
+		assert.True(t, IsRight(result))
+	})
+
+	t.Run("SequenceSeq is equivalent to TraverseSeq with Identity", func(t *testing.T) {
+		mkInput := func() []Result[int] {
+			return []Result[int]{Of(10), Of(20), Of(30)}
+		}
+
+		collected1 := F.Pipe1(SequenceSeq(slices.Values(mkInput())), Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+		collected2 := F.Pipe1(TraverseSeq(F.Identity[Result[int]])(slices.Values(mkInput())), Fold(
+			func(e error) []int { t.Fatal(e); return nil },
+			slices.Collect[int],
+		))
+
+		assert.Equal(t, collected1, collected2)
+	})
+}