fix: expose Empty for Codec

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

v2/AGENTS.md

@@ -151,6 +151,11 @@ func TestFromReaderResult_Success(t *testing.T) {
    - Don't manually handle `(value, error)` tuples when helpers exist
    - Don't use `either.MonadFold` in tests unless necessary
 
+4. **Use Void Type for Unit Values**
+   - Use `function.Void` (or `F.Void`) instead of `struct{}`
+   - Use `function.VOID` (or `F.VOID`) instead of `struct{}{}`
+   - Example: `Empty[F.Void, F.Void, any](lazy.Of(pair.MakePair(F.VOID, F.VOID)))`
+
 ### Error Handling
 
 1. **In Production Code**

v2/assert/from_test.go

@@ -29,7 +29,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
 				// Return a Reader that always passes
-				return result.Of[Reader](func(t *testing.T) bool {
+				return result.Of(func(t *testing.T) bool {
 					return true
 				})
 			}
@@ -46,7 +46,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		// Create a ReaderIOResult that returns a successful Equal assertion
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
-				return result.Of[Reader](Equal(42)(42))
+				return result.Of(Equal(42)(42))
 			}
 		}
 
@@ -80,7 +80,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		// Create a ReaderIOResult that returns a failing assertion
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
-				return result.Of[Reader](Equal(42)(43))
+				return result.Of(Equal(42)(43))
 			}
 		}
 
@@ -100,7 +100,7 @@ func TestFromReaderIOResult(t *testing.T) {
 				contextUsed = true
 			}
 			return func() result.Result[Reader] {
-				return result.Of[Reader](func(t *testing.T) bool {
+				return result.Of(func(t *testing.T) bool {
 					return true
 				})
 			}
@@ -118,7 +118,7 @@ func TestFromReaderIOResult(t *testing.T) {
 		// Create a ReaderIOResult that returns NoError assertion
 		ri := func(ctx context.Context) func() result.Result[Reader] {
 			return func() result.Result[Reader] {
-				return result.Of[Reader](NoError(nil))
+				return result.Of(NoError(nil))
 			}
 		}
 
@@ -139,7 +139,7 @@ func TestFromReaderIOResult(t *testing.T) {
 					ArrayLength[int](3)(arr),
 					ArrayContains(2)(arr),
 				})
-				return result.Of[Reader](assertions)
+				return result.Of(assertions)
 			}
 		}
 
@@ -297,7 +297,7 @@ func TestFromReaderIO(t *testing.T) {
 		// Create a ReaderIO with Result assertions
 		ri := func(ctx context.Context) func() Reader {
 			return func() Reader {
-				successResult := result.Of[int](42)
+				successResult := result.Of(42)
 				return Success(successResult)
 			}
 		}
@@ -338,7 +338,7 @@ func TestFromReaderIOResultIntegration(t *testing.T) {
 				}
 
 				// Return a successful assertion
-				return result.Of[Reader](Equal("test")("test"))
+				return result.Of(Equal("test")("test"))
 			}
 		}
 

v2/cli/apply.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -388,8 +387,8 @@ func generateApplyHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/bind.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -266,8 +265,8 @@ func generateBindHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n", pkg)
 

v2/cli/di.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -189,8 +188,8 @@ func generateDIHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/either.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -148,8 +147,8 @@ func generateEitherHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/header.go

@@ -18,7 +18,6 @@ package cli
 import (
 	"fmt"
 	"os"
-	"time"
 )
 
 func writePackage(f *os.File, pkg string) {
@@ -26,6 +25,6 @@ func writePackage(f *os.File, pkg string) {
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 }

v2/cli/identity.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -62,8 +61,8 @@ func generateIdentityHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/io.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	A "github.com/IBM/fp-go/v2/array"
 	C "github.com/urfave/cli/v3"
@@ -71,8 +70,8 @@ func generateIOHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/ioeither.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	A "github.com/IBM/fp-go/v2/array"
 	C "github.com/urfave/cli/v3"
@@ -219,8 +218,8 @@ func generateIOEitherHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 
@@ -234,8 +233,7 @@ import (
 
 	// some header
 	fmt.Fprintln(fg, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(fg, "// This file was generated by robots at")
-	fmt.Fprintf(fg, "// %s\n", time.Now())
+	fmt.Fprintln(fg, "// This file was generated by robots.")
 
 	fmt.Fprintf(fg, "package generic\n\n")
 

v2/cli/iooption.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	A "github.com/IBM/fp-go/v2/array"
 	C "github.com/urfave/cli/v3"
@@ -76,8 +75,8 @@ func generateIOOptionHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/option.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -148,8 +147,8 @@ func generateOptionHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/cli/pipe.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -378,8 +377,8 @@ func generatePipeHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n", pkg)
 

v2/cli/reader.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -118,8 +117,8 @@ func generateReaderHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 
@@ -131,8 +130,7 @@ import (
 
 	// some header
 	fmt.Fprintln(fg, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(fg, "// This file was generated by robots at")
-	fmt.Fprintf(fg, "// %s\n", time.Now())
+	fmt.Fprintln(fg, "// This file was generated by robots.")
 
 	fmt.Fprintf(fg, "package generic\n\n")
 

v2/cli/readerioeither.go

@@ -21,7 +21,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -233,8 +232,8 @@ func generateReaderIOEitherHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 
@@ -246,8 +245,7 @@ import (
 
 	// some header
 	fmt.Fprintln(fg, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(fg, "// This file was generated by robots at")
-	fmt.Fprintf(fg, "// %s\n", time.Now())
+	fmt.Fprintln(fg, "// This file was generated by robots.")
 
 	fmt.Fprintf(fg, "package generic\n\n")
 

v2/cli/tuple.go

@@ -22,7 +22,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"time"
 
 	C "github.com/urfave/cli/v3"
 )
@@ -399,8 +398,8 @@ func generateTupleHelpers(filename string, count int) error {
 
 	// some header
 	fmt.Fprintln(f, "// Code generated by go generate; DO NOT EDIT.")
-	fmt.Fprintln(f, "// This file was generated by robots at")
-	fmt.Fprintf(f, "// %s\n\n", time.Now())
+	fmt.Fprintln(f, "// This file was generated by robots.")
+	fmt.Fprintln(f)
 
 	fmt.Fprintf(f, "package %s\n\n", pkg)
 

v2/constant/const.go

@@ -13,6 +13,37 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package constant provides the Const functor, a phantom type that ignores its second type parameter.
+//
+// The Const functor is a fundamental building block in functional programming that wraps a value
+// of type E while having a phantom type parameter A. This makes it useful for:
+//   - Accumulating values during traversals (e.g., collecting metadata)
+//   - Implementing optics (lenses, prisms) where you need to track information
+//   - Building applicative functors that combine values using a semigroup
+//
+// # The Const Functor
+//
+// Const[E, A] wraps a value of type E and has a phantom type parameter A that doesn't affect
+// the runtime value. This allows it to participate in functor and applicative operations while
+// maintaining the wrapped value unchanged.
+//
+// # Key Properties
+//
+//   - Map operations ignore the function and preserve the wrapped value
+//   - Ap operations combine wrapped values using a semigroup
+//   - The phantom type A allows type-safe composition with other functors
+//
+// # Example Usage
+//
+//	// Accumulate string values
+//	c1 := Make[string, int]("hello")
+//	c2 := Make[string, int]("world")
+//
+//	// Map doesn't change the wrapped value
+//	mapped := Map[string, int, string](strconv.Itoa)(c1)  // Still contains "hello"
+//
+//	// Ap combines values using a semigroup
+//	combined := Ap[string, int, int](S.Monoid)(c1)(c2)  // Contains "helloworld"
 package constant
 
 import (
@@ -21,36 +52,209 @@ import (
 	S "github.com/IBM/fp-go/v2/semigroup"
 )
 
+// Const is a functor that wraps a value of type E with a phantom type parameter A.
+//
+// The Const functor is useful for accumulating values during traversals or implementing
+// optics. The type parameter A is phantom - it doesn't affect the runtime value but allows
+// the type to participate in functor and applicative operations.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value (the actual data)
+//   - A: The phantom type parameter (not stored, only used for type-level operations)
+//
+// Example:
+//
+//	// Create a Const that wraps a string
+//	c := Make[string, int]("metadata")
+//
+//	// The int type parameter is phantom - no int value is stored
+//	value := Unwrap(c)  // "metadata"
 type Const[E, A any] struct {
 	value E
 }
 
+// Make creates a Const value wrapping the given value.
+//
+// This is the primary constructor for Const values. The second type parameter A
+// is phantom and must be specified explicitly when needed for type inference.
+//
+// Type Parameters:
+//   - E: The type of the value to wrap
+//   - A: The phantom type parameter
+//
+// Parameters:
+//   - e: The value to wrap
+//
+// Returns:
+//   - A Const[E, A] wrapping the value
+//
+// Example:
+//
+//	c := Make[string, int]("hello")
+//	value := Unwrap(c)  // "hello"
 func Make[E, A any](e E) Const[E, A] {
 	return Const[E, A]{value: e}
 }
 
+// Unwrap extracts the wrapped value from a Const.
+//
+// This is the inverse of Make, retrieving the actual value stored in the Const.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value
+//   - A: The phantom type parameter
+//
+// Parameters:
+//   - c: The Const to unwrap
+//
+// Returns:
+//   - The wrapped value of type E
+//
+// Example:
+//
+//	c := Make[string, int]("world")
+//	value := Unwrap(c)  // "world"
 func Unwrap[E, A any](c Const[E, A]) E {
 	return c.value
 }
 
+// Of creates a Const containing the monoid's empty value, ignoring the input.
+//
+// This implements the Applicative's "pure" operation for Const. It creates a Const
+// wrapping the monoid's identity element, regardless of the input value.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value (must have a monoid)
+//   - A: The input type (ignored)
+//
+// Parameters:
+//   - m: The monoid providing the empty value
+//
+// Returns:
+//   - A function that ignores its input and returns Const[E, A] with the empty value
+//
+// Example:
+//
+//	import S "github.com/IBM/fp-go/v2/string"
+//
+//	of := Of[string, int](S.Monoid)
+//	c := of(42)  // Const[string, int] containing ""
+//	value := Unwrap(c)  // ""
 func Of[E, A any](m M.Monoid[E]) func(A) Const[E, A] {
 	return F.Constant1[A](Make[E, A](m.Empty()))
 }
 
+// MonadMap applies a function to the phantom type parameter without changing the wrapped value.
+//
+// This implements the Functor's map operation for Const. Since the type parameter A is phantom,
+// the function is never actually called - the wrapped value E remains unchanged.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - fa: The Const to map over
+//   - _: The function to apply (ignored)
+//
+// Returns:
+//   - A Const[E, B] with the same wrapped value
+//
+// Example:
+//
+//	c := Make[string, int]("hello")
+//	mapped := MonadMap(c, func(i int) string { return strconv.Itoa(i) })
+//	// mapped still contains "hello", function was never called
 func MonadMap[E, A, B any](fa Const[E, A], _ func(A) B) Const[E, B] {
 	return Make[E, B](fa.value)
 }
 
+// MonadAp combines two Const values using a semigroup.
+//
+// This implements the Applicative's ap operation for Const. It combines the wrapped
+// values from both Const instances using the provided semigroup, ignoring the function
+// type in the first argument.
+//
+// Type Parameters:
+//   - E: The type of the wrapped values (must have a semigroup)
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - s: The semigroup for combining wrapped values
+//
+// Returns:
+//   - A function that takes two Const values and combines their wrapped values
+//
+// Example:
+//
+//	import S "github.com/IBM/fp-go/v2/string"
+//
+//	ap := MonadAp[string, int, int](S.Monoid)
+//	c1 := Make[string, func(int) int]("hello")
+//	c2 := Make[string, int]("world")
+//	result := ap(c1, c2)  // Const containing "helloworld"
 func MonadAp[E, A, B any](s S.Semigroup[E]) func(fab Const[E, func(A) B], fa Const[E, A]) Const[E, B] {
 	return func(fab Const[E, func(A) B], fa Const[E, A]) Const[E, B] {
 		return Make[E, B](s.Concat(fab.value, fa.value))
 	}
 }
 
+// Map applies a function to the phantom type parameter without changing the wrapped value.
+//
+// This is the curried version of MonadMap, providing a more functional programming style.
+// The function is never actually called since A is a phantom type.
+//
+// Type Parameters:
+//   - E: The type of the wrapped value
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - f: The function to apply (ignored)
+//
+// Returns:
+//   - A function that transforms Const[E, A] to Const[E, B]
+//
+// Example:
+//
+//	import F "github.com/IBM/fp-go/v2/function"
+//
+//	c := Make[string, int]("data")
+//	mapped := F.Pipe1(c, Map[string, int, string](strconv.Itoa))
+//	// mapped still contains "data"
 func Map[E, A, B any](f func(A) B) func(fa Const[E, A]) Const[E, B] {
 	return F.Bind2nd(MonadMap[E, A, B], f)
 }
 
+// Ap combines Const values using a semigroup in a curried style.
+//
+// This is the curried version of MonadAp, providing data-last style for better composition.
+// It combines the wrapped values from both Const instances using the provided semigroup.
+//
+// Type Parameters:
+//   - E: The type of the wrapped values (must have a semigroup)
+//   - A: The input phantom type
+//   - B: The output phantom type
+//
+// Parameters:
+//   - s: The semigroup for combining wrapped values
+//
+// Returns:
+//   - A curried function for combining Const values
+//
+// Example:
+//
+//	import (
+//	    F "github.com/IBM/fp-go/v2/function"
+//	    S "github.com/IBM/fp-go/v2/string"
+//	)
+//
+//	c1 := Make[string, int]("hello")
+//	c2 := Make[string, func(int) int]("world")
+//	result := F.Pipe1(c1, Ap[string, int, int](S.Monoid)(c2))
+//	// result contains "helloworld"
 func Ap[E, A, B any](s S.Semigroup[E]) func(fa Const[E, A]) func(fab Const[E, func(A) B]) Const[E, B] {
 	monadap := MonadAp[E, A, B](s)
 	return func(fa Const[E, A]) func(fab Const[E, func(A) B]) Const[E, B] {

v2/constant/const_test.go

@@ -16,25 +16,340 @@
 package constant
 
 import (
+	"strconv"
 	"testing"
 
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/utils"
+	N "github.com/IBM/fp-go/v2/number"
 	S "github.com/IBM/fp-go/v2/string"
 
 	"github.com/stretchr/testify/assert"
 )
 
-func TestMap(t *testing.T) {
-	fa := Make[string, int]("foo")
-	assert.Equal(t, fa, F.Pipe1(fa, Map[string](utils.Double)))
+// TestMake tests the Make constructor
+func TestMake(t *testing.T) {
+	t.Run("creates Const with string value", func(t *testing.T) {
+		c := Make[string, int]("hello")
+		assert.Equal(t, "hello", Unwrap(c))
+	})
+
+	t.Run("creates Const with int value", func(t *testing.T) {
+		c := Make[int, string](42)
+		assert.Equal(t, 42, Unwrap(c))
+	})
+
+	t.Run("creates Const with struct value", func(t *testing.T) {
+		type Config struct {
+			Name string
+			Port int
+		}
+		cfg := Config{Name: "server", Port: 8080}
+		c := Make[Config, bool](cfg)
+		assert.Equal(t, cfg, Unwrap(c))
+	})
 }
 
+// TestUnwrap tests extracting values from Const
+func TestUnwrap(t *testing.T) {
+	t.Run("unwraps string value", func(t *testing.T) {
+		c := Make[string, int]("world")
+		value := Unwrap(c)
+		assert.Equal(t, "world", value)
+	})
+
+	t.Run("unwraps empty string", func(t *testing.T) {
+		c := Make[string, int]("")
+		value := Unwrap(c)
+		assert.Equal(t, "", value)
+	})
+
+	t.Run("unwraps zero value", func(t *testing.T) {
+		c := Make[int, string](0)
+		value := Unwrap(c)
+		assert.Equal(t, 0, value)
+	})
+}
+
+// TestOf tests the Of function
 func TestOf(t *testing.T) {
-	assert.Equal(t, Make[string, int](""), Of[string, int](S.Monoid)(1))
+	t.Run("creates Const with monoid empty value", func(t *testing.T) {
+		of := Of[string, int](S.Monoid)
+		c := of(42)
+		assert.Equal(t, "", Unwrap(c))
+	})
+
+	t.Run("ignores input value", func(t *testing.T) {
+		of := Of[string, int](S.Monoid)
+		c1 := of(1)
+		c2 := of(100)
+		assert.Equal(t, Unwrap(c1), Unwrap(c2))
+	})
+
+	t.Run("works with int monoid", func(t *testing.T) {
+		of := Of[int, string](N.MonoidSum[int]())
+		c := of("ignored")
+		assert.Equal(t, 0, Unwrap(c))
+	})
 }
 
-func TestAp(t *testing.T) {
-	fab := Make[string, int]("bar")
-	assert.Equal(t, Make[string, int]("foobar"), Ap[string, int, int](S.Monoid)(fab)(Make[string, func(int) int]("foo")))
+// TestMap tests the Map function
+func TestMap(t *testing.T) {
+	t.Run("preserves wrapped value", func(t *testing.T) {
+		fa := Make[string, int]("foo")
+		result := F.Pipe1(fa, Map[string](utils.Double))
+		assert.Equal(t, "foo", Unwrap(result))
+	})
+
+	t.Run("changes phantom type", func(t *testing.T) {
+		fa := Make[string, int]("data")
+		fb := Map[string, int, string](strconv.Itoa)(fa)
+		// Value unchanged, but type changed from Const[string, int] to Const[string, string]
+		assert.Equal(t, "data", Unwrap(fb))
+	})
+
+	t.Run("function is never called", func(t *testing.T) {
+		called := false
+		fa := Make[string, int]("test")
+		fb := Map[string, int, string](func(i int) string {
+			called = true
+			return strconv.Itoa(i)
+		})(fa)
+		assert.False(t, called, "Map function should not be called")
+		assert.Equal(t, "test", Unwrap(fb))
+	})
+}
+
+// TestMonadMap tests the MonadMap function
+func TestMonadMap(t *testing.T) {
+	t.Run("preserves wrapped value", func(t *testing.T) {
+		fa := Make[string, int]("original")
+		fb := MonadMap(fa, func(i int) string { return strconv.Itoa(i) })
+		assert.Equal(t, "original", Unwrap(fb))
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		fa := Make[int, string](42)
+		fb := MonadMap(fa, func(s string) bool { return len(s) > 0 })
+		assert.Equal(t, 42, Unwrap(fb))
+	})
+}
+
+// TestAp tests the Ap function
+func TestAp(t *testing.T) {
+	t.Run("combines string values", func(t *testing.T) {
+		fab := Make[string, int]("bar")
+		fa := Make[string, func(int) int]("foo")
+		result := Ap[string, int, int](S.Monoid)(fab)(fa)
+		assert.Equal(t, "foobar", Unwrap(result))
+	})
+
+	t.Run("combines int values with sum", func(t *testing.T) {
+		fab := Make[int, string](10)
+		fa := Make[int, func(string) string](5)
+		result := Ap[int, string, string](N.SemigroupSum[int]())(fab)(fa)
+		assert.Equal(t, 15, Unwrap(result))
+	})
+
+	t.Run("combines int values with product", func(t *testing.T) {
+		fab := Make[int, bool](3)
+		fa := Make[int, func(bool) bool](4)
+		result := Ap[int, bool, bool](N.SemigroupProduct[int]())(fab)(fa)
+		assert.Equal(t, 12, Unwrap(result))
+	})
+}
+
+// TestMonadAp tests the MonadAp function
+func TestMonadAp(t *testing.T) {
+	t.Run("combines values using semigroup", func(t *testing.T) {
+		ap := MonadAp[string, int, int](S.Monoid)
+		fab := Make[string, func(int) int]("hello")
+		fa := Make[string, int]("world")
+		result := ap(fab, fa)
+		assert.Equal(t, "helloworld", Unwrap(result))
+	})
+
+	t.Run("works with empty strings", func(t *testing.T) {
+		ap := MonadAp[string, int, int](S.Monoid)
+		fab := Make[string, func(int) int]("")
+		fa := Make[string, int]("test")
+		result := ap(fab, fa)
+		assert.Equal(t, "test", Unwrap(result))
+	})
+}
+
+// TestMonoid tests the Monoid function
+func TestMonoid(t *testing.T) {
+	t.Run("always returns constant value", func(t *testing.T) {
+		m := Monoid(42)
+		assert.Equal(t, 42, m.Concat(1, 2))
+		assert.Equal(t, 42, m.Concat(100, 200))
+		assert.Equal(t, 42, m.Empty())
+	})
+
+	t.Run("works with strings", func(t *testing.T) {
+		m := Monoid("constant")
+		assert.Equal(t, "constant", m.Concat("a", "b"))
+		assert.Equal(t, "constant", m.Empty())
+	})
+
+	t.Run("works with structs", func(t *testing.T) {
+		type Point struct{ X, Y int }
+		p := Point{X: 1, Y: 2}
+		m := Monoid(p)
+		assert.Equal(t, p, m.Concat(Point{X: 3, Y: 4}, Point{X: 5, Y: 6}))
+		assert.Equal(t, p, m.Empty())
+	})
+
+	t.Run("satisfies monoid laws", func(t *testing.T) {
+		m := Monoid(10)
+
+		// Left identity: Concat(Empty(), x) = x (both return constant)
+		assert.Equal(t, 10, m.Concat(m.Empty(), 5))
+
+		// Right identity: Concat(x, Empty()) = x (both return constant)
+		assert.Equal(t, 10, m.Concat(5, m.Empty()))
+
+		// Associativity: Concat(Concat(x, y), z) = Concat(x, Concat(y, z))
+		left := m.Concat(m.Concat(1, 2), 3)
+		right := m.Concat(1, m.Concat(2, 3))
+		assert.Equal(t, left, right)
+		assert.Equal(t, 10, left)
+	})
+}
+
+// TestConstFunctorLaws tests functor laws for Const
+func TestConstFunctorLaws(t *testing.T) {
+	t.Run("identity law", func(t *testing.T) {
+		// map id = id
+		fa := Make[string, int]("test")
+		mapped := Map[string, int, int](F.Identity[int])(fa)
+		assert.Equal(t, Unwrap(fa), Unwrap(mapped))
+	})
+
+	t.Run("composition law", func(t *testing.T) {
+		// map (g . f) = map g . map f
+		fa := Make[string, int]("data")
+		f := func(i int) string { return strconv.Itoa(i) }
+		g := func(s string) bool { return len(s) > 0 }
+
+		// map (g . f)
+		composed := Map[string, int, bool](func(i int) bool { return g(f(i)) })(fa)
+
+		// map g . map f
+		intermediate := F.Pipe1(fa, Map[string, int, string](f))
+		chained := Map[string, string, bool](g)(intermediate)
+
+		assert.Equal(t, Unwrap(composed), Unwrap(chained))
+	})
+}
+
+// TestConstApplicativeLaws tests applicative laws for Const
+func TestConstApplicativeLaws(t *testing.T) {
+	t.Run("identity law", func(t *testing.T) {
+		// For Const, ap combines the wrapped values using the semigroup
+		// ap (of id) v combines empty (from of) with v's value
+		v := Make[string, int]("value")
+		ofId := Of[string, func(int) int](S.Monoid)(F.Identity[int])
+		result := Ap[string, int, int](S.Monoid)(v)(ofId)
+		// Result combines "" (from Of) with "value" using string monoid
+		assert.Equal(t, "value", Unwrap(result))
+	})
+
+	t.Run("homomorphism law", func(t *testing.T) {
+		// ap (of f) (of x) = of (f x)
+		f := func(i int) string { return strconv.Itoa(i) }
+		x := 42
+
+		ofF := Of[string, func(int) string](S.Monoid)(f)
+		ofX := Of[string, int](S.Monoid)(x)
+		left := Ap[string, int, string](S.Monoid)(ofX)(ofF)
+
+		right := Of[string, string](S.Monoid)(f(x))
+
+		assert.Equal(t, Unwrap(left), Unwrap(right))
+	})
+}
+
+// TestConstEdgeCases tests edge cases
+func TestConstEdgeCases(t *testing.T) {
+	t.Run("empty string values", func(t *testing.T) {
+		c := Make[string, int]("")
+		assert.Equal(t, "", Unwrap(c))
+
+		mapped := Map[string, int, string](strconv.Itoa)(c)
+		assert.Equal(t, "", Unwrap(mapped))
+	})
+
+	t.Run("zero values", func(t *testing.T) {
+		c := Make[int, string](0)
+		assert.Equal(t, 0, Unwrap(c))
+	})
+
+	t.Run("nil pointer", func(t *testing.T) {
+		var ptr *int
+		c := Make[*int, string](ptr)
+		assert.Nil(t, Unwrap(c))
+	})
+
+	t.Run("multiple map operations", func(t *testing.T) {
+		c := Make[string, int]("original")
+		// Chain multiple map operations
+		step1 := Map[string, int, string](strconv.Itoa)(c)
+		step2 := Map[string, string, bool](func(s string) bool { return len(s) > 0 })(step1)
+		result := Map[string, bool, int](func(b bool) int {
+			if b {
+				return 1
+			}
+			return 0
+		})(step2)
+		assert.Equal(t, "original", Unwrap(result))
+	})
+}
+
+// BenchmarkMake benchmarks the Make constructor
+func BenchmarkMake(b *testing.B) {
+	b.ResetTimer()
+	for b.Loop() {
+		_ = Make[string, int]("test")
+	}
+}
+
+// BenchmarkUnwrap benchmarks the Unwrap function
+func BenchmarkUnwrap(b *testing.B) {
+	c := Make[string, int]("test")
+	b.ResetTimer()
+	for b.Loop() {
+		_ = Unwrap(c)
+	}
+}
+
+// BenchmarkMap benchmarks the Map function
+func BenchmarkMap(b *testing.B) {
+	c := Make[string, int]("test")
+	mapFn := Map[string, int, string](strconv.Itoa)
+	b.ResetTimer()
+	for b.Loop() {
+		_ = mapFn(c)
+	}
+}
+
+// BenchmarkAp benchmarks the Ap function
+func BenchmarkAp(b *testing.B) {
+	fab := Make[string, int]("hello")
+	fa := Make[string, func(int) int]("world")
+	apFn := Ap[string, int, int](S.Monoid)
+	b.ResetTimer()
+	for b.Loop() {
+		_ = apFn(fab)(fa)
+	}
+}
+
+// BenchmarkMonoid benchmarks the Monoid function
+func BenchmarkMonoid(b *testing.B) {
+	m := Monoid(42)
+	b.ResetTimer()
+	for b.Loop() {
+		_ = m.Concat(1, 2)
+	}
 }

v2/constant/monoid.go

@@ -1,3 +1,18 @@
+// 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 constant
 
 import (
@@ -5,7 +20,47 @@ import (
 	M "github.com/IBM/fp-go/v2/monoid"
 )
 
-// Monoid returns a [M.Monoid] that returns a constant value in all operations
+// Monoid creates a monoid that always returns a constant value.
+//
+// This creates a trivial monoid where both the Concat operation and Empty
+// always return the same constant value, regardless of inputs. This is useful
+// for testing, placeholder implementations, or when you need a monoid instance
+// but the actual combining behavior doesn't matter.
+//
+// # Monoid Laws
+//
+// The constant monoid satisfies all monoid laws trivially:
+//   - Associativity: Concat(Concat(x, y), z) = Concat(x, Concat(y, z)) - always returns 'a'
+//   - Left Identity: Concat(Empty(), x) = x - both return 'a'
+//   - Right Identity: Concat(x, Empty()) = x - both return 'a'
+//
+// Type Parameters:
+//   - A: The type of the constant value
+//
+// Parameters:
+//   - a: The constant value to return in all operations
+//
+// Returns:
+//   - A Monoid[A] that always returns the constant value
+//
+// Example:
+//
+//	// Create a monoid that always returns 42
+//	m := Monoid(42)
+//	result := m.Concat(1, 2)  // 42
+//	empty := m.Empty()         // 42
+//
+//	// Useful for testing or placeholder implementations
+//	type Config struct {
+//	    Timeout int
+//	}
+//	defaultConfig := Monoid(Config{Timeout: 30})
+//	config := defaultConfig.Concat(Config{Timeout: 10}, Config{Timeout: 20})
+//	// config is Config{Timeout: 30}
+//
+// See also:
+//   - function.Constant2: The underlying constant function
+//   - M.MakeMonoid: The monoid constructor
 func Monoid[A any](a A) M.Monoid[A] {
 	return M.MakeMonoid(function.Constant2[A, A](a), a)
 }

v2/effect/common_test.go

@@ -26,7 +26,7 @@ type TestContext struct {
 
 // runEffect is a helper function to run an effect with a context and return the result
 func runEffect[C, A any](eff Effect[C, A], ctx C) (A, error) {
-	ioResult := Provide[A, C](ctx)(eff)
+	ioResult := Provide[A](ctx)(eff)
 	readerResult := RunSync(ioResult)
 	return readerResult(context.Background())
 }

v2/effect/dependencies_test.go

@@ -44,11 +44,11 @@ func TestLocal(t *testing.T) {
 		}
 
 		// Apply Local to transform the context
-		kleisli := Local[string, OuterContext, InnerContext](accessor)
+		kleisli := Local[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
 		// Run with OuterContext
-		ioResult := Provide[string, OuterContext](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
@@ -70,11 +70,11 @@ func TestLocal(t *testing.T) {
 			return InnerContext{Value: outer.Value + " transformed"}
 		}
 
-		kleisli := Local[string, OuterContext, InnerContext](accessor)
+		kleisli := Local[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
 		// Run with OuterContext
-		ioResult := Provide[string, OuterContext](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "original",
 			Number: 100,
 		})(outerEffect)
@@ -93,10 +93,10 @@ func TestLocal(t *testing.T) {
 			return InnerContext{Value: outer.Value}
 		}
 
-		kleisli := Local[string, OuterContext, InnerContext](accessor)
+		kleisli := Local[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
-		ioResult := Provide[string, OuterContext](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
@@ -122,12 +122,12 @@ func TestLocal(t *testing.T) {
 		level3Effect := Of[Level3]("deep result")
 
 		// Transform Level2 -> Level3
-		local23 := Local[string, Level2, Level3](func(l2 Level2) Level3 {
+		local23 := Local[string](func(l2 Level2) Level3 {
 			return Level3{C: l2.B + "-c"}
 		})
 
 		// Transform Level1 -> Level2
-		local12 := Local[string, Level1, Level2](func(l1 Level1) Level2 {
+		local12 := Local[string](func(l1 Level1) Level2 {
 			return Level2{B: l1.A + "-b"}
 		})
 
@@ -136,7 +136,7 @@ func TestLocal(t *testing.T) {
 		level1Effect := local12(level2Effect)
 
 		// Run with Level1 context
-		ioResult := Provide[string, Level1](Level1{A: "a"})(level1Effect)
+		ioResult := Provide[string](Level1{A: "a"})(level1Effect)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -165,11 +165,11 @@ func TestLocal(t *testing.T) {
 			return app.DB
 		}
 
-		kleisli := Local[string, AppConfig, DatabaseConfig](accessor)
+		kleisli := Local[string](accessor)
 		appEffect := kleisli(dbEffect)
 
 		// Run with full AppConfig
-		ioResult := Provide[string, AppConfig](AppConfig{
+		ioResult := Provide[string](AppConfig{
 			DB: DatabaseConfig{
 				Host:     "localhost",
 				Port:     5432,
@@ -195,21 +195,21 @@ func TestContramap(t *testing.T) {
 		}
 
 		// Test Local
-		localKleisli := Local[int, OuterContext, InnerContext](accessor)
+		localKleisli := Local[int](accessor)
 		localEffect := localKleisli(innerEffect)
 
 		// Test Contramap
-		contramapKleisli := Contramap[int, OuterContext, InnerContext](accessor)
+		contramapKleisli := Contramap[int](accessor)
 		contramapEffect := contramapKleisli(innerEffect)
 
 		outerCtx := OuterContext{Value: "test", Number: 100}
 
 		// Run both
-		localIO := Provide[int, OuterContext](outerCtx)(localEffect)
+		localIO := Provide[int](outerCtx)(localEffect)
 		localReader := RunSync(localIO)
 		localResult, localErr := localReader(context.Background())
 
-		contramapIO := Provide[int, OuterContext](outerCtx)(contramapEffect)
+		contramapIO := Provide[int](outerCtx)(contramapEffect)
 		contramapReader := RunSync(contramapIO)
 		contramapResult, contramapErr := contramapReader(context.Background())
 
@@ -225,10 +225,10 @@ func TestContramap(t *testing.T) {
 			return InnerContext{Value: outer.Value + " modified"}
 		}
 
-		kleisli := Contramap[string, OuterContext, InnerContext](accessor)
+		kleisli := Contramap[string](accessor)
 		outerEffect := kleisli(innerEffect)
 
-		ioResult := Provide[string, OuterContext](OuterContext{
+		ioResult := Provide[string](OuterContext{
 			Value:  "original",
 			Number: 50,
 		})(outerEffect)
@@ -247,10 +247,10 @@ func TestContramap(t *testing.T) {
 			return InnerContext{Value: outer.Value}
 		}
 
-		kleisli := Contramap[int, OuterContext, InnerContext](accessor)
+		kleisli := Contramap[int](accessor)
 		outerEffect := kleisli(innerEffect)
 
-		ioResult := Provide[int, OuterContext](OuterContext{
+		ioResult := Provide[int](OuterContext{
 			Value:  "test",
 			Number: 42,
 		})(outerEffect)
@@ -278,12 +278,12 @@ func TestLocalAndContramapInteroperability(t *testing.T) {
 		effect3 := Of[Config3]("result")
 
 		// Use Local for first transformation
-		local23 := Local[string, Config2, Config3](func(c2 Config2) Config3 {
+		local23 := Local[string](func(c2 Config2) Config3 {
 			return Config3{Info: c2.Data}
 		})
 
 		// Use Contramap for second transformation
-		contramap12 := Contramap[string, Config1, Config2](func(c1 Config1) Config2 {
+		contramap12 := Contramap[string](func(c1 Config1) Config2 {
 			return Config2{Data: c1.Value}
 		})
 
@@ -292,7 +292,7 @@ func TestLocalAndContramapInteroperability(t *testing.T) {
 		effect1 := contramap12(effect2)
 
 		// Run
-		ioResult := Provide[string, Config1](Config1{Value: "test"})(effect1)
+		ioResult := Provide[string](Config1{Value: "test"})(effect1)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -326,7 +326,7 @@ func TestLocalEffectK(t *testing.T) {
 		appEffect := transform(dbEffect)
 
 		// Run with AppConfig
-		ioResult := Provide[string, AppConfig](AppConfig{
+		ioResult := Provide[string](AppConfig{
 			ConfigPath: "/etc/app.conf",
 		})(appEffect)
 		readerResult := RunSync(ioResult)
@@ -356,7 +356,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[string](failingTransform)
 		outerEffect := transform(innerEffect)
 
-		ioResult := Provide[string, OuterCtx](OuterCtx{Path: "test"})(outerEffect)
+		ioResult := Provide[string](OuterCtx{Path: "test"})(outerEffect)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -384,7 +384,7 @@ func TestLocalEffectK(t *testing.T) {
 		transformK := LocalEffectK[string](transform)
 		outerEffect := transformK(innerEffect)
 
-		ioResult := Provide[string, OuterCtx](OuterCtx{Path: "test"})(outerEffect)
+		ioResult := Provide[string](OuterCtx{Path: "test"})(outerEffect)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -417,7 +417,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[string](loadConfigEffect)
 		appEffect := transform(configEffect)
 
-		ioResult := Provide[string, AppContext](AppContext{
+		ioResult := Provide[string](AppContext{
 			ConfigFile: "config.json",
 		})(appEffect)
 		readerResult := RunSync(ioResult)
@@ -456,7 +456,7 @@ func TestLocalEffectK(t *testing.T) {
 		level1Effect := transform12(level2Effect)
 
 		// Run with Level1 context
-		ioResult := Provide[string, Level1](Level1{A: "a"})(level1Effect)
+		ioResult := Provide[string](Level1{A: "a"})(level1Effect)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -497,7 +497,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[string](transformWithContext)
 		appEffect := transform(dbEffect)
 
-		ioResult := Provide[string, AppConfig](AppConfig{
+		ioResult := Provide[string](AppConfig{
 			Environment: "prod",
 			DBHost:      "localhost",
 			DBPort:      5432,
@@ -534,14 +534,14 @@ func TestLocalEffectK(t *testing.T) {
 		outerEffect := transform(innerEffect)
 
 		// Test with invalid config
-		ioResult := Provide[string, RawConfig](RawConfig{APIKey: ""})(outerEffect)
+		ioResult := Provide[string](RawConfig{APIKey: ""})(outerEffect)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
 		assert.Error(t, err)
 
 		// Test with valid config
-		ioResult2 := Provide[string, RawConfig](RawConfig{APIKey: "valid-key"})(outerEffect)
+		ioResult2 := Provide[string](RawConfig{APIKey: "valid-key"})(outerEffect)
 		readerResult2 := RunSync(ioResult2)
 		result, err2 := readerResult2(context.Background())
 
@@ -569,7 +569,7 @@ func TestLocalEffectK(t *testing.T) {
 		})
 
 		// Use Local for second transformation (pure)
-		local12 := Local[string, Level1, Level2](func(l1 Level1) Level2 {
+		local12 := Local[string](func(l1 Level1) Level2 {
 			return Level2{Data: l1.Value}
 		})
 
@@ -578,7 +578,7 @@ func TestLocalEffectK(t *testing.T) {
 		effect1 := local12(effect2)
 
 		// Run
-		ioResult := Provide[string, Level1](Level1{Value: "test"})(effect1)
+		ioResult := Provide[string](Level1{Value: "test"})(effect1)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -610,7 +610,7 @@ func TestLocalEffectK(t *testing.T) {
 		transform := LocalEffectK[int](complexTransform)
 		outerEffect := transform(innerEffect)
 
-		ioResult := Provide[int, OuterCtx](OuterCtx{Multiplier: 3})(outerEffect)
+		ioResult := Provide[int](OuterCtx{Multiplier: 3})(outerEffect)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 

v2/effect/effect_additional_test.go

@@ -138,7 +138,7 @@ func TestChain_Success(t *testing.T) {
 	t.Run("sequences two effects", func(t *testing.T) {
 		eff := F.Pipe1(
 			Of[TestConfig](42),
-			Chain[TestConfig](func(x int) Effect[TestConfig, string] {
+			Chain(func(x int) Effect[TestConfig, string] {
 				return Of[TestConfig](strconv.Itoa(x))
 			}),
 		)
@@ -149,10 +149,10 @@ func TestChain_Success(t *testing.T) {
 	t.Run("chains multiple effects", func(t *testing.T) {
 		eff := F.Pipe2(
 			Of[TestConfig](10),
-			Chain[TestConfig](func(x int) Effect[TestConfig, int] {
+			Chain(func(x int) Effect[TestConfig, int] {
 				return Of[TestConfig](x + 5)
 			}),
-			Chain[TestConfig](func(x int) Effect[TestConfig, int] {
+			Chain(func(x int) Effect[TestConfig, int] {
 				return Of[TestConfig](x * 2)
 			}),
 		)
@@ -166,7 +166,7 @@ func TestChain_Failure(t *testing.T) {
 		testErr := errors.New("first error")
 		eff := F.Pipe1(
 			Fail[TestConfig, int](testErr),
-			Chain[TestConfig](func(x int) Effect[TestConfig, string] {
+			Chain(func(x int) Effect[TestConfig, string] {
 				return Of[TestConfig]("should not execute")
 			}),
 		)
@@ -178,7 +178,7 @@ func TestChain_Failure(t *testing.T) {
 		testErr := errors.New("second error")
 		eff := F.Pipe1(
 			Of[TestConfig](42),
-			Chain[TestConfig](func(x int) Effect[TestConfig, string] {
+			Chain(func(x int) Effect[TestConfig, string] {
 				return Fail[TestConfig, string](testErr)
 			}),
 		)
@@ -503,7 +503,7 @@ func TestTap_Success(t *testing.T) {
 		log := []string{}
 		eff := F.Pipe1(
 			Of[TestConfig](42),
-			Tap[TestConfig](func(x int) Effect[TestConfig, any] {
+			Tap(func(x int) Effect[TestConfig, any] {
 				log = append(log, fmt.Sprintf("tapped: %d", x))
 				return Of[TestConfig, any](nil)
 			}),
@@ -517,11 +517,11 @@ func TestTap_Success(t *testing.T) {
 		log := []string{}
 		eff := F.Pipe2(
 			Of[TestConfig](10),
-			Tap[TestConfig](func(x int) Effect[TestConfig, any] {
+			Tap(func(x int) Effect[TestConfig, any] {
 				log = append(log, "first")
 				return Of[TestConfig, any](nil)
 			}),
-			Tap[TestConfig](func(x int) Effect[TestConfig, any] {
+			Tap(func(x int) Effect[TestConfig, any] {
 				log = append(log, "second")
 				return Of[TestConfig, any](nil)
 			}),
@@ -538,7 +538,7 @@ func TestTap_Failure(t *testing.T) {
 		executed := false
 		eff := F.Pipe1(
 			Fail[TestConfig, int](testErr),
-			Tap[TestConfig](func(x int) Effect[TestConfig, any] {
+			Tap(func(x int) Effect[TestConfig, any] {
 				executed = true
 				return Of[TestConfig, any](nil)
 			}),
@@ -620,7 +620,7 @@ func TestRead_Success(t *testing.T) {
 		// Create an effect that uses the context's Multiplier
 		eff := F.Pipe1(
 			Of[TestConfig](10),
-			ChainReaderK[TestConfig](func(x int) reader.Reader[TestConfig, int] {
+			ChainReaderK(func(x int) reader.Reader[TestConfig, int] {
 				return func(cfg TestConfig) int {
 					return x * cfg.Multiplier
 				}

v2/effect/effect_test.go

@@ -641,8 +641,8 @@ func TestChainThunkK_Integration(t *testing.T) {
 
 		computation := F.Pipe3(
 			Of[TestConfig](5),
-			ChainReaderK[TestConfig](addMultiplier),
-			ChainReaderIOK[TestConfig](logValue),
+			ChainReaderK(addMultiplier),
+			ChainReaderIOK(logValue),
 			ChainThunkK[TestConfig](processThunk),
 		)
 		outcome := computation(testConfig)(context.Background())()

v2/effect/run_test.go

@@ -28,7 +28,7 @@ func TestProvide(t *testing.T) {
 		ctx := TestContext{Value: "test-value"}
 		eff := Of[TestContext]("result")
 
-		ioResult := Provide[string, TestContext](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -45,7 +45,7 @@ func TestProvide(t *testing.T) {
 		cfg := Config{Host: "localhost", Port: 8080}
 		eff := Of[Config]("connected")
 
-		ioResult := Provide[string, Config](cfg)(eff)
+		ioResult := Provide[string](cfg)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -58,7 +58,7 @@ func TestProvide(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Fail[TestContext, string](expectedErr)
 
-		ioResult := Provide[string, TestContext](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -74,7 +74,7 @@ func TestProvide(t *testing.T) {
 		ctx := SimpleContext{ID: 42}
 		eff := Of[SimpleContext](100)
 
-		ioResult := Provide[int, SimpleContext](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -89,7 +89,7 @@ func TestProvide(t *testing.T) {
 			return Of[TestContext]("result")
 		})(Of[TestContext](42))
 
-		ioResult := Provide[string, TestContext](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -104,7 +104,7 @@ func TestProvide(t *testing.T) {
 			return "mapped"
 		})(Of[TestContext](42))
 
-		ioResult := Provide[string, TestContext](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -118,7 +118,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Of[TestContext](42)
 
-		ioResult := Provide[int, TestContext](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -130,7 +130,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Of[TestContext]("hello")
 
-		ioResult := Provide[string, TestContext](ctx)(eff)
+		ioResult := Provide[string](ctx)(eff)
 		readerResult := RunSync(ioResult)
 
 		bgCtx := context.Background()
@@ -145,7 +145,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Fail[TestContext, int](expectedErr)
 
-		ioResult := Provide[int, TestContext](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		_, err := readerResult(context.Background())
 
@@ -162,7 +162,7 @@ func TestRunSync(t *testing.T) {
 			return Of[TestContext](x + 10)
 		})(Of[TestContext](5)))
 
-		ioResult := Provide[int, TestContext](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -174,7 +174,7 @@ func TestRunSync(t *testing.T) {
 		ctx := TestContext{Value: "test"}
 		eff := Of[TestContext](42)
 
-		ioResult := Provide[int, TestContext](ctx)(eff)
+		ioResult := Provide[int](ctx)(eff)
 		readerResult := RunSync(ioResult)
 
 		// Run multiple times
@@ -200,7 +200,7 @@ func TestRunSync(t *testing.T) {
 		user := User{Name: "Alice", Age: 30}
 		eff := Of[TestContext](user)
 
-		ioResult := Provide[User, TestContext](ctx)(eff)
+		ioResult := Provide[User](ctx)(eff)
 		readerResult := RunSync(ioResult)
 		result, err := readerResult(context.Background())
 
@@ -222,7 +222,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 		eff := Of[AppConfig]("API call successful")
 
 		// Provide config and run
-		result, err := RunSync(Provide[string, AppConfig](cfg)(eff))(context.Background())
+		result, err := RunSync(Provide[string](cfg)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "API call successful", result)
@@ -238,7 +238,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 
 		eff := Fail[AppConfig, string](expectedErr)
 
-		_, err := RunSync(Provide[string, AppConfig](cfg)(eff))(context.Background())
+		_, err := RunSync(Provide[string](cfg)(eff))(context.Background())
 
 		assert.Error(t, err)
 		assert.Equal(t, expectedErr, err)
@@ -253,7 +253,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 			return Of[TestContext](x * 2)
 		})(Of[TestContext](21)))
 
-		result, err := RunSync(Provide[string, TestContext](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[string](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "final", result)
@@ -281,7 +281,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 			return State{X: x}
 		})(Of[TestContext](10)))
 
-		result, err := RunSync(Provide[State, TestContext](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[State](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, 10, result.X)
@@ -300,11 +300,11 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 		innerEff := Of[InnerCtx]("inner result")
 
 		// Transform context
-		transformedEff := Local[string, OuterCtx, InnerCtx](func(outer OuterCtx) InnerCtx {
+		transformedEff := Local[string](func(outer OuterCtx) InnerCtx {
 			return InnerCtx{Data: outer.Value + "-transformed"}
 		})(innerEff)
 
-		result, err := RunSync(Provide[string, OuterCtx](outerCtx)(transformedEff))(context.Background())
+		result, err := RunSync(Provide[string](outerCtx)(transformedEff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, "inner result", result)
@@ -318,7 +318,7 @@ func TestProvideAndRunSyncIntegration(t *testing.T) {
 			return Of[TestContext](x * 2)
 		})(input)
 
-		result, err := RunSync(Provide[[]int, TestContext](ctx)(eff))(context.Background())
+		result, err := RunSync(Provide[[]int](ctx)(eff))(context.Background())
 
 		assert.NoError(t, err)
 		assert.Equal(t, []int{2, 4, 6, 8, 10}, result)

v2/either/either_test.go

@@ -379,7 +379,7 @@ func TestMonadChainLeft(t *testing.T) {
 func TestChainLeft(t *testing.T) {
 	t.Run("Curried function transforms Left value", func(t *testing.T) {
 		// Create a reusable error handler
-		handleNotFound := ChainLeft[error, string](func(err error) Either[string, int] {
+		handleNotFound := ChainLeft(func(err error) Either[string, int] {
 			if err.Error() == "not found" {
 				return Right[string](0)
 			}
@@ -391,7 +391,7 @@ func TestChainLeft(t *testing.T) {
 	})
 
 	t.Run("Curried function with Right value", func(t *testing.T) {
-		handler := ChainLeft[error, string](func(err error) Either[string, int] {
+		handler := ChainLeft(func(err error) Either[string, int] {
 			return Left[int]("should not be called")
 		})
 
@@ -401,7 +401,7 @@ func TestChainLeft(t *testing.T) {
 
 	t.Run("Use in pipeline with Pipe", func(t *testing.T) {
 		// Create error transformer
-		toStringError := ChainLeft[int, string](func(code int) Either[string, string] {
+		toStringError := ChainLeft(func(code int) Either[string, string] {
 			return Left[string](fmt.Sprintf("Error: %d", code))
 		})
 
@@ -414,12 +414,12 @@ func TestChainLeft(t *testing.T) {
 
 	t.Run("Compose multiple ChainLeft operations", func(t *testing.T) {
 		// First handler: convert error to string
-		handler1 := ChainLeft[error, string](func(err error) Either[string, int] {
+		handler1 := ChainLeft(func(err error) Either[string, int] {
 			return Left[int](err.Error())
 		})
 
 		// Second handler: add prefix to string error
-		handler2 := ChainLeft[string, string](func(s string) Either[string, int] {
+		handler2 := ChainLeft(func(s string) Either[string, int] {
 			return Left[int]("Handled: " + s)
 		})
 

v2/either/types.go

@@ -55,5 +55,7 @@ type (
 	// It's commonly used for filtering and conditional operations.
 	Predicate[A any] = predicate.Predicate[A]
 
+	// Pair represents a tuple of two values of types L and R.
+	// It's commonly used to return multiple values from functions or to group related data.
 	Pair[L, R any] = pair.Pair[L, R]
 )

v2/monoid/types.go

@@ -0,0 +1,52 @@
+// 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 monoid
+
+import "github.com/IBM/fp-go/v2/function"
+
+// Void is an alias for function.Void, representing the unit type.
+//
+// The Void type (also known as Unit in functional programming) has exactly one value,
+// making it useful for representing the absence of meaningful information. It's similar
+// to void in other languages, but as a value rather than the absence of a value.
+//
+// This type alias is provided in the monoid package for convenience when working with
+// VoidMonoid and other monoid operations that may use the unit type.
+//
+// Common use cases:
+//   - As a return type for functions that perform side effects but don't return meaningful data
+//   - As a placeholder type parameter when a type is required but no data needs to be passed
+//   - In monoid operations where you need to track that operations occurred without caring about results
+//
+// See also:
+//   - function.Void: The underlying type definition
+//   - function.VOID: The single inhabitant of the Void type
+//   - VoidMonoid: A monoid instance for the Void type
+//
+// Example:
+//
+//	// Function that performs an action but returns no meaningful data
+//	func logMessage(msg string) Void {
+//	    fmt.Println(msg)
+//	    return function.VOID
+//	}
+//
+//	// Using Void in monoid operations
+//	m := VoidMonoid()
+//	result := m.Concat(function.VOID, function.VOID)  // function.VOID
+type (
+	Void = function.Void
+)

v2/monoid/void.go

@@ -0,0 +1,65 @@
+// 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 monoid
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	S "github.com/IBM/fp-go/v2/semigroup"
+)
+
+// VoidMonoid creates a Monoid for the Void (unit) type.
+//
+// The Void type has exactly one value (function.VOID), making it trivial to define
+// a monoid. This monoid uses the Last semigroup, which always returns the second
+// argument, though since all Void values are identical, the choice of semigroup
+// doesn't affect the result.
+//
+// This monoid is useful in contexts where:
+//   - A monoid instance is required but no meaningful data needs to be combined
+//   - You need to track that an operation occurred without caring about its result
+//   - Building generic abstractions that work with any monoid, including the trivial case
+//
+// # Monoid Laws
+//
+// The VoidMonoid satisfies all monoid laws trivially:
+//   - Associativity: Concat(Concat(x, y), z) = Concat(x, Concat(y, z)) - always VOID
+//   - Left Identity: Concat(Empty(), x) = x - always VOID
+//   - Right Identity: Concat(x, Empty()) = x - always VOID
+//
+// Returns:
+//   - A Monoid[Void] instance
+//
+// Example:
+//
+//	m := VoidMonoid()
+//	result := m.Concat(function.VOID, function.VOID)  // function.VOID
+//	empty := m.Empty()                                 // function.VOID
+//
+//	// Useful for tracking operations without data
+//	type Action = func() Void
+//	actions := []Action{
+//	    func() Void { fmt.Println("Action 1"); return function.VOID },
+//	    func() Void { fmt.Println("Action 2"); return function.VOID },
+//	}
+//	// Execute all actions and combine results
+//	results := A.Map(func(a Action) Void { return a() })(actions)
+//	_ = ConcatAll(m)(results)  // All actions executed, result is VOID
+func VoidMonoid() Monoid[Void] {
+	return MakeMonoid(
+		S.Last[Void]().Concat,
+		function.VOID,
+	)
+}

v2/monoid/void_test.go

@@ -0,0 +1,292 @@
+// 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 monoid
+
+import (
+	"testing"
+
+	"github.com/IBM/fp-go/v2/function"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestVoidMonoid_Basic tests basic VoidMonoid functionality
+func TestVoidMonoid_Basic(t *testing.T) {
+	m := VoidMonoid()
+
+	// Test Empty returns VOID
+	empty := m.Empty()
+	assert.Equal(t, function.VOID, empty)
+
+	// Test Concat returns VOID (since all Void values are identical)
+	result := m.Concat(function.VOID, function.VOID)
+	assert.Equal(t, function.VOID, result)
+}
+
+// TestVoidMonoid_Laws verifies VoidMonoid satisfies monoid laws
+func TestVoidMonoid_Laws(t *testing.T) {
+	m := VoidMonoid()
+
+	// Since Void has only one value, we test with that value
+	v := function.VOID
+
+	// Left Identity: Concat(Empty(), x) = x
+	t.Run("left identity", func(t *testing.T) {
+		result := m.Concat(m.Empty(), v)
+		assert.Equal(t, v, result, "Left identity law failed")
+	})
+
+	// Right Identity: Concat(x, Empty()) = x
+	t.Run("right identity", func(t *testing.T) {
+		result := m.Concat(v, m.Empty())
+		assert.Equal(t, v, result, "Right identity law failed")
+	})
+
+	// Associativity: Concat(Concat(x, y), z) = Concat(x, Concat(y, z))
+	t.Run("associativity", func(t *testing.T) {
+		left := m.Concat(m.Concat(v, v), v)
+		right := m.Concat(v, m.Concat(v, v))
+		assert.Equal(t, left, right, "Associativity law failed")
+	})
+
+	// All results should be VOID
+	t.Run("all operations return VOID", func(t *testing.T) {
+		assert.Equal(t, function.VOID, m.Concat(v, v))
+		assert.Equal(t, function.VOID, m.Empty())
+		assert.Equal(t, function.VOID, m.Concat(m.Empty(), v))
+		assert.Equal(t, function.VOID, m.Concat(v, m.Empty()))
+	})
+}
+
+// TestVoidMonoid_ConcatAll tests combining multiple Void values
+func TestVoidMonoid_ConcatAll(t *testing.T) {
+	m := VoidMonoid()
+	concatAll := ConcatAll(m)
+
+	tests := []struct {
+		name     string
+		input    []Void
+		expected Void
+	}{
+		{
+			name:     "empty slice",
+			input:    []Void{},
+			expected: function.VOID,
+		},
+		{
+			name:     "single element",
+			input:    []Void{function.VOID},
+			expected: function.VOID,
+		},
+		{
+			name:     "multiple elements",
+			input:    []Void{function.VOID, function.VOID, function.VOID},
+			expected: function.VOID,
+		},
+		{
+			name:     "many elements",
+			input:    make([]Void, 100),
+			expected: function.VOID,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Initialize slice with VOID values
+			for i := range tt.input {
+				tt.input[i] = function.VOID
+			}
+			result := concatAll(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+// TestVoidMonoid_Fold tests the Fold function with VoidMonoid
+func TestVoidMonoid_Fold(t *testing.T) {
+	m := VoidMonoid()
+	fold := Fold(m)
+
+	// Fold should behave identically to ConcatAll
+	voids := []Void{function.VOID, function.VOID, function.VOID}
+	result := fold(voids)
+	assert.Equal(t, function.VOID, result)
+
+	// Empty fold
+	emptyResult := fold([]Void{})
+	assert.Equal(t, function.VOID, emptyResult)
+}
+
+// TestVoidMonoid_Reverse tests that Reverse doesn't affect VoidMonoid
+func TestVoidMonoid_Reverse(t *testing.T) {
+	m := VoidMonoid()
+	reversed := Reverse(m)
+
+	// Since all Void values are identical, reverse should have no effect
+	v := function.VOID
+
+	assert.Equal(t, m.Concat(v, v), reversed.Concat(v, v))
+	assert.Equal(t, m.Empty(), reversed.Empty())
+
+	// Test identity laws still hold
+	assert.Equal(t, v, reversed.Concat(reversed.Empty(), v))
+	assert.Equal(t, v, reversed.Concat(v, reversed.Empty()))
+}
+
+// TestVoidMonoid_ToSemigroup tests conversion to Semigroup
+func TestVoidMonoid_ToSemigroup(t *testing.T) {
+	m := VoidMonoid()
+	sg := ToSemigroup(m)
+
+	// Should work as a semigroup
+	result := sg.Concat(function.VOID, function.VOID)
+	assert.Equal(t, function.VOID, result)
+
+	// Verify it's the same underlying operation
+	assert.Equal(t, m.Concat(function.VOID, function.VOID), sg.Concat(function.VOID, function.VOID))
+}
+
+// TestVoidMonoid_FunctionMonoid tests VoidMonoid with FunctionMonoid
+func TestVoidMonoid_FunctionMonoid(t *testing.T) {
+	m := VoidMonoid()
+	funcMonoid := FunctionMonoid[string](m)
+
+	// Create functions that return Void
+	f1 := func(s string) Void { return function.VOID }
+	f2 := func(s string) Void { return function.VOID }
+
+	// Combine functions
+	combined := funcMonoid.Concat(f1, f2)
+
+	// Test combined function
+	result := combined("test")
+	assert.Equal(t, function.VOID, result)
+
+	// Test empty function
+	emptyFunc := funcMonoid.Empty()
+	assert.Equal(t, function.VOID, emptyFunc("anything"))
+}
+
+// TestVoidMonoid_PracticalUsage demonstrates practical usage patterns
+func TestVoidMonoid_PracticalUsage(t *testing.T) {
+	m := VoidMonoid()
+
+	// Simulate tracking that operations occurred without caring about results
+	type Action func() Void
+
+	actions := []Action{
+		func() Void { return function.VOID }, // Action 1
+		func() Void { return function.VOID }, // Action 2
+		func() Void { return function.VOID }, // Action 3
+	}
+
+	// Execute all actions and collect results
+	results := make([]Void, len(actions))
+	for i, action := range actions {
+		results[i] = action()
+	}
+
+	// Combine all results (all are VOID)
+	finalResult := ConcatAll(m)(results)
+	assert.Equal(t, function.VOID, finalResult)
+}
+
+// TestVoidMonoid_EdgeCases tests edge cases
+func TestVoidMonoid_EdgeCases(t *testing.T) {
+	m := VoidMonoid()
+
+	t.Run("multiple concatenations", func(t *testing.T) {
+		// Chain multiple Concat operations
+		result := m.Concat(
+			m.Concat(
+				m.Concat(function.VOID, function.VOID),
+				function.VOID,
+			),
+			function.VOID,
+		)
+		assert.Equal(t, function.VOID, result)
+	})
+
+	t.Run("concat with empty", func(t *testing.T) {
+		// Various combinations with Empty()
+		assert.Equal(t, function.VOID, m.Concat(m.Empty(), m.Empty()))
+		assert.Equal(t, function.VOID, m.Concat(m.Concat(m.Empty(), function.VOID), m.Empty()))
+	})
+
+	t.Run("large slice", func(t *testing.T) {
+		// Test with a large number of elements
+		largeSlice := make([]Void, 10000)
+		for i := range largeSlice {
+			largeSlice[i] = function.VOID
+		}
+		result := ConcatAll(m)(largeSlice)
+		assert.Equal(t, function.VOID, result)
+	})
+}
+
+// TestVoidMonoid_TypeSafety verifies type safety
+func TestVoidMonoid_TypeSafety(t *testing.T) {
+	m := VoidMonoid()
+
+	// Verify it implements Monoid interface
+	var _ Monoid[Void] = m
+
+	// Verify Empty returns correct type
+	empty := m.Empty()
+	var _ Void = empty
+
+	// Verify Concat returns correct type
+	result := m.Concat(function.VOID, function.VOID)
+	var _ Void = result
+}
+
+// BenchmarkVoidMonoid_Concat benchmarks the Concat operation
+func BenchmarkVoidMonoid_Concat(b *testing.B) {
+	m := VoidMonoid()
+	v := function.VOID
+
+	b.ResetTimer()
+	for b.Loop() {
+		_ = m.Concat(v, v)
+	}
+}
+
+// BenchmarkVoidMonoid_ConcatAll benchmarks combining multiple Void values
+func BenchmarkVoidMonoid_ConcatAll(b *testing.B) {
+	m := VoidMonoid()
+	concatAll := ConcatAll(m)
+
+	voids := make([]Void, 1000)
+	for i := range voids {
+		voids[i] = function.VOID
+	}
+
+	b.ResetTimer()
+	for b.Loop() {
+		_ = concatAll(voids)
+	}
+}
+
+// BenchmarkVoidMonoid_Empty benchmarks the Empty operation
+func BenchmarkVoidMonoid_Empty(b *testing.B) {
+	m := VoidMonoid()
+
+	b.ResetTimer()
+	for b.Loop() {
+		_ = m.Empty()
+	}
+}
+
+// Made with Bob

v2/optics/codec/alt_test.go

@@ -62,7 +62,7 @@ func TestMonadAltBasicFunctionality(t *testing.T) {
 
 		assert.True(t, either.IsRight(result), "should successfully decode with first codec")
 
-		value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+		value := either.GetOrElse(reader.Of[validation.Errors](""))(result)
 		assert.Equal(t, "HELLO", value)
 	})
 
@@ -105,7 +105,7 @@ func TestMonadAltBasicFunctionality(t *testing.T) {
 
 		assert.True(t, either.IsRight(result), "should successfully decode with second codec")
 
-		value := either.GetOrElse(reader.Of[validation.Errors, int](0))(result)
+		value := either.GetOrElse(reader.Of[validation.Errors](0))(result)
 		assert.Equal(t, -5, value)
 	})
 
@@ -302,19 +302,19 @@ func TestAltOperator(t *testing.T) {
 		// Test with "42" - should use base codec
 		result1 := pipeline.Decode("42")
 		assert.True(t, either.IsRight(result1))
-		value1 := either.GetOrElse(reader.Of[validation.Errors, int](0))(result1)
+		value1 := either.GetOrElse(reader.Of[validation.Errors](0))(result1)
 		assert.Equal(t, 42, value1)
 
 		// Test with "100" - should use fallback1
 		result2 := pipeline.Decode("100")
 		assert.True(t, either.IsRight(result2))
-		value2 := either.GetOrElse(reader.Of[validation.Errors, int](0))(result2)
+		value2 := either.GetOrElse(reader.Of[validation.Errors](0))(result2)
 		assert.Equal(t, 100, value2)
 
 		// Test with "999" - should use fallback2
 		result3 := pipeline.Decode("999")
 		assert.True(t, either.IsRight(result3))
-		value3 := either.GetOrElse(reader.Of[validation.Errors, int](0))(result3)
+		value3 := either.GetOrElse(reader.Of[validation.Errors](0))(result3)
 		assert.Equal(t, 999, value3)
 	})
 }
@@ -449,7 +449,7 @@ func TestAltRoundTrip(t *testing.T) {
 		decodeResult := altCodec.Decode(original)
 		require.True(t, either.IsRight(decodeResult))
 
-		decoded := either.GetOrElse(reader.Of[validation.Errors, string](""))(decodeResult)
+		decoded := either.GetOrElse(reader.Of[validation.Errors](""))(decodeResult)
 
 		// Encode
 		encoded := altCodec.Encode(decoded)
@@ -487,7 +487,7 @@ func TestAltRoundTrip(t *testing.T) {
 		decodeResult := altCodec.Decode(original)
 		require.True(t, either.IsRight(decodeResult))
 
-		decoded := either.GetOrElse(reader.Of[validation.Errors, string](""))(decodeResult)
+		decoded := either.GetOrElse(reader.Of[validation.Errors](""))(decodeResult)
 
 		// Encode (uses first codec's encoder, which is identity)
 		encoded := altCodec.Encode(decoded)
@@ -619,7 +619,7 @@ func TestAltMonoid(t *testing.T) {
 			result := combined.Decode("input")
 
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, int](0))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](0))(result)
 			assert.Equal(t, 10, value, "first success should win")
 		})
 
@@ -628,7 +628,7 @@ func TestAltMonoid(t *testing.T) {
 			result := combined.Decode("42")
 
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, int](0))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](0))(result)
 			assert.Equal(t, 42, value)
 		})
 
@@ -637,7 +637,7 @@ func TestAltMonoid(t *testing.T) {
 			result := combined.Decode("invalid")
 
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, int](-1))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](-1))(result)
 			assert.Equal(t, 0, value, "should use default zero value")
 		})
 	})
@@ -768,21 +768,21 @@ func TestAltMonoid(t *testing.T) {
 		t.Run("uses primary when it succeeds", func(t *testing.T) {
 			result := combined.Decode("primary")
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](""))(result)
 			assert.Equal(t, "from primary", value)
 		})
 
 		t.Run("uses secondary when primary fails", func(t *testing.T) {
 			result := combined.Decode("secondary")
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](""))(result)
 			assert.Equal(t, "from secondary", value)
 		})
 
 		t.Run("uses default when both fail", func(t *testing.T) {
 			result := combined.Decode("other")
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, string](""))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](""))(result)
 			assert.Equal(t, "default", value)
 		})
 	})
@@ -841,7 +841,7 @@ func TestAltMonoid(t *testing.T) {
 			result := combined.Decode("input")
 
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, int](-1))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](-1))(result)
 			// Empty (0) comes first, so it wins
 			assert.Equal(t, 0, value)
 		})
@@ -852,7 +852,7 @@ func TestAltMonoid(t *testing.T) {
 			result := combined.Decode("input")
 
 			assert.True(t, either.IsRight(result))
-			value := either.GetOrElse(reader.Of[validation.Errors, int](-1))(result)
+			value := either.GetOrElse(reader.Of[validation.Errors](-1))(result)
 			assert.Equal(t, 10, value, "codec1 should win")
 		})
 
@@ -867,8 +867,8 @@ func TestAltMonoid(t *testing.T) {
 			assert.True(t, either.IsRight(resultLeft))
 			assert.True(t, either.IsRight(resultRight))
 
-			valueLeft := either.GetOrElse(reader.Of[validation.Errors, int](-1))(resultLeft)
-			valueRight := either.GetOrElse(reader.Of[validation.Errors, int](-1))(resultRight)
+			valueLeft := either.GetOrElse(reader.Of[validation.Errors](-1))(resultLeft)
+			valueRight := either.GetOrElse(reader.Of[validation.Errors](-1))(resultRight)
 
 			// Both should return 10 (first success)
 			assert.Equal(t, valueLeft, valueRight)

v2/optics/codec/bind.go

@@ -0,0 +1,299 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/optics/codec/validate"
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/semigroup"
+)
+
+// ApSL creates an applicative sequencing operator for codecs using a lens.
+//
+// This function implements the "ApS" (Applicative Sequencing) pattern for codecs,
+// allowing you to build up complex codecs by combining a base codec with a field
+// accessed through a lens. It's particularly useful for building struct codecs
+// field-by-field in a composable way.
+//
+// The function combines:
+//   - Encoding: Extracts the field value using the lens, encodes it with fa, and
+//     combines it with the base encoding using the monoid
+//   - Validation: Validates the field using the lens and combines the validation
+//     with the base validation
+//
+// # Type Parameters
+//
+//   - S: The source struct type (what we're building a codec for)
+//   - T: The field type accessed by the lens
+//   - O: The output type for encoding (must have a monoid)
+//   - I: The input type for decoding
+//
+// # Parameters
+//
+//   - m: A Monoid[O] for combining encoded outputs
+//   - l: A Lens[S, T] that focuses on a specific field in S
+//   - fa: A Type[T, O, I] codec for the field type T
+//
+// # Returns
+//
+// An Operator[S, S, O, I] that transforms a base codec by adding the field
+// specified by the lens.
+//
+// # How It Works
+//
+// 1. **Encoding**: When encoding a value of type S:
+//   - Extract the field T using l.Get
+//   - Encode T to O using fa.Encode
+//   - Combine with the base encoding using the monoid
+//
+// 2. **Validation**: When validating input I:
+//   - Validate the field using fa.Validate through the lens
+//   - Combine with the base validation
+//
+// 3. **Type Checking**: Preserves the base type checker
+//
+// # Example
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec"
+//	    "github.com/IBM/fp-go/v2/optics/lens"
+//	    S "github.com/IBM/fp-go/v2/string"
+//	)
+//
+//	type Person struct {
+//	    Name string
+//	    Age  int
+//	}
+//
+//	// Lenses for Person fields
+//	nameLens := lens.MakeLens(
+//	    func(p *Person) string { return p.Name },
+//	    func(p *Person, name string) *Person { p.Name = name; return p },
+//	)
+//
+//	// Build a Person codec field by field
+//	personCodec := F.Pipe1(
+//	    codec.Struct[Person]("Person"),
+//	    codec.ApSL(S.Monoid, nameLens, codec.String),
+//	    // ... add more fields
+//	)
+//
+// # Use Cases
+//
+//   - Building struct codecs incrementally
+//   - Composing codecs for nested structures
+//   - Creating type-safe serialization/deserialization
+//   - Implementing Do-notation style codec construction
+//
+// # Notes
+//
+//   - The monoid determines how encoded outputs are combined
+//   - The lens must be total (handle all cases safely)
+//   - This is typically used with other ApS functions to build complete codecs
+//   - The name is automatically generated for debugging purposes
+//
+// See also:
+//   - validate.ApSL: The underlying validation combinator
+//   - reader.ApplicativeMonoid: The monoid-based applicative instance
+//   - Lens: The optic for accessing struct fields
+func ApSL[S, T, O, I any](
+	m Monoid[O],
+	l Lens[S, T],
+	fa Type[T, O, I],
+) Operator[S, S, O, I] {
+	name := fmt.Sprintf("ApS[%s x %s]", l, fa)
+	rm := reader.ApplicativeMonoid[S](m)
+
+	encConcat := F.Pipe1(
+		F.Flow2(
+			l.Get,
+			fa.Encode,
+		),
+		semigroup.AppendTo(rm),
+	)
+
+	valConcat := validate.ApSL(l, fa.Validate)
+
+	return func(t Type[S, O, I]) Type[S, O, I] {
+
+		return MakeType(
+			name,
+			t.Is,
+			F.Pipe1(
+				t.Validate,
+				valConcat,
+			),
+			encConcat(t.Encode),
+		)
+	}
+}
+
+// ApSO creates an applicative sequencing operator for codecs using an optional.
+//
+// This function implements the "ApS" (Applicative Sequencing) pattern for codecs
+// with optional fields, allowing you to build up complex codecs by combining a base
+// codec with a field that may or may not be present. It's particularly useful for
+// building struct codecs with optional fields in a composable way.
+//
+// The function combines:
+//   - Encoding: Attempts to extract the optional field value, encodes it if present,
+//     and combines it with the base encoding using the monoid. If the field is absent,
+//     only the base encoding is used.
+//   - Validation: Validates the optional field and combines the validation with the
+//     base validation using applicative semantics (error accumulation).
+//
+// # Type Parameters
+//
+//   - S: The source struct type (what we're building a codec for)
+//   - T: The optional field type accessed by the optional
+//   - O: The output type for encoding (must have a monoid)
+//   - I: The input type for decoding
+//
+// # Parameters
+//
+//   - m: A Monoid[O] for combining encoded outputs
+//   - o: An Optional[S, T] that focuses on a field in S that may not exist
+//   - fa: A Type[T, O, I] codec for the optional field type T
+//
+// # Returns
+//
+// An Operator[S, S, O, I] that transforms a base codec by adding the optional field
+// specified by the optional.
+//
+// # How It Works
+//
+// 1. **Encoding**: When encoding a value of type S:
+//   - Try to extract the optional field T using o.GetOption
+//   - If present (Some(T)): Encode T to O using fa.Encode and combine with base using monoid
+//   - If absent (None): Return only the base encoding unchanged
+//
+// 2. **Validation**: When validating input I:
+//   - Validate the optional field using fa.Validate through o.Set
+//   - Combine with the base validation using applicative semantics
+//   - Accumulates all validation errors from both base and field
+//
+// 3. **Type Checking**: Preserves the base type checker
+//
+// # Difference from ApSL
+//
+// Unlike ApSL which works with required fields via Lens, ApSO handles optional fields:
+//   - ApSL: Field always exists, always encoded
+//   - ApSO: Field may not exist, only encoded when present
+//   - ApSO uses Optional.GetOption which returns Option[T]
+//   - ApSO gracefully handles missing fields without errors
+//
+// # Example
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec"
+//	    "github.com/IBM/fp-go/v2/optics/optional"
+//	    S "github.com/IBM/fp-go/v2/string"
+//	)
+//
+//	type Person struct {
+//	    Name      string
+//	    Nickname  *string  // Optional field
+//	}
+//
+//	// Optional for Person.Nickname
+//	nicknameOpt := optional.MakeOptional(
+//	    func(p Person) option.Option[string] {
+//	        if p.Nickname != nil {
+//	            return option.Some(*p.Nickname)
+//	        }
+//	        return option.None[string]()
+//	    },
+//	    func(p Person, nick string) Person {
+//	        p.Nickname = &nick
+//	        return p
+//	    },
+//	)
+//
+//	// Build a Person codec with optional nickname
+//	personCodec := F.Pipe1(
+//	    codec.Struct[Person]("Person"),
+//	    codec.ApSO(S.Monoid, nicknameOpt, codec.String),
+//	)
+//
+//	// Encoding with nickname present
+//	p1 := Person{Name: "Alice", Nickname: ptr("Ali")}
+//	encoded1 := personCodec.Encode(p1)  // Includes nickname
+//
+//	// Encoding with nickname absent
+//	p2 := Person{Name: "Bob", Nickname: nil}
+//	encoded2 := personCodec.Encode(p2)  // No nickname in output
+//
+// # Use Cases
+//
+//   - Building struct codecs with optional/nullable fields
+//   - Handling pointer fields that may be nil
+//   - Composing codecs for structures with optional nested data
+//   - Creating flexible serialization that omits absent fields
+//
+// # Notes
+//
+//   - The monoid determines how encoded outputs are combined when field is present
+//   - When the optional field is absent, encoding returns base encoding unchanged
+//   - Validation still accumulates errors even for optional fields
+//   - The name is automatically generated for debugging purposes
+//
+// # See Also
+//
+//   - ApSL: For required fields using Lens
+//   - validate.ApS: The underlying validation combinator
+//   - Optional: The optic for accessing optional fields
+func ApSO[S, T, O, I any](
+	m Monoid[O],
+	o Optional[S, T],
+	fa Type[T, O, I],
+) Operator[S, S, O, I] {
+	name := fmt.Sprintf("ApS[%s x %s]", o, fa)
+
+	encConcat := F.Flow2(
+		o.GetOption,
+		option.Map(F.Flow2(
+			fa.Encode,
+			semigroup.AppendTo(m),
+		)),
+	)
+
+	valConcat := validate.ApS(o.Set, fa.Validate)
+
+	return func(t Type[S, O, I]) Type[S, O, I] {
+
+		return MakeType(
+			name,
+			t.Is,
+			F.Pipe1(
+				t.Validate,
+				valConcat,
+			),
+			func(s S) O {
+				to := t.Encode(s)
+				return F.Pipe2(
+					encConcat(s),
+					option.Flap[O](to),
+					option.GetOrElse(lazy.Of(to)),
+				)
+			},
+		)
+	}
+}

v2/optics/codec/bind_test.go

@@ -0,0 +1,816 @@
+// 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 (
+	"strconv"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	"github.com/IBM/fp-go/v2/optics/lens"
+	"github.com/IBM/fp-go/v2/optics/optional"
+	"github.com/IBM/fp-go/v2/option"
+	S "github.com/IBM/fp-go/v2/string"
+	"github.com/stretchr/testify/assert"
+)
+
+// Test types for ApSL
+type Person struct {
+	Name string
+	Age  int
+}
+
+func TestApSL_EncodingCombination(t *testing.T) {
+	t.Run("combines encodings using monoid", func(t *testing.T) {
+		// Create a lens for Person.Name
+		nameLens := lens.MakeLens(
+			func(p Person) string { return p.Name },
+			func(p Person, name string) Person {
+				return Person{Name: name, Age: p.Age}
+			},
+		)
+
+		// Create base codec that encodes to "Person:"
+		baseCodec := MakeType(
+			"Person",
+			func(i any) validation.Result[Person] {
+				if p, ok := i.(Person); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[Person](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected Person",
+					},
+				}))
+			},
+			func(i any) Decode[Context, Person] {
+				return func(ctx Context) validation.Validation[Person] {
+					if p, ok := i.(Person); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[Person](i, "expected Person")(ctx)
+				}
+			},
+			func(p Person) string { return "Person:" },
+		)
+
+		// Create field codec for Name
+		nameCodec := MakeType(
+			"Name",
+			func(i any) validation.Result[string] {
+				if s, ok := i.(string); ok {
+					return validation.ToResult(validation.Success(s))
+				}
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected string",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					if s, ok := i.(string); ok {
+						return validation.Success(s)
+					}
+					return validation.FailureWithMessage[string](i, "expected string")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSL to combine encodings
+		operator := ApSL(S.Monoid, nameLens, nameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test encoding - should concatenate base encoding with field encoding
+		person := Person{Name: "Alice", Age: 30}
+		encoded := enhancedCodec.Encode(person)
+
+		// The monoid concatenates: base encoding + field encoding
+		// Note: The order depends on how the monoid is applied in ApSL
+		assert.Contains(t, encoded, "Person:")
+		assert.Contains(t, encoded, "Alice")
+	})
+}
+
+func TestApSL_ValidationCombination(t *testing.T) {
+	t.Run("validates field through lens", func(t *testing.T) {
+		// Create a lens for Person.Age
+		ageLens := lens.MakeLens(
+			func(p Person) int { return p.Age },
+			func(p Person, age int) Person {
+				return Person{Name: p.Name, Age: age}
+			},
+		)
+
+		// Create base codec that always succeeds
+		baseCodec := MakeType(
+			"Person",
+			func(i any) validation.Result[Person] {
+				if p, ok := i.(Person); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[Person](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected Person",
+					},
+				}))
+			},
+			func(i any) Decode[Context, Person] {
+				return func(ctx Context) validation.Validation[Person] {
+					if p, ok := i.(Person); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[Person](i, "expected Person")(ctx)
+				}
+			},
+			func(p Person) string { return "" },
+		)
+
+		// Create field codec for Age that validates positive numbers
+		ageCodec := MakeType(
+			"Age",
+			func(i any) validation.Result[int] {
+				if n, ok := i.(int); ok {
+					if n > 0 {
+						return validation.ToResult(validation.Success(n))
+					}
+					return validation.ToResult(validation.Failures[int](validation.Errors{
+						&validation.ValidationError{
+							Value:    n,
+							Messsage: "age must be positive",
+						},
+					}))
+				}
+				return validation.ToResult(validation.Failures[int](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected int",
+					},
+				}))
+			},
+			func(i any) Decode[Context, int] {
+				return func(ctx Context) validation.Validation[int] {
+					if n, ok := i.(int); ok {
+						if n > 0 {
+							return validation.Success(n)
+						}
+						return validation.FailureWithMessage[int](n, "age must be positive")(ctx)
+					}
+					return validation.FailureWithMessage[int](i, "expected int")(ctx)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		// Apply ApSL
+		operator := ApSL(S.Monoid, ageLens, ageCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test with invalid age (negative) - field validation should fail
+		invalidPerson := Person{Name: "Charlie", Age: -5}
+		invalidResult := enhancedCodec.Decode(invalidPerson)
+		assert.True(t, either.IsLeft(invalidResult), "Should fail with negative age")
+
+		// Extract and verify we have errors
+		errors := either.MonadFold(invalidResult,
+			F.Identity[validation.Errors],
+			func(Person) validation.Errors { return nil },
+		)
+		assert.NotEmpty(t, errors, "Should have validation errors")
+	})
+}
+
+func TestApSL_TypeChecking(t *testing.T) {
+	t.Run("preserves base type checker", func(t *testing.T) {
+		// Create a lens for Person.Name
+		nameLens := lens.MakeLens(
+			func(p Person) string { return p.Name },
+			func(p Person, name string) Person {
+				return Person{Name: name, Age: p.Age}
+			},
+		)
+
+		// Create base codec with type checker
+		baseCodec := MakeType(
+			"Person",
+			func(i any) validation.Result[Person] {
+				if p, ok := i.(Person); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[Person](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected Person",
+					},
+				}))
+			},
+			func(i any) Decode[Context, Person] {
+				return func(ctx Context) validation.Validation[Person] {
+					if p, ok := i.(Person); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[Person](i, "expected Person")(ctx)
+				}
+			},
+			func(p Person) string { return "" },
+		)
+
+		// Create field codec
+		nameCodec := MakeType(
+			"Name",
+			func(i any) validation.Result[string] {
+				if s, ok := i.(string); ok {
+					return validation.ToResult(validation.Success(s))
+				}
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected string",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					if s, ok := i.(string); ok {
+						return validation.Success(s)
+					}
+					return validation.FailureWithMessage[string](i, "expected string")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSL
+		operator := ApSL(S.Monoid, nameLens, nameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test type checking with valid type
+		person := Person{Name: "Eve", Age: 22}
+		isResult := enhancedCodec.Is(person)
+		assert.True(t, either.IsRight(isResult), "Should accept Person type")
+
+		// Test type checking with invalid type
+		invalidResult := enhancedCodec.Is("not a person")
+		assert.True(t, either.IsLeft(invalidResult), "Should reject non-Person type")
+	})
+}
+
+func TestApSL_Naming(t *testing.T) {
+	t.Run("generates descriptive name", func(t *testing.T) {
+		// Create a lens for Person.Name
+		nameLens := lens.MakeLens(
+			func(p Person) string { return p.Name },
+			func(p Person, name string) Person {
+				return Person{Name: name, Age: p.Age}
+			},
+		)
+
+		// Create base codec
+		baseCodec := MakeType(
+			"Person",
+			func(i any) validation.Result[Person] {
+				if p, ok := i.(Person); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[Person](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected Person",
+					},
+				}))
+			},
+			func(i any) Decode[Context, Person] {
+				return func(ctx Context) validation.Validation[Person] {
+					if p, ok := i.(Person); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[Person](i, "expected Person")(ctx)
+				}
+			},
+			func(p Person) string { return "" },
+		)
+
+		// Create field codec
+		nameCodec := MakeType(
+			"Name",
+			func(i any) validation.Result[string] {
+				if s, ok := i.(string); ok {
+					return validation.ToResult(validation.Success(s))
+				}
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected string",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					if s, ok := i.(string); ok {
+						return validation.Success(s)
+					}
+					return validation.FailureWithMessage[string](i, "expected string")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSL
+		operator := ApSL(S.Monoid, nameLens, nameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Check that the name includes ApS
+		name := enhancedCodec.Name()
+		assert.Contains(t, name, "ApS", "Name should contain 'ApS'")
+	})
+}
+
+func TestApSL_ErrorAccumulation(t *testing.T) {
+	t.Run("accumulates validation errors", func(t *testing.T) {
+		// Create a lens for Person.Age
+		ageLens := lens.MakeLens(
+			func(p Person) int { return p.Age },
+			func(p Person, age int) Person {
+				return Person{Name: p.Name, Age: age}
+			},
+		)
+
+		// Create base codec that fails validation
+		baseCodec := MakeType(
+			"Person",
+			func(i any) validation.Result[Person] {
+				return validation.ToResult(validation.Failures[Person](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "base validation error",
+					},
+				}))
+			},
+			func(i any) Decode[Context, Person] {
+				return func(ctx Context) validation.Validation[Person] {
+					return validation.FailureWithMessage[Person](i, "base validation error")(ctx)
+				}
+			},
+			func(p Person) string { return "" },
+		)
+
+		// Create field codec that also fails
+		ageCodec := MakeType(
+			"Age",
+			func(i any) validation.Result[int] {
+				return validation.ToResult(validation.Failures[int](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "age validation error",
+					},
+				}))
+			},
+			func(i any) Decode[Context, int] {
+				return func(ctx Context) validation.Validation[int] {
+					return validation.FailureWithMessage[int](i, "age validation error")(ctx)
+				}
+			},
+			strconv.Itoa,
+		)
+
+		// Apply ApSL
+		operator := ApSL(S.Monoid, ageLens, ageCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test validation - should accumulate errors
+		person := Person{Name: "Dave", Age: 30}
+		result := enhancedCodec.Decode(person)
+
+		// Should fail
+		assert.True(t, either.IsLeft(result), "Should fail validation")
+
+		// Extract errors
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(Person) validation.Errors { return nil },
+		)
+
+		// Should have errors from both base and field validation
+		assert.NotEmpty(t, errors, "Should have validation errors")
+	})
+}
+
+// Test types for ApSO
+type PersonWithNickname struct {
+	Name     string
+	Nickname *string
+}
+
+func TestApSO_EncodingWithPresentField(t *testing.T) {
+	t.Run("encodes optional field when present", func(t *testing.T) {
+		// Create an optional for PersonWithNickname.Nickname
+		nicknameOpt := optional.MakeOptional(
+			func(p PersonWithNickname) option.Option[string] {
+				if p.Nickname != nil {
+					return option.Some(*p.Nickname)
+				}
+				return option.None[string]()
+			},
+			func(p PersonWithNickname, nick string) PersonWithNickname {
+				p.Nickname = &nick
+				return p
+			},
+		)
+
+		// Create base codec that encodes to "Person:"
+		baseCodec := MakeType(
+			"PersonWithNickname",
+			func(i any) validation.Result[PersonWithNickname] {
+				if p, ok := i.(PersonWithNickname); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[PersonWithNickname](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected PersonWithNickname",
+					},
+				}))
+			},
+			func(i any) Decode[Context, PersonWithNickname] {
+				return func(ctx Context) validation.Validation[PersonWithNickname] {
+					if p, ok := i.(PersonWithNickname); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[PersonWithNickname](i, "expected PersonWithNickname")(ctx)
+				}
+			},
+			func(p PersonWithNickname) string { return "Person:" },
+		)
+
+		// Create field codec for Nickname
+		nicknameCodec := MakeType(
+			"Nickname",
+			func(i any) validation.Result[string] {
+				if s, ok := i.(string); ok {
+					return validation.ToResult(validation.Success(s))
+				}
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected string",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					if s, ok := i.(string); ok {
+						return validation.Success(s)
+					}
+					return validation.FailureWithMessage[string](i, "expected string")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSO to combine encodings
+		operator := ApSO(S.Monoid, nicknameOpt, nicknameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test encoding with nickname present
+		nickname := "Ali"
+		person := PersonWithNickname{Name: "Alice", Nickname: &nickname}
+		encoded := enhancedCodec.Encode(person)
+
+		// Should include both base and nickname
+		assert.Contains(t, encoded, "Person:")
+		assert.Contains(t, encoded, "Ali")
+	})
+}
+
+func TestApSO_EncodingWithAbsentField(t *testing.T) {
+	t.Run("omits optional field when absent", func(t *testing.T) {
+		// Create an optional for PersonWithNickname.Nickname
+		nicknameOpt := optional.MakeOptional(
+			func(p PersonWithNickname) option.Option[string] {
+				if p.Nickname != nil {
+					return option.Some(*p.Nickname)
+				}
+				return option.None[string]()
+			},
+			func(p PersonWithNickname, nick string) PersonWithNickname {
+				p.Nickname = &nick
+				return p
+			},
+		)
+
+		// Create base codec
+		baseCodec := MakeType(
+			"PersonWithNickname",
+			func(i any) validation.Result[PersonWithNickname] {
+				if p, ok := i.(PersonWithNickname); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[PersonWithNickname](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected PersonWithNickname",
+					},
+				}))
+			},
+			func(i any) Decode[Context, PersonWithNickname] {
+				return func(ctx Context) validation.Validation[PersonWithNickname] {
+					if p, ok := i.(PersonWithNickname); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[PersonWithNickname](i, "expected PersonWithNickname")(ctx)
+				}
+			},
+			func(p PersonWithNickname) string { return "Person:Bob" },
+		)
+
+		// Create field codec
+		nicknameCodec := MakeType(
+			"Nickname",
+			func(i any) validation.Result[string] {
+				if s, ok := i.(string); ok {
+					return validation.ToResult(validation.Success(s))
+				}
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected string",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					if s, ok := i.(string); ok {
+						return validation.Success(s)
+					}
+					return validation.FailureWithMessage[string](i, "expected string")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSO
+		operator := ApSO(S.Monoid, nicknameOpt, nicknameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test encoding with nickname absent
+		person := PersonWithNickname{Name: "Bob", Nickname: nil}
+		encoded := enhancedCodec.Encode(person)
+
+		// Should only have base encoding
+		assert.Equal(t, "Person:Bob", encoded)
+	})
+}
+
+func TestApSO_TypeChecking(t *testing.T) {
+	t.Run("preserves base type checker", func(t *testing.T) {
+		// Create an optional for PersonWithNickname.Nickname
+		nicknameOpt := optional.MakeOptional(
+			func(p PersonWithNickname) option.Option[string] {
+				if p.Nickname != nil {
+					return option.Some(*p.Nickname)
+				}
+				return option.None[string]()
+			},
+			func(p PersonWithNickname, nick string) PersonWithNickname {
+				p.Nickname = &nick
+				return p
+			},
+		)
+
+		// Create base codec with type checker
+		baseCodec := MakeType(
+			"PersonWithNickname",
+			func(i any) validation.Result[PersonWithNickname] {
+				if p, ok := i.(PersonWithNickname); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[PersonWithNickname](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected PersonWithNickname",
+					},
+				}))
+			},
+			func(i any) Decode[Context, PersonWithNickname] {
+				return func(ctx Context) validation.Validation[PersonWithNickname] {
+					if p, ok := i.(PersonWithNickname); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[PersonWithNickname](i, "expected PersonWithNickname")(ctx)
+				}
+			},
+			func(p PersonWithNickname) string { return "" },
+		)
+
+		// Create field codec
+		nicknameCodec := MakeType(
+			"Nickname",
+			func(i any) validation.Result[string] {
+				if s, ok := i.(string); ok {
+					return validation.ToResult(validation.Success(s))
+				}
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected string",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					if s, ok := i.(string); ok {
+						return validation.Success(s)
+					}
+					return validation.FailureWithMessage[string](i, "expected string")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSO
+		operator := ApSO(S.Monoid, nicknameOpt, nicknameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test type checking with valid type
+		nickname := "Eve"
+		person := PersonWithNickname{Name: "Eve", Nickname: &nickname}
+		isResult := enhancedCodec.Is(person)
+		assert.True(t, either.IsRight(isResult), "Should accept PersonWithNickname type")
+
+		// Test type checking with invalid type
+		invalidResult := enhancedCodec.Is("not a person")
+		assert.True(t, either.IsLeft(invalidResult), "Should reject non-PersonWithNickname type")
+	})
+}
+
+func TestApSO_Naming(t *testing.T) {
+	t.Run("generates descriptive name", func(t *testing.T) {
+		// Create an optional for PersonWithNickname.Nickname
+		nicknameOpt := optional.MakeOptional(
+			func(p PersonWithNickname) option.Option[string] {
+				if p.Nickname != nil {
+					return option.Some(*p.Nickname)
+				}
+				return option.None[string]()
+			},
+			func(p PersonWithNickname, nick string) PersonWithNickname {
+				p.Nickname = &nick
+				return p
+			},
+		)
+
+		// Create base codec
+		baseCodec := MakeType(
+			"PersonWithNickname",
+			func(i any) validation.Result[PersonWithNickname] {
+				if p, ok := i.(PersonWithNickname); ok {
+					return validation.ToResult(validation.Success(p))
+				}
+				return validation.ToResult(validation.Failures[PersonWithNickname](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected PersonWithNickname",
+					},
+				}))
+			},
+			func(i any) Decode[Context, PersonWithNickname] {
+				return func(ctx Context) validation.Validation[PersonWithNickname] {
+					if p, ok := i.(PersonWithNickname); ok {
+						return validation.Success(p)
+					}
+					return validation.FailureWithMessage[PersonWithNickname](i, "expected PersonWithNickname")(ctx)
+				}
+			},
+			func(p PersonWithNickname) string { return "" },
+		)
+
+		// Create field codec
+		nicknameCodec := MakeType(
+			"Nickname",
+			func(i any) validation.Result[string] {
+				if s, ok := i.(string); ok {
+					return validation.ToResult(validation.Success(s))
+				}
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "expected string",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					if s, ok := i.(string); ok {
+						return validation.Success(s)
+					}
+					return validation.FailureWithMessage[string](i, "expected string")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSO
+		operator := ApSO(S.Monoid, nicknameOpt, nicknameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Check that the name includes ApS
+		name := enhancedCodec.Name()
+		assert.Contains(t, name, "ApS", "Name should contain 'ApS'")
+	})
+}
+
+func TestApSO_ErrorAccumulation(t *testing.T) {
+	t.Run("accumulates validation errors", func(t *testing.T) {
+		// Create an optional for PersonWithNickname.Nickname
+		nicknameOpt := optional.MakeOptional(
+			func(p PersonWithNickname) option.Option[string] {
+				if p.Nickname != nil {
+					return option.Some(*p.Nickname)
+				}
+				return option.None[string]()
+			},
+			func(p PersonWithNickname, nick string) PersonWithNickname {
+				p.Nickname = &nick
+				return p
+			},
+		)
+
+		// Create base codec that fails validation
+		baseCodec := MakeType(
+			"PersonWithNickname",
+			func(i any) validation.Result[PersonWithNickname] {
+				return validation.ToResult(validation.Failures[PersonWithNickname](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "base validation error",
+					},
+				}))
+			},
+			func(i any) Decode[Context, PersonWithNickname] {
+				return func(ctx Context) validation.Validation[PersonWithNickname] {
+					return validation.FailureWithMessage[PersonWithNickname](i, "base validation error")(ctx)
+				}
+			},
+			func(p PersonWithNickname) string { return "" },
+		)
+
+		// Create field codec that also fails
+		nicknameCodec := MakeType(
+			"Nickname",
+			func(i any) validation.Result[string] {
+				return validation.ToResult(validation.Failures[string](validation.Errors{
+					&validation.ValidationError{
+						Value:    i,
+						Messsage: "nickname validation error",
+					},
+				}))
+			},
+			func(i any) Decode[Context, string] {
+				return func(ctx Context) validation.Validation[string] {
+					return validation.FailureWithMessage[string](i, "nickname validation error")(ctx)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Apply ApSO
+		operator := ApSO(S.Monoid, nicknameOpt, nicknameCodec)
+		enhancedCodec := operator(baseCodec)
+
+		// Test validation with present nickname - should accumulate errors
+		nickname := "Dave"
+		person := PersonWithNickname{Name: "Dave", Nickname: &nickname}
+		result := enhancedCodec.Decode(person)
+
+		// Should fail
+		assert.True(t, either.IsLeft(result), "Should fail validation")
+
+		// Extract errors
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(PersonWithNickname) validation.Errors { return nil },
+		)
+
+		// Should have errors from both base and field validation
+		assert.NotEmpty(t, errors, "Should have validation errors")
+	})
+}

v2/optics/codec/codec.go

@@ -11,6 +11,7 @@ import (
 	"github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/lazy"
+	"github.com/IBM/fp-go/v2/optics/codec/validate"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
 	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/reader"
@@ -100,7 +101,7 @@ func (t *typeImpl[A, O, I]) Is(i any) Result[A] {
 //	stringToInt := codec.MakeType(...)  // Type[int, string, string]
 //	intToPositive := codec.MakeType(...) // Type[PositiveInt, int, int]
 //	composed := codec.Pipe(intToPositive)(stringToInt) // Type[PositiveInt, string, string]
-func Pipe[A, B, O, I any](ab Type[B, A, A]) func(Type[A, O, I]) Type[B, O, I] {
+func Pipe[O, I, A, B any](ab Type[B, A, A]) func(Type[A, O, I]) Type[B, O, I] {
 	return func(this Type[A, O, I]) Type[B, O, I] {
 		return MakeType(
 			fmt.Sprintf("Pipe(%s, %s)", this.Name(), ab.Name()),
@@ -747,3 +748,114 @@ func FromRefinement[A, B any](refinement Refinement[A, B]) Type[B, A, A] {
 		refinement.ReverseGet,
 	)
 }
+
+// Empty creates a Type codec that ignores input during decoding and uses a default value,
+// and ignores the value during encoding, using a default output.
+//
+// This codec is useful for:
+//   - Providing default values for optional fields
+//   - Creating placeholder codecs in generic contexts
+//   - Implementing constant codecs that always produce the same value
+//   - Building codecs for phantom types or unit-like types
+//
+// The codec uses a lazily-evaluated Pair[O, A] to provide both the default output
+// for encoding and the default value for decoding. The lazy evaluation ensures that
+// the defaults are only computed when needed.
+//
+// # 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, but is ignored)
+//
+// # Parameters
+//
+//   - e: A Lazy[Pair[O, A]] that provides the default values:
+//   - pair.Head(e()): The default output value O used during encoding
+//   - pair.Tail(e()): The default decoded value A used during decoding
+//
+// # Returns
+//
+//   - A Type[A, O, I] that:
+//   - Decode: Always succeeds and returns the default value A, ignoring input I
+//   - Encode: Always returns the default output O, ignoring the input value A
+//   - Is: Checks if a value is of type A (standard type checking)
+//   - Name: Returns "Empty"
+//
+// # Behavior
+//
+// Decoding:
+//   - Ignores the input value completely
+//   - Always succeeds with validation.Success
+//   - Returns the default value from pair.Tail(e())
+//
+// Encoding:
+//   - Ignores the input value completely
+//   - Always returns the default output from pair.Head(e())
+//
+// # Example Usage
+//
+// Creating a codec with default values:
+//
+//	// Create a codec that always decodes to 42 and encodes to "default"
+//	defaultCodec := codec.Empty[int, string, any](lazy.Of(pair.MakePair("default", 42)))
+//
+//	// Decode always returns 42, regardless of input
+//	result := defaultCodec.Decode("anything")     // Success: Right(42)
+//	result = defaultCodec.Decode(123)             // Success: Right(42)
+//	result = defaultCodec.Decode(nil)             // Success: Right(42)
+//
+//	// Encode always returns "default", regardless of input
+//	encoded := defaultCodec.Encode(100)           // Returns: "default"
+//	encoded = defaultCodec.Encode(0)              // Returns: "default"
+//
+// Using with struct fields for default values:
+//
+//	type Config struct {
+//	    Timeout int
+//	    Retries int
+//	}
+//
+//	// Codec that provides default retries value
+//	defaultRetries := codec.Empty[int, int, any](lazy.Of(pair.MakePair(3, 3)))
+//
+//	configCodec := F.Pipe2(
+//	    codec.Struct[Config]("Config"),
+//	    codec.ApSL(S.Monoid, timeoutLens, codec.Int()),
+//	    codec.ApSL(S.Monoid, retriesLens, defaultRetries),
+//	)
+//
+// Creating a unit-like codec:
+//
+//	// Codec for a unit type that always produces Void
+//	unitCodec := codec.Empty[function.Void, function.Void, any](
+//	    lazy.Of(pair.MakePair(function.VOID, function.VOID)),
+//	)
+//
+// # Use Cases
+//
+//   - Default values: Provide fallback values when decoding optional fields
+//   - Constant codecs: Always produce the same value regardless of input
+//   - Placeholder codecs: Use in generic contexts where a codec is required but not used
+//   - Unit types: Encode/decode unit-like types that carry no information
+//   - Testing: Create simple codecs for testing codec composition
+//
+// # Notes
+//
+//   - The lazy evaluation of the Pair ensures defaults are only computed when needed
+//   - Both encoding and decoding always succeed (no validation errors)
+//   - The input values are completely ignored in both directions
+//   - The Is method still performs standard type checking for type A
+//   - This codec is useful in applicative composition where some fields have defaults
+//
+// See also:
+//   - Id: For identity codecs that preserve values
+//   - MakeType: For creating custom codecs with validation logic
+func Empty[A, O, I any](e Lazy[Pair[O, A]]) Type[A, O, I] {
+	return MakeType(
+		"Empty",
+		Is[A](),
+		validate.OfLazy[I](F.Pipe1(e, lazy.Map(pair.Tail[O, A]))),
+		reader.OfLazy[A](F.Pipe1(e, lazy.Map(pair.Head[O, A]))),
+	)
+}

v2/optics/codec/codec_test.go

@@ -7,9 +7,11 @@ import (
 
 	"github.com/IBM/fp-go/v2/either"
 	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
 	"github.com/IBM/fp-go/v2/optics/prism"
 	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -19,12 +21,7 @@ func TestString(t *testing.T) {
 		stringType := String()
 		result := stringType.Decode("hello")
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) string { return "" },
-			F.Identity[string],
-		)
-		assert.Equal(t, "hello", value)
+		assert.Equal(t, validation.Of("hello"), result)
 	})
 
 	t.Run("fails to decode non-string", func(t *testing.T) {
@@ -57,12 +54,7 @@ func TestString(t *testing.T) {
 		stringType := String()
 		result := stringType.Decode("")
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) string { return "error" },
-			F.Identity[string],
-		)
-		assert.Equal(t, "", value)
+		assert.Equal(t, validation.Of(""), result)
 	})
 }
 
@@ -71,12 +63,7 @@ func TestInt(t *testing.T) {
 		intType := Int()
 		result := intType.Decode(42)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) int { return 0 },
-			F.Identity[int],
-		)
-		assert.Equal(t, 42, value)
+		assert.Equal(t, validation.Of(42), result)
 	})
 
 	t.Run("fails to decode string as int", func(t *testing.T) {
@@ -109,24 +96,14 @@ func TestInt(t *testing.T) {
 		intType := Int()
 		result := intType.Decode(-42)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) int { return 0 },
-			F.Identity[int],
-		)
-		assert.Equal(t, -42, value)
+		assert.Equal(t, validation.Of(-42), result)
 	})
 
 	t.Run("decodes zero", func(t *testing.T) {
 		intType := Int()
 		result := intType.Decode(0)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) int { return -1 },
-			F.Identity[int],
-		)
-		assert.Equal(t, 0, value)
+		assert.Equal(t, validation.Of(0), result)
 	})
 }
 
@@ -135,24 +112,14 @@ func TestBool(t *testing.T) {
 		boolType := Bool()
 		result := boolType.Decode(true)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) bool { return false },
-			F.Identity[bool],
-		)
-		assert.Equal(t, true, value)
+		assert.Equal(t, validation.Of(true), result)
 	})
 
 	t.Run("decodes false", func(t *testing.T) {
 		boolType := Bool()
 		result := boolType.Decode(false)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) bool { return true },
-			F.Identity[bool],
-		)
-		assert.Equal(t, false, value)
+		assert.Equal(t, validation.Of(false), result)
 	})
 
 	t.Run("fails to decode int as bool", func(t *testing.T) {
@@ -189,36 +156,21 @@ func TestArray(t *testing.T) {
 		intArray := Array(Int())
 		result := intArray.Decode([]int{1, 2, 3})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []int { return nil },
-			F.Identity[[]int],
-		)
-		assert.Equal(t, []int{1, 2, 3}, value)
+		assert.Equal(t, validation.Of([]int{1, 2, 3}), result)
 	})
 
 	t.Run("decodes valid string array", func(t *testing.T) {
 		stringArray := Array(String())
 		result := stringArray.Decode([]string{"a", "b", "c"})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []string { return nil },
-			F.Identity[[]string],
-		)
-		assert.Equal(t, []string{"a", "b", "c"}, value)
+		assert.Equal(t, validation.Of([]string{"a", "b", "c"}), result)
 	})
 
 	t.Run("decodes empty array", func(t *testing.T) {
 		intArray := Array(Int())
 		result := intArray.Decode([]int{})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []int { return nil },
-			F.Identity[[]int],
-		)
-		assert.Equal(t, []int{}, value)
+		assert.Equal(t, validation.Of([]int{}), result)
 	})
 
 	t.Run("fails when array contains invalid element", func(t *testing.T) {
@@ -256,12 +208,7 @@ func TestArray(t *testing.T) {
 		nestedArray := Array(Array(Int()))
 		result := nestedArray.Decode([][]int{{1, 2}, {3, 4}})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) [][]int { return nil },
-			F.Identity[[][]int],
-		)
-		assert.Equal(t, [][]int{{1, 2}, {3, 4}}, value)
+		assert.Equal(t, validation.Of([][]int{{1, 2}, {3, 4}}), result)
 	})
 
 	t.Run("fails to decode non-iterable", func(t *testing.T) {
@@ -275,12 +222,7 @@ func TestArray(t *testing.T) {
 		boolArray := Array(Bool())
 		result := boolArray.Decode([]bool{true, false, true})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []bool { return nil },
-			F.Identity[[]bool],
-		)
-		assert.Equal(t, []bool{true, false, true}, value)
+		assert.Equal(t, validation.Of([]bool{true, false, true}), result)
 	})
 
 	t.Run("collects multiple validation errors", func(t *testing.T) {
@@ -360,24 +302,14 @@ func TestTranscodeArray(t *testing.T) {
 		intTranscode := TranscodeArray(Int())
 		result := intTranscode.Decode([]any{1, 2, 3})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []int { return nil },
-			F.Identity[[]int],
-		)
-		assert.Equal(t, []int{1, 2, 3}, value)
+		assert.Equal(t, validation.Of([]int{1, 2, 3}), result)
 	})
 
 	t.Run("decodes valid string array from string slice", func(t *testing.T) {
 		stringTranscode := TranscodeArray(String())
 		result := stringTranscode.Decode([]any{"a", "b", "c"})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []string { return nil },
-			F.Identity[[]string],
-		)
-		assert.Equal(t, []string{"a", "b", "c"}, value)
+		assert.Equal(t, validation.Of([]string{"a", "b", "c"}), result)
 	})
 
 	t.Run("decodes empty array", func(t *testing.T) {
@@ -411,24 +343,14 @@ func TestTranscodeArray(t *testing.T) {
 		nestedTranscode := TranscodeArray(TranscodeArray(Int()))
 		result := nestedTranscode.Decode([][]any{{1, 2}, {3, 4}})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) [][]int { return nil },
-			F.Identity[[][]int],
-		)
-		assert.Equal(t, [][]int{{1, 2}, {3, 4}}, value)
+		assert.Equal(t, validation.Of([][]int{{1, 2}, {3, 4}}), result)
 	})
 
 	t.Run("decodes array of bools", func(t *testing.T) {
 		boolTranscode := TranscodeArray(Bool())
 		result := boolTranscode.Decode([]any{true, false, true})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []bool { return nil },
-			F.Identity[[]bool],
-		)
-		assert.Equal(t, []bool{true, false, true}, value)
+		assert.Equal(t, validation.Of([]bool{true, false, true}), result)
 	})
 
 	t.Run("encodes empty array", func(t *testing.T) {
@@ -481,12 +403,7 @@ func TestTranscodeArrayWithTransformation(t *testing.T) {
 		arrayTranscode := TranscodeArray(stringToInt)
 		result := arrayTranscode.Decode([]string{"a", "bb", "ccc"})
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) []int { return nil },
-			F.Identity[[]int],
-		)
-		assert.Equal(t, []int{1, 2, 3}, value)
+		assert.Equal(t, validation.Of([]int{1, 2, 3}), result)
 	})
 
 	t.Run("encodes int slice to string slice", func(t *testing.T) {
@@ -1358,24 +1275,14 @@ func TestId(t *testing.T) {
 		idCodec := Id[string]()
 		result := idCodec.Decode("hello")
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) string { return "" },
-			F.Identity[string],
-		)
-		assert.Equal(t, "hello", value)
+		assert.Equal(t, validation.Of("hello"), result)
 	})
 
 	t.Run("decodes int successfully", func(t *testing.T) {
 		idCodec := Id[int]()
 		result := idCodec.Decode(42)
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) int { return 0 },
-			F.Identity[int],
-		)
-		assert.Equal(t, 42, value)
+		assert.Equal(t, validation.Of(42), result)
 	})
 
 	t.Run("encodes with identity function", func(t *testing.T) {
@@ -1431,13 +1338,7 @@ func TestId(t *testing.T) {
 		person := Person{Name: "Alice", Age: 30}
 
 		result := idCodec.Decode(person)
-		assert.True(t, either.IsRight(result))
-
-		value := either.MonadFold(result,
-			func(validation.Errors) Person { return Person{} },
-			F.Identity[Person],
-		)
-		assert.Equal(t, person, value)
+		assert.Equal(t, validation.Of(person), result)
 
 		encoded := idCodec.Encode(person)
 		assert.Equal(t, person, encoded)
@@ -1450,13 +1351,7 @@ func TestIdWithTranscodeArray(t *testing.T) {
 		arrayCodec := TranscodeArray(intId)
 
 		result := arrayCodec.Decode([]int{1, 2, 3, 4, 5})
-		assert.True(t, either.IsRight(result))
-
-		value := either.MonadFold(result,
-			func(validation.Errors) []int { return nil },
-			F.Identity[[]int],
-		)
-		assert.Equal(t, []int{1, 2, 3, 4, 5}, value)
+		assert.Equal(t, validation.Of([]int{1, 2, 3, 4, 5}), result)
 	})
 
 	t.Run("Id codec encodes array with identity", func(t *testing.T) {
@@ -1473,13 +1368,7 @@ func TestIdWithTranscodeArray(t *testing.T) {
 
 		input := [][]int{{1, 2}, {3, 4}, {5}}
 		result := nestedCodec.Decode(input)
-		assert.True(t, either.IsRight(result))
-
-		value := either.MonadFold(result,
-			func(validation.Errors) [][]int { return nil },
-			F.Identity[[][]int],
-		)
-		assert.Equal(t, input, value)
+		assert.Equal(t, validation.Of(input), result)
 	})
 }
 
@@ -1748,7 +1637,7 @@ func TestFromRefinementComposition(t *testing.T) {
 	positiveCodec := FromRefinement(positiveIntPrism)
 
 	// Compose with Int codec using Pipe
-	composed := Pipe[int, int, int, any](positiveCodec)(Int())
+	composed := Pipe[int, any](positiveCodec)(Int())
 
 	t.Run("ComposedDecodeValid", func(t *testing.T) {
 		result := composed.Decode(42)
@@ -1849,3 +1738,416 @@ func TestFromRefinementValidationContext(t *testing.T) {
 		assert.Equal(t, -5, err.Value)
 	})
 }
+
+// TestEmpty_Success tests that Empty always succeeds during decoding
+func TestEmpty_Success(t *testing.T) {
+	t.Run("decodes any input to default value", func(t *testing.T) {
+		defaultCodec := Empty[int, string, any](lazy.Of(pair.MakePair("default", 42)))
+
+		// Test with various input types
+		testCases := []struct {
+			name  string
+			input any
+		}{
+			{"string input", "anything"},
+			{"int input", 123},
+			{"nil input", nil},
+			{"bool input", true},
+			{"struct input", struct{ X int }{X: 10}},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				result := defaultCodec.Decode(tc.input)
+
+				assert.Equal(t, validation.Of(42), result)
+			})
+		}
+	})
+
+	t.Run("always returns same default value", func(t *testing.T) {
+		defaultCodec := Empty[string, string, any](lazy.Of(pair.MakePair("output", "default")))
+
+		result1 := defaultCodec.Decode(123)
+		result2 := defaultCodec.Decode("different")
+		result3 := defaultCodec.Decode(nil)
+
+		assert.True(t, either.IsRight(result1))
+		assert.True(t, either.IsRight(result2))
+		assert.True(t, either.IsRight(result3))
+
+		value1 := either.MonadFold(result1, func(validation.Errors) string { return "" }, F.Identity[string])
+		value2 := either.MonadFold(result2, func(validation.Errors) string { return "" }, F.Identity[string])
+		value3 := either.MonadFold(result3, func(validation.Errors) string { return "" }, F.Identity[string])
+
+		assert.Equal(t, "default", value1)
+		assert.Equal(t, "default", value2)
+		assert.Equal(t, "default", value3)
+	})
+}
+
+// TestEmpty_Encoding tests that Empty always uses default output during encoding
+func TestEmpty_Encoding(t *testing.T) {
+	t.Run("encodes any value to default output", func(t *testing.T) {
+		defaultCodec := Empty[int, string, any](lazy.Of(pair.MakePair("default", 42)))
+
+		// Test with various input values
+		testCases := []struct {
+			name  string
+			input int
+		}{
+			{"zero value", 0},
+			{"positive value", 100},
+			{"negative value", -50},
+			{"default value", 42},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				encoded := defaultCodec.Encode(tc.input)
+				assert.Equal(t, "default", encoded)
+			})
+		}
+	})
+
+	t.Run("always returns same default output", func(t *testing.T) {
+		defaultCodec := Empty[string, int, any](lazy.Of(pair.MakePair(999, "ignored")))
+
+		encoded1 := defaultCodec.Encode("value1")
+		encoded2 := defaultCodec.Encode("value2")
+		encoded3 := defaultCodec.Encode("")
+
+		assert.Equal(t, 999, encoded1)
+		assert.Equal(t, 999, encoded2)
+		assert.Equal(t, 999, encoded3)
+	})
+}
+
+// TestEmpty_Name tests that Empty has correct name
+func TestEmpty_Name(t *testing.T) {
+	t.Run("has name 'Empty'", func(t *testing.T) {
+		defaultCodec := Empty[int, int, any](lazy.Of(pair.MakePair(0, 0)))
+		assert.Equal(t, "Empty", defaultCodec.Name())
+	})
+}
+
+// TestEmpty_TypeChecking tests that Empty performs standard type checking
+func TestEmpty_TypeChecking(t *testing.T) {
+	t.Run("Is checks for correct type", func(t *testing.T) {
+		defaultCodec := Empty[int, string, any](lazy.Of(pair.MakePair("default", 42)))
+
+		// Should succeed for int
+		result := defaultCodec.Is(100)
+		assert.True(t, either.IsRight(result))
+
+		// Should fail for non-int
+		result = defaultCodec.Is("not an int")
+		assert.True(t, either.IsLeft(result))
+	})
+
+	t.Run("Is checks for string type", func(t *testing.T) {
+		defaultCodec := Empty[string, string, any](lazy.Of(pair.MakePair("out", "in")))
+
+		// Should succeed for string
+		result := defaultCodec.Is("hello")
+		assert.True(t, either.IsRight(result))
+
+		// Should fail for non-string
+		result = defaultCodec.Is(123)
+		assert.True(t, either.IsLeft(result))
+	})
+}
+
+// TestEmpty_LazyEvaluation tests that the Pair parameter allows dynamic values
+func TestEmpty_LazyEvaluation(t *testing.T) {
+	t.Run("lazy pair allows dynamic values", func(t *testing.T) {
+		counter := 0
+		lazyPair := func() pair.Pair[int, int] {
+			counter++
+			return pair.MakePair(counter, counter*10)
+		}
+
+		defaultCodec := Empty[int, int, any](lazyPair)
+
+		// Each decode can get a different value if the lazy function is dynamic
+		result1 := defaultCodec.Decode("input1")
+		value1 := either.MonadFold(result1,
+			func(validation.Errors) int { return 0 },
+			F.Identity[int],
+		)
+
+		result2 := defaultCodec.Decode("input2")
+		value2 := either.MonadFold(result2,
+			func(validation.Errors) int { return 0 },
+			F.Identity[int],
+		)
+
+		// Values can be different if lazy function produces different results
+		assert.True(t, value1 > 0)
+		assert.True(t, value2 > 0)
+	})
+}
+
+// TestEmpty_WithStructs tests Empty with struct types
+func TestEmpty_WithStructs(t *testing.T) {
+	type Config struct {
+		Timeout int
+		Retries int
+	}
+
+	t.Run("provides default struct value", func(t *testing.T) {
+		defaultConfig := Config{Timeout: 30, Retries: 3}
+		defaultCodec := Empty[Config, Config, any](lazy.Of(pair.MakePair(defaultConfig, defaultConfig)))
+
+		result := defaultCodec.Decode("anything")
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) Config { return Config{} },
+			F.Identity[Config],
+		)
+		assert.Equal(t, 30, value.Timeout)
+		assert.Equal(t, 3, value.Retries)
+	})
+
+	t.Run("encodes to default struct", func(t *testing.T) {
+		defaultConfig := Config{Timeout: 30, Retries: 3}
+		inputConfig := Config{Timeout: 60, Retries: 5}
+
+		defaultCodec := Empty[Config, Config, any](lazy.Of(pair.MakePair(defaultConfig, defaultConfig)))
+
+		encoded := defaultCodec.Encode(inputConfig)
+		assert.Equal(t, 30, encoded.Timeout)
+		assert.Equal(t, 3, encoded.Retries)
+	})
+}
+
+// TestEmpty_WithPointers tests Empty with pointer types
+func TestEmpty_WithPointers(t *testing.T) {
+	t.Run("provides default pointer value", func(t *testing.T) {
+		defaultValue := 42
+		defaultCodec := Empty[*int, *int, any](lazy.Of(pair.MakePair(&defaultValue, &defaultValue)))
+
+		result := defaultCodec.Decode("anything")
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) *int { return nil },
+			F.Identity[*int],
+		)
+		require.NotNil(t, value)
+		assert.Equal(t, 42, *value)
+	})
+
+	t.Run("provides nil pointer as default", func(t *testing.T) {
+		var nilPtr *int
+		defaultCodec := Empty[*int, *int, any](lazy.Of(pair.MakePair(nilPtr, nilPtr)))
+
+		result := defaultCodec.Decode("anything")
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) *int { return new(int) },
+			F.Identity[*int],
+		)
+		assert.Nil(t, value)
+	})
+}
+
+// TestEmpty_WithSlices tests Empty with slice types
+func TestEmpty_WithSlices(t *testing.T) {
+	t.Run("provides default slice value", func(t *testing.T) {
+		defaultSlice := []int{1, 2, 3}
+		defaultCodec := Empty[[]int, []int, any](lazy.Of(pair.MakePair(defaultSlice, defaultSlice)))
+
+		result := defaultCodec.Decode("anything")
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) []int { return nil },
+			F.Identity[[]int],
+		)
+		assert.Equal(t, []int{1, 2, 3}, value)
+	})
+
+	t.Run("provides empty slice as default", func(t *testing.T) {
+		emptySlice := []int{}
+		defaultCodec := Empty[[]int, []int, any](lazy.Of(pair.MakePair(emptySlice, emptySlice)))
+
+		result := defaultCodec.Decode("anything")
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) []int { return nil },
+			F.Identity[[]int],
+		)
+		assert.Equal(t, []int{}, value)
+	})
+}
+
+// TestEmpty_DifferentInputOutput tests Empty with different input and output types
+func TestEmpty_DifferentInputOutput(t *testing.T) {
+	t.Run("decodes to int, encodes to string", func(t *testing.T) {
+		defaultCodec := Empty[int, string, any](lazy.Of(pair.MakePair("default-output", 42)))
+
+		// Decode always returns 42
+		result := defaultCodec.Decode("any input")
+		assert.Equal(t, validation.Of(42), result)
+
+		// Encode always returns "default-output"
+		encoded := defaultCodec.Encode(100)
+		assert.Equal(t, "default-output", encoded)
+	})
+
+	t.Run("decodes to string, encodes to int", func(t *testing.T) {
+		defaultCodec := Empty[string, int, any](lazy.Of(pair.MakePair(999, "default-value")))
+
+		// Decode always returns "default-value"
+		result := defaultCodec.Decode(123)
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) string { return "" },
+			F.Identity[string],
+		)
+		assert.Equal(t, "default-value", value)
+
+		// Encode always returns 999
+		encoded := defaultCodec.Encode("any string")
+		assert.Equal(t, 999, encoded)
+	})
+}
+
+// TestEmpty_EdgeCases tests edge cases for Empty
+func TestEmpty_EdgeCases(t *testing.T) {
+	t.Run("with zero values", func(t *testing.T) {
+		defaultCodec := Empty[int, int, any](lazy.Of(pair.MakePair(0, 0)))
+
+		result := defaultCodec.Decode("anything")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) int { return -1 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 0, value)
+
+		encoded := defaultCodec.Encode(100)
+		assert.Equal(t, 0, encoded)
+	})
+
+	t.Run("with empty string", func(t *testing.T) {
+		defaultCodec := Empty[string, string, any](lazy.Of(pair.MakePair("", "")))
+
+		result := defaultCodec.Decode("non-empty")
+		assert.True(t, either.IsRight(result))
+		value := either.MonadFold(result,
+			func(validation.Errors) string { return "error" },
+			F.Identity[string],
+		)
+		assert.Equal(t, "", value)
+
+		encoded := defaultCodec.Encode("non-empty")
+		assert.Equal(t, "", encoded)
+	})
+
+	t.Run("with false boolean", func(t *testing.T) {
+		defaultCodec := Empty[bool, bool, any](lazy.Of(pair.MakePair(false, false)))
+
+		result := defaultCodec.Decode(true)
+		assert.Equal(t, validation.Of(false), result)
+
+		encoded := defaultCodec.Encode(true)
+		assert.Equal(t, false, encoded)
+	})
+}
+
+// TestEmpty_Integration tests Empty in composition scenarios
+func TestEmpty_Integration(t *testing.T) {
+	t.Run("composes with other codecs using Pipe", func(t *testing.T) {
+		// Create a codec that always provides a default int
+		defaultIntCodec := Empty[int, int, any](lazy.Of(pair.MakePair(42, 42)))
+
+		// Create a refinement that only accepts positive integers
+		positiveIntPrism := prism.MakePrismWithName(
+			func(n int) option.Option[int] {
+				if n > 0 {
+					return option.Some(n)
+				}
+				return option.None[int]()
+			},
+			func(n int) int { return n },
+			"PositiveInt",
+		)
+
+		positiveCodec := FromRefinement(positiveIntPrism)
+
+		// Compose: always decode to 42, then validate it's positive
+		composed := Pipe[int, any](positiveCodec)(defaultIntCodec)
+
+		// Should succeed because 42 is positive
+		result := composed.Decode("anything")
+		assert.Equal(t, validation.Of(42), result)
+	})
+
+	t.Run("used as placeholder in generic contexts", func(t *testing.T) {
+		// Empty can be used where a codec is required but not actually used
+		unitCodec := Empty[Void, Void, any](
+			lazy.Of(pair.MakePair(F.VOID, F.VOID)),
+		)
+
+		result := unitCodec.Decode("ignored")
+		assert.Equal(t, validation.Of(F.VOID), result)
+
+		encoded := unitCodec.Encode(F.VOID)
+		assert.Equal(t, F.VOID, encoded)
+	})
+}
+
+// TestEmpty_RoundTrip tests that Empty maintains consistency
+func TestEmpty_RoundTrip(t *testing.T) {
+	t.Run("decode then encode returns default output", func(t *testing.T) {
+		defaultCodec := Empty[int, string, any](lazy.Of(pair.MakePair("output", 42)))
+
+		// Decode
+		result := defaultCodec.Decode("input")
+		require.True(t, either.IsRight(result))
+
+		decoded := either.MonadFold(result,
+			func(validation.Errors) int { return 0 },
+			F.Identity[int],
+		)
+
+		// Encode
+		encoded := defaultCodec.Encode(decoded)
+
+		// Should get default output, not related to decoded value
+		assert.Equal(t, "output", encoded)
+	})
+
+	t.Run("multiple round trips are consistent", func(t *testing.T) {
+		defaultCodec := Empty[int, int, any](lazy.Of(pair.MakePair(100, 50)))
+
+		// First round trip
+		result1 := defaultCodec.Decode("input1")
+		decoded1 := either.MonadFold(result1,
+			func(validation.Errors) int { return 0 },
+			F.Identity[int],
+		)
+		encoded1 := defaultCodec.Encode(decoded1)
+
+		// Second round trip
+		result2 := defaultCodec.Decode("input2")
+		decoded2 := either.MonadFold(result2,
+			func(validation.Errors) int { return 0 },
+			F.Identity[int],
+		)
+		encoded2 := defaultCodec.Encode(decoded2)
+
+		// All decoded values should be the same
+		assert.Equal(t, 50, decoded1)
+		assert.Equal(t, 50, decoded2)
+
+		// All encoded values should be the same
+		assert.Equal(t, 100, encoded1)
+		assert.Equal(t, 100, encoded2)
+	})
+}

v2/optics/codec/codecs_test.go

@@ -159,7 +159,7 @@ func TestURL(t *testing.T) {
 
 func TestDate(t *testing.T) {
 
-	getOrElseNull := either.GetOrElse(reader.Of[validation.Errors, time.Time](time.Time{}))
+	getOrElseNull := either.GetOrElse(reader.Of[validation.Errors](time.Time{}))
 
 	t.Run("ISO 8601 date format", func(t *testing.T) {
 		dateCodec := Date("2006-01-02")

v2/optics/codec/decode/bind_test.go

@@ -19,12 +19,7 @@ func TestDo(t *testing.T) {
 		decoder := Do[string](State{})
 		result := decoder("input")
 
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) State { return State{} },
-			F.Identity[State],
-		)
-		assert.Equal(t, State{}, value)
+		assert.Equal(t, validation.Of(State{}), result)
 	})
 
 	t.Run("creates decoder with initialized state", func(t *testing.T) {
@@ -79,12 +74,7 @@ func TestBind(t *testing.T) {
 		)
 
 		result := decoder("input")
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) State { return State{} },
-			F.Identity[State],
-		)
-		assert.Equal(t, State{x: 42, y: 10}, value)
+		assert.Equal(t, validation.Of(State{x: 42, y: 10}), result)
 	})
 
 	t.Run("propagates failure", func(t *testing.T) {
@@ -216,12 +206,7 @@ func TestLet(t *testing.T) {
 		)
 
 		result := decoder("input")
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(validation.Errors) State { return State{} },
-			F.Identity[State],
-		)
-		assert.Equal(t, State{x: 60, y: 10, z: 20}, value)
+		assert.Equal(t, validation.Of(State{x: 60, y: 10, z: 20}), result)
 	})
 }
 

v2/optics/codec/decode/monad.go

@@ -18,6 +18,47 @@ func Of[I, A any](a A) Decode[I, A] {
 	return readereither.Of[I, Errors](a)
 }
 
+// OfLazy converts a lazy computation into a Decode that ignores its input.
+// The resulting Decode will evaluate the lazy computation when executed and wrap
+// the result in a successful validation, regardless of the input provided.
+//
+// This function is intended solely for deferring the computation of a value, NOT for
+// representing side effects. The lazy computation should be a pure function that
+// produces the same result each time it's called (referential transparency). For
+// operations with side effects, use appropriate effect types like IO or IOResult.
+//
+// This is useful for lifting deferred computations into the Decode context without
+// requiring access to the input, while maintaining the validation wrapper for consistency.
+//
+// Type Parameters:
+//   - I: The input type (ignored by the resulting Decode)
+//   - A: The result type produced by the lazy computation
+//
+// Parameters:
+//   - fa: A lazy computation that produces a value of type A (must be pure, no side effects)
+//
+// Returns:
+//   - A Decode that ignores its input, evaluates the lazy computation, and wraps the result in Validation[A]
+//
+// Example:
+//
+//	lazyValue := func() int { return 42 }
+//	decoder := decode.OfLazy[string](lazyValue)
+//	result := decoder("any input") // validation.Success(42)
+//
+// Example - Deferring expensive computation:
+//
+//	expensiveCalc := func() Config {
+//	    // Expensive but pure computation here
+//	    return computeDefaultConfig()
+//	}
+//	decoder := decode.OfLazy[map[string]any](expensiveCalc)
+//	// Computation is deferred until the Decode is executed
+//	result := decoder(inputData) // validation.Success(config)
+func OfLazy[I, A any](fa Lazy[A]) Decode[I, A] {
+	return readereither.OfLazy[I, Errors](fa)
+}
+
 // Left creates a Decode that always fails with the given validation errors.
 // This is the dual of Of - while Of lifts a success value, Left lifts failure errors
 // into the Decode context.

v2/optics/codec/decode/monad_test.go

@@ -51,6 +51,108 @@ func TestOf(t *testing.T) {
 	})
 }
 
+// TestOfLazy tests the OfLazy function
+func TestOfLazy(t *testing.T) {
+	t.Run("evaluates lazy computation ignoring input", func(t *testing.T) {
+		lazyValue := func() int { return 42 }
+		decoder := OfLazy[string](lazyValue)
+		res := decoder("any input")
+
+		assert.Equal(t, validation.Of(42), res)
+	})
+
+	t.Run("defers computation until Decode is executed", func(t *testing.T) {
+		executed := false
+		lazyComputation := func() string {
+			executed = true
+			return "computed"
+		}
+		decoder := OfLazy[string](lazyComputation)
+
+		// Computation should not be executed yet
+		assert.False(t, executed, "lazy computation should not be executed during Decode creation")
+
+		// Execute the Decode
+		res := decoder("input")
+
+		// Now computation should be executed
+		assert.True(t, executed, "lazy computation should be executed when Decode runs")
+		assert.Equal(t, validation.Of("computed"), res)
+	})
+
+	t.Run("evaluates lazy computation each time Decode is called", func(t *testing.T) {
+		counter := 0
+		lazyCounter := func() int {
+			counter++
+			return counter
+		}
+		decoder := OfLazy[string](lazyCounter)
+
+		// First execution
+		res1 := decoder("input")
+		assert.Equal(t, validation.Of(1), res1)
+
+		// Second execution
+		res2 := decoder("input")
+		assert.Equal(t, validation.Of(2), res2)
+
+		// Third execution
+		res3 := decoder("input")
+		assert.Equal(t, validation.Of(3), res3)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		lazyString := func() string { return "hello" }
+		decoder1 := OfLazy[int](lazyString)
+		assert.Equal(t, validation.Of("hello"), decoder1(123))
+
+		lazySlice := func() []int { return []int{1, 2, 3} }
+		decoder2 := OfLazy[string](lazySlice)
+		assert.Equal(t, validation.Of([]int{1, 2, 3}), decoder2("input"))
+
+		type Person struct {
+			Name string
+			Age  int
+		}
+		lazyStruct := func() Person { return Person{Name: "Alice", Age: 30} }
+		decoder3 := OfLazy[map[string]any](lazyStruct)
+		assert.Equal(t, validation.Of(Person{Name: "Alice", Age: 30}), decoder3(map[string]any{}))
+	})
+
+	t.Run("can be composed with other Decode operations", func(t *testing.T) {
+		lazyValue := func() int { return 10 }
+		decoder := MonadMap(
+			OfLazy[string](lazyValue),
+			func(x int) int { return x * 2 },
+		)
+		res := decoder("input")
+		assert.Equal(t, validation.Of(20), res)
+	})
+
+	t.Run("ignores input completely", func(t *testing.T) {
+		lazyValue := func() string { return "constant" }
+		decoder := OfLazy[string](lazyValue)
+
+		// Different inputs should produce same result
+		res1 := decoder("input1")
+		res2 := decoder("input2")
+
+		assert.Equal(t, validation.Of("constant"), res1)
+		assert.Equal(t, validation.Of("constant"), res2)
+		assert.Equal(t, res1, res2)
+	})
+
+	t.Run("always wraps result in success validation", func(t *testing.T) {
+		lazyValue := func() int { return 42 }
+		decoder := OfLazy[string](lazyValue)
+		res := decoder("input")
+
+		// Verify it's a successful validation
+		assert.True(t, either.IsRight(res))
+		assert.Equal(t, validation.Of(42), res)
+	})
+}
+
 // TestLeft tests the Left function
 func TestLeft(t *testing.T) {
 	t.Run("creates decoder that always fails", func(t *testing.T) {

v2/optics/codec/either_test.go

@@ -70,7 +70,7 @@ func TestEitherEncode(t *testing.T) {
 
 // TestEitherDecode tests decoding/validation of Either values
 func TestEitherDecode(t *testing.T) {
-	getOrElseNull := either.GetOrElse(reader.Of[validation.Errors, either.Either[string, int]](either.Left[int]("")))
+	getOrElseNull := either.GetOrElse(reader.Of[validation.Errors](either.Left[int]("")))
 
 	// Create codecs that both work with string input
 	stringCodec := Id[string]()

v2/optics/codec/types.go

@@ -2,6 +2,7 @@ package codec
 
 import (
 	"github.com/IBM/fp-go/v2/endomorphism"
+	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/formatting"
 	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/monoid"
@@ -10,12 +11,15 @@ 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/lens"
+	"github.com/IBM/fp-go/v2/optics/optional"
 	"github.com/IBM/fp-go/v2/optics/prism"
 	"github.com/IBM/fp-go/v2/option"
 	"github.com/IBM/fp-go/v2/pair"
 	"github.com/IBM/fp-go/v2/reader"
 	"github.com/IBM/fp-go/v2/readerresult"
 	"github.com/IBM/fp-go/v2/result"
+	"github.com/IBM/fp-go/v2/semigroup"
 )
 
 type (
@@ -338,4 +342,156 @@ type (
 	//   - ApplicativeMonoid: Combines successful results using inner monoid
 	//   - AlternativeMonoid: Combines applicative and alternative behaviors
 	Monoid[A any] = monoid.Monoid[A]
+
+	// Lens is an optic that focuses on a specific field within a product type S.
+	// It provides a way to get and set a field of type A within a structure of type S.
+	//
+	// A Lens[S, A] represents a relationship between a source type S and a focus type A,
+	// where the focus always exists (unlike Optional which may not exist).
+	//
+	// Lens operations:
+	//   - Get: Extract the field value A from structure S
+	//   - Set: Update the field value A in structure S, returning a new S
+	//
+	// Lens laws:
+	//   1. GetSet: If you get a value and then set it back, nothing changes
+	//      Set(Get(s))(s) = s
+	//   2. SetGet: If you set a value, you can get it back
+	//      Get(Set(a)(s)) = a
+	//   3. SetSet: Setting twice is the same as setting once with the final value
+	//      Set(b)(Set(a)(s)) = Set(b)(s)
+	//
+	// In the codec context, lenses are used with ApSL to build codecs for struct fields:
+	//   - Extract field values for encoding
+	//   - Update field values during validation
+	//   - Compose codec operations on nested structures
+	//
+	// Example:
+	//   type Person struct { Name string; Age int }
+	//
+	//   nameLens := lens.MakeLens(
+	//       func(p Person) string { return p.Name },
+	//       func(p Person, name string) Person { p.Name = name; return p },
+	//   )
+	//
+	//   // Use with ApSL to build a codec
+	//   personCodec := F.Pipe1(
+	//       codec.Struct[Person]("Person"),
+	//       codec.ApSL(S.Monoid, nameLens, codec.String),
+	//   )
+	//
+	// See also:
+	//   - ApSL: Applicative sequencing with lens
+	//   - Optional: For fields that may not exist
+	Lens[S, A any] = lens.Lens[S, A]
+
+	// Optional is an optic that focuses on a field within a product type S that may not exist.
+	// It provides a way to get and set an optional field of type A within a structure of type S.
+	//
+	// An Optional[S, A] represents a relationship between a source type S and a focus type A,
+	// where the focus may or may not be present (unlike Lens where it always exists).
+	//
+	// Optional operations:
+	//   - GetOption: Try to extract the field value, returning Option[A]
+	//   - Set: Update the field value if it exists, returning a new S
+	//
+	// Optional laws:
+	//   1. GetSet (No-op on None): If GetOption returns None, Set has no effect
+	//      GetOption(s) = None => Set(a)(s) = s
+	//   2. SetGet (Get what you Set): If GetOption returns Some, you can get back what you set
+	//      GetOption(s) = Some(_) => GetOption(Set(a)(s)) = Some(a)
+	//   3. SetSet (Last Set Wins): Setting twice is the same as setting once with the final value
+	//      Set(b)(Set(a)(s)) = Set(b)(s)
+	//
+	// In the codec context, optionals are used with ApSO to build codecs for optional fields:
+	//   - Extract optional field values for encoding (only if present)
+	//   - Update optional field values during validation
+	//   - Handle nullable or pointer fields gracefully
+	//   - Compose codec operations on structures with optional data
+	//
+	// Example:
+	//   type Person struct {
+	//       Name     string
+	//       Nickname *string  // Optional field
+	//   }
+	//
+	//   nicknameOpt := optional.MakeOptional(
+	//       func(p Person) option.Option[string] {
+	//           if p.Nickname != nil {
+	//               return option.Some(*p.Nickname)
+	//           }
+	//           return option.None[string]()
+	//       },
+	//       func(p Person, nick string) Person {
+	//           p.Nickname = &nick
+	//           return p
+	//       },
+	//   )
+	//
+	//   // Use with ApSO to build a codec with optional field
+	//   personCodec := F.Pipe1(
+	//       codec.Struct[Person]("Person"),
+	//       codec.ApSO(S.Monoid, nicknameOpt, codec.String),
+	//   )
+	//
+	//   // Encoding omits the field when absent
+	//   p1 := Person{Name: "Alice", Nickname: nil}
+	//   encoded := personCodec.Encode(p1)  // No nickname in output
+	//
+	// See also:
+	//   - ApSO: Applicative sequencing with optional
+	//   - Lens: For fields that always exist
+	Optional[S, A any] = optional.Optional[S, A]
+
+	// Semigroup represents an algebraic structure with an associative binary operation.
+	//
+	// A Semigroup[A] provides:
+	//   - Concat(A, A): Combines two values associatively
+	//
+	// Semigroup law:
+	//   - Associativity: Concat(Concat(a, b), c) = Concat(a, Concat(b, c))
+	//
+	// Unlike Monoid, Semigroup does not require an identity element (Empty).
+	// This makes it more general but less powerful for certain operations.
+	//
+	// In the codec context, semigroups are used to:
+	//   - Combine validation errors
+	//   - Merge partial results
+	//   - Aggregate codec outputs
+	//
+	// Example semigroups:
+	//   - String concatenation (without empty string)
+	//   - Array concatenation (without empty array)
+	//   - Error accumulation
+	//
+	// Note: Every Monoid is also a Semigroup, but not every Semigroup is a Monoid.
+	Semigroup[A any] = semigroup.Semigroup[A]
+
+	// Void represents a unit type with a single value.
+	//
+	// Void is used instead of struct{} to represent:
+	//   - Unit values in functional programming
+	//   - Placeholder types where no meaningful value is needed
+	//   - Return types for functions that produce no useful result
+	//
+	// The single value of type Void is VOID (function.VOID).
+	//
+	// Usage:
+	//   - Use function.Void (or F.Void) as the type
+	//   - Use function.VOID (or F.VOID) as the value
+	//
+	// Example:
+	//   unitCodec := codec.Empty[F.Void, F.Void, any](
+	//       lazy.Of(pair.MakePair(F.VOID, F.VOID)),
+	//   )
+	//
+	// Benefits over struct{}:
+	//   - More explicit intent (unit type vs empty struct)
+	//   - Consistent with functional programming conventions
+	//   - Better semantic meaning in type signatures
+	//
+	// See also:
+	//   - function.VOID: The single value of type Void
+	//   - Empty: Codec function that uses Void for unit types
+	Void = function.Void
 )

v2/optics/codec/validate/bind_test.go

@@ -170,12 +170,7 @@ func TestLet(t *testing.T) {
 		)
 
 		result := validator("input")(nil)
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(Errors) State { return State{} },
-			F.Identity[State],
-		)
-		assert.Equal(t, State{x: 5, computed: 10}, value)
+		assert.Equal(t, validation.Of(State{x: 5, computed: 10}), result)
 	})
 
 	t.Run("preserves failure", func(t *testing.T) {
@@ -218,12 +213,7 @@ func TestLet(t *testing.T) {
 		)
 
 		result := validator("input")(nil)
-		assert.True(t, either.IsRight(result))
-		value := either.MonadFold(result,
-			func(Errors) State { return State{} },
-			F.Identity[State],
-		)
-		assert.Equal(t, State{x: 60, y: 10, z: 20}, value)
+		assert.Equal(t, validation.Of(State{x: 60, y: 10, z: 20}), result)
 	})
 }
 

v2/optics/codec/validate/from_test.go

@@ -39,7 +39,7 @@ func TestFromReaderResult_Success(t *testing.T) {
 		}
 
 		// Convert to Validate
-		validator := FromReaderResult[int, string](successRR)
+		validator := FromReaderResult(successRR)
 
 		// Execute the validator
 		validationResult := validator(42)(nil)
@@ -53,7 +53,7 @@ func TestFromReaderResult_Success(t *testing.T) {
 		parseIntRR := result.Eitherize1(strconv.Atoi)
 
 		// Convert to Validate
-		validator := FromReaderResult[string, int](parseIntRR)
+		validator := FromReaderResult(parseIntRR)
 
 		// Execute with valid input
 		validationResult := validator("123")(nil)
@@ -74,7 +74,7 @@ func TestFromReaderResult_Success(t *testing.T) {
 		}
 
 		// Convert to Validate
-		validator := FromReaderResult[string, User](createUserRR)
+		validator := FromReaderResult(createUserRR)
 
 		// Execute the validator
 		validationResult := validator("Alice")(nil)
@@ -88,7 +88,7 @@ func TestFromReaderResult_Success(t *testing.T) {
 			return result.Of(input * 2)
 		}
 
-		validator := FromReaderResult[int, int](successRR)
+		validator := FromReaderResult(successRR)
 		validationResult := validator(21)(Context{})
 
 		assert.Equal(t, validation.Success(42), validationResult)
@@ -99,7 +99,7 @@ func TestFromReaderResult_Success(t *testing.T) {
 			return result.Of(input + " processed")
 		}
 
-		validator := FromReaderResult[string, string](successRR)
+		validator := FromReaderResult(successRR)
 		ctx := Context{
 			{Key: "user", Type: "User"},
 			{Key: "name", Type: "string"},
@@ -122,7 +122,7 @@ func TestFromReaderResult_Failure(t *testing.T) {
 		}
 
 		// Convert to Validate
-		validator := FromReaderResult[string, int](failureRR)
+		validator := FromReaderResult(failureRR)
 
 		// Execute the validator
 		validationResult := validator("invalid")(nil)
@@ -147,7 +147,7 @@ func TestFromReaderResult_Failure(t *testing.T) {
 			return result.Left[string](originalErr)
 		}
 
-		validator := FromReaderResult[int, string](failureRR)
+		validator := FromReaderResult(failureRR)
 		validationResult := validator(42)(nil)
 
 		assert.True(t, either.IsLeft(validationResult))
@@ -166,7 +166,7 @@ func TestFromReaderResult_Failure(t *testing.T) {
 			return result.Left[int](errors.New("conversion failed"))
 		}
 
-		validator := FromReaderResult[string, int](failureRR)
+		validator := FromReaderResult(failureRR)
 		ctx := Context{
 			{Key: "user", Type: "User"},
 			{Key: "age", Type: "int"},
@@ -213,7 +213,7 @@ func TestFromReaderResult_Failure(t *testing.T) {
 					return result.Left[int](tc.err)
 				}
 
-				validator := FromReaderResult[string, int](failureRR)
+				validator := FromReaderResult(failureRR)
 				validationResult := validator(tc.input)(nil)
 
 				assert.True(t, either.IsLeft(validationResult))
@@ -251,7 +251,7 @@ func TestFromReaderResult_Integration(t *testing.T) {
 
 		// Combine validators
 		validator := F.Pipe1(
-			FromReaderResult[string, int](parseIntRR),
+			FromReaderResult(parseIntRR),
 			Chain(validatePositive),
 		)
 
@@ -273,8 +273,8 @@ func TestFromReaderResult_Integration(t *testing.T) {
 
 		// Convert and map to double the value
 		validator := F.Pipe1(
-			FromReaderResult[string, int](parseIntRR),
-			Map[string, int, int](func(n int) int { return n * 2 }),
+			FromReaderResult(parseIntRR),
+			Map[string](func(n int) int { return n * 2 }),
 		)
 
 		validationResult := validator("21")(nil)
@@ -294,7 +294,7 @@ func TestFromReaderResult_Integration(t *testing.T) {
 			Bind(func(p int) func(State) State {
 				return func(s State) State { s.parsed = p; return s }
 			}, func(s State) Validate[string, int] {
-				return FromReaderResult[string, int](parseIntRR)
+				return FromReaderResult(parseIntRR)
 			}),
 			Let[string](func(v bool) func(State) State {
 				return func(s State) State { s.valid = v; return s }
@@ -315,7 +315,7 @@ func TestFromReaderResult_EdgeCases(t *testing.T) {
 			return result.Of(input)
 		}
 
-		validator := FromReaderResult[int, int](successRR)
+		validator := FromReaderResult(successRR)
 		validationResult := validator(42)(nil)
 
 		assert.True(t, either.IsRight(validationResult))
@@ -326,7 +326,7 @@ func TestFromReaderResult_EdgeCases(t *testing.T) {
 			return result.Of(input)
 		}
 
-		validator := FromReaderResult[string, string](identityRR)
+		validator := FromReaderResult(identityRR)
 		validationResult := validator("")(nil)
 
 		assert.Equal(t, validation.Success(""), validationResult)
@@ -337,7 +337,7 @@ func TestFromReaderResult_EdgeCases(t *testing.T) {
 			return result.Of(input)
 		}
 
-		validator := FromReaderResult[int, int](identityRR)
+		validator := FromReaderResult(identityRR)
 		validationResult := validator(0)(nil)
 
 		assert.Equal(t, validation.Success(0), validationResult)
@@ -352,7 +352,7 @@ func TestFromReaderResult_EdgeCases(t *testing.T) {
 			return result.Of(&Data{Value: input})
 		}
 
-		validator := FromReaderResult[int, *Data](createDataRR)
+		validator := FromReaderResult(createDataRR)
 		validationResult := validator(42)(nil)
 
 		assert.True(t, either.IsRight(validationResult))
@@ -372,7 +372,7 @@ func TestFromReaderResult_EdgeCases(t *testing.T) {
 			return result.Of([]string{input, input})
 		}
 
-		validator := FromReaderResult[string, []string](splitRR)
+		validator := FromReaderResult(splitRR)
 		validationResult := validator("test")(nil)
 
 		assert.Equal(t, validation.Success([]string{"test", "test"}), validationResult)
@@ -383,7 +383,7 @@ func TestFromReaderResult_EdgeCases(t *testing.T) {
 			return result.Of(map[string]int{input: len(input)})
 		}
 
-		validator := FromReaderResult[string, map[string]int](createMapRR)
+		validator := FromReaderResult(createMapRR)
 		validationResult := validator("hello")(nil)
 
 		assert.Equal(t, validation.Success(map[string]int{"hello": 5}), validationResult)
@@ -398,7 +398,7 @@ func TestFromReaderResult_TypeSafety(t *testing.T) {
 			return result.Of(fmt.Sprintf("%d", input))
 		}
 
-		validator := FromReaderResult[int, string](intToStringRR)
+		validator := FromReaderResult(intToStringRR)
 
 		// This should compile and work correctly
 		validationResult := validator(42)(nil)
@@ -409,7 +409,7 @@ func TestFromReaderResult_TypeSafety(t *testing.T) {
 		// This test verifies that the output type is preserved
 		stringToIntRR := result.Eitherize1(strconv.Atoi)
 
-		validator := FromReaderResult[string, int](stringToIntRR)
+		validator := FromReaderResult(stringToIntRR)
 		validationResult := validator("42")(nil)
 
 		// The result should be Validation[int]
@@ -428,7 +428,7 @@ func TestFromReaderResult_TypeSafety(t *testing.T) {
 			return Output{Result: val}, nil
 		})
 
-		validator := FromReaderResult[Input, Output](transformRR)
+		validator := FromReaderResult(transformRR)
 		validationResult := validator(Input{Value: "42"})(nil)
 
 		assert.Equal(t, validation.Success(Output{Result: 42}), validationResult)
@@ -441,7 +441,7 @@ func BenchmarkFromReaderResult_Success(b *testing.B) {
 		return result.Of(input * 2)
 	}
 
-	validator := FromReaderResult[int, int](successRR)
+	validator := FromReaderResult(successRR)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -455,7 +455,7 @@ func BenchmarkFromReaderResult_Failure(b *testing.B) {
 		return result.Left[int](errors.New("error"))
 	}
 
-	validator := FromReaderResult[int, int](failureRR)
+	validator := FromReaderResult(failureRR)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -469,7 +469,7 @@ func BenchmarkFromReaderResult_WithContext(b *testing.B) {
 		return result.Of(input * 2)
 	}
 
-	validator := FromReaderResult[int, int](successRR)
+	validator := FromReaderResult(successRR)
 	ctx := Context{
 		{Key: "user", Type: "User"},
 		{Key: "age", Type: "int"},

v2/optics/codec/validate/monad_test.go

@@ -26,7 +26,7 @@ func TestMonadChainLeft(t *testing.T) {
 		handler := func(errs Errors) Validate[string, int] {
 			for _, err := range errs {
 				if err.Messsage == "validation failed" {
-					return Of[string, int](0) // recover with default
+					return Of[string](0) // recover with default
 				}
 			}
 			return func(input string) Reader[Context, Validation[int]] {
@@ -43,7 +43,7 @@ func TestMonadChainLeft(t *testing.T) {
 	})
 
 	t.Run("preserves success values unchanged", func(t *testing.T) {
-		successValidator := Of[string, int](42)
+		successValidator := Of[string](42)
 
 		handler := func(errs Errors) Validate[string, int] {
 			return func(input string) Reader[Context, Validation[int]] {
@@ -145,7 +145,7 @@ func TestMonadChainLeft(t *testing.T) {
 		}
 
 		handler := func(errs Errors) Validate[Config, string] {
-			return Of[Config, string]("default-value")
+			return Of[Config]("default-value")
 		}
 
 		validator := MonadChainLeft(failingValidator, handler)
@@ -194,7 +194,7 @@ func TestMonadChainLeft(t *testing.T) {
 		}
 
 		handler := func(errs Errors) Validate[string, int] {
-			return Of[string, int](42)
+			return Of[string](42)
 		}
 
 		// MonadChainLeft - direct application
@@ -229,7 +229,7 @@ func TestMonadChainLeft(t *testing.T) {
 			// Check if we can recover
 			for _, err := range errs {
 				if err.Messsage == "error1" {
-					return Of[string, int](100) // recover
+					return Of[string](100) // recover
 				}
 			}
 			return func(input string) Reader[Context, Validation[int]] {
@@ -248,12 +248,12 @@ func TestMonadChainLeft(t *testing.T) {
 	})
 
 	t.Run("does not call handler on success", func(t *testing.T) {
-		successValidator := Of[string, int](42)
+		successValidator := Of[string](42)
 		handlerCalled := false
 
 		handler := func(errs Errors) Validate[string, int] {
 			handlerCalled = true
-			return Of[string, int](0)
+			return Of[string](0)
 		}
 
 		validator := MonadChainLeft(successValidator, handler)
@@ -267,9 +267,9 @@ func TestMonadChainLeft(t *testing.T) {
 // TestMonadAlt tests the MonadAlt function
 func TestMonadAlt(t *testing.T) {
 	t.Run("returns first validator when it succeeds", func(t *testing.T) {
-		validator1 := Of[string, int](42)
+		validator1 := Of[string](42)
 		validator2 := func() Validate[string, int] {
-			return Of[string, int](100)
+			return Of[string](100)
 		}
 
 		result := MonadAlt(validator1, validator2)("input")(nil)
@@ -285,7 +285,7 @@ func TestMonadAlt(t *testing.T) {
 			}
 		}
 		fallback := func() Validate[string, int] {
-			return Of[string, int](42)
+			return Of[string](42)
 		}
 
 		result := MonadAlt(failing, fallback)("input")(nil)
@@ -328,11 +328,11 @@ func TestMonadAlt(t *testing.T) {
 	})
 
 	t.Run("does not evaluate second validator when first succeeds", func(t *testing.T) {
-		validator1 := Of[string, int](42)
+		validator1 := Of[string](42)
 		evaluated := false
 		validator2 := func() Validate[string, int] {
 			evaluated = true
-			return Of[string, int](100)
+			return Of[string](100)
 		}
 
 		result := MonadAlt(validator1, validator2)("input")(nil)
@@ -349,7 +349,7 @@ func TestMonadAlt(t *testing.T) {
 			}
 		}
 		fallback := func() Validate[string, string] {
-			return Of[string, string]("fallback")
+			return Of[string]("fallback")
 		}
 
 		result := MonadAlt(failing, fallback)("input")(nil)
@@ -374,7 +374,7 @@ func TestMonadAlt(t *testing.T) {
 			}
 		}
 		succeeding := func() Validate[string, int] {
-			return Of[string, int](42)
+			return Of[string](42)
 		}
 
 		// Chain: try failing1, then failing2, then succeeding
@@ -395,7 +395,7 @@ func TestMonadAlt(t *testing.T) {
 			}
 		}
 		fallback := func() Validate[Config, string] {
-			return Of[Config, string]("default")
+			return Of[Config]("default")
 		}
 
 		result := MonadAlt(failing, fallback)(Config{Port: 9999})(nil)
@@ -458,9 +458,9 @@ func TestMonadAlt(t *testing.T) {
 // TestAlt tests the Alt function
 func TestAlt(t *testing.T) {
 	t.Run("returns first validator when it succeeds", func(t *testing.T) {
-		validator1 := Of[string, int](42)
+		validator1 := Of[string](42)
 		validator2 := func() Validate[string, int] {
-			return Of[string, int](100)
+			return Of[string](100)
 		}
 
 		withAlt := Alt(validator2)
@@ -477,7 +477,7 @@ func TestAlt(t *testing.T) {
 			}
 		}
 		fallback := func() Validate[string, int] {
-			return Of[string, int](42)
+			return Of[string](42)
 		}
 
 		withAlt := Alt(fallback)
@@ -522,11 +522,11 @@ func TestAlt(t *testing.T) {
 	})
 
 	t.Run("does not evaluate second validator when first succeeds", func(t *testing.T) {
-		validator1 := Of[string, int](42)
+		validator1 := Of[string](42)
 		evaluated := false
 		validator2 := func() Validate[string, int] {
 			evaluated = true
-			return Of[string, int](100)
+			return Of[string](100)
 		}
 
 		withAlt := Alt(validator2)
@@ -553,7 +553,7 @@ func TestAlt(t *testing.T) {
 			}
 		}
 		succeeding := func() Validate[string, int] {
-			return Of[string, int](42)
+			return Of[string](42)
 		}
 
 		// Use F.Pipe to chain alternatives
@@ -576,7 +576,7 @@ func TestAlt(t *testing.T) {
 			}
 		}
 		fallback := func() Validate[string, int] {
-			return Of[string, int](42)
+			return Of[string](42)
 		}
 
 		// Alt - curried for pipelines
@@ -592,9 +592,9 @@ func TestAlt(t *testing.T) {
 // TestMonadAltAndAltEquivalence tests that MonadAlt and Alt are equivalent
 func TestMonadAltAndAltEquivalence(t *testing.T) {
 	t.Run("both produce same results for success", func(t *testing.T) {
-		validator1 := Of[string, int](42)
+		validator1 := Of[string](42)
 		validator2 := func() Validate[string, int] {
-			return Of[string, int](100)
+			return Of[string](100)
 		}
 
 		resultMonadAlt := MonadAlt(validator1, validator2)("input")(nil)
@@ -612,7 +612,7 @@ func TestMonadAltAndAltEquivalence(t *testing.T) {
 			}
 		}
 		fallback := func() Validate[string, int] {
-			return Of[string, int](42)
+			return Of[string](42)
 		}
 
 		resultMonadAlt := MonadAlt(failing, fallback)("input")(nil)

v2/optics/codec/validate/monoid_test.go

@@ -15,7 +15,7 @@ import (
 // TestAlternativeMonoid tests the AlternativeMonoid function
 func TestAlternativeMonoid(t *testing.T) {
 	t.Run("with string monoid", func(t *testing.T) {
-		m := AlternativeMonoid[string, string](S.Monoid)
+		m := AlternativeMonoid[string](S.Monoid)
 
 		t.Run("empty returns validator that succeeds with empty string", func(t *testing.T) {
 			empty := m.Empty()
@@ -25,8 +25,8 @@ func TestAlternativeMonoid(t *testing.T) {
 		})
 
 		t.Run("concat combines successful validators using monoid", func(t *testing.T) {
-			validator1 := Of[string, string]("Hello")
-			validator2 := Of[string, string](" World")
+			validator1 := Of[string]("Hello")
+			validator2 := Of[string](" World")
 
 			combined := m.Concat(validator1, validator2)
 			result := combined("input")(nil)
@@ -42,7 +42,7 @@ func TestAlternativeMonoid(t *testing.T) {
 					})
 				}
 			}
-			succeeding := Of[string, string]("fallback")
+			succeeding := Of[string]("fallback")
 
 			combined := m.Concat(failing, succeeding)
 			result := combined("input")(nil)
@@ -85,7 +85,7 @@ func TestAlternativeMonoid(t *testing.T) {
 		})
 
 		t.Run("concat with empty preserves validator", func(t *testing.T) {
-			validator := Of[string, string]("test")
+			validator := Of[string]("test")
 			empty := m.Empty()
 
 			result1 := m.Concat(validator, empty)("input")(nil)
@@ -110,7 +110,7 @@ func TestAlternativeMonoid(t *testing.T) {
 			func(a, b int) int { return a + b },
 			0,
 		)
-		m := AlternativeMonoid[string, int](intMonoid)
+		m := AlternativeMonoid[string](intMonoid)
 
 		t.Run("empty returns validator with zero", func(t *testing.T) {
 			empty := m.Empty()
@@ -124,8 +124,8 @@ func TestAlternativeMonoid(t *testing.T) {
 		})
 
 		t.Run("concat combines decoded values when both succeed", func(t *testing.T) {
-			validator1 := Of[string, int](10)
-			validator2 := Of[string, int](32)
+			validator1 := Of[string](10)
+			validator2 := Of[string](32)
 
 			combined := m.Concat(validator1, validator2)
 			result := combined("input")(nil)
@@ -145,7 +145,7 @@ func TestAlternativeMonoid(t *testing.T) {
 					})
 				}
 			}
-			succeeding := Of[string, int](42)
+			succeeding := Of[string](42)
 
 			combined := m.Concat(failing, succeeding)
 			result := combined("input")(nil)
@@ -158,10 +158,10 @@ func TestAlternativeMonoid(t *testing.T) {
 		})
 
 		t.Run("multiple concat operations", func(t *testing.T) {
-			validator1 := Of[string, int](1)
-			validator2 := Of[string, int](2)
-			validator3 := Of[string, int](3)
-			validator4 := Of[string, int](4)
+			validator1 := Of[string](1)
+			validator2 := Of[string](2)
+			validator3 := Of[string](3)
+			validator4 := Of[string](4)
 
 			combined := m.Concat(m.Concat(m.Concat(validator1, validator2), validator3), validator4)
 			result := combined("input")(nil)
@@ -175,11 +175,11 @@ func TestAlternativeMonoid(t *testing.T) {
 	})
 
 	t.Run("satisfies monoid laws", func(t *testing.T) {
-		m := AlternativeMonoid[string, string](S.Monoid)
+		m := AlternativeMonoid[string](S.Monoid)
 
-		validator1 := Of[string, string]("a")
-		validator2 := Of[string, string]("b")
-		validator3 := Of[string, string]("c")
+		validator1 := Of[string]("a")
+		validator2 := Of[string]("b")
+		validator3 := Of[string]("c")
 
 		t.Run("left identity", func(t *testing.T) {
 			result := m.Concat(m.Empty(), validator1)("input")(nil)
@@ -222,7 +222,7 @@ func TestAlternativeMonoid(t *testing.T) {
 func TestAltMonoid(t *testing.T) {
 	t.Run("with default value as zero", func(t *testing.T) {
 		m := AltMonoid(func() Validate[string, int] {
-			return Of[string, int](0)
+			return Of[string](0)
 		})
 
 		t.Run("empty returns the provided zero validator", func(t *testing.T) {
@@ -233,8 +233,8 @@ func TestAltMonoid(t *testing.T) {
 		})
 
 		t.Run("concat returns first validator when it succeeds", func(t *testing.T) {
-			validator1 := Of[string, int](42)
-			validator2 := Of[string, int](100)
+			validator1 := Of[string](42)
+			validator2 := Of[string](100)
 
 			combined := m.Concat(validator1, validator2)
 			result := combined("input")(nil)
@@ -250,7 +250,7 @@ func TestAltMonoid(t *testing.T) {
 					})
 				}
 			}
-			succeeding := Of[string, int](42)
+			succeeding := Of[string](42)
 
 			combined := m.Concat(failing, succeeding)
 			result := combined("input")(nil)
@@ -341,7 +341,7 @@ func TestAltMonoid(t *testing.T) {
 
 	t.Run("chaining multiple fallbacks", func(t *testing.T) {
 		m := AltMonoid(func() Validate[string, string] {
-			return Of[string, string]("default")
+			return Of[string]("default")
 		})
 
 		primary := func(input string) Reader[Context, Validation[string]] {
@@ -358,7 +358,7 @@ func TestAltMonoid(t *testing.T) {
 				})
 			}
 		}
-		tertiary := Of[string, string]("tertiary value")
+		tertiary := Of[string]("tertiary value")
 
 		combined := m.Concat(m.Concat(primary, secondary), tertiary)
 		result := combined("input")(nil)
@@ -369,14 +369,14 @@ func TestAltMonoid(t *testing.T) {
 	t.Run("difference from AlternativeMonoid", func(t *testing.T) {
 		// AltMonoid - first success wins
 		altM := AltMonoid(func() Validate[string, int] {
-			return Of[string, int](0)
+			return Of[string](0)
 		})
 
 		// AlternativeMonoid - combines successes
-		altMonoid := AlternativeMonoid[string, int](N.MonoidSum[int]())
+		altMonoid := AlternativeMonoid[string](N.MonoidSum[int]())
 
-		validator1 := Of[string, int](10)
-		validator2 := Of[string, int](32)
+		validator1 := Of[string](10)
+		validator2 := Of[string](32)
 
 		// AltMonoid: returns first success (10)
 		result1 := altM.Concat(validator1, validator2)("input")(nil)

v2/optics/codec/validate/validate.go

@@ -160,6 +160,109 @@ func Of[I, A any](a A) Validate[I, A] {
 	return reader.Of[I](decode.Of[Context](a))
 }
 
+// OfLazy creates a Validate that defers the computation of a value until needed.
+//
+// This function lifts a lazy computation into the validation context. The computation
+// is deferred until the validator is actually executed, allowing for efficient handling
+// of expensive operations or values that may not always be needed.
+//
+// **IMPORTANT**: The lazy function MUST be pure (referentially transparent). It should
+// always return the same value when called and must not perform side effects. For
+// computations with side effects, use IO or IOEither types instead.
+//
+// # Type Parameters
+//
+//   - I: The input type (not used, but required for type consistency)
+//   - A: The type of the value produced by the lazy computation
+//
+// # Parameters
+//
+//   - fa: A lazy computation that produces a value of type A. This function is called
+//     each time the validator is executed.
+//
+// # Returns
+//
+// A Validate[I, A] that ignores its input and returns a successful validation containing
+// the lazily computed value.
+//
+// # Purity Requirements
+//
+// The lazy function MUST be pure:
+//   - Always returns the same result for the same (lack of) input
+//   - No side effects (no I/O, no mutation, no randomness)
+//   - Deterministic and referentially transparent
+//
+// For side effects, use:
+//   - IO types for effectful computations
+//   - IOEither for effectful computations that may fail
+//
+// # Example: Deferring Expensive Computation
+//
+//	import (
+//	    "github.com/IBM/fp-go/v2/optics/codec/validate"
+//	    "github.com/IBM/fp-go/v2/optics/codec/validation"
+//	)
+//
+//	// Expensive computation deferred until needed
+//	expensiveValue := validate.OfLazy[string, int](func() int {
+//	    // This computation only runs when the validator is executed
+//	    return computeExpensiveValue()
+//	})
+//
+//	result := expensiveValue("any input")(nil)
+//	// result is validation.Success(computed value)
+//
+// # Example: Lazy Default Value
+//
+//	// Provide a default value that's only computed if needed
+//	withDefault := validate.OfLazy[Config, Config](func() Config {
+//	    return loadDefaultConfig()
+//	})
+//
+//	// Use in a validation pipeline
+//	validator := F.Pipe1(
+//	    validateFromFile,
+//	    validate.Alt(func() validate.Validate[string, Config] {
+//	        return withDefault
+//	    }),
+//	)
+//	// Default config only loaded if file validation fails
+//
+// # Example: Composition with Other Validators
+//
+//	// Combine lazy value with validation logic
+//	lazyValidator := F.Pipe1(
+//	    validate.OfLazy[string, int](func() int { return 42 }),
+//	    validate.Chain(func(n int) validate.Validate[string, string] {
+//	        return func(input string) validate.Reader[validation.Context, validation.Validation[string]] {
+//	            return func(ctx validation.Context) validation.Validation[string] {
+//	                if len(input) > n {
+//	                    return validation.FailureWithMessage[string](input, "too long")(ctx)
+//	                }
+//	                return validation.Success(input)
+//	            }
+//	        }
+//	    }),
+//	)
+//
+// # Notes
+//
+//   - The lazy function is evaluated each time the validator is executed
+//   - The input value I is ignored; the validator succeeds regardless of input
+//   - The result is always wrapped in a successful validation
+//   - This is useful for deferring expensive computations or providing lazy defaults
+//   - The lazy function must be pure - no side effects allowed
+//   - For side effects, use IO or IOEither types instead
+//
+// # See Also
+//
+//   - Of: For non-lazy values
+//   - decode.OfLazy: The underlying decode operation
+//   - reader.Of: The reader lifting operation
+func OfLazy[I, A any](fa Lazy[A]) Validate[I, A] {
+	return reader.Of[I](decode.OfLazy[Context](fa))
+}
+
 // MonadMap applies a function to the successful result of a validation.
 //
 // This is the functor map operation for Validate. It transforms the success value

v2/optics/codec/validate/validate_test.go

@@ -1274,3 +1274,139 @@ func TestOrElse(t *testing.T) {
 		}
 	})
 }
+
+// TestOfLazy tests the OfLazy function
+func TestOfLazy(t *testing.T) {
+	t.Run("evaluates lazy computation", func(t *testing.T) {
+		// Create a validator with a lazy value
+		validator := OfLazy[string, int](func() int {
+			return 42
+		})
+
+		result := validator("any input")(nil)
+		assert.Equal(t, validation.Success(42), result)
+	})
+
+	t.Run("defers execution until called", func(t *testing.T) {
+		executed := false
+		validator := OfLazy[string, int](func() int {
+			executed = true
+			return 100
+		})
+
+		// Lazy function not executed yet
+		assert.False(t, executed)
+
+		// Execute the validator
+		result := validator("input")(nil)
+
+		// Now it should be executed
+		assert.True(t, executed)
+		assert.Equal(t, validation.Success(100), result)
+	})
+
+	t.Run("evaluates on each call", func(t *testing.T) {
+		callCount := 0
+		validator := OfLazy[string, int](func() int {
+			callCount++
+			return callCount
+		})
+
+		// First call
+		result1 := validator("input")(nil)
+		assert.Equal(t, validation.Success(1), result1)
+
+		// Second call - evaluates again
+		result2 := validator("input")(nil)
+		assert.Equal(t, validation.Success(2), result2)
+
+		// Third call
+		result3 := validator("input")(nil)
+		assert.Equal(t, validation.Success(3), result3)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		// String type
+		stringValidator := OfLazy[int, string](func() string {
+			return "hello"
+		})
+		result := stringValidator(42)(nil)
+		assert.Equal(t, validation.Success("hello"), result)
+
+		// Struct type
+		type Config struct {
+			Host string
+			Port int
+		}
+		configValidator := OfLazy[string, Config](func() Config {
+			return Config{Host: "localhost", Port: 8080}
+		})
+		result2 := configValidator("input")(nil)
+		assert.Equal(t, validation.Success(Config{Host: "localhost", Port: 8080}), result2)
+
+		// Slice type
+		sliceValidator := OfLazy[string, []int](func() []int {
+			return []int{1, 2, 3}
+		})
+		result3 := sliceValidator("input")(nil)
+		assert.Equal(t, validation.Success([]int{1, 2, 3}), result3)
+	})
+
+	t.Run("composes with other validators", func(t *testing.T) {
+		// Create a lazy validator that produces a number
+		lazyValue := OfLazy[string, int](func() int {
+			return 42
+		})
+
+		// Map to transform the value
+		validator := MonadMap(lazyValue, func(n int) int {
+			return n * 2
+		})
+
+		result := validator("any input")(nil)
+		assert.Equal(t, validation.Success(84), result)
+	})
+
+	t.Run("ignores input value", func(t *testing.T) {
+		validator := OfLazy[string, int](func() int {
+			return 999
+		})
+
+		// Different inputs should produce the same result
+		result1 := validator("input1")(nil)
+		result2 := validator("input2")(nil)
+		result3 := validator("")(nil)
+
+		assert.Equal(t, validation.Success(999), result1)
+		assert.Equal(t, validation.Success(999), result2)
+		assert.Equal(t, validation.Success(999), result3)
+	})
+
+	t.Run("always wraps in success validation", func(t *testing.T) {
+		validator := OfLazy[string, int](func() int {
+			return 42
+		})
+
+		result := validator("input")(nil)
+
+		// Verify it's a Right (success)
+		assert.True(t, E.IsRight(result))
+
+		// Extract and verify the value
+		value, _ := E.Unwrap(result)
+		assert.Equal(t, 42, value)
+	})
+
+	t.Run("works with context", func(t *testing.T) {
+		validator := OfLazy[string, string](func() string {
+			return "validated"
+		})
+
+		ctx := validation.Context{
+			{Key: "field", Type: "string"},
+		}
+
+		result := validator("input")(ctx)
+		assert.Equal(t, validation.Success("validated"), result)
+	})
+}

v2/optics/prism/prisms_test.go

@@ -1405,7 +1405,7 @@ func TestFromResult(t *testing.T) {
 	t.Run("extract from successful result", func(t *testing.T) {
 		prism := FromResult[int]()
 
-		success := result.Of[int](42)
+		success := result.Of(42)
 		extracted := prism.GetOption(success)
 
 		assert.True(t, O.IsSome(extracted))
@@ -1435,7 +1435,7 @@ func TestFromResult(t *testing.T) {
 	t.Run("works with string type", func(t *testing.T) {
 		prism := FromResult[string]()
 
-		success := result.Of[string]("hello")
+		success := result.Of("hello")
 		extracted := prism.GetOption(success)
 
 		assert.True(t, O.IsSome(extracted))
@@ -1451,7 +1451,7 @@ func TestFromResult(t *testing.T) {
 		prism := FromResult[Person]()
 
 		person := Person{Name: "Alice", Age: 30}
-		success := result.Of[Person](person)
+		success := result.Of(person)
 		extracted := prism.GetOption(success)
 
 		assert.True(t, O.IsSome(extracted))
@@ -1465,9 +1465,9 @@ func TestFromResult(t *testing.T) {
 func TestFromResultWithSet(t *testing.T) {
 	t.Run("set on successful result", func(t *testing.T) {
 		prism := FromResult[int]()
-		setter := Set[result.Result[int], int](200)
+		setter := Set[result.Result[int]](200)
 
-		success := result.Of[int](42)
+		success := result.Of(42)
 		updated := setter(prism)(success)
 
 		// Verify the value was updated
@@ -1478,7 +1478,7 @@ func TestFromResultWithSet(t *testing.T) {
 
 	t.Run("set on error result leaves it unchanged", func(t *testing.T) {
 		prism := FromResult[int]()
-		setter := Set[result.Result[int], int](200)
+		setter := Set[result.Result[int]](200)
 
 		failure := E.Left[int](errors.New("test error"))
 		updated := setter(prism)(failure)
@@ -1527,13 +1527,13 @@ func TestFromResultComposition(t *testing.T) {
 		composed := Compose[result.Result[int]](positivePrism)(FromResult[int]())
 
 		// Test with positive number
-		success := result.Of[int](42)
+		success := result.Of(42)
 		extracted := composed.GetOption(success)
 		assert.True(t, O.IsSome(extracted))
 		assert.Equal(t, 42, O.GetOrElse(F.Constant(-1))(extracted))
 
 		// Test with negative number
-		negativeSuccess := result.Of[int](-5)
+		negativeSuccess := result.Of(-5)
 		extracted = composed.GetOption(negativeSuccess)
 		assert.True(t, O.IsNone(extracted))
 
@@ -1705,7 +1705,7 @@ func TestParseJSONWithSet(t *testing.T) {
 		originalJSON := []byte(`{"name":"Alice","age":30}`)
 		newPerson := Person{Name: "Bob", Age: 25}
 
-		setter := Set[[]byte, Person](newPerson)
+		setter := Set[[]byte](newPerson)
 		updatedJSON := setter(prism)(originalJSON)
 
 		// Parse the updated JSON
@@ -1722,7 +1722,7 @@ func TestParseJSONWithSet(t *testing.T) {
 		invalidJSON := []byte(`{invalid}`)
 		newPerson := Person{Name: "Charlie", Age: 35}
 
-		setter := Set[[]byte, Person](newPerson)
+		setter := Set[[]byte](newPerson)
 		result := setter(prism)(invalidJSON)
 
 		// Should return original unchanged since it couldn't be parsed

v2/reader/reader.go

@@ -227,6 +227,51 @@ func Of[R, A any](a A) Reader[R, A] {
 	return function.Constant1[R](a)
 }
 
+// OfLazy converts a lazy computation into a Reader that ignores its environment.
+// The resulting Reader will evaluate the lazy computation when executed, regardless
+// of the environment provided.
+//
+// This function is intended solely for deferring the computation of a value, NOT for
+// representing side effects. The lazy computation should be a pure function that
+// produces the same result each time it's called (referential transparency). For
+// operations with side effects, use appropriate effect types like IO or IOEither.
+//
+// This is useful for lifting deferred computations into the Reader context without
+// requiring access to the environment.
+//
+// Type Parameters:
+//   - R: The environment type (ignored by the resulting Reader)
+//   - A: The result type produced by the lazy computation
+//
+// Parameters:
+//   - fa: A lazy computation that produces a value of type A (must be pure, no side effects)
+//
+// Returns:
+//   - A Reader that ignores its environment and evaluates the lazy computation
+//
+// Example:
+//
+//	type Config struct { Host string }
+//	lazyValue := func() int { return 42 }
+//	r := reader.OfLazy[Config](lazyValue)
+//	result := r(Config{Host: "localhost"}) // 42
+//
+// Example - Deferring expensive computation:
+//
+//	type Env struct { Debug bool }
+//	expensiveCalc := func() string {
+//	    // Expensive but pure computation here
+//	    return "computed result"
+//	}
+//	r := reader.OfLazy[Env](expensiveCalc)
+//	// Computation is deferred until the Reader is executed
+//	result := r(Env{Debug: true}) // "computed result"
+func OfLazy[R, A any](fa Lazy[A]) Reader[R, A] {
+	return func(_ R) A {
+		return fa()
+	}
+}
+
 // MonadChain sequences two Reader computations where the second depends on the result of the first.
 // Both computations share the same environment.
 // This is the monadic bind operation (flatMap).

v2/reader/reader_test.go

@@ -92,6 +92,91 @@ func TestOf(t *testing.T) {
 	assert.Equal(t, "constant", result)
 }
 
+func TestOfLazy(t *testing.T) {
+	t.Run("evaluates lazy computation ignoring environment", func(t *testing.T) {
+		lazyValue := func() int { return 42 }
+		r := OfLazy[Config](lazyValue)
+		result := r(Config{Host: "localhost", Port: 8080})
+		assert.Equal(t, 42, result)
+	})
+
+	t.Run("defers computation until Reader is executed", func(t *testing.T) {
+		executed := false
+		lazyComputation := func() string {
+			executed = true
+			return "computed"
+		}
+		r := OfLazy[Config](lazyComputation)
+
+		// Computation should not be executed yet
+		assert.False(t, executed, "lazy computation should not be executed during Reader creation")
+
+		// Execute the Reader
+		result := r(Config{Host: "localhost"})
+
+		// Now computation should be executed
+		assert.True(t, executed, "lazy computation should be executed when Reader runs")
+		assert.Equal(t, "computed", result)
+	})
+
+	t.Run("evaluates lazy computation each time Reader is called", func(t *testing.T) {
+		counter := 0
+		lazyCounter := func() int {
+			counter++
+			return counter
+		}
+		r := OfLazy[Config](lazyCounter)
+
+		// First execution
+		result1 := r(Config{Host: "localhost"})
+		assert.Equal(t, 1, result1)
+
+		// Second execution
+		result2 := r(Config{Host: "localhost"})
+		assert.Equal(t, 2, result2)
+
+		// Third execution
+		result3 := r(Config{Host: "localhost"})
+		assert.Equal(t, 3, result3)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		lazyString := func() string { return "hello" }
+		r1 := OfLazy[Config](lazyString)
+		assert.Equal(t, "hello", r1(Config{}))
+
+		lazySlice := func() []int { return []int{1, 2, 3} }
+		r2 := OfLazy[Config](lazySlice)
+		assert.Equal(t, []int{1, 2, 3}, r2(Config{}))
+
+		lazyStruct := func() Config { return Config{Host: "test", Port: 9000} }
+		r3 := OfLazy[string](lazyStruct)
+		assert.Equal(t, Config{Host: "test", Port: 9000}, r3("ignored"))
+	})
+
+	t.Run("can be composed with other Reader operations", func(t *testing.T) {
+		lazyValue := func() int { return 10 }
+		r := F.Pipe1(
+			OfLazy[Config](lazyValue),
+			Map[Config](func(x int) int { return x * 2 }),
+		)
+		result := r(Config{Host: "localhost"})
+		assert.Equal(t, 20, result)
+	})
+
+	t.Run("ignores environment completely", func(t *testing.T) {
+		lazyValue := func() string { return "constant" }
+		r := OfLazy[Config](lazyValue)
+
+		// Different environments should produce same result
+		config1 := Config{Host: "host1", Port: 8080}
+		config2 := Config{Host: "host2", Port: 9090}
+
+		assert.Equal(t, "constant", r(config1))
+		assert.Equal(t, "constant", r(config2))
+	})
+}
+
 func TestChain(t *testing.T) {
 	config := Config{Port: 8080}
 	getPort := Asks(func(c Config) int { return c.Port })

v2/reader/types.go

@@ -103,4 +103,6 @@ type (
 
 	// Seq represents an iterator sequence over values of type T.
 	Seq[T any] = iter.Seq[T]
+
+	Lazy[A any] = func() A
 )

v2/readereither/reader.go

@@ -23,6 +23,7 @@ import (
 	"github.com/IBM/fp-go/v2/internal/fromreader"
 	"github.com/IBM/fp-go/v2/internal/functor"
 	"github.com/IBM/fp-go/v2/internal/readert"
+	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/reader"
 )
 
@@ -46,6 +47,13 @@ func Right[E, L, A any](r A) ReaderEither[E, L, A] {
 	return eithert.Right(reader.Of[E, Either[L, A]], r)
 }
 
+func OfLazy[E, L, A any](r Lazy[A]) ReaderEither[E, L, A] {
+	return reader.OfLazy[E](function.Pipe1(
+		r,
+		lazy.Map(ET.Of[L, A]),
+	))
+}
+
 func FromReader[L, E, A any](r Reader[E, A]) ReaderEither[E, L, A] {
 	return RightReader[L](r)
 }

v2/readereither/types.go

@@ -23,10 +23,14 @@ import (
 )
 
 type (
+	// Lazy represents a deferred computation that produces a value of type A.
+	Lazy[A any] = lazy.Lazy[A]
+
 	// Option represents an optional value that may or may not be present.
 	Option[A any] = option.Option[A]
 
 	// Either represents a value of one of two possible types (a disjoint union).
+	// An instance of Either is either Left (representing an error) or Right (representing a success).
 	Either[E, A any] = either.Either[E, A]
 
 	// Reader represents a computation that depends on an environment R and produces a value A.
@@ -34,9 +38,9 @@ type (
 
 	// ReaderEither represents a computation that depends on an environment R and can fail
 	// with an error E or succeed with a value A.
-	// It combines Reader (dependency injection) with Either (error handling).
-
+	// It combines the Reader monad (for dependency injection) with the Either monad (for error handling).
 	ReaderEither[R, E, A any] = Reader[R, Either[E, A]]
+
 	// Kleisli represents a Kleisli arrow for the ReaderEither monad.
 	// It's a function from A to ReaderEither[R, E, B], used for composing operations that
 	// depend on an environment and may fail.
@@ -44,7 +48,6 @@ type (
 
 	// Operator represents a function that transforms one ReaderEither into another.
 	// It takes a ReaderEither[R, E, A] and produces a ReaderEither[R, E, B].
+	// This is commonly used for lifting functions into the ReaderEither context.
 	Operator[R, E, A, B any] = Kleisli[R, E, ReaderEither[R, E, A], B]
-
-	Lazy[A any] = lazy.Lazy[A]
 )

v2/readerio/profunctor_test.go

@@ -630,7 +630,7 @@ func TestLocalIOK(t *testing.T) {
 		}
 
 		// Compose using LocalIOK
-		adapted := LocalIOK[string, SimpleConfig, string](loadConfig)(useConfig)
+		adapted := LocalIOK[string](loadConfig)(useConfig)
 		result := adapted("config.json")()
 
 		assert.Equal(t, "localhost:8080", result)
@@ -650,7 +650,7 @@ func TestLocalIOK(t *testing.T) {
 			return io.Of(fmt.Sprintf("Processed: %d", n))
 		}
 
-		adapted := LocalIOK[string, int, string](loadData)(processData)
+		adapted := LocalIOK[string](loadData)(processData)
 		result := adapted("test")()
 
 		assert.Equal(t, "Processed: 40", result)
@@ -679,8 +679,8 @@ func TestLocalIOK(t *testing.T) {
 		}
 
 		// Compose transformations
-		step1 := LocalIOK[string, UserEnv, int](loadUser)(formatUser)
-		step2 := LocalIOK[string, int, string](parseID)(step1)
+		step1 := LocalIOK[string](loadUser)(formatUser)
+		step2 := LocalIOK[string](parseID)(step1)
 
 		result := step2("42")()
 		assert.Equal(t, "User ID: 42", result)
@@ -704,7 +704,7 @@ func TestLocalIOK(t *testing.T) {
 			return io.Of(fmt.Sprintf("Connected to %s:%d", cfg.Host, cfg.Port))
 		}
 
-		adapted := LocalIOK[string, DatabaseConfig, AppConfig](extractDB)(connectDB)
+		adapted := LocalIOK[string](extractDB)(connectDB)
 		result := adapted(AppConfig{
 			Database: DatabaseConfig{Host: "", Port: 5432},
 		})()
@@ -735,8 +735,8 @@ func TestLocalIOK(t *testing.T) {
 		}
 
 		// Compose the pipeline
-		step1 := LocalIOK[string, SimpleConfig, string](parseConfig)(useConfig)
-		step2 := LocalIOK[string, string, ConfigFile](readFile)(step1)
+		step1 := LocalIOK[string](parseConfig)(useConfig)
+		step2 := LocalIOK[string](readFile)(step1)
 
 		result := step2(ConfigFile{Path: "app.json"})()
 		assert.Equal(t, "Using example.com:9000", result)

v2/readerioeither/profunctor_test.go

@@ -149,7 +149,7 @@ func TestLocalIOK(t *testing.T) {
 		}
 
 		// Compose using LocalIOK
-		adapted := LocalIOK[string, string, SimpleConfig, string](loadConfig)(useConfig)
+		adapted := LocalIOK[string, string](loadConfig)(useConfig)
 		result := adapted("config.json")()
 
 		assert.Equal(t, E.Of[string]("Port: 8080"), result)
@@ -169,7 +169,7 @@ func TestLocalIOK(t *testing.T) {
 			return IOE.Of[string]("Processed: " + strconv.Itoa(n))
 		}
 
-		adapted := LocalIOK[string, string, int, string](loadData)(processData)
+		adapted := LocalIOK[string, string](loadData)(processData)
 		result := adapted("test")()
 
 		assert.Equal(t, E.Of[string]("Processed: 40"), result)
@@ -188,7 +188,7 @@ func TestLocalIOK(t *testing.T) {
 			return IOE.Left[string]("operation failed")
 		}
 
-		adapted := LocalIOK[string, string, SimpleConfig, string](loadConfig)(failingOperation)
+		adapted := LocalIOK[string, string](loadConfig)(failingOperation)
 		result := adapted("config.json")()
 
 		assert.Equal(t, E.Left[string]("operation failed"), result)
@@ -216,8 +216,8 @@ func TestLocalIOK(t *testing.T) {
 		}
 
 		// Compose transformations
-		step1 := LocalIOK[string, string, SimpleConfig, int](loadConfig)(formatConfig)
-		step2 := LocalIOK[string, string, int, string](parseID)(step1)
+		step1 := LocalIOK[string, string](loadConfig)(formatConfig)
+		step2 := LocalIOK[string, string](parseID)(step1)
 
 		result := step2("42")()
 		assert.Equal(t, E.Of[string]("Port: 8042"), result)
@@ -243,7 +243,7 @@ func TestLocalIOEitherK(t *testing.T) {
 		}
 
 		// Compose using LocalIOEitherK
-		adapted := LocalIOEitherK[string, SimpleConfig, string, string](loadConfig)(useConfig)
+		adapted := LocalIOEitherK[string](loadConfig)(useConfig)
 
 		// Success case
 		result := adapted("config.json")()
@@ -265,7 +265,7 @@ func TestLocalIOEitherK(t *testing.T) {
 			return IOE.Of[string]("Port: " + strconv.Itoa(cfg.Port))
 		}
 
-		adapted := LocalIOEitherK[string, SimpleConfig, string, string](loadConfig)(useConfig)
+		adapted := LocalIOEitherK[string](loadConfig)(useConfig)
 		result := adapted("missing.json")()
 
 		// Error from loadConfig should propagate
@@ -282,7 +282,7 @@ func TestLocalIOEitherK(t *testing.T) {
 			return IOE.Left[string]("operation failed")
 		}
 
-		adapted := LocalIOEitherK[string, SimpleConfig, string, string](loadConfig)(failingOperation)
+		adapted := LocalIOEitherK[string](loadConfig)(failingOperation)
 		result := adapted("config.json")()
 
 		// Error from ReaderIOEither should propagate
@@ -317,8 +317,8 @@ func TestLocalIOEitherK(t *testing.T) {
 		}
 
 		// Compose transformations
-		step1 := LocalIOEitherK[string, SimpleConfig, int, string](loadConfig)(formatConfig)
-		step2 := LocalIOEitherK[string, int, string, string](parseID)(step1)
+		step1 := LocalIOEitherK[string](loadConfig)(formatConfig)
+		step2 := LocalIOEitherK[string](parseID)(step1)
 
 		// Success case
 		result := step2("42")()
@@ -364,8 +364,8 @@ func TestLocalIOEitherK(t *testing.T) {
 		}
 
 		// Compose the pipeline
-		step1 := LocalIOEitherK[string, SimpleConfig, string, string](parseConfig)(useConfig)
-		step2 := LocalIOEitherK[string, string, ConfigFile, string](readFile)(step1)
+		step1 := LocalIOEitherK[string](parseConfig)(useConfig)
+		step2 := LocalIOEitherK[string](readFile)(step1)
 
 		// Success case
 		result := step2(ConfigFile{Path: "app.json"})()

v2/readeriooption/array_test.go

@@ -32,7 +32,7 @@ func TestTraverseArray_AllSuccess(t *testing.T) {
 	}
 
 	input := []int{1, 2, 3, 4, 5}
-	result := TraverseArray[context.Context](double)(input)
+	result := TraverseArray(double)(input)
 
 	expected := O.Of([]int{2, 4, 6, 8, 10})
 	assert.Equal(t, expected, result(context.Background())())
@@ -48,7 +48,7 @@ func TestTraverseArray_OneFailure(t *testing.T) {
 	}
 
 	input := []int{1, 2, 3, 4, 5}
-	result := TraverseArray[context.Context](failOnThree)(input)
+	result := TraverseArray(failOnThree)(input)
 
 	expected := O.None[[]int]()
 	assert.Equal(t, expected, result(context.Background())())
@@ -61,7 +61,7 @@ func TestTraverseArray_EmptyArray(t *testing.T) {
 	}
 
 	input := []int{}
-	result := TraverseArray[context.Context](double)(input)
+	result := TraverseArray(double)(input)
 
 	expected := O.Of([]int{})
 	assert.Equal(t, expected, result(context.Background())())
@@ -82,7 +82,7 @@ func TestTraverseArray_WithEnvironment(t *testing.T) {
 	}
 
 	input := []int{1, 2, 3}
-	result := TraverseArray[Config](multiply)(input)
+	result := TraverseArray(multiply)(input)
 
 	cfg := Config{Multiplier: 10}
 	expected := O.Of([]int{10, 20, 30})
@@ -105,7 +105,7 @@ func TestTraverseArray_ChainedOperation(t *testing.T) {
 
 	result := F.Pipe1(
 		Of[Config]([]int{1, 2, 3, 4}),
-		Chain(TraverseArray[Config](multiplyByFactor)),
+		Chain(TraverseArray(multiplyByFactor)),
 	)
 
 	cfg := Config{Factor: 5}
@@ -120,7 +120,7 @@ func TestTraverseArrayWithIndex_AllSuccess(t *testing.T) {
 	}
 
 	input := []string{"a", "b", "c"}
-	result := TraverseArrayWithIndex[context.Context](addIndex)(input)
+	result := TraverseArrayWithIndex(addIndex)(input)
 
 	expected := O.Of([]string{"0:a", "1:b", "2:c"})
 	assert.Equal(t, expected, result(context.Background())())
@@ -136,7 +136,7 @@ func TestTraverseArrayWithIndex_OneFailure(t *testing.T) {
 	}
 
 	input := []string{"a", "b", "c"}
-	result := TraverseArrayWithIndex[context.Context](failOnIndex)(input)
+	result := TraverseArrayWithIndex(failOnIndex)(input)
 
 	expected := O.None[[]string]()
 	assert.Equal(t, expected, result(context.Background())())
@@ -149,7 +149,7 @@ func TestTraverseArrayWithIndex_EmptyArray(t *testing.T) {
 	}
 
 	input := []string{}
-	result := TraverseArrayWithIndex[context.Context](addIndex)(input)
+	result := TraverseArrayWithIndex(addIndex)(input)
 
 	expected := O.Of([]string{})
 	assert.Equal(t, expected, result(context.Background())())
@@ -170,7 +170,7 @@ func TestTraverseArrayWithIndex_WithEnvironment(t *testing.T) {
 	}
 
 	input := []string{"a", "b", "c"}
-	result := TraverseArrayWithIndex[Config](formatWithIndex)(input)
+	result := TraverseArrayWithIndex(formatWithIndex)(input)
 
 	cfg := Config{Prefix: "item-"}
 	expected := O.Of([]string{"item-0:a", "item-1:b", "item-2:c"})
@@ -184,7 +184,7 @@ func TestTraverseArrayWithIndex_IndexUsedInLogic(t *testing.T) {
 	}
 
 	input := []int{10, 20, 30, 40}
-	result := TraverseArrayWithIndex[context.Context](multiplyByIndex)(input)
+	result := TraverseArrayWithIndex(multiplyByIndex)(input)
 
 	// 10*0=0, 20*1=20, 30*2=60, 40*3=120
 	expected := O.Of([]int{0, 20, 60, 120})
@@ -216,7 +216,7 @@ func TestTraverseArray_ComplexType(t *testing.T) {
 		{ID: 3, Name: "Charlie"},
 	}
 
-	result := TraverseArray[context.Context](loadProfile)(users)
+	result := TraverseArray(loadProfile)(users)
 
 	expected := O.Of([]UserProfile{
 		{UserID: 1, DisplayName: "Profile: Alice"},
@@ -247,12 +247,12 @@ func TestTraverseArray_ConditionalFailure(t *testing.T) {
 
 	// With MaxValue=3, should fail on 4 and 5
 	cfg1 := Config{MaxValue: 3}
-	result1 := TraverseArray[Config](validateAndDouble)(input)
+	result1 := TraverseArray(validateAndDouble)(input)
 	assert.Equal(t, O.None[[]int](), result1(cfg1)())
 
 	// With MaxValue=10, all should succeed
 	cfg2 := Config{MaxValue: 10}
-	result2 := TraverseArray[Config](validateAndDouble)(input)
+	result2 := TraverseArray(validateAndDouble)(input)
 	expected := O.Of([]int{2, 4, 6, 8, 10})
 	assert.Equal(t, expected, result2(cfg2)())
 }

v2/readeriooption/reader_test.go

@@ -67,7 +67,7 @@ func TestFromReader(t *testing.T) {
 		return cfg.Value * 2
 	}
 
-	ro := FromReader[Config](r)
+	ro := FromReader(r)
 	cfg := Config{Value: 21}
 	result := ro(cfg)()
 
@@ -83,7 +83,7 @@ func TestSomeReader(t *testing.T) {
 		return cfg.Value * 2
 	}
 
-	ro := SomeReader[Config](r)
+	ro := SomeReader(r)
 	cfg := Config{Value: 21}
 	result := ro(cfg)()
 

v2/readerioresult/reader_test.go

@@ -349,7 +349,7 @@ func TestLocalIOK(t *testing.T) {
 		}
 
 		// Compose using LocalIOK
-		adapted := LocalIOK[string, SimpleConfig, string](loadConfig)(useConfig)
+		adapted := LocalIOK[string](loadConfig)(useConfig)
 		res := adapted("config.json")()
 
 		assert.Equal(t, result.Of("Port: 8080"), res)
@@ -371,7 +371,7 @@ func TestLocalIOK(t *testing.T) {
 			}
 		}
 
-		adapted := LocalIOK[string, int, string](loadData)(processData)
+		adapted := LocalIOK[string](loadData)(processData)
 		res := adapted("test")()
 
 		assert.Equal(t, result.Of("Processed: 40"), res)
@@ -392,7 +392,7 @@ func TestLocalIOK(t *testing.T) {
 			}
 		}
 
-		adapted := LocalIOK[string, SimpleConfig, string](loadConfig)(failingOperation)
+		adapted := LocalIOK[string](loadConfig)(failingOperation)
 		res := adapted("config.json")()
 
 		assert.True(t, result.IsLeft(res))
@@ -424,7 +424,7 @@ func TestLocalIOEitherK(t *testing.T) {
 		}
 
 		// Compose using LocalIOEitherK
-		adapted := LocalIOEitherK[string, SimpleConfig, string](loadConfig)(useConfig)
+		adapted := LocalIOEitherK[string](loadConfig)(useConfig)
 
 		// Success case
 		res := adapted("config.json")()
@@ -448,7 +448,7 @@ func TestLocalIOEitherK(t *testing.T) {
 			}
 		}
 
-		adapted := LocalIOEitherK[string, SimpleConfig, string](loadConfig)(useConfig)
+		adapted := LocalIOEitherK[string](loadConfig)(useConfig)
 		res := adapted("missing.json")()
 
 		// Error from loadConfig should propagate
@@ -469,7 +469,7 @@ func TestLocalIOEitherK(t *testing.T) {
 			}
 		}
 
-		adapted := LocalIOEitherK[string, SimpleConfig, string](loadConfig)(failingOperation)
+		adapted := LocalIOEitherK[string](loadConfig)(failingOperation)
 		res := adapted("config.json")()
 
 		// Error from ReaderIOResult should propagate
@@ -502,7 +502,7 @@ func TestLocalIOResultK(t *testing.T) {
 		}
 
 		// Compose using LocalIOResultK
-		adapted := LocalIOResultK[string, SimpleConfig, string](loadConfig)(useConfig)
+		adapted := LocalIOResultK[string](loadConfig)(useConfig)
 
 		// Success case
 		res := adapted("config.json")()
@@ -526,7 +526,7 @@ func TestLocalIOResultK(t *testing.T) {
 			}
 		}
 
-		adapted := LocalIOResultK[string, SimpleConfig, string](loadConfig)(useConfig)
+		adapted := LocalIOResultK[string](loadConfig)(useConfig)
 		res := adapted("missing.json")()
 
 		// Error from loadConfig should propagate
@@ -562,8 +562,8 @@ func TestLocalIOResultK(t *testing.T) {
 		}
 
 		// Compose transformations
-		step1 := LocalIOResultK[string, SimpleConfig, int](loadConfig)(formatConfig)
-		step2 := LocalIOResultK[string, int, string](parseID)(step1)
+		step1 := LocalIOResultK[string](loadConfig)(formatConfig)
+		step2 := LocalIOResultK[string](parseID)(step1)
 
 		// Success case
 		res := step2("test")()
@@ -607,8 +607,8 @@ func TestLocalIOResultK(t *testing.T) {
 		}
 
 		// Compose the pipeline
-		step1 := LocalIOResultK[string, SimpleConfig, string](parseConfig)(useConfig)
-		step2 := LocalIOResultK[string, string, ConfigFile](readFile)(step1)
+		step1 := LocalIOResultK[string](parseConfig)(useConfig)
+		step2 := LocalIOResultK[string](readFile)(step1)
 
 		// Success case
 		res := step2(ConfigFile{Path: "app.json"})()
@@ -764,7 +764,7 @@ func TestTap(t *testing.T) {
 
 		pipeline := F.Pipe1(
 			Of[Config](42),
-			Tap[Config](tapFunc),
+			Tap(tapFunc),
 		)
 
 		res := pipeline(Config{})()
@@ -783,12 +783,12 @@ func TestTap(t *testing.T) {
 
 		pipelineWithTap := F.Pipe1(
 			Of[Config](42),
-			Tap[Config](sideEffect),
+			Tap(sideEffect),
 		)
 
 		pipelineWithChainFirst := F.Pipe1(
 			Of[Config](42),
-			ChainFirst[Config](sideEffect),
+			ChainFirst(sideEffect),
 		)
 
 		resultTap := pipelineWithTap(Config{})()
@@ -896,7 +896,7 @@ func TestTapReaderK(t *testing.T) {
 
 		pipeline := F.Pipe1(
 			Of[Config](42),
-			TapReaderK[Config](tapFunc),
+			TapReaderK(tapFunc),
 		)
 
 		res := pipeline(Config{multiplier: 2})()
@@ -965,7 +965,7 @@ func TestChainLeft(t *testing.T) {
 	t.Run("Right passes through unchanged", func(t *testing.T) {
 		pipeline := F.Pipe1(
 			Right[Config](42),
-			ChainLeft[Config](func(err error) ReaderIOResult[Config, int] {
+			ChainLeft(func(err error) ReaderIOResult[Config, int] {
 				return Left[Config, int](errors.New("should not run"))
 			}),
 		)
@@ -980,7 +980,7 @@ func TestChainLeft(t *testing.T) {
 
 		pipeline := F.Pipe1(
 			Left[Config, int](originalError),
-			ChainLeft[Config](func(err error) ReaderIOResult[Config, int] {
+			ChainLeft(func(err error) ReaderIOResult[Config, int] {
 				return Left[Config, int](newError)
 			}),
 		)
@@ -992,7 +992,7 @@ func TestChainLeft(t *testing.T) {
 	t.Run("Left recovers to Right", func(t *testing.T) {
 		pipeline := F.Pipe1(
 			Left[Config, int](errors.New("error")),
-			ChainLeft[Config](func(err error) ReaderIOResult[Config, int] {
+			ChainLeft(func(err error) ReaderIOResult[Config, int] {
 				return Right[Config](99)
 			}),
 		)
@@ -1010,7 +1010,7 @@ func TestTapLeft(t *testing.T) {
 
 		pipeline := F.Pipe1(
 			Right[Config](42),
-			TapLeft[int, Config](func(err error) ReaderIOResult[Config, string] {
+			TapLeft[int](func(err error) ReaderIOResult[Config, string] {
 				return func(cfg Config) IOResult[string] {
 					return func() Result[string] {
 						tapped = true
@@ -1031,7 +1031,7 @@ func TestTapLeft(t *testing.T) {
 
 		pipeline := F.Pipe1(
 			Left[Config, int](originalError),
-			TapLeft[int, Config](func(err error) ReaderIOResult[Config, string] {
+			TapLeft[int](func(err error) ReaderIOResult[Config, string] {
 				return func(cfg Config) IOResult[string] {
 					return func() Result[string] {
 						tapped = true

v2/readerreaderioeither/reader_test.go

@@ -91,7 +91,7 @@ func TestMonadMapTo(t *testing.T) {
 func TestChain(t *testing.T) {
 	g := F.Pipe1(
 		Of[OuterConfig, InnerConfig, error](1),
-		Chain[OuterConfig, InnerConfig, error](func(v int) ReaderReaderIOEither[OuterConfig, InnerConfig, error, string] {
+		Chain(func(v int) ReaderReaderIOEither[OuterConfig, InnerConfig, error, string] {
 			return Of[OuterConfig, InnerConfig, error](fmt.Sprintf("%d", v))
 		}),
 	)
@@ -193,7 +193,7 @@ func TestFromEither(t *testing.T) {
 
 	t.Run("Left", func(t *testing.T) {
 		err := errors.New("test error")
-		result := FromEither[OuterConfig, InnerConfig, error, int](E.Left[int](err))
+		result := FromEither[OuterConfig, InnerConfig](E.Left[int](err))
 		assert.Equal(t, E.Left[int](err), result(OuterConfig{})(InnerConfig{})())
 	})
 }
@@ -239,7 +239,7 @@ func TestLeftIO(t *testing.T) {
 func TestFromIOEither(t *testing.T) {
 	t.Run("Right", func(t *testing.T) {
 		ioe := IOE.Right[error](42)
-		result := FromIOEither[OuterConfig, InnerConfig, error](ioe)
+		result := FromIOEither[OuterConfig, InnerConfig](ioe)
 		assert.Equal(t, E.Right[error](42), result(OuterConfig{})(InnerConfig{})())
 	})
 
@@ -344,7 +344,7 @@ func TestFromPredicate(t *testing.T) {
 	})
 
 	t.Run("Predicate false", func(t *testing.T) {
-		result := FromPredicate[OuterConfig, InnerConfig, error](isPositive, onFalse)(-5)
+		result := FromPredicate[OuterConfig, InnerConfig](isPositive, onFalse)(-5)
 		expected := E.Left[int](fmt.Errorf("not positive: -5"))
 		assert.Equal(t, expected, result(OuterConfig{})(InnerConfig{})())
 	})
@@ -391,7 +391,7 @@ func TestRead(t *testing.T) {
 func TestChainEitherK(t *testing.T) {
 	g := F.Pipe1(
 		Of[OuterConfig, InnerConfig, error](1),
-		ChainEitherK[OuterConfig, InnerConfig, error](func(v int) E.Either[error, string] {
+		ChainEitherK[OuterConfig, InnerConfig](func(v int) E.Either[error, string] {
 			return E.Right[error](fmt.Sprintf("%d", v))
 		}),
 	)

v2/readerresult/reader.go

@@ -29,6 +29,7 @@ import (
 	"github.com/IBM/fp-go/v2/lazy"
 	"github.com/IBM/fp-go/v2/option"
 	"github.com/IBM/fp-go/v2/reader"
+	"github.com/IBM/fp-go/v2/readereither"
 	"github.com/IBM/fp-go/v2/result"
 )
 
@@ -279,6 +280,49 @@ func Of[R, A any](a A) ReaderResult[R, A] {
 	return readert.MonadOf[ReaderResult[R, A]](ET.Of[error, A], a)
 }
 
+// OfLazy converts a lazy computation into a ReaderResult that ignores its environment.
+// The resulting ReaderResult will evaluate the lazy computation when executed and wrap
+// the result in a successful Result, regardless of the environment provided.
+//
+// This function is intended solely for deferring the computation of a value, NOT for
+// representing side effects. The lazy computation should be a pure function that
+// produces the same result each time it's called (referential transparency). For
+// operations with side effects, use appropriate effect types like IO or IOResult.
+//
+// This is useful for lifting deferred computations into the ReaderResult context without
+// requiring access to the environment, while maintaining the Result wrapper for consistency.
+//
+// Type Parameters:
+//   - R: The environment type (ignored by the resulting ReaderResult)
+//   - A: The result type produced by the lazy computation
+//
+// Parameters:
+//   - r: A lazy computation that produces a value of type A (must be pure, no side effects)
+//
+// Returns:
+//   - A ReaderResult that ignores its environment, evaluates the lazy computation, and wraps the result in Result[A]
+//
+// Example:
+//
+//	type Config struct { Host string }
+//	lazyValue := func() int { return 42 }
+//	rr := readerresult.OfLazy[Config](lazyValue)
+//	result := rr(Config{Host: "localhost"}) // result.Of(42)
+//
+// Example - Deferring expensive computation:
+//
+//	type Env struct { Debug bool }
+//	expensiveCalc := func() string {
+//	    // Expensive but pure computation here
+//	    return "computed result"
+//	}
+//	rr := readerresult.OfLazy[Env](expensiveCalc)
+//	// Computation is deferred until the ReaderResult is executed
+//	result := rr(Env{Debug: true}) // result.Of("computed result")
+func OfLazy[R, A any](r Lazy[A]) ReaderResult[R, A] {
+	return readereither.OfLazy[R, error](r)
+}
+
 // MonadAp applies a function wrapped in a ReaderResult to a value wrapped in a ReaderResult.
 // Both computations share the same environment. This is useful for combining independent
 // computations that don't depend on each other's results.

v2/readerresult/reader_test.go

@@ -77,6 +77,101 @@ func TestOf(t *testing.T) {
 	assert.Equal(t, result.Of(42), rr(defaultContext))
 }
 
+func TestOfLazy(t *testing.T) {
+	t.Run("evaluates lazy computation ignoring environment", func(t *testing.T) {
+		lazyValue := func() int { return 42 }
+		rr := OfLazy[MyContext](lazyValue)
+		res := rr(defaultContext)
+		assert.Equal(t, result.Of(42), res)
+	})
+
+	t.Run("defers computation until ReaderResult is executed", func(t *testing.T) {
+		executed := false
+		lazyComputation := func() string {
+			executed = true
+			return "computed"
+		}
+		rr := OfLazy[MyContext](lazyComputation)
+
+		// Computation should not be executed yet
+		assert.False(t, executed, "lazy computation should not be executed during ReaderResult creation")
+
+		// Execute the ReaderResult
+		res := rr(defaultContext)
+
+		// Now computation should be executed
+		assert.True(t, executed, "lazy computation should be executed when ReaderResult runs")
+		assert.Equal(t, result.Of("computed"), res)
+	})
+
+	t.Run("evaluates lazy computation each time ReaderResult is called", func(t *testing.T) {
+		counter := 0
+		lazyCounter := func() int {
+			counter++
+			return counter
+		}
+		rr := OfLazy[MyContext](lazyCounter)
+
+		// First execution
+		res1 := rr(defaultContext)
+		assert.Equal(t, result.Of(1), res1)
+
+		// Second execution
+		res2 := rr(defaultContext)
+		assert.Equal(t, result.Of(2), res2)
+
+		// Third execution
+		res3 := rr(defaultContext)
+		assert.Equal(t, result.Of(3), res3)
+	})
+
+	t.Run("works with different types", func(t *testing.T) {
+		lazyString := func() string { return "hello" }
+		rr1 := OfLazy[MyContext](lazyString)
+		assert.Equal(t, result.Of("hello"), rr1(defaultContext))
+
+		lazySlice := func() []int { return []int{1, 2, 3} }
+		rr2 := OfLazy[MyContext](lazySlice)
+		assert.Equal(t, result.Of([]int{1, 2, 3}), rr2(defaultContext))
+
+		lazyStruct := func() MyContext { return "test" }
+		rr3 := OfLazy[string](lazyStruct)
+		assert.Equal(t, result.Of(MyContext("test")), rr3("ignored"))
+	})
+
+	t.Run("can be composed with other ReaderResult operations", func(t *testing.T) {
+		lazyValue := func() int { return 10 }
+		rr := F.Pipe1(
+			OfLazy[MyContext](lazyValue),
+			Map[MyContext](func(x int) int { return x * 2 }),
+		)
+		res := rr(defaultContext)
+		assert.Equal(t, result.Of(20), res)
+	})
+
+	t.Run("ignores environment completely", func(t *testing.T) {
+		lazyValue := func() string { return "constant" }
+		rr := OfLazy[MyContext](lazyValue)
+
+		// Different environments should produce same result
+		ctx1 := MyContext("context1")
+		ctx2 := MyContext("context2")
+
+		assert.Equal(t, result.Of("constant"), rr(ctx1))
+		assert.Equal(t, result.Of("constant"), rr(ctx2))
+	})
+
+	t.Run("always wraps result in success", func(t *testing.T) {
+		lazyValue := func() int { return 42 }
+		rr := OfLazy[MyContext](lazyValue)
+		res := rr(defaultContext)
+
+		// Verify it's a successful Result
+		assert.True(t, result.IsRight(res))
+		assert.Equal(t, result.Of(42), res)
+	})
+}
+
 func TestFromReader(t *testing.T) {
 	r := func(ctx MyContext) string { return string(ctx) }
 	rr := FromReader(r)

v2/samples/README.md

@@ -6,9 +6,62 @@ This folder is meant to contain examples that illustrate how to use the library.
 
 [![introduction to fp-go](presentation/cover.jpg)](https://www.youtube.com/watch?v=Jif3jL6DRdw "introduction to fp-go")
 
-### References
+## External Documentation References
 
-- [Ryan's Blog](https://rlee.dev/practical-guide-to-fp-ts-part-1) - practical introduction into FP concepts
-- [Investigate Functional Programming Concepts in Go](https://betterprogramming.pub/investigate-functional-programming-concepts-in-go-1dada09bc913) - discussion around FP concepts in golang
-- [Investigating the I/O Monad in Go](https://medium.com/better-programming/investigating-the-i-o-monad-in-go-3c0fabbb4b3d) - a closer look at I/O monads in golang
-- 
+### Official Documentation
+
+- [API Documentation](https://pkg.go.dev/github.com/IBM/fp-go/v2) - Complete API reference
+- [Go 1.24 Release Notes](https://tip.golang.org/doc/go1.24) - Information about generic type aliases
+- [Go Blog: Generating code](https://go.dev/blog/generate) - Using `go generate`
+- [Go Context Package](https://pkg.go.dev/context) - Standard library context documentation
+
+### Functional Programming Concepts
+
+#### Introductory Resources
+- [Ryan's Blog](https://rlee.dev/practical-guide-to-fp-ts-part-1) - Practical introduction into FP concepts
+- [Investigate Functional Programming Concepts in Go](https://betterprogramming.pub/investigate-functional-programming-concepts-in-go-1dada09bc913) - Discussion around FP concepts in golang
+- [Investigating the I/O Monad in Go](https://medium.com/better-programming/investigating-the-i-o-monad-in-go-3c0fabbb4b3d) - A closer look at I/O monads in golang
+- [Professor Frisby's Mostly Adequate Guide](https://github.com/MostlyAdequate/mostly-adequate-guide) - Comprehensive FP guide
+- [mostly-adequate-fp-ts](https://github.com/ChuckJonas/mostly-adequate-fp-ts/) - TypeScript companion to Frisby's guide
+
+#### Currying and Function Composition
+- [Mostly Adequate Guide - Ch. 4: Currying](https://mostly-adequate.gitbook.io/mostly-adequate-guide/ch04) - Excellent introduction with clear examples
+- [Curry and Function Composition](https://medium.com/javascript-scene/curry-and-function-composition-2c208d774983) by Eric Elliott
+- [Why Curry Helps](https://hughfdjackson.com/javascript/why-curry-helps/) - Practical benefits of currying
+
+### Haskell and Type Theory
+
+- [Haskell Wiki - Currying](https://wiki.haskell.org/Currying) - Comprehensive explanation of currying in Haskell
+- [Learn You a Haskell - Higher Order Functions](http://learnyouahaskell.com/higher-order-functions) - Introduction to currying and partial application
+- [Haskell's Prelude](https://hackage.haskell.org/package/base/docs/Prelude.html) - Standard library showing data-last convention
+- [Haskell Pair Type](https://hackage.haskell.org/package/TypeCompose-0.9.14/docs/Data-Pair.html) - Haskell definition of Pair
+- [Haskell Lens Library](https://hackage.haskell.org/package/lens) - Pioneering optics library
+
+### Optics
+
+- [Introduction to optics: lenses and prisms](https://medium.com/@gcanti/introduction-to-optics-lenses-and-prisms-3230e73bfcfe) by Giulio Canti - Excellent introduction to optics concepts
+- [Lenses in Functional Programming](https://www.schoolofhaskell.com/school/to-infinity-and-beyond/pick-of-the-week/a-little-lens-starter-tutorial) - Tutorial on lens fundamentals
+- [Profunctor Optics: The Categorical View](https://bartoszmilewski.com/2017/07/07/profunctor-optics-the-categorical-view/) by Bartosz Milewski - Deep dive into the theory
+- [Why Optics?](https://www.tweag.io/blog/2022-01-06-optics-vs-lenses/) - Discussion of benefits and use cases
+
+### Related Libraries
+
+- [fp-ts](https://github.com/gcanti/fp-ts) - TypeScript library that inspired fp-go
+- [fp-ts Documentation](https://gcanti.github.io/fp-ts/) - TypeScript library documentation
+- [fp-ts Issue #1238](https://github.com/gcanti/fp-ts/issues/1238) - Real-world examples of data-last refactoring
+- [urfave/cli/v3](https://github.com/urfave/cli) - Underlying CLI framework
+
+### Project Resources
+
+- [GitHub Repository](https://github.com/IBM/fp-go) - Source code and issues
+- [Coverage Status](https://coveralls.io/github/IBM/fp-go?branch=main) - Test coverage reports
+- [Go Report Card](https://goreportcard.com/report/github.com/IBM/fp-go/v2) - Code quality metrics
+- [Apache License 2.0](https://github.com/IBM/fp-go/blob/main/LICENSE) - Project license
+
+### Internal Documentation
+
+- [DESIGN.md](../DESIGN.md) - Design philosophy and patterns
+- [IDIOMATIC_COMPARISON.md](../IDIOMATIC_COMPARISON.md) - Performance comparison between standard and idiomatic packages
+- [Optics Overview](../optics/README.md) - Complete guide to lenses, prisms, and other optics
+- [CLI Package](../cli/README.md) - Command-line interface utilities
+- [ReaderResult Package](../idiomatic/context/readerresult/README.md) - Context-aware result handling

v2/semigroup/semigroup.go

@@ -123,3 +123,78 @@ func Last[A any]() Semigroup[A] {
 func ToMagma[A any](s Semigroup[A]) M.Magma[A] {
 	return s
 }
+
+// ConcatWith creates a curried version of the Concat operation with the left argument fixed first.
+// It returns a function that takes the left operand and returns another function that takes
+// the right operand and performs the concatenation.
+//
+// This is useful for partial application and function composition patterns.
+//
+// # Type Parameters
+//
+//   - A: The type of elements in the semigroup
+//
+// # Parameters
+//
+//   - s: The semigroup to use for concatenation
+//
+// # Returns
+//
+//   - func(A) func(A) A: A curried function that takes left then right operand
+//
+// # Example Usage
+//
+//	import N "github.com/IBM/fp-go/v2/number"
+//	sum := N.SemigroupSum[int]()
+//	concatWith := ConcatWith(sum)
+//	add5 := concatWith(5)
+//	result := add5(3)  // 5 + 3 = 8
+//
+// # See Also
+//
+//   - AppendTo: Similar but fixes the right argument first
+func ConcatWith[A any](s Semigroup[A]) func(A) func(A) A {
+	return func(l A) func(A) A {
+		return func(r A) A {
+			return s.Concat(l, r)
+		}
+	}
+}
+
+// AppendTo creates a curried version of the Concat operation with the right argument fixed first.
+// It returns a function that takes the right operand and returns another function that takes
+// the left operand and performs the concatenation.
+//
+// This is useful for partial application where you want to fix the second argument first,
+// which is common in append-style operations.
+//
+// # Type Parameters
+//
+//   - A: The type of elements in the semigroup
+//
+// # Parameters
+//
+//   - s: The semigroup to use for concatenation
+//
+// # Returns
+//
+//   - func(A) func(A) A: A curried function that takes right then left operand
+//
+// # Example Usage
+//
+//	import S "github.com/IBM/fp-go/v2/string"
+//	strConcat := S.Semigroup
+//	appendTo := AppendTo(strConcat)
+//	addSuffix := appendTo("!")
+//	result := addSuffix("Hello")  // "Hello" + "!" = "Hello!"
+//
+// # See Also
+//
+//   - ConcatWith: Similar but fixes the left argument first
+func AppendTo[A any](s Semigroup[A]) func(A) func(A) A {
+	return func(r A) func(A) A {
+		return func(l A) A {
+			return s.Concat(l, r)
+		}
+	}
+}

v2/semigroup/semigroup_test.go

@@ -444,3 +444,261 @@ func BenchmarkFunctionSemigroup(b *testing.B) {
 		combined("hello")
 	}
 }
+
+// Test ConcatWith function
+func TestConcatWith(t *testing.T) {
+	t.Run("with integer addition", func(t *testing.T) {
+		add := MakeSemigroup(func(a, b int) int { return a + b })
+		concatWith := ConcatWith(add)
+
+		// Fix left operand to 5
+		add5 := concatWith(5)
+		assert.Equal(t, 8, add5(3))   // 5 + 3 = 8
+		assert.Equal(t, 15, add5(10)) // 5 + 10 = 15
+		assert.Equal(t, 5, add5(0))   // 5 + 0 = 5
+	})
+
+	t.Run("with string concatenation", func(t *testing.T) {
+		concat := MakeSemigroup(func(a, b string) string { return a + b })
+		concatWith := ConcatWith(concat)
+
+		// Fix left operand to "Hello, "
+		greet := concatWith("Hello, ")
+		assert.Equal(t, "Hello, World", greet("World"))
+		assert.Equal(t, "Hello, Bob", greet("Bob"))
+		assert.Equal(t, "Hello, ", greet(""))
+	})
+
+	t.Run("with subtraction (non-commutative)", func(t *testing.T) {
+		sub := MakeSemigroup(func(a, b int) int { return a - b })
+		concatWith := ConcatWith(sub)
+
+		// Fix left operand to 10
+		subtract10 := concatWith(10)
+		assert.Equal(t, 7, subtract10(3))   // 10 - 3 = 7
+		assert.Equal(t, 5, subtract10(5))   // 10 - 5 = 5
+		assert.Equal(t, -5, subtract10(15)) // 10 - 15 = -5
+	})
+
+	t.Run("with First semigroup", func(t *testing.T) {
+		first := First[int]()
+		concatWith := ConcatWith(first)
+
+		// Fix left operand to 42
+		always42 := concatWith(42)
+		assert.Equal(t, 42, always42(1))
+		assert.Equal(t, 42, always42(100))
+		assert.Equal(t, 42, always42(0))
+	})
+
+	t.Run("with Last semigroup", func(t *testing.T) {
+		last := Last[string]()
+		concatWith := ConcatWith(last)
+
+		// Fix left operand to "first"
+		alwaysSecond := concatWith("first")
+		assert.Equal(t, "second", alwaysSecond("second"))
+		assert.Equal(t, "other", alwaysSecond("other"))
+		assert.Equal(t, "", alwaysSecond(""))
+	})
+
+	t.Run("currying behavior", func(t *testing.T) {
+		mul := MakeSemigroup(func(a, b int) int { return a * b })
+		concatWith := ConcatWith(mul)
+
+		// Create multiple partially applied functions
+		double := concatWith(2)
+		triple := concatWith(3)
+		quadruple := concatWith(4)
+
+		assert.Equal(t, 10, double(5))    // 2 * 5 = 10
+		assert.Equal(t, 15, triple(5))    // 3 * 5 = 15
+		assert.Equal(t, 20, quadruple(5)) // 4 * 5 = 20
+	})
+
+	t.Run("with complex types", func(t *testing.T) {
+		type Point struct {
+			X, Y int
+		}
+
+		pointAdd := MakeSemigroup(func(a, b Point) Point {
+			return Point{X: a.X + b.X, Y: a.Y + b.Y}
+		})
+		concatWith := ConcatWith(pointAdd)
+
+		// Fix left operand to origin offset
+		offset := concatWith(Point{X: 10, Y: 20})
+		assert.Equal(t, Point{X: 15, Y: 25}, offset(Point{X: 5, Y: 5}))
+		assert.Equal(t, Point{X: 10, Y: 20}, offset(Point{X: 0, Y: 0}))
+	})
+}
+
+// Test AppendTo function
+func TestAppendTo(t *testing.T) {
+	t.Run("with integer addition", func(t *testing.T) {
+		add := MakeSemigroup(func(a, b int) int { return a + b })
+		appendTo := AppendTo(add)
+
+		// Fix right operand to 5
+		addTo5 := appendTo(5)
+		assert.Equal(t, 8, addTo5(3))   // 3 + 5 = 8
+		assert.Equal(t, 15, addTo5(10)) // 10 + 5 = 15
+		assert.Equal(t, 5, addTo5(0))   // 0 + 5 = 5
+	})
+
+	t.Run("with string concatenation", func(t *testing.T) {
+		concat := MakeSemigroup(func(a, b string) string { return a + b })
+		appendTo := AppendTo(concat)
+
+		// Fix right operand to "!"
+		addExclamation := appendTo("!")
+		assert.Equal(t, "Hello!", addExclamation("Hello"))
+		assert.Equal(t, "World!", addExclamation("World"))
+		assert.Equal(t, "!", addExclamation(""))
+	})
+
+	t.Run("with subtraction (non-commutative)", func(t *testing.T) {
+		sub := MakeSemigroup(func(a, b int) int { return a - b })
+		appendTo := AppendTo(sub)
+
+		// Fix right operand to 3
+		subtract3 := appendTo(3)
+		assert.Equal(t, 7, subtract3(10))  // 10 - 3 = 7
+		assert.Equal(t, 2, subtract3(5))   // 5 - 3 = 2
+		assert.Equal(t, -3, subtract3(0))  // 0 - 3 = -3
+		assert.Equal(t, -8, subtract3(-5)) // -5 - 3 = -8
+	})
+
+	t.Run("with First semigroup", func(t *testing.T) {
+		first := First[string]()
+		appendTo := AppendTo(first)
+
+		// Fix right operand to "second"
+		alwaysFirst := appendTo("second")
+		assert.Equal(t, "first", alwaysFirst("first"))
+		assert.Equal(t, "other", alwaysFirst("other"))
+		assert.Equal(t, "", alwaysFirst(""))
+	})
+
+	t.Run("with Last semigroup", func(t *testing.T) {
+		last := Last[int]()
+		appendTo := AppendTo(last)
+
+		// Fix right operand to 42
+		always42 := appendTo(42)
+		assert.Equal(t, 42, always42(1))
+		assert.Equal(t, 42, always42(100))
+		assert.Equal(t, 42, always42(0))
+	})
+
+	t.Run("currying behavior", func(t *testing.T) {
+		mul := MakeSemigroup(func(a, b int) int { return a * b })
+		appendTo := AppendTo(mul)
+
+		// Create multiple partially applied functions
+		multiplyBy2 := appendTo(2)
+		multiplyBy3 := appendTo(3)
+		multiplyBy4 := appendTo(4)
+
+		assert.Equal(t, 10, multiplyBy2(5)) // 5 * 2 = 10
+		assert.Equal(t, 15, multiplyBy3(5)) // 5 * 3 = 15
+		assert.Equal(t, 20, multiplyBy4(5)) // 5 * 4 = 20
+	})
+
+	t.Run("with complex types", func(t *testing.T) {
+		type Point struct {
+			X, Y int
+		}
+
+		pointAdd := MakeSemigroup(func(a, b Point) Point {
+			return Point{X: a.X + b.X, Y: a.Y + b.Y}
+		})
+		appendTo := AppendTo(pointAdd)
+
+		// Fix right operand to offset
+		addOffset := appendTo(Point{X: 10, Y: 20})
+		assert.Equal(t, Point{X: 15, Y: 25}, addOffset(Point{X: 5, Y: 5}))
+		assert.Equal(t, Point{X: 10, Y: 20}, addOffset(Point{X: 0, Y: 0}))
+	})
+}
+
+// Test ConcatWith vs AppendTo difference
+func TestConcatWithVsAppendTo(t *testing.T) {
+	t.Run("demonstrates order difference with non-commutative operation", func(t *testing.T) {
+		sub := MakeSemigroup(func(a, b int) int { return a - b })
+
+		concatWith := ConcatWith(sub)
+		appendTo := AppendTo(sub)
+
+		// ConcatWith fixes left operand first
+		subtract10From := concatWith(10)      // 10 - x
+		assert.Equal(t, 7, subtract10From(3)) // 10 - 3 = 7
+
+		// AppendTo fixes right operand first
+		subtract3From := appendTo(3)          // x - 3
+		assert.Equal(t, 7, subtract3From(10)) // 10 - 3 = 7
+
+		// Same result but different partial application order
+		assert.Equal(t, subtract10From(3), subtract3From(10))
+	})
+
+	t.Run("demonstrates order difference with string concatenation", func(t *testing.T) {
+		concat := MakeSemigroup(func(a, b string) string { return a + b })
+
+		concatWith := ConcatWith(concat)
+		appendTo := AppendTo(concat)
+
+		// ConcatWith: prefix is fixed
+		addPrefix := concatWith("Hello, ")
+		assert.Equal(t, "Hello, World", addPrefix("World"))
+
+		// AppendTo: suffix is fixed
+		addSuffix := appendTo("!")
+		assert.Equal(t, "Hello!", addSuffix("Hello"))
+
+		// Different results due to different order
+		assert.NotEqual(t, addPrefix("test"), addSuffix("test"))
+	})
+}
+
+// Test composition with ConcatWith and AppendTo
+func TestConcatWithAppendToComposition(t *testing.T) {
+	t.Run("composing multiple operations", func(t *testing.T) {
+		add := MakeSemigroup(func(a, b int) int { return a + b })
+
+		// Create a pipeline: add 5, then add 3
+		concatWith := ConcatWith(add)
+		appendTo := AppendTo(add)
+
+		add5 := concatWith(5)
+		add3 := appendTo(3)
+
+		// Apply both operations
+		result := add3(add5(2)) // (2 + 5) + 3 = 10
+		assert.Equal(t, 10, result)
+	})
+}
+
+// Benchmark ConcatWith
+func BenchmarkConcatWith(b *testing.B) {
+	add := MakeSemigroup(func(a, b int) int { return a + b })
+	concatWith := ConcatWith(add)
+	add5 := concatWith(5)
+
+	b.ResetTimer()
+	for b.Loop() {
+		add5(3)
+	}
+}
+
+// Benchmark AppendTo
+func BenchmarkAppendTo(b *testing.B) {
+	add := MakeSemigroup(func(a, b int) int { return a + b })
+	appendTo := AppendTo(add)
+	addTo5 := appendTo(5)
+
+	b.ResetTimer()
+	for b.Loop() {
+		addTo5(3)
+	}
+}