fix: OrElse

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

v2/.bob/mcp.json

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

v2/effect/bind.go

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

v2/effect/bind_test.go

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

v2/effect/dependencies_test.go

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

v2/effect/effect_test.go

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

v2/effect/monoid_test.go

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

v2/effect/retry_test.go

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

v2/effect/run_test.go

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

v2/effect/traverse_test.go

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

v2/either/either.go

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

v2/either/either_test.go

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

v2/ioeither/ioeither.go

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

v2/ioeither/ioeither_test.go

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

v2/ioref/ioref.go

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

v2/ioref/ioref_test.go

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

v2/ioref/types.go

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

v2/optics/codec/codec_test.go

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

v2/optics/codec/decode/OrElse_ChainLeft_explanation.md

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

v2/optics/codec/decode/bind.go

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

v2/optics/codec/decode/bind_test.go

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

v2/optics/codec/decode/monad.go

@@ -50,6 +50,17 @@ func Chain[I, A, B any](f Kleisli[I, A, B]) Operator[I, A, B] {
 	)
 }
 
+func ChainLeft[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return readert.Chain[Decode[I, A]](
+		validation.ChainLeft,
+		f,
+	)
+}
+
+func OrElse[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return ChainLeft(f)
+}
+
 // MonadMap transforms the decoded value using the provided function.
 // This is the functor map operation that applies a transformation to successful decode results.
 //

v2/optics/codec/decode/monad_test.go

@@ -382,3 +382,400 @@ func TestFunctorLaws(t *testing.T) {
 		assert.Equal(t, right(input), left(input))
 	})
 }
+
+// TestChainLeft tests the ChainLeft function
+func TestChainLeft(t *testing.T) {
+	t.Run("transforms failures while preserving successes", func(t *testing.T) {
+		// Create a failing decoder
+		failingDecoder := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{Value: input, Messsage: "decode failed"},
+			})
+		}
+
+		// Handler that recovers from specific errors
+		handler := ChainLeft(func(errs Errors) Decode[string, int] {
+			for _, err := range errs {
+				if err.Messsage == "decode failed" {
+					return Of[string](0) // recover with default
+				}
+			}
+			return func(input string) Validation[int] {
+				return either.Left[int](errs)
+			}
+		})
+
+		decoder := handler(failingDecoder)
+		res := decoder("input")
+
+		assert.Equal(t, validation.Of(0), res, "Should recover from failure")
+	})
+
+	t.Run("preserves success values unchanged", func(t *testing.T) {
+		successDecoder := Of[string](42)
+
+		handler := ChainLeft(func(errs Errors) Decode[string, int] {
+			return func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Messsage: "should not be called"},
+				})
+			}
+		})
+
+		decoder := handler(successDecoder)
+		res := decoder("input")
+
+		assert.Equal(t, validation.Of(42), res, "Success should pass through unchanged")
+	})
+
+	t.Run("aggregates errors when transformation also fails", func(t *testing.T) {
+		failingDecoder := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "original error"},
+			})
+		}
+
+		handler := ChainLeft(func(errs Errors) Decode[string, string] {
+			return func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Messsage: "additional error"},
+				})
+			}
+		})
+
+		decoder := handler(failingDecoder)
+		res := decoder("input")
+
+		assert.True(t, either.IsLeft(res))
+		errors := either.MonadFold(res,
+			func(e Errors) Errors { return e },
+			func(string) Errors { return nil },
+		)
+		assert.Len(t, errors, 2, "Should aggregate both errors")
+
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "original error")
+		assert.Contains(t, messages, "additional error")
+	})
+
+	t.Run("adds context to errors", func(t *testing.T) {
+		failingDecoder := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{Value: input, Messsage: "invalid format"},
+			})
+		}
+
+		addContext := ChainLeft(func(errs Errors) Decode[string, int] {
+			return func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{
+						Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+						Messsage: "failed to decode user age",
+					},
+				})
+			}
+		})
+
+		decoder := addContext(failingDecoder)
+		res := decoder("abc")
+
+		assert.True(t, either.IsLeft(res))
+		errors := either.MonadFold(res,
+			func(e Errors) Errors { return e },
+			func(int) Errors { return nil },
+		)
+		assert.Len(t, errors, 2, "Should have both original and context errors")
+	})
+
+	t.Run("can be composed in pipeline", func(t *testing.T) {
+		failingDecoder := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{Value: input, Messsage: "error1"},
+			})
+		}
+
+		handler1 := ChainLeft(func(errs Errors) Decode[string, int] {
+			return func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Messsage: "error2"},
+				})
+			}
+		})
+
+		handler2 := ChainLeft(func(errs Errors) Decode[string, int] {
+			// Check if we can recover
+			for _, err := range errs {
+				if err.Messsage == "error1" {
+					return Of[string](100) // recover
+				}
+			}
+			return func(input string) Validation[int] {
+				return either.Left[int](errs)
+			}
+		})
+
+		// Compose handlers
+		decoder := handler2(handler1(failingDecoder))
+		res := decoder("input")
+
+		// Should recover because error1 is present
+		assert.Equal(t, validation.Of(100), res)
+	})
+
+	t.Run("works with different input types", func(t *testing.T) {
+		type Config struct {
+			Port int
+		}
+
+		failingDecoder := func(cfg Config) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: cfg.Port, Messsage: "invalid port"},
+			})
+		}
+
+		handler := ChainLeft(func(errs Errors) Decode[Config, string] {
+			return Of[Config]("default-value")
+		})
+
+		decoder := handler(failingDecoder)
+		res := decoder(Config{Port: 9999})
+
+		assert.Equal(t, validation.Of("default-value"), res)
+	})
+}
+
+// TestOrElse tests the OrElse function
+func TestOrElse(t *testing.T) {
+	t.Run("OrElse is equivalent to ChainLeft - Success case", func(t *testing.T) {
+		successDecoder := Of[string](42)
+
+		handler := func(errs Errors) Decode[string, int] {
+			return func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Messsage: "should not be called"},
+				})
+			}
+		}
+
+		// Test with OrElse
+		resultOrElse := OrElse(handler)(successDecoder)("input")
+		// Test with ChainLeft
+		resultChainLeft := ChainLeft(handler)(successDecoder)("input")
+
+		assert.Equal(t, resultChainLeft, resultOrElse, "OrElse and ChainLeft should produce identical results for Success")
+		assert.Equal(t, validation.Of(42), resultOrElse)
+	})
+
+	t.Run("OrElse is equivalent to ChainLeft - Failure recovery", func(t *testing.T) {
+		failingDecoder := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{Value: input, Messsage: "not found"},
+			})
+		}
+
+		handler := func(errs Errors) Decode[string, int] {
+			for _, err := range errs {
+				if err.Messsage == "not found" {
+					return Of[string](0) // recover with default
+				}
+			}
+			return func(input string) Validation[int] {
+				return either.Left[int](errs)
+			}
+		}
+
+		// Test with OrElse
+		resultOrElse := OrElse(handler)(failingDecoder)("input")
+		// Test with ChainLeft
+		resultChainLeft := ChainLeft(handler)(failingDecoder)("input")
+
+		assert.Equal(t, resultChainLeft, resultOrElse, "OrElse and ChainLeft should produce identical results for recovery")
+		assert.Equal(t, validation.Of(0), resultOrElse)
+	})
+
+	t.Run("OrElse is equivalent to ChainLeft - Error aggregation", func(t *testing.T) {
+		failingDecoder := func(input string) Validation[string] {
+			return either.Left[string](validation.Errors{
+				{Value: input, Messsage: "original error"},
+			})
+		}
+
+		handler := func(errs Errors) Decode[string, string] {
+			return func(input string) Validation[string] {
+				return either.Left[string](validation.Errors{
+					{Messsage: "additional error"},
+				})
+			}
+		}
+
+		// Test with OrElse
+		resultOrElse := OrElse(handler)(failingDecoder)("input")
+		// Test with ChainLeft
+		resultChainLeft := ChainLeft(handler)(failingDecoder)("input")
+
+		assert.Equal(t, resultChainLeft, resultOrElse, "OrElse and ChainLeft should produce identical results for error aggregation")
+
+		// Verify both aggregate errors
+		assert.True(t, either.IsLeft(resultOrElse))
+		errors := either.MonadFold(resultOrElse,
+			func(e Errors) Errors { return e },
+			func(string) Errors { return nil },
+		)
+		assert.Len(t, errors, 2, "Should aggregate both errors")
+		messages := make([]string, len(errors))
+		for i, err := range errors {
+			messages[i] = err.Messsage
+		}
+		assert.Contains(t, messages, "original error")
+		assert.Contains(t, messages, "additional error")
+	})
+
+	t.Run("OrElse semantic meaning - fallback decoder", func(t *testing.T) {
+		// OrElse provides a semantic name for fallback/alternative decoding
+		// It reads naturally: "try this decoder, or else try this alternative"
+
+		primaryDecoder := func(input string) Validation[int] {
+			if input == "valid" {
+				return validation.Of(42)
+			}
+			return either.Left[int](validation.Errors{
+				{Value: input, Messsage: "primary decode failed"},
+			})
+		}
+
+		// Use OrElse to provide a fallback: if decoding fails, use default value
+		withDefault := OrElse(func(errs Errors) Decode[string, int] {
+			return Of[string](0) // default to 0 if decoding fails
+		})
+
+		decoder := withDefault(primaryDecoder)
+
+		// Test success case
+		resSuccess := decoder("valid")
+		assert.Equal(t, validation.Of(42), resSuccess, "Should use primary decoder on success")
+
+		// Test fallback case
+		resFallback := decoder("invalid")
+		assert.Equal(t, validation.Of(0), resFallback, "OrElse provides fallback value")
+	})
+
+	t.Run("OrElse in pipeline composition", func(t *testing.T) {
+		failingDecoder := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{Value: input, Messsage: "database error"},
+			})
+		}
+
+		addContext := OrElse(func(errs Errors) Decode[string, int] {
+			return func(input string) Validation[int] {
+				return either.Left[int](validation.Errors{
+					{Messsage: "context added"},
+				})
+			}
+		})
+
+		recoverFromNotFound := OrElse(func(errs Errors) Decode[string, int] {
+			for _, err := range errs {
+				if err.Messsage == "not found" {
+					return Of[string](0)
+				}
+			}
+			return func(input string) Validation[int] {
+				return either.Left[int](errs)
+			}
+		})
+
+		// Test error aggregation in pipeline
+		decoder1 := recoverFromNotFound(addContext(failingDecoder))
+		res1 := decoder1("input")
+
+		assert.True(t, either.IsLeft(res1))
+		errors := either.MonadFold(res1,
+			func(e Errors) Errors { return e },
+			func(int) Errors { return nil },
+		)
+		// Errors accumulate through the pipeline
+		assert.Greater(t, len(errors), 1, "Should aggregate errors from pipeline")
+
+		// Test recovery in pipeline
+		failingDecoder2 := func(input string) Validation[int] {
+			return either.Left[int](validation.Errors{
+				{Value: input, Messsage: "not found"},
+			})
+		}
+
+		decoder2 := recoverFromNotFound(addContext(failingDecoder2))
+		res2 := decoder2("input")
+
+		assert.Equal(t, validation.Of(0), res2, "Should recover from 'not found' error")
+	})
+
+	t.Run("OrElse vs ChainLeft - identical behavior verification", func(t *testing.T) {
+		// Create various test scenarios
+		scenarios := []struct {
+			name    string
+			decoder Decode[string, int]
+			handler func(Errors) Decode[string, int]
+		}{
+			{
+				name:    "Success value",
+				decoder: Of[string](42),
+				handler: func(errs Errors) Decode[string, int] {
+					return func(input string) Validation[int] {
+						return either.Left[int](validation.Errors{{Messsage: "error"}})
+					}
+				},
+			},
+			{
+				name: "Failure with recovery",
+				decoder: func(input string) Validation[int] {
+					return either.Left[int](validation.Errors{{Messsage: "error"}})
+				},
+				handler: func(errs Errors) Decode[string, int] {
+					return Of[string](0)
+				},
+			},
+			{
+				name: "Failure with error transformation",
+				decoder: func(input string) Validation[int] {
+					return either.Left[int](validation.Errors{{Messsage: "error1"}})
+				},
+				handler: func(errs Errors) Decode[string, int] {
+					return func(input string) Validation[int] {
+						return either.Left[int](validation.Errors{{Messsage: "error2"}})
+					}
+				},
+			},
+			{
+				name: "Multiple errors aggregation",
+				decoder: func(input string) Validation[int] {
+					return either.Left[int](validation.Errors{
+						{Messsage: "error1"},
+						{Messsage: "error2"},
+					})
+				},
+				handler: func(errs Errors) Decode[string, int] {
+					return func(input string) Validation[int] {
+						return either.Left[int](validation.Errors{
+							{Messsage: "error3"},
+							{Messsage: "error4"},
+						})
+					}
+				},
+			},
+		}
+
+		for _, scenario := range scenarios {
+			t.Run(scenario.name, func(t *testing.T) {
+				resultOrElse := OrElse(scenario.handler)(scenario.decoder)("test-input")
+				resultChainLeft := ChainLeft(scenario.handler)(scenario.decoder)("test-input")
+
+				assert.Equal(t, resultChainLeft, resultOrElse,
+					"OrElse and ChainLeft must produce identical results for: %s", scenario.name)
+			})
+		}
+	})
+}

v2/optics/codec/decode/types.go

@@ -1,30 +1,222 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package decode
 
 import (
+	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
 	"github.com/IBM/fp-go/v2/reader"
 )
 
 type (
+	Errors = validation.Errors
 
 	// Validation represents the result of a validation operation that may contain
 	// validation errors or a successfully validated value of type A.
+	// This is an alias for validation.Validation[A], which is Either[Errors, A].
+	//
+	// In the decode context:
+	//   - Left(Errors): Decoding failed with one or more validation errors
+	//   - Right(A): Successfully decoded value of type A
+	//
+	// Example:
+	//
+	//	// Success case
+	//	valid := validation.Success(42)  // Right(42)
+	//
+	//	// Failure case
+	//	invalid := validation.Failures[int](validation.Errors{
+	//	    &validation.ValidationError{Messsage: "invalid format"},
+	//	})  // Left([...])
 	Validation[A any] = validation.Validation[A]
 
 	// Reader represents a computation that depends on an environment R and produces a value A.
+	// This is an alias for reader.Reader[R, A], which is func(R) A.
+	//
+	// In the decode context, Reader is used to access the input data being decoded.
+	// The environment R is typically the raw input (e.g., JSON, string, bytes) that
+	// needs to be decoded into a structured type A.
+	//
+	// Example:
+	//
+	//	// A reader that extracts a field from a map
+	//	getField := func(data map[string]any) string {
+	//	    return data["name"].(string)
+	//	}  // Reader[map[string]any, string]
 	Reader[R, A any] = reader.Reader[R, A]
 
 	// Decode is a function that decodes input I to type A with validation.
-	// It returns a Validation result directly.
+	// It combines the Reader pattern (for accessing input) with Validation (for error handling).
+	//
+	// Type: func(I) Validation[A]
+	//
+	// A Decode function:
+	//  1. Takes raw input of type I (e.g., JSON, string, bytes)
+	//  2. Attempts to decode/parse it into type A
+	//  3. Returns a Validation[A] with either:
+	//     - Success(A): Successfully decoded value
+	//     - Failures(Errors): Validation errors describing what went wrong
+	//
+	// This type is the foundation of the decode package, enabling composable,
+	// type-safe decoding with comprehensive error reporting.
+	//
+	// Example:
+	//
+	//	// Decode a string to an integer
+	//	decodeInt := func(input string) Validation[int] {
+	//	    n, err := strconv.Atoi(input)
+	//	    if err != nil {
+	//	        return validation.Failures[int](validation.Errors{
+	//	            &validation.ValidationError{
+	//	                Value:    input,
+	//	                Messsage: "not a valid integer",
+	//	                Cause:    err,
+	//	            },
+	//	        })
+	//	    }
+	//	    return validation.Success(n)
+	//	}  // Decode[string, int]
+	//
+	//	result := decodeInt("42")  // Success(42)
+	//	result := decodeInt("abc") // Failures([...])
 	Decode[I, A any] = Reader[I, Validation[A]]
 
 	// Kleisli represents a function from A to a decoded B given input type I.
 	// It's a Reader that takes an input A and produces a Decode[I, B] function.
 	// This enables composition of decoding operations in a functional style.
+	//
+	// Type: func(A) Decode[I, B]
+	//       which expands to: func(A) func(I) Validation[B]
+	//
+	// Kleisli arrows are the fundamental building blocks for composing decoders.
+	// They allow you to chain decoding operations where each step can:
+	//  1. Depend on the result of the previous step (the A parameter)
+	//  2. Access the original input (the I parameter via Decode)
+	//  3. Fail with validation errors (via Validation[B])
+	//
+	// This is particularly useful for:
+	//  - Conditional decoding based on previously decoded values
+	//  - Multi-stage decoding pipelines
+	//  - Dependent field validation
+	//
+	// Example:
+	//
+	//	// Decode a user, then decode their age based on their type
+	//	decodeAge := func(userType string) Decode[map[string]any, int] {
+	//	    return func(data map[string]any) Validation[int] {
+	//	        if userType == "admin" {
+	//	            // Admins must be 18+
+	//	            age := data["age"].(int)
+	//	            if age < 18 {
+	//	                return validation.Failures[int](/* error */)
+	//	            }
+	//	            return validation.Success(age)
+	//	        }
+	//	        // Regular users can be any age
+	//	        return validation.Success(data["age"].(int))
+	//	    }
+	//	}  // Kleisli[map[string]any, string, int]
+	//
+	//	// Use with Chain to compose decoders
+	//	decoder := F.Pipe2(
+	//	    decodeUserType,           // Decode[map[string]any, string]
+	//	    Chain(decodeAge),         // Chains with Kleisli
+	//	    Map(func(age int) User {  // Transform to final type
+	//	        return User{Age: age}
+	//	    }),
+	//	)
 	Kleisli[I, A, B any] = Reader[A, Decode[I, B]]
 
 	// Operator represents a decoding transformation that takes a decoded A and produces a decoded B.
 	// It's a specialized Kleisli arrow for composing decode operations where the input is already decoded.
 	// This allows chaining multiple decode transformations together.
+	//
+	// Type: func(Decode[I, A]) Decode[I, B]
+	//
+	// Operators are higher-order functions that transform one decoder into another.
+	// They are the result of partially applying functions like Map, Chain, and Ap,
+	// making them ideal for use in composition pipelines with F.Pipe.
+	//
+	// Key characteristics:
+	//  - Takes a Decode[I, A] as input
+	//  - Returns a Decode[I, B] as output
+	//  - Preserves the input type I (the raw data being decoded)
+	//  - Transforms the output type from A to B
+	//
+	// Common operators:
+	//  - Map(f): Transforms successful decode results
+	//  - Chain(f): Sequences dependent decode operations
+	//  - Ap(fa): Applies function decoders to value decoders
+	//
+	// Example:
+	//
+	//	// Create reusable operators
+	//	toString := Map(func(n int) string {
+	//	    return strconv.Itoa(n)
+	//	})  // Operator[string, int, string]
+	//
+	//	validatePositive := Chain(func(n int) Decode[string, int] {
+	//	    return func(input string) Validation[int] {
+	//	        if n <= 0 {
+	//	            return validation.Failures[int](/* error */)
+	//	        }
+	//	        return validation.Success(n)
+	//	    }
+	//	})  // Operator[string, int, int]
+	//
+	//	// Compose operators in a pipeline
+	//	decoder := F.Pipe2(
+	//	    decodeInt,          // Decode[string, int]
+	//	    validatePositive,   // Operator[string, int, int]
+	//	    toString,           // Operator[string, int, string]
+	//	)  // Decode[string, string]
+	//
+	//	result := decoder("42")  // Success("42")
+	//	result := decoder("-5")  // Failures([...])
 	Operator[I, A, B any] = Kleisli[I, Decode[I, A], B]
+
+	// Endomorphism represents a function from a type to itself: func(A) A.
+	// This is an alias for endomorphism.Endomorphism[A].
+	//
+	// In the decode context, endomorphisms are used with LetL to transform
+	// decoded values using pure functions that don't change the type.
+	//
+	// Endomorphisms are useful for:
+	//  - Normalizing data (e.g., trimming strings, rounding numbers)
+	//  - Applying business rules (e.g., clamping values to ranges)
+	//  - Data sanitization (e.g., removing special characters)
+	//
+	// Example:
+	//
+	//	// Normalize a string by trimming and lowercasing
+	//	normalize := func(s string) string {
+	//	    return strings.ToLower(strings.TrimSpace(s))
+	//	}  // Endomorphism[string]
+	//
+	//	// Clamp an integer to a range
+	//	clamp := func(n int) int {
+	//	    if n < 0 { return 0 }
+	//	    if n > 100 { return 100 }
+	//	    return n
+	//	}  // Endomorphism[int]
+	//
+	//	// Use with LetL to transform decoded values
+	//	decoder := F.Pipe1(
+	//	    decodeString,
+	//	    LetL(nameLens, normalize),
+	//	)
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
 )

v2/optics/codec/either.go

@@ -0,0 +1,277 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+
+	"github.com/IBM/fp-go/v2/array"
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validate"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+)
+
+// encodeEither creates an encoder for Either[A, B] values.
+//
+// This function produces an encoder that handles both Left and Right cases of an Either value.
+// It uses the provided codecs to encode the Left (A) and Right (B) values respectively.
+//
+// # Type Parameters
+//
+//   - A: The type of the Left value
+//   - B: The type of the Right value
+//   - O: The output type after encoding
+//   - I: The input type for validation (not used in encoding)
+//
+// # Parameters
+//
+//   - leftItem: The codec for encoding Left values of type A
+//   - rightItem: The codec for encoding Right values of type B
+//
+// # Returns
+//
+// An Encode function that takes an Either[A, B] and returns O by encoding
+// either the Left or Right value using the appropriate codec.
+//
+// # Example
+//
+//	stringCodec := String()
+//	intCodec := Int()
+//	encoder := encodeEither(stringCodec, intCodec)
+//
+//	// Encode a Left value
+//	leftResult := encoder(either.Left[int]("error"))
+//	// leftResult contains the encoded string "error"
+//
+//	// Encode a Right value
+//	rightResult := encoder(either.Right[string](42))
+//	// rightResult contains the encoded int 42
+//
+// # Notes
+//
+//   - Uses either.Fold to pattern match on the Either value
+//   - Left values are encoded using leftItem.Encode
+//   - Right values are encoded using rightItem.Encode
+func encodeEither[A, B, O, I any](
+	leftItem Type[A, O, I],
+	rightItem Type[B, O, I],
+) Encode[either.Either[A, B], O] {
+	return either.Fold(
+		leftItem.Encode,
+		rightItem.Encode,
+	)
+}
+
+// validateEither creates a validator for Either[A, B] values.
+//
+// This function produces a validator that attempts to validate the input as both
+// a Left (A) and Right (B) value. The validation strategy is:
+//  1. First, try to validate as a Right value (B)
+//  2. If Right validation succeeds, return Either.Right[A](B)
+//  3. If Right validation fails, try to validate as a Left value (A)
+//  4. If Left validation succeeds, return Either.Left[B](A)
+//  5. If both validations fail, concatenate all errors from both attempts
+//
+// This approach ensures that the validator tries both branches and provides
+// comprehensive error information when both fail.
+//
+// # Type Parameters
+//
+//   - A: The type of the Left value
+//   - B: The type of the Right value
+//   - O: The output type after encoding (not used in validation)
+//   - I: The input type to validate
+//
+// # Parameters
+//
+//   - leftItem: The codec for validating Left values of type A
+//   - rightItem: The codec for validating Right values of type B
+//
+// # Returns
+//
+// A Validate function that takes an input I and returns a Decode function.
+// The Decode function takes a Context and returns a Validation[Either[A, B]].
+//
+// # Validation Logic
+//
+// The validator follows this decision tree:
+//
+//	Input I
+//	  |
+//	  +--> Validate as Right (B)
+//	         |
+//	         +-- Success --> Return Either.Right[A](B)
+//	         |
+//	         +-- Failure --> Validate as Left (A)
+//	                           |
+//	                           +-- Success --> Return Either.Left[B](A)
+//	                           |
+//	                           +-- Failure --> Return all errors (Left + Right)
+//
+// # Example
+//
+//	stringCodec := String()
+//	intCodec := Int()
+//	validator := validateEither(stringCodec, intCodec)
+//
+//	// Validate a string (will succeed as Left)
+//	result1 := validator("hello")(validation.Context{})
+//	// result1 is Success(Either.Left[int]("hello"))
+//
+//	// Validate an int (will succeed as Right)
+//	result2 := validator(42)(validation.Context{})
+//	// result2 is Success(Either.Right[string](42))
+//
+//	// Validate something that's neither (will fail with both errors)
+//	result3 := validator([]int{1, 2, 3})(validation.Context{})
+//	// result3 is Failure with errors from both string and int validation
+//
+// # Notes
+//
+//   - Prioritizes Right validation over Left validation
+//   - Accumulates errors from both branches when both fail
+//   - Uses the validation context to provide detailed error messages
+//   - The validator is lazy: it only evaluates Left if Right fails
+func validateEither[A, B, O, I any](
+	leftItem Type[A, O, I],
+	rightItem Type[B, O, I],
+) Validate[I, either.Either[A, B]] {
+
+	return func(i I) Decode[Context, either.Either[A, B]] {
+		valRight := rightItem.Validate(i)
+		valLeft := leftItem.Validate(i)
+
+		return func(ctx Context) Validation[either.Either[A, B]] {
+
+			resRight := valRight(ctx)
+
+			return either.Fold(
+				func(rightErrors validate.Errors) Validation[either.Either[A, B]] {
+					resLeft := valLeft(ctx)
+					return either.Fold(
+						func(leftErrors validate.Errors) Validation[either.Either[A, B]] {
+							return validation.Failures[either.Either[A, B]](array.Concat(leftErrors)(rightErrors))
+						},
+						F.Flow2(either.Left[B, A], validation.Of),
+					)(resLeft)
+				},
+				F.Flow2(either.Right[A, B], validation.Of),
+			)(resRight)
+		}
+	}
+}
+
+// Either creates a codec for Either[A, B] values.
+//
+// This function constructs a complete codec that can encode, decode, and validate
+// Either values. An Either represents a value that can be one of two types: Left (A)
+// or Right (B). This is commonly used for error handling, where Left represents an
+// error and Right represents a success value.
+//
+// The codec handles both branches of the Either type using the provided codecs for
+// each branch. During validation, it attempts to validate the input as both types
+// and succeeds if either validation passes.
+//
+// # Type Parameters
+//
+//   - A: The type of the Left value
+//   - B: The type of the Right value
+//   - O: The output type after encoding
+//   - I: The input type for validation
+//
+// # Parameters
+//
+//   - leftItem: The codec for handling Left values of type A
+//   - rightItem: The codec for handling Right values of type B
+//
+// # Returns
+//
+// A Type[either.Either[A, B], O, I] that can encode, decode, and validate Either values.
+//
+// # Codec Behavior
+//
+// Encoding:
+//   - Left values are encoded using leftItem.Encode
+//   - Right values are encoded using rightItem.Encode
+//
+// Validation:
+//   - First attempts to validate as Right (B)
+//   - If Right fails, attempts to validate as Left (A)
+//   - If both fail, returns all accumulated errors
+//   - If either succeeds, returns the corresponding Either value
+//
+// Type Checking:
+//   - Uses Is[either.Either[A, B]]() to verify the value is an Either
+//
+// Naming:
+//   - The codec name is "Either[<leftName>, <rightName>]"
+//   - Example: "Either[string, int]"
+//
+// # Example
+//
+//	// Create a codec for Either[string, int]
+//	stringCodec := String()
+//	intCodec := Int()
+//	eitherCodec := Either(stringCodec, intCodec)
+//
+//	// Encode a Left value
+//	leftEncoded := eitherCodec.Encode(either.Left[int]("error"))
+//	// leftEncoded contains the encoded string
+//
+//	// Encode a Right value
+//	rightEncoded := eitherCodec.Encode(either.Right[string](42))
+//	// rightEncoded contains the encoded int
+//
+//	// Decode/validate an input
+//	result := eitherCodec.Decode("hello")
+//	// result is Success(Either.Left[int]("hello"))
+//
+//	result2 := eitherCodec.Decode(42)
+//	// result2 is Success(Either.Right[string](42))
+//
+//	// Get the codec name
+//	name := eitherCodec.Name()
+//	// name is "Either[string, int]"
+//
+// # Use Cases
+//
+//   - Error handling: Either[Error, Value]
+//   - Alternative values: Either[DefaultValue, CustomValue]
+//   - Union types: Either[TypeA, TypeB]
+//   - Validation results: Either[ValidationError, ValidatedValue]
+//
+// # Notes
+//
+//   - The codec prioritizes Right validation over Left validation
+//   - Both branches must have compatible encoding output types (O)
+//   - Both branches must have compatible validation input types (I)
+//   - The codec name includes the names of both branch codecs
+//   - This is a building block for more complex sum types
+func Either[A, B, O, I any](
+	leftItem Type[A, O, I],
+	rightItem Type[B, O, I],
+) Type[either.Either[A, B], O, I] {
+	name := fmt.Sprintf("Either[%s, %s]", leftItem.Name(), rightItem.Name())
+	isEither := Is[either.Either[A, B]]()
+
+	return MakeType(
+		name,
+		isEither,
+		validateEither(leftItem, rightItem),
+		encodeEither(leftItem, rightItem),
+	)
+}

v2/optics/codec/either_test.go

@@ -0,0 +1,347 @@
+// Copyright (c) 2023 - 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codec
+
+import (
+	"fmt"
+	"strconv"
+	"testing"
+
+	"github.com/IBM/fp-go/v2/either"
+	F "github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/optics/codec/validation"
+	"github.com/IBM/fp-go/v2/reader"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestEitherWithIdentityCodecs tests the Either function with identity codecs
+// where both branches have the same output and input types
+func TestEitherWithIdentityCodecs(t *testing.T) {
+	t.Run("creates codec with correct name", func(t *testing.T) {
+		// The Either function is designed for cases where both branches encode to the same type
+		// For example, both encode to string or both encode to JSON
+
+		// Create codecs that both encode to string
+		stringToString := Id[string]()
+		intToString := IntFromString()
+
+		eitherCodec := Either(stringToString, intToString)
+
+		assert.Equal(t, "Either[string, IntFromString]", eitherCodec.Name())
+	})
+}
+
+// TestEitherEncode tests encoding of Either values
+func TestEitherEncode(t *testing.T) {
+	// Create codecs that both encode to string
+	stringToString := Id[string]()
+	intToString := IntFromString()
+
+	eitherCodec := Either(stringToString, intToString)
+
+	t.Run("encodes Left value", func(t *testing.T) {
+		leftValue := either.Left[int]("hello")
+		encoded := eitherCodec.Encode(leftValue)
+
+		assert.Equal(t, "hello", encoded)
+	})
+
+	t.Run("encodes Right value", func(t *testing.T) {
+		rightValue := either.Right[string](42)
+		encoded := eitherCodec.Encode(rightValue)
+
+		assert.Equal(t, "42", encoded)
+	})
+}
+
+// TestEitherDecode tests decoding/validation of Either values
+func TestEitherDecode(t *testing.T) {
+	getOrElseNull := either.GetOrElse(reader.Of[validation.Errors, either.Either[string, int]](either.Left[int]("")))
+
+	// Create codecs that both work with string input
+	stringCodec := Id[string]()
+	intFromString := IntFromString()
+
+	eitherCodec := Either(stringCodec, intFromString)
+
+	t.Run("decodes integer string as Right", func(t *testing.T) {
+		result := eitherCodec.Decode("42")
+
+		assert.True(t, either.IsRight(result), "should successfully decode integer string")
+
+		value := getOrElseNull(result)
+		assert.True(t, either.IsRight(value), "should be Right")
+
+		rightValue := either.MonadFold(value,
+			func(string) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 42, rightValue)
+	})
+
+	t.Run("decodes non-integer string as Left", func(t *testing.T) {
+		result := eitherCodec.Decode("hello")
+
+		assert.True(t, either.IsRight(result), "should successfully decode string")
+
+		value := getOrElseNull(result)
+		assert.True(t, either.IsLeft(value), "should be Left")
+
+		leftValue := either.MonadFold(value,
+			F.Identity[string],
+			func(int) string { return "" },
+		)
+		assert.Equal(t, "hello", leftValue)
+	})
+}
+
+// TestEitherValidation tests validation behavior
+func TestEitherValidation(t *testing.T) {
+	t.Run("validates with custom codecs", func(t *testing.T) {
+		// Create a codec that only accepts non-empty strings
+		nonEmptyString := MakeType(
+			"NonEmptyString",
+			func(u any) either.Either[error, string] {
+				s, ok := u.(string)
+				if !ok || len(s) == 0 {
+					return either.Left[string](fmt.Errorf("not a non-empty string"))
+				}
+				return either.Of[error](s)
+			},
+			func(s string) Decode[Context, string] {
+				return func(c Context) Validation[string] {
+					if len(s) == 0 {
+						return validation.FailureWithMessage[string](s, "must not be empty")(c)
+					}
+					return validation.Success(s)
+				}
+			},
+			F.Identity[string],
+		)
+
+		// Create a codec that only accepts positive integers from strings
+		positiveIntFromString := MakeType(
+			"PositiveInt",
+			func(u any) either.Either[error, int] {
+				i, ok := u.(int)
+				if !ok || i <= 0 {
+					return either.Left[int](fmt.Errorf("not a positive integer"))
+				}
+				return either.Of[error](i)
+			},
+			func(s string) Decode[Context, int] {
+				return func(c Context) Validation[int] {
+					var n int
+					_, err := fmt.Sscanf(s, "%d", &n)
+					if err != nil {
+						return validation.FailureWithError[int](s, "expected integer string")(err)(c)
+					}
+					if n <= 0 {
+						return validation.FailureWithMessage[int](n, "must be positive")(c)
+					}
+					return validation.Success(n)
+				}
+			},
+			func(n int) string {
+				return fmt.Sprintf("%d", n)
+			},
+		)
+
+		eitherCodec := Either(nonEmptyString, positiveIntFromString)
+
+		// Valid non-empty string
+		validLeft := eitherCodec.Decode("hello")
+		assert.True(t, either.IsRight(validLeft))
+
+		// Valid positive integer
+		validRight := eitherCodec.Decode("42")
+		assert.True(t, either.IsRight(validRight))
+
+		// Invalid empty string - should fail both validations
+		invalidEmpty := eitherCodec.Decode("")
+		assert.True(t, either.IsLeft(invalidEmpty))
+
+		// Invalid zero - should fail Right validation, succeed as Left
+		zeroResult := eitherCodec.Decode("0")
+		// "0" is a valid non-empty string, so it should succeed as Left
+		assert.True(t, either.IsRight(zeroResult))
+	})
+}
+
+// TestEitherRoundTrip tests encoding and decoding round trips
+func TestEitherRoundTrip(t *testing.T) {
+	stringCodec := Id[string]()
+	intFromString := IntFromString()
+
+	eitherCodec := Either(stringCodec, intFromString)
+
+	t.Run("round-trip Left value", func(t *testing.T) {
+		original := "hello"
+
+		// Decode
+		decodeResult := eitherCodec.Decode(original)
+		require.True(t, either.IsRight(decodeResult))
+
+		decoded := either.MonadFold(decodeResult,
+			func(validation.Errors) either.Either[string, int] { return either.Left[int]("") },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Encode
+		encoded := eitherCodec.Encode(decoded)
+
+		// Verify
+		assert.Equal(t, original, encoded)
+	})
+
+	t.Run("round-trip Right value", func(t *testing.T) {
+		original := "42"
+
+		// Decode
+		decodeResult := eitherCodec.Decode(original)
+		require.True(t, either.IsRight(decodeResult))
+
+		decoded := either.MonadFold(decodeResult,
+			func(validation.Errors) either.Either[string, int] { return either.Right[string](0) },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Encode
+		encoded := eitherCodec.Encode(decoded)
+
+		// Verify
+		assert.Equal(t, original, encoded)
+	})
+}
+
+// TestEitherPrioritization tests that Right validation is prioritized over Left
+func TestEitherPrioritization(t *testing.T) {
+	stringCodec := Id[string]()
+	intFromString := IntFromString()
+
+	eitherCodec := Either(stringCodec, intFromString)
+
+	t.Run("prioritizes Right over Left when both could succeed", func(t *testing.T) {
+		// "42" can be validated as both string (Left) and int (Right)
+		// The codec should prioritize Right
+		result := eitherCodec.Decode("42")
+
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) either.Either[string, int] { return either.Left[int]("") },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Should be Right because int validation succeeds and is prioritized
+		assert.True(t, either.IsRight(value))
+
+		rightValue := either.MonadFold(value,
+			func(string) int { return 0 },
+			F.Identity[int],
+		)
+		assert.Equal(t, 42, rightValue)
+	})
+
+	t.Run("falls back to Left when Right fails", func(t *testing.T) {
+		// "hello" can only be validated as string (Left), not as int (Right)
+		result := eitherCodec.Decode("hello")
+
+		assert.True(t, either.IsRight(result))
+
+		value := either.MonadFold(result,
+			func(validation.Errors) either.Either[string, int] { return either.Left[int]("") },
+			F.Identity[either.Either[string, int]],
+		)
+
+		// Should be Left because int validation failed
+		assert.True(t, either.IsLeft(value))
+
+		leftValue := either.MonadFold(value,
+			F.Identity[string],
+			func(int) string { return "" },
+		)
+		assert.Equal(t, "hello", leftValue)
+	})
+}
+
+// TestEitherErrorAccumulation tests that errors from both branches are accumulated
+func TestEitherErrorAccumulation(t *testing.T) {
+	// Create codecs with specific validation rules that will both fail
+	nonEmptyString := MakeType(
+		"NonEmptyString",
+		func(u any) either.Either[error, string] {
+			s, ok := u.(string)
+			if !ok || len(s) == 0 {
+				return either.Left[string](fmt.Errorf("not a non-empty string"))
+			}
+			return either.Of[error](s)
+		},
+		func(s string) Decode[Context, string] {
+			return func(c Context) Validation[string] {
+				if len(s) == 0 {
+					return validation.FailureWithMessage[string](s, "must not be empty")(c)
+				}
+				return validation.Success(s)
+			}
+		},
+		F.Identity[string],
+	)
+
+	positiveIntFromString := MakeType(
+		"PositiveInt",
+		func(u any) either.Either[error, int] {
+			i, ok := u.(int)
+			if !ok || i <= 0 {
+				return either.Left[int](fmt.Errorf("not a positive integer"))
+			}
+			return either.Of[error](i)
+		},
+		func(s string) Decode[Context, int] {
+			return func(c Context) Validation[int] {
+				var n int
+				_, err := fmt.Sscanf(s, "%d", &n)
+				if err != nil {
+					return validation.FailureWithError[int](s, "expected integer string")(err)(c)
+				}
+				if n <= 0 {
+					return validation.FailureWithMessage[int](n, "must be positive")(c)
+				}
+				return validation.Success(n)
+			}
+		},
+		strconv.Itoa,
+	)
+
+	eitherCodec := Either(nonEmptyString, positiveIntFromString)
+
+	t.Run("accumulates errors from both branches when both fail", func(t *testing.T) {
+		// Empty string will fail both validations
+		result := eitherCodec.Decode("")
+
+		assert.True(t, either.IsLeft(result))
+
+		errors := either.MonadFold(result,
+			F.Identity[validation.Errors],
+			func(either.Either[string, int]) validation.Errors { return nil },
+		)
+
+		require.NotNil(t, errors)
+		// Should have errors from both string and int validation attempts
+		assert.NotEmpty(t, errors)
+	})
+}

v2/optics/codec/validate/bind.go

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

v2/optics/codec/validate/bind_test.go

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

v2/optics/codec/validate/types.go

@@ -16,6 +16,7 @@
 package validate
 
 import (
+	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/optics/codec/decode"
 	"github.com/IBM/fp-go/v2/optics/codec/validation"
@@ -90,25 +91,72 @@ type (
 	//   "at user.address.zipCode: expected string, got number"
 	Context = validation.Context
 
-	Decode[I, A any] = decode.Decode[I, A]
-
-	// Validate is a function that validates input I to produce type A with full context tracking.
+	// Decode represents a decoding operation that transforms input I into output A
+	// within a validation context.
 	//
 	// Type structure:
-	//   Validate[I, A] = Reader[I, Decode[Context, A]]
+	//   Decode[I, A] = Reader[Context, Validation[A]]
 	//
 	// This means:
-	//  1. Takes an input of type I
-	//  2. Returns a Reader that depends on validation Context
-	//  3. That Reader produces a Validation[A] (Either[Errors, A])
+	//   1. Takes a validation Context (path through nested structures)
+	//   2. Returns a Validation[A] (Either[Errors, A])
 	//
-	// The layered structure enables:
-	//   - Access to the input value being validated
-	//   - Context tracking through nested structures
-	//   - Error accumulation with detailed paths
-	//   - Composition with other validators
+	// Decode is used as the foundation for validation operations, providing:
+	//   - Context-aware error reporting with detailed paths
+	//   - Error accumulation across multiple validations
+	//   - Composable validation logic
+	//
+	// The Decode type is typically not used directly but through the Validate type,
+	// which adds an additional Reader layer for accessing the input value.
+	//
+	// Example:
+	//   decoder := func(ctx Context) Validation[int] {
+	//     // Perform validation and return result
+	//     return validation.Success(42)
+	//   }
+	//   // decoder is a Decode[any, int]
+	Decode[I, A any] = decode.Decode[I, A]
+
+	// Validate represents a composable validator that transforms input I to output A
+	// with comprehensive error tracking and context propagation.
+	//
+	// # Type Structure
+	//
+	//   Validate[I, A] = Reader[I, Decode[Context, A]]
+	//                  = Reader[I, Reader[Context, Validation[A]]]
+	//                  = func(I) func(Context) Either[Errors, A]
+	//
+	// This three-layer structure provides:
+	//   1. Input access: The outer Reader[I, ...] gives access to the input value I
+	//   2. Context tracking: The middle Reader[Context, ...] tracks the validation path
+	//   3. Error handling: The inner Validation[A] accumulates errors or produces value A
+	//
+	// # Purpose
+	//
+	// Validate is the core type for building type-safe, composable validators that:
+	//   - Transform and validate data from one type to another
+	//   - Track the path through nested structures for detailed error messages
+	//   - Accumulate multiple validation errors instead of failing fast
+	//   - Compose with other validators using functional patterns
+	//
+	// # Key Features
+	//
+	//   - Context-aware: Automatically tracks validation path (e.g., "user.address.zipCode")
+	//   - Error accumulation: Collects all validation errors, not just the first one
+	//   - Type-safe: Leverages Go's type system to ensure correctness
+	//   - Composable: Validators can be combined using Map, Chain, Ap, and other operators
+	//
+	// # Algebraic Structure
+	//
+	// Validate forms several algebraic structures:
+	//   - Functor: Transform successful results with Map
+	//   - Applicative: Combine independent validators in parallel with Ap
+	//   - Monad: Chain dependent validators sequentially with Chain
+	//
+	// # Example Usage
+	//
+	// Basic validator:
 	//
-	// Example usage:
 	//   validatePositive := func(n int) Reader[Context, Validation[int]] {
 	//     return func(ctx Context) Validation[int] {
 	//       if n > 0 {
@@ -119,10 +167,33 @@ type (
 	//   }
 	//   // validatePositive is a Validate[int, int]
 	//
-	// The Validate type forms:
-	//   - A Functor: Can map over successful results
-	//   - An Applicative: Can combine validators in parallel
-	//   - A Monad: Can chain dependent validations
+	// Composing validators:
+	//
+	//   // Transform the result of a validator
+	//   doubled := Map[int, int, int](func(x int) int { return x * 2 })(validatePositive)
+	//
+	//   // Chain dependent validations
+	//   validateRange := func(n int) Validate[int, int] {
+	//     return func(input int) Reader[Context, Validation[int]] {
+	//       return func(ctx Context) Validation[int] {
+	//         if n <= 100 {
+	//           return validation.Success(n)
+	//         }
+	//         return validation.FailureWithMessage[int](n, "must be <= 100")(ctx)
+	//       }
+	//     }
+	//   }
+	//   combined := Chain(validateRange)(validatePositive)
+	//
+	// # Integration
+	//
+	// Validate integrates with the broader optics/codec ecosystem:
+	//   - Works with Decode for decoding operations
+	//   - Uses Validation for error handling
+	//   - Leverages Context for detailed error reporting
+	//   - Composes with other codec types for complete encode/decode pipelines
+	//
+	// See the package documentation for more examples and patterns.
 	Validate[I, A any] = Reader[I, Decode[Context, A]]
 
 	// Errors is a collection of validation errors that occurred during validation.
@@ -174,4 +245,30 @@ type (
 	//   // toUpper is an Operator[string, string, string]
 	//   // It can be applied to any string validator to uppercase the result
 	Operator[I, A, B any] = Kleisli[I, Validate[I, A], B]
+
+	// Endomorphism represents a function from a type to itself.
+	//
+	// Type: Endomorphism[A] = func(A) A
+	//
+	// An endomorphism is a morphism (structure-preserving map) where the source
+	// and target are the same type. In simpler terms, it's a function that takes
+	// a value of type A and returns a value of the same type A.
+	//
+	// Endomorphisms are useful for:
+	//   - Transformations that preserve type (e.g., string normalization)
+	//   - Composable updates and modifications
+	//   - Building pipelines of same-type transformations
+	//   - Implementing the Monoid pattern (composition as binary operation)
+	//
+	// Endomorphisms form a Monoid under function composition:
+	//   - Identity: func(a A) A { return a }
+	//   - Concat: func(f, g Endomorphism[A]) Endomorphism[A] {
+	//       return func(a A) A { return f(g(a)) }
+	//     }
+	//
+	// Example:
+	//   trim := strings.TrimSpace      // Endomorphism[string]
+	//   lower := strings.ToLower       // Endomorphism[string]
+	//   normalize := compose(trim, lower)  // Endomorphism[string]
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
 )

v2/optics/codec/validate/validate.go

@@ -309,6 +309,225 @@ func Chain[I, A, B any](f Kleisli[I, A, B]) Operator[I, A, B] {
 	)
 }
 
+// ChainLeft sequences a computation on the failure (Left) channel of a validation.
+//
+// This function operates on the error path of validation, allowing you to transform,
+// enrich, or recover from validation failures. It's the dual of Chain - while Chain
+// operates on success values, ChainLeft operates on error values.
+//
+// # Key Behavior
+//
+// **Critical difference from standard Either operations**: This validation-specific
+// implementation **aggregates errors** using the Errors monoid. When the transformation
+// function returns a failure, both the original errors AND the new errors are combined,
+// ensuring comprehensive error reporting.
+//
+//  1. **Success Pass-Through**: If validation succeeds, the handler is never called and
+//     the success value passes through unchanged.
+//
+//  2. **Error Recovery**: The handler can recover from failures by returning a successful
+//     validation, converting Left to Right.
+//
+//  3. **Error Aggregation**: When the handler also returns a failure, both the original
+//     errors and the new errors are combined using the Errors monoid.
+//
+//  4. **Input Access**: The handler returns a Validate[I, A] function, giving it access
+//     to the original input value I for context-aware error handling.
+//
+// # Type Parameters
+//
+//   - I: The input type
+//   - A: The type of the validation result
+//
+// # Parameters
+//
+//   - f: A Kleisli arrow that takes Errors and returns a Validate[I, A]. This function
+//     is called only when validation fails, receiving the accumulated errors.
+//
+// # Returns
+//
+// An Operator[I, A, A] that transforms validators by handling their error cases.
+//
+// # Example: Error Recovery
+//
+//	// Validator that may fail
+//	validatePositive := func(n int) Reader[validation.Context, validation.Validation[int]] {
+//	    return func(ctx validation.Context) validation.Validation[int] {
+//	        if n > 0 {
+//	            return validation.Success(n)
+//	        }
+//	        return validation.FailureWithMessage[int](n, "must be positive")(ctx)
+//	    }
+//	}
+//
+//	// Recover from specific errors with a default value
+//	withDefault := ChainLeft(func(errs Errors) Validate[int, int] {
+//	    for _, err := range errs {
+//	        if err.Messsage == "must be positive" {
+//	            return Of[int](0) // recover with default
+//	        }
+//	    }
+//	    return func(input int) Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            return either.Left[int](errs)
+//	        }
+//	    }
+//	})
+//
+//	validator := withDefault(validatePositive)
+//	result := validator(-5)(nil)
+//	// Result: Success(0) - recovered from failure
+//
+// # Example: Error Context Addition
+//
+//	// Add contextual information to errors
+//	addContext := ChainLeft(func(errs Errors) Validate[string, int] {
+//	    return func(input string) Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            return either.Left[int](validation.Errors{
+//	                {
+//	                    Context:  validation.Context{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+//	                    Messsage: "failed to validate user age",
+//	                },
+//	            })
+//	        }
+//	    }
+//	})
+//
+//	validator := addContext(someValidator)
+//	// Errors will include both original error and context
+//
+// # Example: Input-Dependent Recovery
+//
+//	// Recover with different defaults based on input
+//	smartDefault := ChainLeft(func(errs Errors) Validate[string, int] {
+//	    return func(input string) Reader[validation.Context, validation.Validation[int]] {
+//	        return func(ctx validation.Context) validation.Validation[int] {
+//	            // Use input to determine appropriate default
+//	            if strings.Contains(input, "http") {
+//	                return validation.Of(80)
+//	            }
+//	            if strings.Contains(input, "https") {
+//	                return validation.Of(443)
+//	            }
+//	            return validation.Of(8080)
+//	        }
+//	    }
+//	})
+//
+// # Notes
+//
+//   - Errors are accumulated, not replaced - this ensures no validation failures are lost
+//   - The handler has access to both the errors and the original input
+//   - Success values bypass the handler completely
+//   - This enables sophisticated error handling strategies including recovery, enrichment, and transformation
+//   - Use OrElse as a semantic alias when emphasizing fallback/alternative logic
+func ChainLeft[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return readert.Chain[Validate[I, A]](
+		decode.ChainLeft,
+		f,
+	)
+}
+
+// OrElse provides an alternative validation when the primary validation fails.
+//
+// This is a semantic alias for ChainLeft with identical behavior. The name "OrElse"
+// emphasizes the intent of providing fallback or alternative validation logic, making
+// code more readable when that's the primary use case.
+//
+// # Relationship to ChainLeft
+//
+// **OrElse and ChainLeft are functionally identical** - they produce exactly the same
+// results for all inputs. The choice between them is purely about code readability:
+//
+//   - Use **OrElse** when emphasizing fallback/alternative validation logic
+//   - Use **ChainLeft** when emphasizing technical error channel transformation
+//
+// Both maintain the critical property of **error aggregation**, ensuring all validation
+// failures are preserved and reported together.
+//
+// # Type Parameters
+//
+//   - I: The input type
+//   - A: The type of the validation result
+//
+// # Parameters
+//
+//   - f: A Kleisli arrow that takes Errors and returns a Validate[I, A]. This function
+//     is called only when validation fails, receiving the accumulated errors.
+//
+// # Returns
+//
+// An Operator[I, A, A] that transforms validators by providing alternative validation.
+//
+// # Example: Fallback Validation
+//
+//	// Primary validator that may fail
+//	validateFromConfig := func(key string) Reader[validation.Context, validation.Validation[string]] {
+//	    return func(ctx validation.Context) validation.Validation[string] {
+//	        // Try to get value from config
+//	        if value, ok := config[key]; ok {
+//	            return validation.Success(value)
+//	        }
+//	        return validation.FailureWithMessage[string](key, "not found in config")(ctx)
+//	    }
+//	}
+//
+//	// Use OrElse for semantic clarity - "try config, or else use environment"
+//	withEnvFallback := OrElse(func(errs Errors) Validate[string, string] {
+//	    return func(key string) Reader[validation.Context, validation.Validation[string]] {
+//	        return func(ctx validation.Context) validation.Validation[string] {
+//	            if value := os.Getenv(key); value != "" {
+//	                return validation.Success(value)
+//	            }
+//	            return either.Left[string](errs) // propagate original errors
+//	        }
+//	    }
+//	})
+//
+//	validator := withEnvFallback(validateFromConfig)
+//	result := validator("DATABASE_URL")(nil)
+//	// Tries config first, falls back to environment variable
+//
+// # Example: Default Value on Failure
+//
+//	// Provide a default value when validation fails
+//	withDefault := OrElse(func(errs Errors) Validate[int, int] {
+//	    return Of[int](0) // default to 0 on any failure
+//	})
+//
+//	validator := withDefault(someValidator)
+//	result := validator(input)(nil)
+//	// Always succeeds, using default value if validation fails
+//
+// # Example: Pipeline with Multiple Fallbacks
+//
+//	// Build a validation pipeline with multiple fallback strategies
+//	validator := F.Pipe2(
+//	    validateFromDatabase,
+//	    OrElse(func(errs Errors) Validate[string, Config] {
+//	        // Try cache as first fallback
+//	        return validateFromCache
+//	    }),
+//	    OrElse(func(errs Errors) Validate[string, Config] {
+//	        // Use default config as final fallback
+//	        return Of[string](defaultConfig)
+//	    }),
+//	)
+//	// Tries database, then cache, then default
+//
+// # Notes
+//
+//   - Identical behavior to ChainLeft - they are aliases
+//   - Errors are accumulated when transformations fail
+//   - Success values pass through unchanged
+//   - The handler has access to both errors and original input
+//   - Choose OrElse for better readability when providing alternatives
+//   - See ChainLeft documentation for detailed behavior and additional examples
+func OrElse[I, A any](f Kleisli[I, Errors, A]) Operator[I, A, A] {
+	return ChainLeft(f)
+}
+
 // MonadAp applies a validator containing a function to a validator containing a value.
 //
 // This is the applicative apply operation for Validate. It allows you to apply

v2/optics/codec/validate/validate_test.go

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

v2/optics/codec/validation/OrElse_explanation.md

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

v2/optics/codec/validation/bind.go

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

v2/optics/codec/validation/bind_test.go

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

v2/optics/codec/validation/monad.go

@@ -1,10 +1,14 @@
 package validation
 
 import (
+	"github.com/IBM/fp-go/v2/array"
 	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/function"
 	"github.com/IBM/fp-go/v2/internal/applicative"
 )
 
+var errorsMonoid = ErrorsMonoid()
+
 // Of creates a successful validation result containing the given value.
 // This is the pure/return operation for the Validation monad.
 //
@@ -28,37 +32,376 @@ func Of[A any](a A) Validation[A] {
 //		return func(age int) User { return User{name, age} }
 //	}))(validateName))(validateAge)
 func Ap[B, A any](fa Validation[A]) Operator[func(A) B, B] {
-	return either.ApV[B, A](ErrorsMonoid())(fa)
+	return either.ApV[B, A](errorsMonoid)(fa)
 }
 
+// MonadAp applies a validation containing a function to a validation containing a value.
+// This is the applicative apply operation that **accumulates errors** from both validations.
+//
+// **Key behavior**: Unlike Either's MonadAp which fails fast (returns first error),
+// this validation-specific implementation **accumulates all errors** using the Errors monoid.
+// When both the function validation and value validation fail, all errors from both are combined.
+//
+// This error accumulation is the defining characteristic of the Validation applicative,
+// making it ideal for scenarios where you want to collect all validation failures at once
+// rather than stopping at the first error.
+//
+// Behavior:
+//   - Both succeed: applies the function to the value → Success(result)
+//   - Function fails, value succeeds: returns function's errors → Failure(func errors)
+//   - Function succeeds, value fails: returns value's errors → Failure(value errors)
+//   - Both fail: **combines all errors** → Failure(func errors + value errors)
+//
+// This is particularly useful for:
+//   - Form validation: collect all field errors at once
+//   - Configuration validation: report all invalid settings together
+//   - Data validation: accumulate all constraint violations
+//   - Multi-field validation: validate independent fields in parallel
+//
+// Example - Both succeed:
+//
+//	double := func(x int) int { return x * 2 }
+//	result := MonadAp(Of(double), Of(21))
+//	// Result: Success(42)
+//
+// Example - Error accumulation (key feature):
+//
+//	funcValidation := Failures[func(int) int](Errors{
+//	    &ValidationError{Messsage: "function error"},
+//	})
+//	valueValidation := Failures[int](Errors{
+//	    &ValidationError{Messsage: "value error"},
+//	})
+//	result := MonadAp(funcValidation, valueValidation)
+//	// Result: Failure with BOTH errors: ["function error", "value error"]
+//
+// Example - Validating multiple fields:
+//
+//	type User struct {
+//	    Name string
+//	    Age  int
+//	}
+//
+//	makeUser := func(name string) func(int) User {
+//	    return func(age int) User { return User{name, age} }
+//	}
+//
+//	nameValidation := validateName("ab")  // Fails: too short
+//	ageValidation := validateAge(16)      // Fails: too young
+//
+//	// First apply name
+//	step1 := MonadAp(Of(makeUser), nameValidation)
+//	// Then apply age
+//	result := MonadAp(step1, ageValidation)
+//	// Result contains ALL validation errors from both fields
 func MonadAp[B, A any](fab Validation[func(A) B], fa Validation[A]) Validation[B] {
-	return either.MonadApV[B, A](ErrorsMonoid())(fab, fa)
+	return either.MonadApV[B, A](errorsMonoid)(fab, fa)
 }
 
 // Map transforms the value inside a successful validation using the provided function.
 // If the validation is a failure, the errors are preserved unchanged.
 // This is the functor map operation for Validation.
 //
-// Example:
+// Map is used for transforming successful values without changing the validation context.
+// It's the most basic operation for working with validated values and forms the foundation
+// for more complex validation pipelines.
+//
+// Behavior:
+//   - Success: applies function to value → Success(f(value))
+//   - Failure: preserves errors unchanged → Failure(same errors)
+//
+// This is useful for:
+//   - Type transformations: converting validated values to different types
+//   - Value transformations: normalizing, formatting, or computing derived values
+//   - Pipeline composition: chaining multiple transformations
+//   - Preserving validation context: errors pass through unchanged
+//
+// Example - Transform successful value:
 //
 //	doubled := Map(func(x int) int { return x * 2 })(Of(21))
 //	// Result: Success(42)
+//
+// Example - Failure preserved:
+//
+//	result := Map(func(x int) int { return x * 2 })(
+//	    Failures[int](Errors{&ValidationError{Messsage: "invalid"}}),
+//	)
+//	// Result: Failure with same error: ["invalid"]
+//
+// Example - Type transformation:
+//
+//	toString := Map(func(x int) string { return fmt.Sprintf("%d", x) })
+//	result := toString(Of(42))
+//	// Result: Success("42")
+//
+// Example - Chaining transformations:
+//
+//	result := F.Pipe3(
+//	    Of(5),
+//	    Map(func(x int) int { return x + 10 }),  // 15
+//	    Map(func(x int) int { return x * 2 }),   // 30
+//	    Map(func(x int) string { return fmt.Sprintf("%d", x) }),  // "30"
+//	)
+//	// Result: Success("30")
 func Map[A, B any](f func(A) B) Operator[A, B] {
 	return either.Map[Errors](f)
 }
 
+// MonadMap transforms the value inside a successful validation using the provided function.
+// If the validation is a failure, the errors are preserved unchanged.
+// This is the non-curried version of [Map].
+//
+// MonadMap is useful when you have both the validation and the transformation function
+// available at the same time, rather than needing to create a reusable operator.
+//
+// Behavior:
+//   - Success: applies function to value → Success(f(value))
+//   - Failure: preserves errors unchanged → Failure(same errors)
+//
+// Example - Transform successful value:
+//
+//	result := MonadMap(Of(21), func(x int) int { return x * 2 })
+//	// Result: Success(42)
+//
+// Example - Failure preserved:
+//
+//	result := MonadMap(
+//	    Failures[int](Errors{&ValidationError{Messsage: "invalid"}}),
+//	    func(x int) int { return x * 2 },
+//	)
+//	// Result: Failure with same error: ["invalid"]
+//
+// Example - Type transformation:
+//
+//	result := MonadMap(Of(42), func(x int) string {
+//	    return fmt.Sprintf("Value: %d", x)
+//	})
+//	// Result: Success("Value: 42")
+//
+// Example - Computing derived values:
+//
+//	type User struct { FirstName, LastName string }
+//	result := MonadMap(
+//	    Of(User{"John", "Doe"}),
+//	    func(u User) string { return u.FirstName + " " + u.LastName },
+//	)
+//	// Result: Success("John Doe")
 func MonadMap[A, B any](fa Validation[A], f func(A) B) Validation[B] {
 	return either.MonadMap(fa, f)
 }
 
+// Chain is the curried version of [MonadChain].
+// Sequences two validation computations where the second depends on the first.
+//
+// Example:
+//
+//	validatePositive := func(x int) Validation[int] {
+//	    if x > 0 { return Success(x) }
+//	    return Failure("must be positive")
+//	}
+//	result := Chain(validatePositive)(Success(42)) // Success(42)
 func Chain[A, B any](f Kleisli[A, B]) Operator[A, B] {
 	return either.Chain(f)
 }
 
+// MonadChain sequences two validation computations where the second depends on the first.
+// If the first validation fails, returns the failure without executing the second.
+// This is the monadic bind operation for Validation.
+//
+// Example:
+//
+//	result := MonadChain(
+//	    Success(42),
+//	    func(x int) Validation[string] {
+//	        return Success(fmt.Sprintf("Value: %d", x))
+//	    },
+//	) // Success("Value: 42")
 func MonadChain[A, B any](fa Validation[A], f Kleisli[A, B]) Validation[B] {
 	return either.MonadChain(fa, f)
 }
 
+// chainErrors is an internal helper that chains error transformations while accumulating errors.
+// When the transformation function f returns a failure, it concatenates the original errors (e1)
+// with the new errors (e2) using the Errors monoid, ensuring all validation errors are preserved.
+func chainErrors[A any](f Kleisli[Errors, A]) func(Errors) Validation[A] {
+	return func(e1 Errors) Validation[A] {
+		return either.MonadFold(
+			f(e1),
+			function.Flow2(array.Concat(e1), either.Left[A]),
+			Of[A],
+		)
+	}
+}
+
+// ChainLeft is the curried version of [MonadChainLeft].
+// Returns a function that transforms validation failures while preserving successes.
+//
+// Unlike the standard Either ChainLeft which replaces errors, this validation-specific
+// implementation **aggregates errors** using the Errors monoid. When the transformation
+// function returns a failure, both the original errors and the new errors are combined,
+// ensuring no validation errors are lost.
+//
+// This is particularly useful for:
+//   - Error recovery with fallback validation
+//   - Adding contextual information to existing errors
+//   - Transforming error types while preserving all error details
+//   - Building error handling pipelines that accumulate failures
+//
+// Key behavior:
+//   - Success values pass through unchanged
+//   - When transforming failures, if the transformation also fails, **all errors are aggregated**
+//   - If the transformation succeeds, it recovers from the original failure
+//
+// Example - Error recovery with aggregation:
+//
+//	recoverFromNotFound := ChainLeft(func(errs Errors) Validation[int] {
+//	    // Check if this is a "not found" error
+//	    for _, err := range errs {
+//	        if err.Messsage == "not found" {
+//	            return Success(0) // recover with default
+//	        }
+//	    }
+//	    // Add context to existing errors
+//	    return Failures[int](Errors{
+//	        &ValidationError{Messsage: "recovery failed"},
+//	    })
+//	    // Result will contain BOTH original errors AND "recovery failed"
+//	})
+//
+//	result := recoverFromNotFound(Failures[int](Errors{
+//	    &ValidationError{Messsage: "database error"},
+//	}))
+//	// Result contains: ["database error", "recovery failed"]
+//
+// Example - Adding context to errors:
+//
+//	addContext := ChainLeft(func(errs Errors) Validation[string] {
+//	    // Add contextual information
+//	    return Failures[string](Errors{
+//	        &ValidationError{
+//	            Messsage: "validation failed in user.email field",
+//	        },
+//	    })
+//	    // Original errors are preserved and new context is added
+//	})
+//
+//	result := F.Pipe1(
+//	    Failures[string](Errors{
+//	        &ValidationError{Messsage: "invalid format"},
+//	    }),
+//	    addContext,
+//	)
+//	// Result contains: ["invalid format", "validation failed in user.email field"]
+//
+// Example - Success values pass through:
+//
+//	handler := ChainLeft(func(errs Errors) Validation[int] {
+//	    return Failures[int](Errors{
+//	        &ValidationError{Messsage: "never called"},
+//	    })
+//	})
+//	result := handler(Success(42)) // Success(42) - unchanged
+func ChainLeft[A any](f Kleisli[Errors, A]) Operator[A, A] {
+	return either.Fold(
+		chainErrors(f),
+		Of[A],
+	)
+}
+
+// MonadChainLeft sequences a computation on the failure (Left) channel of a Validation.
+// If the Validation is a failure, applies the function to transform or recover from the errors.
+// If the Validation is a success, returns the success value unchanged.
+//
+// **Critical difference from Either.MonadChainLeft**: This validation-specific implementation
+// **aggregates errors** using the Errors monoid. When the transformation function returns a
+// failure, both the original errors and the new errors are combined, ensuring comprehensive
+// error reporting.
+//
+// This is the dual of [MonadChain] - while Chain operates on success values, ChainLeft
+// operates on failure values. It's particularly useful for:
+//   - Error recovery: converting specific errors into successful values
+//   - Error enrichment: adding context or transforming error messages
+//   - Fallback logic: providing alternative validations when the first fails
+//   - Error aggregation: combining multiple validation failures
+//
+// The function parameter receives the collection of validation errors and must return
+// a new Validation[A]. This allows you to:
+//   - Recover by returning Success(value)
+//   - Transform errors by returning Failures(newErrors) - **original errors are preserved**
+//   - Implement conditional error handling based on error content
+//
+// Example - Error recovery:
+//
+//	result := MonadChainLeft(
+//	    Failures[int](Errors{
+//	        &ValidationError{Messsage: "not found"},
+//	    }),
+//	    func(errs Errors) Validation[int] {
+//	        // Check if we can recover
+//	        for _, err := range errs {
+//	            if err.Messsage == "not found" {
+//	                return Success(0) // recover with default value
+//	            }
+//	        }
+//	        return Failures[int](errs) // propagate errors
+//	    },
+//	) // Success(0)
+//
+// Example - Error aggregation (key feature):
+//
+//	result := MonadChainLeft(
+//	    Failures[string](Errors{
+//	        &ValidationError{Messsage: "error 1"},
+//	        &ValidationError{Messsage: "error 2"},
+//	    }),
+//	    func(errs Errors) Validation[string] {
+//	        // Transformation also fails
+//	        return Failures[string](Errors{
+//	            &ValidationError{Messsage: "error 3"},
+//	        })
+//	    },
+//	)
+//	// Result contains ALL errors: ["error 1", "error 2", "error 3"]
+//	// This is different from Either.MonadChainLeft which would only keep "error 3"
+//
+// Example - Adding context to errors:
+//
+//	result := MonadChainLeft(
+//	    Failures[int](Errors{
+//	        &ValidationError{Value: "abc", Messsage: "invalid number"},
+//	    }),
+//	    func(errs Errors) Validation[int] {
+//	        // Add contextual information
+//	        contextErrors := Errors{
+//	            &ValidationError{
+//	                Context:  []ContextEntry{{Key: "user", Type: "User"}, {Key: "age", Type: "int"}},
+//	                Messsage: "failed to parse user age",
+//	            },
+//	        }
+//	        return Failures[int](contextErrors)
+//	    },
+//	)
+//	// Result contains both original error and context:
+//	// ["invalid number", "failed to parse user age"]
+//
+// Example - Success values pass through:
+//
+//	result := MonadChainLeft(
+//	    Success(42),
+//	    func(errs Errors) Validation[int] {
+//	        return Failures[int](Errors{
+//	            &ValidationError{Messsage: "never called"},
+//	        })
+//	    },
+//	) // Success(42) - unchanged
+func MonadChainLeft[A any](fa Validation[A], f Kleisli[Errors, A]) Validation[A] {
+	return either.MonadFold(
+		fa,
+		chainErrors(f),
+		Of[A],
+	)
+}
+
 // Applicative creates an Applicative instance for Validation with error accumulation.
 //
 // This returns a lawful Applicative that accumulates validation errors using the Errors monoid.
@@ -123,6 +466,11 @@ func MonadChain[A, B any](fa Validation[A], f Kleisli[A, B]) Validation[B] {
 //	An Applicative instance with Of, Map, and Ap operations that accumulate errors
 func Applicative[A, B any]() applicative.Applicative[A, B, Validation[A], Validation[B], Validation[func(A) B]] {
 	return either.ApplicativeV[Errors, A, B](
-		ErrorsMonoid(),
+		errorsMonoid,
 	)
 }
+
+//go:inline
+func OrElse[A any](f Kleisli[Errors, A]) Operator[A, A] {
+	return ChainLeft(f)
+}

v2/optics/codec/validation/monoid_test.go

@@ -74,12 +74,7 @@ func TestApplicativeMonoid(t *testing.T) {
 		t.Run("empty returns successful validation with empty string", func(t *testing.T) {
 			empty := m.Empty()
 
-			assert.True(t, either.IsRight(empty))
-			value := either.MonadFold(empty,
-				func(Errors) string { return "ERROR" },
-				F.Identity[string],
-			)
-			assert.Equal(t, "", value)
+			assert.Equal(t, Success(""), empty)
 		})
 
 		t.Run("concat combines successful validations", func(t *testing.T) {
@@ -88,12 +83,7 @@ func TestApplicativeMonoid(t *testing.T) {
 
 			result := m.Concat(v1, v2)
 
-			assert.True(t, either.IsRight(result))
-			value := either.MonadFold(result,
-				func(Errors) string { return "" },
-				F.Identity[string],
-			)
-			assert.Equal(t, "Hello World", value)
+			assert.Equal(t, Success("Hello World"), result)
 		})
 
 		t.Run("concat with failure returns failure", func(t *testing.T) {

v2/optics/codec/validation/types.go

@@ -1,7 +1,23 @@
+// Copyright (c) 2025 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package validation
 
 import (
 	"github.com/IBM/fp-go/v2/either"
+	"github.com/IBM/fp-go/v2/endomorphism"
 	"github.com/IBM/fp-go/v2/monoid"
 	"github.com/IBM/fp-go/v2/reader"
 	"github.com/IBM/fp-go/v2/result"
@@ -9,13 +25,35 @@ import (
 
 type (
 	// Result represents a computation that may succeed with a value of type A or fail with an error.
+	// This is an alias for result.Result[A], which is Either[error, A].
+	//
+	// Used for converting validation results to standard Go error handling patterns.
 	Result[A any] = result.Result[A]
 
 	// Either represents a value that can be one of two types: Left (error) or Right (success).
+	// This is an alias for either.Either[E, A], a disjoint union type.
+	//
+	// In the validation context:
+	//   - Left[E]: Contains error information of type E
+	//   - Right[A]: Contains a successfully validated value of type A
 	Either[E, A any] = either.Either[E, A]
 
 	// ContextEntry represents a single entry in the validation context path.
-	// It tracks the location and type information during nested validation.
+	// It tracks the location and type information during nested validation,
+	// enabling precise error reporting with full path information.
+	//
+	// Fields:
+	//   - Key: The key or field name (e.g., "email", "address", "items[0]")
+	//   - Type: The expected type name (e.g., "string", "int", "User")
+	//   - Actual: The actual value being validated (for error reporting)
+	//
+	// Example:
+	//
+	//	entry := ContextEntry{
+	//	    Key:    "user.email",
+	//	    Type:   "string",
+	//	    Actual: 12345,
+	//	}
 	ContextEntry struct {
 		Key    string // The key or field name (for objects/maps)
 		Type   string // The expected type name
@@ -23,42 +61,200 @@ type (
 	}
 
 	// Context is a stack of ContextEntry values representing the path through
-	// nested structures during validation. Used to provide detailed error messages.
+	// nested structures during validation. Used to provide detailed error messages
+	// that show exactly where in a nested structure a validation failure occurred.
+	//
+	// The context builds up as validation descends into nested structures:
+	//   - [] - root level
+	//   - [{Key: "user"}] - inside user object
+	//   - [{Key: "user"}, {Key: "address"}] - inside user.address
+	//   - [{Key: "user"}, {Key: "address"}, {Key: "zipCode"}] - at user.address.zipCode
+	//
+	// Example:
+	//
+	//	ctx := Context{
+	//	    {Key: "user", Type: "User"},
+	//	    {Key: "address", Type: "Address"},
+	//	    {Key: "zipCode", Type: "string"},
+	//	}
+	//	// Represents path: user.address.zipCode
 	Context = []ContextEntry
 
-	// ValidationError represents a single validation failure with context.
+	// ValidationError represents a single validation failure with full context information.
+	// It implements the error interface and provides detailed information about what failed,
+	// where it failed, and why it failed.
+	//
+	// Fields:
+	//   - Value: The actual value that failed validation
+	//   - Context: The path to the value in nested structures (e.g., user.address.zipCode)
+	//   - Messsage: Human-readable error description
+	//   - Cause: Optional underlying error that caused the validation failure
+	//
+	// The ValidationError type implements:
+	//   - error interface: For standard Go error handling
+	//   - fmt.Formatter: For custom formatting with %v, %+v
+	//   - slog.LogValuer: For structured logging with slog
+	//
+	// Example:
+	//
+	//	err := &ValidationError{
+	//	    Value:    "not-an-email",
+	//	    Context:  []ContextEntry{{Key: "user"}, {Key: "email"}},
+	//	    Messsage: "invalid email format",
+	//	    Cause:    nil,
+	//	}
+	//	fmt.Printf("%v", err)   // at user.email: invalid email format
+	//	fmt.Printf("%+v", err)  // at user.email: invalid email format
+	//	                        //   value: "not-an-email"
 	ValidationError struct {
 		Value    any     // The value that failed validation
 		Context  Context // The path to the value in nested structures
 		Messsage string  // Human-readable error message
-		Cause    error
+		Cause    error   // Optional underlying error cause
 	}
 
 	// Errors is a collection of validation errors.
+	// This type is used to accumulate multiple validation failures,
+	// allowing all errors to be reported at once rather than failing fast.
+	//
+	// Example:
+	//
+	//	errors := Errors{
+	//	    &ValidationError{Value: "", Messsage: "name is required"},
+	//	    &ValidationError{Value: "invalid", Messsage: "invalid email"},
+	//	    &ValidationError{Value: -1, Messsage: "age must be positive"},
+	//	}
 	Errors = []*ValidationError
 
 	// validationErrors wraps a collection of validation errors with an optional root cause.
-	// It provides structured error information for validation failures.
+	// It provides structured error information for validation failures and implements
+	// the error interface for integration with standard Go error handling.
+	//
+	// This type is internal and created via MakeValidationErrors.
+	// It implements:
+	//   - error interface: For standard Go error handling
+	//   - fmt.Formatter: For custom formatting with %v, %+v
+	//   - slog.LogValuer: For structured logging with slog
+	//
+	// Fields:
+	//   - errors: The collection of individual validation errors
+	//   - cause: Optional root cause error
 	validationErrors struct {
 		errors Errors
 		cause  error
 	}
 
 	// Validation represents the result of a validation operation.
-	// Left contains validation errors, Right contains the successfully validated value.
+	// It's an Either type where:
+	//   - Left(Errors): Validation failed with one or more errors
+	//   - Right(A): Successfully validated value of type A
+	//
+	// This type supports applicative operations, allowing multiple validations
+	// to be combined while accumulating all errors rather than failing fast.
+	//
+	// Example:
+	//
+	//	// Success case
+	//	valid := Success(42)  // Right(42)
+	//
+	//	// Failure case
+	//	invalid := Failures[int](Errors{
+	//	    &ValidationError{Messsage: "must be positive"},
+	//	})  // Left([...])
+	//
+	//	// Combining validations (accumulates all errors)
+	//	result := Ap(Ap(Of(func(x int) func(y int) int {
+	//	    return func(y int) int { return x + y }
+	//	}))(validateX))(validateY)
 	Validation[A any] = Either[Errors, A]
 
 	// Reader represents a computation that depends on an environment R and produces a value A.
+	// This is an alias for reader.Reader[R, A], which is func(R) A.
+	//
+	// In the validation context, Reader is used for context-dependent validation operations
+	// where the validation logic needs access to the current validation context path.
+	//
+	// Example:
+	//
+	//	validateWithContext := func(ctx Context) Validation[int] {
+	//	    // Use ctx to provide detailed error messages
+	//	    return Success(42)
+	//	}
 	Reader[R, A any] = reader.Reader[R, A]
 
 	// Kleisli represents a function from A to a validated B.
 	// It's a Reader that takes an input A and produces a Validation[B].
+	// This is the fundamental building block for composable validation operations.
+	//
+	// Type: func(A) Validation[B]
+	//
+	// Kleisli arrows can be composed using Chain/Bind operations to build
+	// complex validation pipelines from simple validation functions.
+	//
+	// Example:
+	//
+	//	validatePositive := func(x int) Validation[int] {
+	//	    if x > 0 {
+	//	        return Success(x)
+	//	    }
+	//	    return Failures[int](/* error */)
+	//	}
+	//
+	//	validateEven := func(x int) Validation[int] {
+	//	    if x%2 == 0 {
+	//	        return Success(x)
+	//	    }
+	//	    return Failures[int](/* error */)
+	//	}
+	//
+	//	// Compose validations
+	//	validatePositiveEven := Chain(validateEven)(Success(42))
 	Kleisli[A, B any] = Reader[A, Validation[B]]
 
 	// Operator represents a validation transformation that takes a validated A and produces a validated B.
-	// It's a specialized Kleisli arrow for composing validation operations.
+	// It's a specialized Kleisli arrow for composing validation operations where the input
+	// is already a Validation[A].
+	//
+	// Type: func(Validation[A]) Validation[B]
+	//
+	// Operators are used to transform and compose validation results, enabling
+	// functional composition of validation pipelines.
+	//
+	// Example:
+	//
+	//	// Transform a validated int to a validated string
+	//	intToString := Map(func(x int) string {
+	//	    return strconv.Itoa(x)
+	//	})  // Operator[int, string]
+	//
+	//	result := intToString(Success(42))  // Success("42")
 	Operator[A, B any] = Kleisli[Validation[A], B]
 
 	// Monoid represents an algebraic structure with an associative binary operation and an identity element.
+	// This is an alias for monoid.Monoid[A].
+	//
+	// In the validation context, monoids are used to combine validation results:
+	//   - ApplicativeMonoid: Combines successful validations using the monoid operation
+	//   - AlternativeMonoid: Provides fallback behavior for failed validations
+	//
+	// Example:
+	//
+	//	import N "github.com/IBM/fp-go/v2/number"
+	//
+	//	intAdd := N.MonoidSum[int]()
+	//	m := ApplicativeMonoid(intAdd)
+	//	result := m.Concat(Success(5), Success(3))  // Success(8)
 	Monoid[A any] = monoid.Monoid[A]
+
+	// Endomorphism represents a function from a type to itself: func(A) A.
+	// This is an alias for endomorphism.Endomorphism[A].
+	//
+	// In the validation context, endomorphisms are used with LetL to transform
+	// values within a validation context using pure functions.
+	//
+	// Example:
+	//
+	//	double := func(x int) int { return x * 2 }  // Endomorphism[int]
+	//	result := LetL(lens, double)(Success(21))   // Success(42)
+	Endomorphism[A any] = endomorphism.Endomorphism[A]
 )

v2/optics/codec/validation/validation_test.go

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

v2/readerioeither/reader.go

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

v2/readerioeither/reader_test.go

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

v2/result/either.go

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

v2/result/either_test.go

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