fix: introduce Kleisli type

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

Author: CarstenLeue

v2/context/readereither/array.go

@@ -18,12 +18,12 @@ package readereither
 import "github.com/IBM/fp-go/v2/readereither"
 
 // TraverseArray transforms an array
-func TraverseArray[A, B any](f func(A) ReaderEither[B]) func([]A) ReaderEither[[]B] {
+func TraverseArray[A, B any](f Kleisli[A, B]) Kleisli[[]A, []B] {
 	return readereither.TraverseArray(f)
 }
 
 // TraverseArrayWithIndex transforms an array
-func TraverseArrayWithIndex[A, B any](f func(int, A) ReaderEither[B]) func([]A) ReaderEither[[]B] {
+func TraverseArrayWithIndex[A, B any](f func(int, A) ReaderEither[B]) Kleisli[[]A, []B] {
 	return readereither.TraverseArrayWithIndex(f)
 }
 

v2/context/readereither/bind.go

@@ -18,6 +18,7 @@ package readereither
 import (
 	"context"
 
+	L "github.com/IBM/fp-go/v2/optics/lens"
 	G "github.com/IBM/fp-go/v2/readereither/generic"
 )
 
@@ -80,31 +81,31 @@ func Do[S any](
 //	)
 func Bind[S1, S2, T any](
 	setter func(T) func(S1) S2,
-	f func(S1) ReaderEither[T],
-) func(ReaderEither[S1]) ReaderEither[S2] {
+	f Kleisli[S1, T],
+) Kleisli[ReaderEither[S1], S2] {
 	return G.Bind[ReaderEither[S1], ReaderEither[S2], ReaderEither[T], context.Context, error, S1, S2, T](setter, f)
 }
 
 // Let attaches the result of a computation to a context [S1] to produce a context [S2]
 func Let[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	f func(S1) T,
-) func(ReaderEither[S1]) ReaderEither[S2] {
+) Kleisli[ReaderEither[S1], S2] {
 	return G.Let[ReaderEither[S1], ReaderEither[S2], context.Context, error, S1, S2, T](setter, f)
 }
 
 // LetTo attaches the a value to a context [S1] to produce a context [S2]
 func LetTo[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	b T,
-) func(ReaderEither[S1]) ReaderEither[S2] {
+) Kleisli[ReaderEither[S1], S2] {
 	return G.LetTo[ReaderEither[S1], ReaderEither[S2], context.Context, error, S1, S2, T](setter, b)
 }
 
 // BindTo initializes a new state [S1] from a value [T]
 func BindTo[S1, T any](
 	setter func(T) S1,
-) func(ReaderEither[T]) ReaderEither[S1] {
+) Kleisli[ReaderEither[T], S1] {
 	return G.BindTo[ReaderEither[S1], ReaderEither[T], context.Context, error, S1, T](setter)
 }
 
@@ -148,6 +149,161 @@ func BindTo[S1, T any](
 func ApS[S1, S2, T any](
 	setter func(T) func(S1) S2,
 	fa ReaderEither[T],
-) func(ReaderEither[S1]) ReaderEither[S2] {
+) Kleisli[ReaderEither[S1], S2] {
 	return G.ApS[ReaderEither[S1], ReaderEither[S2], ReaderEither[T], context.Context, error, S1, S2, T](setter, fa)
 }
+
+// ApSL is a variant of ApS that uses a lens to focus on a specific field in the state.
+// Instead of providing a setter function, you provide a lens that knows how to get and set
+// the field. This is more convenient when working with nested structures.
+//
+// Parameters:
+//   - lens: A lens that focuses on a field of type T within state S
+//   - fa: A ReaderEither computation that produces a value of type T
+//
+// Returns:
+//   - A function that transforms ReaderEither[S] to ReaderEither[S] by setting the focused field
+//
+// Example:
+//
+//	type Person struct {
+//	    Name string
+//	    Age  int
+//	}
+//
+//	ageLens := lens.MakeLens(
+//	    func(p Person) int { return p.Age },
+//	    func(p Person, a int) Person { p.Age = a; return p },
+//	)
+//
+//	getAge := func(ctx context.Context) either.Either[error, int] {
+//	    return either.Right[error](30)
+//	}
+//
+//	result := F.Pipe1(
+//	    readereither.Do(Person{Name: "Alice", Age: 25}),
+//	    readereither.ApSL(ageLens, getAge),
+//	)
+func ApSL[S, T any](
+	lens L.Lens[S, T],
+	fa ReaderEither[T],
+) Kleisli[ReaderEither[S], S] {
+	return ApS(lens.Set, fa)
+}
+
+// BindL is a variant of Bind that uses a lens to focus on a specific field in the state.
+// It combines the lens-based field access with monadic composition, allowing you to:
+// 1. Extract a field value using the lens
+// 2. Use that value in a computation that may fail
+// 3. Update the field with the result
+//
+// Parameters:
+//   - lens: A lens that focuses on a field of type T within state S
+//   - f: A function that takes the current field value and returns a ReaderEither computation
+//
+// Returns:
+//   - A function that transforms ReaderEither[S] to ReaderEither[S]
+//
+// 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 := func(v int) readereither.ReaderEither[int] {
+//	    return func(ctx context.Context) either.Either[error, int] {
+//	        if v >= 100 {
+//	            return either.Left[int](errors.New("value too large"))
+//	        }
+//	        return either.Right[error](v + 1)
+//	    }
+//	}
+//
+//	result := F.Pipe1(
+//	    readereither.Of[error](Counter{Value: 42}),
+//	    readereither.BindL(valueLens, increment),
+//	)
+func BindL[S, T any](
+	lens L.Lens[S, T],
+	f Kleisli[T, T],
+) Kleisli[ReaderEither[S], S] {
+	return Bind[S, S, T](lens.Set, func(s S) ReaderEither[T] {
+		return f(lens.Get(s))
+	})
+}
+
+// LetL is a variant of Let that uses a lens to focus on a specific field in the state.
+// It applies a pure transformation to the focused field without any effects.
+//
+// Parameters:
+//   - lens: A lens that focuses on a field of type T within state S
+//   - f: A pure function that transforms the field value
+//
+// Returns:
+//   - A function that transforms ReaderEither[S] to ReaderEither[S]
+//
+// 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 := func(v int) int { return v * 2 }
+//
+//	result := F.Pipe1(
+//	    readereither.Of[error](Counter{Value: 21}),
+//	    readereither.LetL(valueLens, double),
+//	)
+//	// result when executed will be Right(Counter{Value: 42})
+func LetL[S, T any](
+	lens L.Lens[S, T],
+	f func(T) T,
+) Kleisli[ReaderEither[S], S] {
+	return Let[S, S, T](lens.Set, func(s S) T {
+		return f(lens.Get(s))
+	})
+}
+
+// LetToL is a variant of LetTo that uses a lens to focus on a specific field in the state.
+// It sets the focused field to a constant value.
+//
+// Parameters:
+//   - lens: A lens that focuses on a field of type T within state S
+//   - b: The constant value to set
+//
+// Returns:
+//   - A function that transforms ReaderEither[S] to ReaderEither[S]
+//
+// 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(
+//	    readereither.Of[error](Config{Debug: true, Timeout: 30}),
+//	    readereither.LetToL(debugLens, false),
+//	)
+//	// result when executed will be Right(Config{Debug: false, Timeout: 30})
+func LetToL[S, T any](
+	lens L.Lens[S, T],
+	b T,
+) Kleisli[ReaderEither[S], S] {
+	return LetTo[S, S, T](lens.Set, b)
+}

v2/context/readereither/curry.go

@@ -28,26 +28,26 @@ func Curry0[A any](f func(context.Context) (A, error)) ReaderEither[A] {
 	return readereither.Curry0(f)
 }
 
-func Curry1[T1, A any](f func(context.Context, T1) (A, error)) func(T1) ReaderEither[A] {
+func Curry1[T1, A any](f func(context.Context, T1) (A, error)) Kleisli[T1, A] {
 	return readereither.Curry1(f)
 }
 
-func Curry2[T1, T2, A any](f func(context.Context, T1, T2) (A, error)) func(T1) func(T2) ReaderEither[A] {
+func Curry2[T1, T2, A any](f func(context.Context, T1, T2) (A, error)) func(T1) Kleisli[T2, A] {
 	return readereither.Curry2(f)
 }
 
-func Curry3[T1, T2, T3, A any](f func(context.Context, T1, T2, T3) (A, error)) func(T1) func(T2) func(T3) ReaderEither[A] {
+func Curry3[T1, T2, T3, A any](f func(context.Context, T1, T2, T3) (A, error)) func(T1) func(T2) Kleisli[T3, A] {
 	return readereither.Curry3(f)
 }
 
-func Uncurry1[T1, A any](f func(T1) ReaderEither[A]) func(context.Context, T1) (A, error) {
+func Uncurry1[T1, A any](f Kleisli[T1, A]) func(context.Context, T1) (A, error) {
 	return readereither.Uncurry1(f)
 }
 
-func Uncurry2[T1, T2, A any](f func(T1) func(T2) ReaderEither[A]) func(context.Context, T1, T2) (A, error) {
+func Uncurry2[T1, T2, A any](f func(T1) Kleisli[T2, A]) func(context.Context, T1, T2) (A, error) {
 	return readereither.Uncurry2(f)
 }
 
-func Uncurry3[T1, T2, T3, A any](f func(T1) func(T2) func(T3) ReaderEither[A]) func(context.Context, T1, T2, T3) (A, error) {
+func Uncurry3[T1, T2, T3, A any](f func(T1) func(T2) Kleisli[T3, A]) func(context.Context, T1, T2, T3) (A, error) {
 	return readereither.Uncurry3(f)
 }

v2/context/readereither/from.go

@@ -28,7 +28,7 @@ func From0[A any](f func(context.Context) (A, error)) func() ReaderEither[A] {
 	return readereither.From0(f)
 }
 
-func From1[T1, A any](f func(context.Context, T1) (A, error)) func(T1) ReaderEither[A] {
+func From1[T1, A any](f func(context.Context, T1) (A, error)) Kleisli[T1, A] {
 	return readereither.From1(f)
 }
 

v2/context/readereither/reader.go

@@ -41,11 +41,11 @@ func Map[A, B any](f func(A) B) Operator[A, B] {
 	return readereither.Map[context.Context, error](f)
 }
 
-func MonadChain[A, B any](ma ReaderEither[A], f func(A) ReaderEither[B]) ReaderEither[B] {
+func MonadChain[A, B any](ma ReaderEither[A], f Kleisli[A, B]) ReaderEither[B] {
 	return readereither.MonadChain(ma, f)
 }
 
-func Chain[A, B any](f func(A) ReaderEither[B]) Operator[A, B] {
+func Chain[A, B any](f Kleisli[A, B]) Operator[A, B] {
 	return readereither.Chain(f)
 }
 
@@ -61,11 +61,11 @@ func Ap[A, B any](fa ReaderEither[A]) func(ReaderEither[func(A) B]) ReaderEither
 	return readereither.Ap[B](fa)
 }
 
-func FromPredicate[A any](pred func(A) bool, onFalse func(A) error) func(A) ReaderEither[A] {
+func FromPredicate[A any](pred func(A) bool, onFalse func(A) error) Kleisli[A, A] {
 	return readereither.FromPredicate[context.Context](pred, onFalse)
 }
 
-func OrElse[A any](onLeft func(error) ReaderEither[A]) func(ReaderEither[A]) ReaderEither[A] {
+func OrElse[A any](onLeft Kleisli[error, A]) Kleisli[ReaderEither[A], A] {
 	return readereither.OrElse(onLeft)
 }
 

v2/context/readereither/type.go

@@ -31,5 +31,6 @@ type (
 	// ReaderEither is a specialization of the Reader monad for the typical golang scenario
 	ReaderEither[A any] = readereither.ReaderEither[context.Context, error, A]
 
-	Operator[A, B any] = reader.Reader[ReaderEither[A], ReaderEither[B]]
+	Kleisli[A, B any]  = reader.Reader[A, ReaderEither[B]]
+	Operator[A, B any] = Kleisli[ReaderEither[A], B]
 )