fn: add operations to Result API

fn/result.go

@@ -64,20 +64,37 @@ func (r Result[T]) IsErr() bool {
 	return r.IsRight()
 }
 
-// Map applies a function to the success value if it exists.
+// Map applies an endomorphic function to the success value if it exists.
+//
+// Deprecated: Use MapOk instead.
 func (r Result[T]) Map(f func(T) T) Result[T] {
 	return Result[T]{
 		MapLeft[T, error](f)(r.Either),
 	}
 }
 
-// MapErr applies a function to the error value if it exists.
+// MapOk applies an endomorphic function to the success value if it exists.
+func (r Result[T]) MapOk(f func(T) T) Result[T] {
+	return Result[T]{
+		MapLeft[T, error](f)(r.Either),
+	}
+}
+
+// MapErr applies an endomorphic function to the error value if it exists.
 func (r Result[T]) MapErr(f func(error) error) Result[T] {
 	return Result[T]{
 		MapRight[T](f)(r.Either),
 	}
 }
 
+// MapOk applies a non-endomorphic function to the success value if it exists
+// and returns a Result of the new type.
+func MapOk[A, B any](f func(A) B) func(Result[A]) Result[B] {
+	return func(r Result[A]) Result[B] {
+		return Result[B]{MapLeft[A, error](f)(r.Either)}
+	}
+}
+
 // Option returns the success value as an Option.
 //
 // Deprecated: Use OkToSome instead.
@@ -137,8 +154,22 @@ func (r Result[T]) UnwrapOrFail(t *testing.T) T {
 	return r.left
 }
 
-// FlatMap applies a function that returns a Result to the success value if it
+// FlattenResult takes a nested Result and joins the two functor layers into
-// exists.
+// one.
+func FlattenResult[A any](r Result[Result[A]]) Result[A] {
+	if r.IsErr() {
+		return Err[A](r.right)
+	}
+
+	if r.left.IsErr() {
+		return Err[A](r.left.right)
+	}
+
+	return r.left
+}
+
+// FlatMap applies a kleisli endomorphic function that returns a Result to the
+// success value if it exists.
 func (r Result[T]) FlatMap(f func(T) Result[T]) Result[T] {
 	if r.IsOk() {
 		return r

fn/result_test.go

@@ -2,7 +2,9 @@ package fn
 
 import (
 	"errors"
+	"fmt"
 	"testing"
+	"testing/quick"
 
 	"github.com/stretchr/testify/require"
 )
@@ -17,3 +19,29 @@ func TestOkToSome(t *testing.T) {
 		t, Err[uint8](errors.New("err")).OkToSome(), None[uint8](),
 	)
 }
+
+func TestMapOk(t *testing.T) {
+	inc := func(i int) int {
+		return i + 1
+	}
+	f := func(i int) bool {
+		ok := Ok(i)
+		return MapOk(inc)(ok) == Ok(inc(i))
+	}
+
+	require.NoError(t, quick.Check(f, nil))
+}
+
+func TestFlattenResult(t *testing.T) {
+	f := func(i int) bool {
+		e := fmt.Errorf("error")
+
+		x := FlattenResult(Ok(Ok(i))) == Ok(i)
+		y := FlattenResult(Ok(Err[int](e))) == Err[int](e)
+		z := FlattenResult(Err[Result[int]](e)) == Err[int](e)
+
+		return x && y && z
+	}
+
+	require.NoError(t, quick.Check(f, nil))
+}