fix: try a more efficient implementation of standard interfaces

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

either/core.go

@@ -20,15 +20,17 @@ import (
 )
 
 type (
-	// Either defines a data structure that logically holds either an E or an A. The flag discriminates the cases
-	Either[E, A any] struct {
+	either struct {
 		isLeft bool
 		value  any
 	}
+
+	// Either defines a data structure that logically holds either an E or an A. The flag discriminates the cases
+	Either[E, A any] either
 )
 
 // String prints some debug info for the object
-func (s Either[E, A]) String() string {
+func eitherString(s *either) string {
 	if s.isLeft {
 		return fmt.Sprintf("Left[%T](%v)", s.value, s.value)
 	}
@@ -36,15 +38,25 @@ func (s Either[E, A]) String() string {
 }
 
 // Format prints some debug info for the object
-func (s Either[E, A]) Format(f fmt.State, c rune) {
+func eitherFormat(e *either, f fmt.State, c rune) {
 	switch c {
 	case 's':
-		fmt.Fprint(f, s.String())
+		fmt.Fprint(f, eitherString(e))
 	default:
-		fmt.Fprint(f, s.String())
+		fmt.Fprint(f, eitherString(e))
 	}
 }
 
+// String prints some debug info for the object
+func (s Either[E, A]) String() string {
+	return eitherString((*either)(&s))
+}
+
+// Format prints some debug info for the object
+func (s Either[E, A]) Format(f fmt.State, c rune) {
+	eitherFormat((*either)(&s), f, c)
+}
+
 // IsLeft tests if the [Either] is a left value. Rather use [Fold] if you need to access the values. Inverse is [IsRight].
 func IsLeft[E, A any](val Either[E, A]) bool {
 	return val.isLeft

either/either_test.go

@@ -17,6 +17,7 @@ package either
 
 import (
 	"errors"
+	"fmt"
 	"testing"
 
 	F "github.com/IBM/fp-go/function"
@@ -109,3 +110,13 @@ func TestFromOption(t *testing.T) {
 	assert.Equal(t, Left[int]("none"), FromOption[int](F.Constant("none"))(O.None[int]()))
 	assert.Equal(t, Right[string](1), FromOption[int](F.Constant("none"))(O.Some(1)))
 }
+
+func TestStringer(t *testing.T) {
+	e := Of[error]("foo")
+	exp := "Right[string](foo)"
+
+	assert.Equal(t, exp, e.String())
+
+	var s fmt.Stringer = e
+	assert.Equal(t, exp, s.String())
+}