fix: attempt to expose Monad as an interface

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

either/monad.go

@@ -0,0 +1,43 @@
+// Copyright (c) 2024 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 either
+
+import (
+	"github.com/IBM/fp-go/internal/monad"
+)
+
+type eitherMonad[E, A, B any] struct{}
+
+func (o *eitherMonad[E, A, B]) Of(a A) Either[E, A] {
+	return Of[E, A](a)
+}
+
+func (o *eitherMonad[E, A, B]) Map(f func(A) B) func(Either[E, A]) Either[E, B] {
+	return Map[E, A, B](f)
+}
+
+func (o *eitherMonad[E, A, B]) Chain(f func(A) Either[E, B]) func(Either[E, A]) Either[E, B] {
+	return Chain[E, A, B](f)
+}
+
+func (o *eitherMonad[E, A, B]) Ap(fa Either[E, A]) func(Either[E, func(A) B]) Either[E, B] {
+	return Ap[B, E, A](fa)
+}
+
+// Monad implements the monadic operations for [Either]
+func Monad[E, A, B any]() monad.Monad[A, B, Either[E, A], Either[E, B], Either[E, func(A) B]] {
+	return &eitherMonad[E, A, B]{}
+}