fix: add optics and consistent copyright

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

optics/traversal/traversal.go

@@ -0,0 +1,66 @@
+// Copyright (c) 2023 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 traversal
+
+import (
+	C "github.com/IBM/fp-go/constant"
+	F "github.com/IBM/fp-go/function"
+	G "github.com/IBM/fp-go/optics/traversal/generic"
+)
+
+// Id is the identity constructor of a traversal
+func Id[S, A any]() G.Traversal[S, S, A, A] {
+	return F.Identity[func(S) A]
+}
+
+// Modify applies a transformation function to a traversal
+func Modify[S, A any](f func(A) A) func(sa G.Traversal[S, A, S, A]) func(S) S {
+	return func(sa G.Traversal[S, A, S, A]) func(S) S {
+		return sa(f)
+	}
+}
+
+// Set sets a constant value for all values of the traversal
+func Set[S, A any](a A) func(sa G.Traversal[S, A, S, A]) func(S) S {
+	return Modify[S, A](F.Constant1[A](a))
+}
+
+// FoldMap maps each target to a `Monoid` and combines the result
+func FoldMap[M, S, A any](f func(A) M) func(sa G.Traversal[S, A, C.Const[M, S], C.Const[M, A]]) func(S) M {
+	return G.FoldMap[M, S, A](f)
+}
+
+// Fold maps each target to a `Monoid` and combines the result
+func Fold[S, A any](sa G.Traversal[S, A, C.Const[A, S], C.Const[A, A]]) func(S) A {
+	return G.Fold[S, A](sa)
+}
+
+// GetAll gets all the targets of a traversal
+func GetAll[S, A any](s S) func(sa G.Traversal[S, A, C.Const[[]A, S], C.Const[[]A, A]]) []A {
+	return G.GetAll[[]A, S, A](s)
+}
+
+// Compose composes two traversables
+func Compose[
+	S, A, B, HKTS, HKTA, HKTB any](ab G.Traversal[A, B, HKTA, HKTB]) func(sa G.Traversal[S, A, HKTS, HKTA]) G.Traversal[S, B, HKTS, HKTB] {
+	return G.Compose[
+		G.Traversal[A, B, HKTA, HKTB],
+		G.Traversal[S, A, HKTS, HKTA],
+		G.Traversal[S, B, HKTS, HKTB],
+		S, A, B,
+		HKTS, HKTA, HKTB,
+	](ab)
+}