fix: add missing alt methods and semigroup

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

semigroup/alt.go

@@ -0,0 +1,28 @@
+// 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 semigroup
+
+func AltSemigroup[HKTA any](
+	falt func(HKTA, func() HKTA) HKTA,
+
+) Semigroup[HKTA] {
+
+	return MakeSemigroup(
+		func(first, second HKTA) HKTA {
+			return falt(first, func() HKTA { return second })
+		},
+	)
+}