fix: add Flip function

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

function/flip.go

@@ -0,0 +1,25 @@
+// 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 function
+
+// Flip reverses the order of parameters of a curried function
+func Flip[T1, T2, R any](f func(T1) func(T2) R) func(T2) func(T1) R {
+	return func(t2 T2) func(T1) R {
+		return func(t1 T1) R {
+			return f(t1)(t2)
+		}
+	}
+}

function/flip_test.go

@@ -0,0 +1,36 @@
+// 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 function
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFlip(t *testing.T) {
+
+	x := Curry2(func(a, b string) string {
+		return fmt.Sprintf("%s:%s", a, b)
+	})
+
+	assert.Equal(t, "a:b", x("a")("b"))
+
+	y := Flip(x)
+
+	assert.Equal(t, "b:a", y("a")("b"))
+}