fix: add writer

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

writer/writer_test.go

@@ -0,0 +1,35 @@
+package writer
+
+import (
+	"fmt"
+
+	A "github.com/IBM/fp-go/array"
+	F "github.com/IBM/fp-go/function"
+	M "github.com/IBM/fp-go/monoid"
+	T "github.com/IBM/fp-go/tuple"
+)
+
+func doubleAndLog(data int) Writer[[]string, int] {
+	return func() T.Tuple2[int, []string] {
+		result := data * 2
+		return T.MakeTuple2(result, A.Of(fmt.Sprintf("Doubled %d -> %d", data, result)))
+	}
+}
+
+func ExampleLoggingWriter() {
+
+	m := A.Monoid[string]()
+	s := M.ToSemigroup(m)
+
+	res := F.Pipe3(
+		10,
+		Of[int](m),
+		Chain[int, int](s)(doubleAndLog),
+		Chain[int, int](s)(doubleAndLog),
+	)
+
+	fmt.Println(res())
+
+	// Output: test
+
+}