feat: add ZipWithIndex

stream/zip.go

@@ -0,0 +1,19 @@
+package stream
+
+import (
+	"github.com/primetalk/goio/fun"
+	"github.com/primetalk/goio/io"
+)
+
+// ZipWithIndex prepends the index to each element.
+func ZipWithIndex[A any](as Stream[A]) Stream[fun.Pair[int, A]] {
+	return StateFlatMap(as, 0,
+		func(a A, i int) io.IO[fun.Pair[int, Stream[fun.Pair[int, A]]]] {
+			return io.Lift(
+				fun.NewPair(i+1, Lift(
+					fun.NewPair(i, a),
+				)),
+			)
+		},
+	)
+}

stream/zip_test.go

@@ -0,0 +1,15 @@
+package stream_test
+
+import (
+	"testing"
+
+	"github.com/primetalk/goio/slice"
+	"github.com/primetalk/goio/stream"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestZipWithIndex(t *testing.T) {
+	natsWithIndex := UnsafeStreamToSlice(t, stream.ZipWithIndex(nats10))
+	sNatsWithIndex := slice.ZipWithIndex(slice.Nats(10))
+	assert.ElementsMatch(t, sNatsWithIndex, natsWithIndex)
+}