add simple generic Iterator definition and implementation

lib/funky/funky.go

@@ -1,5 +1,9 @@
+// Package funky provides a few constructs usually present in pure
+// functional programming environments, using Go 1.18 generics.
 package funky
 
+// Maybe (sometimes called `Option`)
+
 type Maybe[V any] interface {
 	IsNothing() bool
 	Value() V
@@ -25,3 +29,44 @@ func (m *maybe[V]) IsNothing() bool {
 func (m *maybe[V]) Value() V {
 	return m.value
 }
+
+// Iterator
+
+type Iterator[V any] interface {
+	Maybe[V]
+	Next() Maybe[V]
+}
+
+type sliceIt[V any] struct {
+	idx   int
+	slice []V
+}
+
+func SliceIterator[V any](sl []V) Iterator[V] {
+	return &sliceIt[V]{slice: sl}
+}
+
+func (it *sliceIt[V]) Next() Maybe[V] {
+	if it.IsNothing() {
+		return Nothing[V]()
+	}
+	v := it.Value()
+	it.idx++
+	return Just(v)
+}
+
+func (it *sliceIt[V]) IsNothing() bool {
+	return it.idx >= len(it.slice)
+}
+
+func (it *sliceIt[V]) Value() V {
+	return it.slice[it.idx]
+}
+
+func Slice[V any](it Iterator[V]) []V {
+	var out []V
+	for m := it.Next(); !m.IsNothing(); m = it.Next() {
+		out = append(out, m.Value())
+	}
+	return out
+}

tests/funky_test.go

@@ -10,6 +10,7 @@ import (
 func TestFunky(tb *tbase.T) {
 	t := testing.SetUp(tb)
 	t.Run("maybe", MaybeTest)
+	t.Run("iterator", IteratorTest)
 }
 
 func MaybeTest(t *testing.T) {
@@ -19,3 +20,11 @@ func MaybeTest(t *testing.T) {
 	t.AssertEqual(i1.IsNothing(), false)
 	t.AssertEqual(i1.Value(), 3)
 }
+
+func IteratorTest(t *testing.T) {
+	sl := []int{3, 4, 5}
+	var it funky.Iterator[int] = funky.SliceIterator[int](sl)
+	s12 := funky.Slice(it)
+	t.AssertEqual(len(s12), 3)
+	t.AssertEqual(s12[1], 4)
+}