Added Iterator interface and two implementation: list itrator and range iterator

it-range.go

@@ -0,0 +1,55 @@
+// it-range.go
+package expr
+
+import "io"
+
+type RangeIterator struct {
+	start   int64
+	end     int64
+	step    int64
+	current int64
+	index   int
+}
+
+func NewRangeIterator(start, end, step int64) *RangeIterator {
+	if step == 0 {
+		panic("Range step must be not zero")
+	}
+	if step < 0 && start < end {
+		panic("When the range's step is less than zero, start must be greater than end")
+	}
+	if step > 0 && start > end {
+		panic("When the range's step is greater than zero, start must be less than end")
+	}
+	return &RangeIterator{start: start, end: end, step: step, current: start, index: 0}
+}
+
+func (it *RangeIterator) Reset() {
+	it.index = 0
+	it.current = it.start
+}
+
+func (it *RangeIterator) Next() (item any, err error) {
+	if it.step > 0 {
+		if it.current < it.end {
+			item = it.current
+			it.current += it.step
+			it.index++
+		} else {
+			err = io.EOF
+		}
+	} else {
+		if it.current > it.end {
+			item = it.current
+			it.current += it.step
+			it.index++
+		} else {
+			err = io.EOF
+		}
+	}
+	return
+}
+
+func (it *RangeIterator) Index() int {
+	return it.index - 1
+}

iterator.go

@@ -0,0 +1,37 @@
+// iterator.go
+package expr
+
+import "io"
+
+type Iterator interface {
+	Reset()
+	Next() (item any, err error) // must return io.EOF after the last item
+	Index() int
+}
+
+type FlatArrayIterator struct {
+	a     []any
+	index int
+}
+
+func NewFlatArrayIterator(array []any) *FlatArrayIterator {
+	return &FlatArrayIterator{a: array, index: 0}
+}
+
+func (it *FlatArrayIterator) Reset() {
+	it.index = 0
+}
+
+func (it *FlatArrayIterator) Next() (item any, err error) {
+	if it.index < len(it.a) {
+		item = it.a[it.index]
+		it.index++
+	} else {
+		err = io.EOF
+	}
+	return
+}
+
+func (it *FlatArrayIterator) Index() int {
+	return it.index - 1
+}