Craig Weber

Go generics iterator sketch

The new Go generics proposal and playground gives us something to play with. Here’s a sketch of what a basic iterator library could look like. It’s based on function types instead of interfaces; I think this gives better ergonomics than interface-based designs, especially if the proposal drops its seemingly arbitrary restrictions for method types.

I’m not sure about returning a pointer to the type as opposed to a (T, bool) tuple. In particular, I suspect this will cause unnecessary allocations, but I haven’t tested at all.

I’d love feedback:


package main

import (

// I made `Iter` a function type instead of an interface because I wanted to
// hang methods off of it such as Iter.Map() and Iter.ForEach()--you can do
// this for functions but strangely not for interfaces--however, this doesn't
// work for methods like Iter.Map() because the proposal bizarrely prohibits
// types not defined on the receiver type. Specifically, we can't specify the
// output type for the map. So instead I made functions that take an iter and
// return an iter. Not a huge loss, especially since Go doesn't support chained
// methods (e.g., `iter.Map(...).Reduce(...)`) very well, esp wrt line
// wrapping).
type Iter(type T) func() *T

func next(type I)(iter Iter(I)) *I { return iter() }

func map_(type I, O)(iter Iter(I), f func(I) O) Iter(O) {
	return func() *O {
		if ptr := next(iter); ptr != nil {
			o := f(*ptr)
			return &o
		return nil

func forEach(type I)(iter Iter(I), f func(I)) {
	for ptr := next(iter); ptr != nil; ptr = next(iter) {

func SliceIter(type T)(slice []T) Iter(T) {
	return func() *T {
		if len(slice) < 1 {
			return nil
		tmp := &slice[0]
		slice = slice[1:]
		return tmp

func main() {
			SliceIter([]string{"hello", "der"}),
		func(s string) { fmt.Println(s) },