experimental promises in go1.18 with generics


async go

a prototype of "promises" in go1.18.

note: this is just an experiment used to test alternate patterns for dealing with asynchronous code in go. i would not recommend adopting this pattern blindly (especially in production environments) until there is a broader consensus in the go community about this. it might turn out that through some hands-on experience that go's native channels/goroutines are a good enough abstraction and there are no gains to be made from building on top of them.


should be just a regular package:

go get -u -v code.nkcmr.net/[email protected]


promises abstract away a lot of details about how asynchronous work is handled. so if you need for something to be async, simply us a promise:

import (

type MyData struct {/* ... */}

func AsyncFetchData(ctx context.Context, dataID int64) async.Promise[MyData] {
    return async.NewPromise(func() (MyData, error) {
        /* ... */
        return myDataFromRemoteServer, nil

func DealWithData(ctx context.Context) {
    myDataPromise := AsyncFetchData(ctx, 451)
    // do other stuff while operation is not settled
    // once your ready to wait for data:
    myData, err := myDataPromise.Await(ctx)
    if err != nil {/* ... */}
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  • Data race in Settled

    Data race in Settled

    	// Settled indicates if a call to Await will cause a blocking behavior, or
    	// if the result will be immediately returned.
    	Settled() bool

    This method

    func (s *syncPromise[T]) Settled() bool {
    	return atomic.LoadInt32(&s.settled) == 1

    will return true once this code has executed

    func (s *syncPromise[T]) resolve(v T) {
    	if atomic.CompareAndSwapInt32(&s.settled, 0, 1) {

    but potentially before this code has executed

    		s.result = result[T]{value: v}

    which means that this code

    func (s *syncPromise[T]) Await(ctx context.Context) (T, error) {
    	select {
    	case <-ctx.Done():
    		var zerov T
    		return zerov, ctx.Err()
    	case <-s.done:
    		return s.result.value, s.result.err

    would block, violating the contract of the interface.

    I think you can solve this in a number of ways... maybe this is one?

    type syncPromise[T any] struct {
    	val  T
    	err  error
    	done chan struct{}
    func NewPromise[T any](fn func() (T, error)) Promise[T] {
    	p := &syncPromise[T]{done: make(chan struct{})}
    	go func() { p.val, p.err = fn(); close(p.done) }()
    	return p
    func (p *syncPromise[T]) Settled() bool {
    	select {
    	case <-p.done:
    		return true
    		return false
    func (p *syncPromise[T]) Await(ctx context.Context) (T, error) {
    	select {
    	case <-p.done:
    		return p.val, p.err
    	case <-ctx.Done():
    		var zero T
    		return zero, ctx.Err()

    edit: golfed it a bit to remove the need for result[T]

    opened by peterbourgon 1
nick comer
programmer, tinkerer, learner.
nick comer
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