JS-style promises in Golang

promises.go

package promises

import (
	"fmt"
	"time"
)

/*
Credits to github.com/Workiva/go-datastructures/blob/master/futures/futures.go
whose interface/API and tests for Futures I generously borrowed, but I restricted my version
to using channels for mutual exclusion and waiting.

Go race detector did not detect any data races for my version

Performance between my version and workiva's version is almost the same

goos: darwin
goarch: amd64
pkg: ...
BenchmarkFutureWorkiva-4          413845              2811 ns/op
BenchmarkMyVersion-4              341008              2947 ns/op
PASS
ok      ...   3.200s
*/

type Promise interface{
  	GetResult() (interface{}, error)
	HasResult() bool
}

type promise struct {
	item      interface{}
	err       error
	completed chan struct{}
}

// GetResult waits until results is available or timeout occurs
func (p *promise) GetResult() (interface{}, error) {
	<-p.completed
	return p.item, p.err
}

// HasResult returns true if promise completed
func (p *promise) HasResult() bool {
	select {
	case <-p.completed:
		return true
	default:
		return false
	}
}

func (p *promise)await(ch <-chan interface{}, timeout time.Duration, sigCh chan<- struct{}) {
  	close(sigCh)
	t := time.NewTimer(timeout)
	select {
	case item := <-ch:
		p.item = item
		t.Stop()
	case <-t.C:
		p.err = fmt.Errorf(`timeout after %f seconds`, timeout.Seconds())
	}
	close(p.completed)
}

func NewPromise(result <-chan interface{}, timeout time.Duration) Promise {
	p := &promise{
		completed: make(chan struct{}),
	}
	sigCh := make(chan struct{})
	go p.await(result, timeout, sigCh)
	<-sigCh
	return p
}