Example of a very basic promises implementation in Go

promises.go

package main

import (
	"errors"
	"log"
	"sync"
	"time"
)

// Promise ...
type Promise struct {
	fn  func() error
	err error
	wg  sync.WaitGroup
}

// Then ...
func (p *Promise) Then(cb func()) *Promise {
	p.wg.Add(1)

	go func(p *Promise) {
		if err := p.fn(); err != nil {
			p.err = err
		}

		p.wg.Done()
	}(p)

	p.wg.Wait()

	if p.err == nil {
		cb()
	}

	return p
}

// Err ...
func (p *Promise) Error(cb func(err error)) *Promise {
	if p.err != nil {
		cb(p.err)
	}

	return p
}

// NewPromise ...
func NewPromise(fn func() error) *Promise {
	return &Promise{fn: fn}
}

func someAsyncFunction() error {
	log.Println("someAsyncFunction()")
	time.Sleep(time.Second * 3)
	return nil
}

func someAsyncFunctionWithError() error {
	log.Println("someAsyncFunctionWithError()")
	return errors.New("some very bad error")
}

func main() {
	promiseThatResolves := NewPromise(someAsyncFunction)

	promiseThatResolves.
		Then(func() {
			log.Println("promise.Then()")
		}).
		Error(func(err error) {
			log.Printf("promise.Error(%v)", err.Error())
		})

	promiseThatErrors := NewPromise(someAsyncFunctionWithError)

	promiseThatErrors.
		Then(func() {
			log.Println("promise.Then()")
		}).
		Error(func(err error) {
			log.Fatalf("promise.Error(%v)", err.Error())
		})
}