Promise

Fast, lightweight, and lock-free promises for Go... the Go way

Features

• Fast and low memory overhead implementation
• Lock-free implementation
• Extensible implementation that can be used to provide typed promises
• Automatic panic recovering
• An API that focuses on Go's idioms, like:
  • Multi return parameters
  • Error being a value
  • The convention of returning errors as the last return parameter
  • The existence of panics and their difference from errors.

Overview

What's a Promise ?

A Promise represents some asynchronous work. It offers ways to get the eventual result of that asynchronous work.
Read more on its Wiki page.

Why in Go ?

Waiting for a goroutine, or returning result from a goroutine, specially with multi return parameters, can get tedious, as it usually requires using more than one variable(a channel with a struct type whose fields correspond to the return parameters, or a sync.WaitGroup with some variables).

But using a promise, it's now possible to wait for the corresponding goroutine to finish and/or access the returned result from it, along with other features, like knowing the status of that goroutine, building computation pipelines, and recovering from panics.

Installation

go get github.com/asmsh/promise

Note: some breaking changes maybe present between each minor version, until the release of v1.0.0, so always check the changelog before updating.

Prerequisites

Go 1.13+

Usage

Basic Usage:

• If you want to return some values from a goroutine:

p := promise.GoRes(func () promise.Res {
	/* do some work, asynchronously */
	return promise.Res{"go", "golang"} // return any values(as elements)
})

• If you don't want to return any values:

p := promise.Go(func () {
	/* do some work, asynchronously */
})

Then use any of p's methods to wait for it, access its result, or create a pipeline.

Examples

• The simplest use case, is to use it as a sync.WaitGroup with only one task:

package main

import "github.com/asmsh/promise"

func main() {
	p := promise.Go(func() {
		/* do some work, asynchronously */
	})

	/* do some other work */

	p.Wait() // wait for the async work to finish

	/* do some other work */
}

The following code is equivalent to the previous, but using only the standard library:

package main

import "sync"

func main() {
	wg := &sync.WaitGroup{}
	wg.Add(1)
	go func(wg *sync.WaitGroup) {
		/* do some work, asynchronously */
		wg.Done()
	}(wg)

	/* do some other work */

	wg.Wait() // wait for the async work to finish

	/* do some other work */
}

• A typical use case, is to use it to return some result from a goroutine:

package main

import (
	"net/http"

	"github.com/asmsh/promise"
)

func main() {
	p := promise.GoRes(func() promise.Res {
		resp, err := http.Get("https://golang.org/")
		return promise.Res{resp, err}
	}).Then(func(res promise.Res, ok bool) promise.Res {
		// uncomment the following and do something with resp..
		//resp := res[0].(*http.Response)
		return nil
	}).Catch(func(err error, res promise.Res, ok bool) promise.Res {
		// handle the error..
		return nil
	})

	/* do some other work */

	p.Wait() // wait for all the work to finish

	/* do some other work */
}

• It can be embedded or extended, to provide typed promises, like the asyncHttp, or the ctxProm examples.

• It provides JavaScript-like Resolver constructor, which can be used like in this example.