Promises, async/await in Go with Channels

Coming from Javascript (Node.js) world and then start learning Go, I get used to using the simple async/await syntax. It’s just so easy for newbie to learn and quickly get familiar with.

On the contrary, Go doesn’t follow the pattern at all. Instead, it introduces [Goroutines](https://tour.golang.org/concurrency/1) and [Channels](https://tour.golang.org/concurrency/2).

However, it isn’t difficult to replicate the async/await pattern with Goroutines and Channels.

Single async/await

First, let’s experiment with a simple use case: await a result from an async function.

// JavaScript
// ---

const longRunningTask = async () => {
	// simulate a long running task
	sleep(3000);
	return Math.floor(Math.random() * Math.floor(100));
};

const r = await longRunningTask();
console.log(r);

// Go
// ---

package main

import (
    "fmt"
    "math/rand"
    "time"
)

func longRunningTask() <-chan int32 {
    r := make(chan int32)

    go func() {
        defer close(r)

        // simulate a long running task
        time.Sleep(time.Second * 3)
        r <- rand.Int31n(100)
    }()

    return r
}

func main() {
    r := <-longRunningTask()
    fmt.Println(r)
}

To declare an async function in Go:

• The return type is <-chan ReturnType.
• Within the function, create a channel by make(chan ReturnType) and return the created channel at the end of the function.
• Start an anonymous goroutine by go func() {…} and implement the function’s logic inside that anonymous function.
• Return the result by sending the value to the channel.
• At the beginning of the anonymous function, add defer close(r) to close the channel once done.

To await the result, simply read the value from the channel by v := <- fn().

Promise.all()

It’s very common that we start multiple async tasks then wait for all of them to finish and gather their results. Doing that is quite simple in both JavaScript and Go.

// JavaScript
// ---

const longRunningTask = async () => {
	// simulate a long running task
	sleep(3000);
	return Math.floor(Math.random() * Math.floor(100));
};

const [a, b, c] = await Promise.all(longRunningTask(), longRunningTask(), longRunningTask());
console.log(a, b, c);

// Go
// ---

package main

import (
    "fmt"
    "math/rand"
    "time"
)

func longRunningTask() <-chan int32 {
    r := make(chan int32)

    go func() {
        defer close(r)

        // simulate a long running task
        time.Sleep(time.Second * 3)
        r <- rand.Int31n(100)
    }()

    return r
}

func main() {
    aCh, bCh, cCh := longRunningTask(), longRunningTask(), longRunningTask()
    a, b, c := <-aCh, <-bCh, <-cCh

    fmt.Println(a, b, c)
}

We have to do it in 2 lines of code and introduce 3 more variables, but it’s clean and simple enough.

We can not do <-longRun(), <-longRun(), <-longRun(), which will execute longRun() one by one instead all in once.

Promise.race()

Sometimes, a piece of data can be received from several sources to avoid high latencies, or there’re cases that multiple results are generated but they’re equivalent and the only first response is consumed. This first-response-win pattern, therefore, is quite popular. Achieving that in both JavaScript and Go is very simple.

// JavaScript
// ---

const one = async () => {
	// simulate a long running task
	sleep(Math.floor(Math.random() * Math.floor(2000)));
	return 1;
};

const two = async () => {
	// simulate a long running task
	sleep(Math.floor(Math.random() * Math.floor(1000)));
	sleep(Math.floor(Math.random() * Math.floor(1000)));
	return 2;
};

const r = await Promise.race(one(), two());
console.log(r);

// Go
// ---

package main

import (
    "fmt"
    "math/rand"
    "time"
)

func one() <-chan int32 {
    r := make(chan int32)

    go func() {
        defer close(r)

        // simulate a long running task
        time.Sleep(time.Millisecond * time.Duration(rand.Int63n(2000)))
        r <- 1
    }()

    return r
}

func two() <-chan int32 {
    r := make(chan int32)

    go func() {
        defer close(r)

        // simulate a long running task
        time.Sleep(time.Millisecond * time.Duration(rand.Int63n(1000)))
        time.Sleep(time.Millisecond * time.Duration(rand.Int63n(1000)))
        r <- 2
    }()

    return r
}

func main() {
    var r int32
    select {
    case r = <-one():
    case r = <-two():
    }

    fmt.Println(r)
}

select-case is the pattern that Go designed specifically for racing channel operations. We can even do more stuff within each case, but we’re focusing only on the result so we just leave them all empty.

Promise.then() and Promise.catch()

Because Go’s error propagation model is very different from JavaScript, there aren’t clean ways to replicate Promise.then() and Promise.catch(). In Go, errors are returned as return values, there aren’t exceptions. Therefore, if your function can fail, you can consider changing your return <-chan ReturnType into <-chan ReturnAndErrorType, which is a [struct](https://tour.golang.org/moretypes/2) holding both the result and error.

Happy coding!