Use Go Channels as Promises and Async/Await
Tags: programming, go, javascript, codenewbie
Published: 2020-11-25 11:49:33

If you’ve ever programmed with JavaScript, you definitely know about `Promise` and `async`/`await`. C#, Java, Python, and some other programming languages apply the same pattern using different names such as `Task` or `Future`.

On the contrary, Go doesn’t follow the pattern at all. Instead, it introduces `goroutines` and `channels`. However, it isn’t difficult to replicate the pattern with `goroutines` and `channels`.

---

# Single async/await

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

```js
// JavaScript
// ---

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

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

```go
// Go
// ---

package main

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

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

	go func() {
		defer close(r)
		
		// simulate a workload
		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.

```js
// JavaScript
// ---

const longRunningTask = async () => {
    // simulate a workload
    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
// Go
// ---

package main

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

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

	go func() {
		defer close(r)
		
		// simulate a workload
		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 `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.

```js
// JavaScript
// ---

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

const two = async () => {
    // simulate a workload
    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
// Go
// ---

package main

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

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

	go func() {
		defer close(r)

		// simulate a workload
		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 workload
		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` holding both the result and error.