Catching return values from goroutines

The below code gives compilation error saying 'unexpected go':

x := go doSomething(arg)

func doSomething(arg int) int{
    ...
    return my_int_value
}

I know, I can fetch the return value if call the function normally, without using goroutine. Or I can use channels etc.

My question is why is it not possible to fetch a return value like this from a goroutine.

The strict answer is that you can do that. It's just probably not a good idea. Here's code that would do that:

var x int
go func() {
    x = doSomething()
}()

This will spawn off a new goroutine which will calculate doSomething() and then assign the result to x. The problem is: how are you going to use x from the original goroutine? You probably want to make sure the spawned goroutine is done with it so that you don't have a race condition. But if you want to do that, you'll need a way to communicate with the goroutine, and if you've got a way to do that, why not just use it to send the value back?

Why is it not possible to fetch a return value from a goroutine assigning it to a variable?

Run goroutine (asynchronously) and fetch return value from function are essentially contradictory actions. When you say go you mean "do it asynchronously" or even simpler: "Go on! Don't wait for the function execution be finished". But when you assign function return value to a variable you are expecting to have this value within the variable. So when you do that x := go doSomething(arg) you are saying: "Go on, don't wait for the function! Wait-wait-wait! I need a returned value be accessible in x var right in the next line below!"

Channels

The most natural way to fetch a value from a goroutine is channels. Channels are the pipes that connect concurrent goroutines. You can send values into channels from one goroutine and receive those values into another goroutine or in a synchronous function. You could easily obtain a value from a goroutine not breaking concurrency using select:

func main() {

    c1 := make(chan string)
    c2 := make(chan string)

    go func() {
        time.Sleep(time.Second * 1)
        c1 <- "one"
    }()
    go func() {
        time.Sleep(time.Second * 2)
        c2 <- "two"
    }()

    for i := 0; i < 2; i++ {
        // Await both of these values
        // simultaneously, printing each one as it arrives.
        select {
        case msg1 := <-c1:
            fmt.Println("received", msg1)
        case msg2 := <-c2:
            fmt.Println("received", msg2)
        } 
    }
}

The example is taken from Go By Example

CSP & message-passing

Go is largerly based on CSP theory. The naive description from above could be precisely outlined in terms of CSP (although I believe it is out of scope of the question). I strongly recommend to familiarize yourself with CSP theory at least because it is RAD. These short quotations give a direction of thinking:

As its name suggests, CSP allows the description of systems in terms of component processes that operate independently, and interact with each other solely through message-passing communication.

In computer science, message passing sends a message to a process and relies on the process and the supporting infrastructure to select and invoke the actual code to run. Message passing differs from conventional programming where a process, subroutine, or function is directly invoked by name.

The idea of the go keyword is that you run the doSomething function asynchronously, and continue the current goroutine without waiting for the result, kind of like executing a command in a Bash shell with an '&' after it. If you want to do

x := doSomething(arg)
// Now do something with x

then you need the current goroutine to block until doSomething finishes. So why not just call doSomething in the current goroutine? There are other options (like, doSomething could post a result to a channel, which the current goroutine receives values from) but simply calling doSomething and assigning the result to a variable is obviously simpler.

It's a design choice by Go creators. There's a whole lot of abstractions/APIs to represent the value of async I/O operations - promise, future, async/await, callback, observable, etc. These abstractions/APIs are inherently tied to the unit of scheduling - coroutines - and these abstractions/APIs dictate how coroutines (or more precisely the return value of async I/O represented by them) can be composed.

Go chose message passing (aka channels) as the abstraction/API to represent the return value of async I/O operations. And of course, goroutines and channels give you a composable tool to implement async I/O operations.

来源：https://stackoverflow.com/questions/20945069/catching-return-values-from-goroutines

标签

concurrency

goroutine