Does a channel return two values?

channel, go

Solution

The boolean variable `ok` returned by a receive operator indicates whether the received value was sent on the channel (true) or is a zero value returned because the channel is closed and empty (false).

The `for` loop terminates when some other part of the Go program closes the `fromServer` or the `fromUser` channel. In that case one of the case statements will set `ok` to true. So if the user closes the connection or the remote server closes the connection, the program will terminate.

http://play.golang.org/p/4fJDkgaa9O:

package main

import "runtime"

func onServer(i int) { println("S:", i) }
func onUser(i int)   { println("U:", i) }

func main() {
    fromServer, fromUser := make(chan int),make(chan int)
    var serverData, userInput int
    var ok bool

    go func() {
        fromServer <- 1
        fromUser <- 1
        close(fromServer)
        runtime.Gosched()
        fromUser <- 2
        close(fromUser)
    }()

    isRunning := true
    for isRunning {
        select {
            case serverData, ok = <-fromServer:
                if ok {
                    onServer(serverData)
                } else {
                    isRunning = false
                }

            case userInput, ok = <-fromUser:
                if ok {
                    onUser(userInput)
                } else {
                    isRunning = false
                }
            }
        }
        println("end")
}

Problem

Original source