Go频道上的Noob

Question

I'm trying to wrap my head around concurrency patterns in Go and was confused by this example from #69

package main

import "fmt"

func fibonacci(c, quit chan int) {
    x, y := 0, 1
    for {
        select {
        case c <- x:
            x, y = y, x+y
        case <-quit:
            fmt.Println("quit")
            return
        }
    }
}

func main() {
    c := make(chan int)
    quit := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            fmt.Println(<-c)
        }
        quit <- 0
    }()
    fibonacci(c, quit)
}

In particular, I don't see how

for i := 0; i < 10; i++ {
    fmt.Println(<-c)
 }

is supposed to work, since all we did was make the channel, and now we "receive" from it 10 times? I tried out other code where I create a channel and then try to receive from it right away and I always get an error, but this seems to work and I can't quite see how. Thanks for any help!

Answer 1

I’m giving you a shorter version of the code above, which I think should be easier to understand. (I explain the differences below.) Consider this:

// http://play.golang.org/p/5CrBSu4wxd
package main

import "fmt"

func fibonacci(c chan int) {
    x, y := 0, 1
    for {
        c <- x
        x, y = y, x+y
    }
}

func main() {
    c := make(chan int)
    go fibonacci(c)

    for i := 0; i < 10; i++ {
        fmt.Println(<-c)
    }
}

This is a more straightforward version because your main function is clearly just printing 10 values from the channel, and then exiting; and there is a background goroutine that is filling the channel as long as a new value is needed.

This alternate version drops the quit channel, because the background goroutine simply dies when main() finishes (no need to kill it explicitly in such a simple example).

Of course this version also kills the use of select{}, which is the topic of #69. But seeing how both versions accomplish the same thing, except killing of the background goroutine, can perhaps be a good aid in understanding what select is doing.

Note, in particular, that if fibonacci() had a time.Sleep() as its first statement, the for loop would hang for that much time, but would eventually work.

Hope this helps!

P.S.: Just realized this version is just a simpler version than #68, so I’m not sure how much it’ll help. Oops. :-)