golang: evaluate variable in callback declaration

Question

I am trying to define a callback in golang:

package main

func main() {
    x, y := "old x ", "old y"
    callback         := func() { print("callback: "        , x        , y       , "\n") }
    callback_bound   := func() { print("callback_bound: "  , x        , y       , "\n") }
    callback_hacked  := func() { print("callback_hacked: " , "old x " , "old y" , "\n") } 

    x, y = "new x ", "new y"

    callback()
    callback_bound()
    callback_hacked()
}

The output is:

callback: new x new y
callback_bound: new x new y
callback_hacked: old x old y

The basic case works, but it leaves the variables unbound, i.e. the values at call-time are used. No doubt, this is useful in some cases, but how do I declare callback_bound so that the values at declaration-time are used and the output becomes:

callback: new x new y
callback_bound: old x old y
callback_hacked: old x old y

peterSO · Accepted Answer · 2014-03-03 02:56:17Z

2

For example,

package main

func callbackXY(x, y string) func() {
    return func() { print("callbackXY: ", x, y, "\n") }
}

func main() {
    x, y := "old x ", "old y"
    callback := callbackXY(x, y)
    x, y = "new x ", "new y"
    callback()
}

Output:

callbackXY: old x old y

Or

package main

func main() {
    x, y := "old x ", "old y"
    callback := func() {}
    {
        x, y := x, y
        callback = func() { print("callbackXY: ", x, y, "\n") }
    }
    x, y = "new x ", "new y"
    callback()
}

Output:

callbackXY: old x old y

edited Mar 3, 2014 at 2:56

answered Mar 3, 2014 at 2:32

peterSO

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Comments

Volker · Accepted Answer · 2014-03-03 00:50:10Z

-1

bound := func(xx,yy int) { return func(){fmt.Println(xx,yy)} }(x,y)

Untested.

Please have a look (or two) at http://golang.org/doc/effective_go.html

answered Mar 3, 2014 at 0:50

Volker

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