Captured Closure (for Loop Variable) in Go

Question

Shouldn't Go compiler capture for...range loop variables as a locally assigned closure variable?

Long Version:

This caused me some confusion in C# too and I was trying to understand it; that why it is fixed in C# 5.0 foreach (reason: the loop variable can not change inside the body of loop) and the reasoning for not fixing it in C# for loops (reason: the loop variable can change inside the body of loop).

Now (to me) for...range loops in Go seems pretty much like foreach loops in C#, but despite the fact that we can not alter those variables (like k and v in for k, v := range m { ... }); still we have to copy them to some local closures first, for them to behave as expected.

What is the reasoning behind this? (I suspect it's because Go treats any for loop the same way; but I'm not sure).

Here is some code to examine described behavior:

func main() {
    lab1() // captured closure is not what is expected
    fmt.Println(" ")

    lab2() // captured closure is not what is expected
    fmt.Println(" ")

    lab3() // captured closure behaves ok
    fmt.Println(" ")
}

func lab3() {
    m := make(map[int32]int32)
    var i int32
    for i = 1; i <= 10; i++ {
        m[i] = i
    }

    l := [](func() (int32, int32)){}
    for k, v := range m {
        kLocal, vLocal := k, v // (C) captures just the right values assigned to k and v
        l = append(l, func() (int32, int32) {
            return kLocal, vLocal
        })
    }

    for _, x := range l {
        k, v := x()
        fmt.Println(k, v)
    }
}

func lab2() {
    m := make(map[int32]int32)
    var i int32
    for i = 1; i <= 10; i++ {
        m[i] = i
    }

    l := [](func() (int32, int32)){}
    for k, v := range m {
        l = append(l, func() (int32, int32) {
            kLocal, vLocal := k, v // (B) captures just the last values assigned to k and v from the range
            return kLocal, vLocal
        })
    }

    for _, x := range l {
        k, v := x()
        fmt.Println(k, v)
    }
}

func lab1() {
    m := make(map[int32]int32)
    var i int32
    for i = 1; i <= 10; i++ {
        m[i] = i
    }

    l := [](func() (int32, int32)){}
    for k, v := range m {
        l = append(l, func() (int32, int32) { return k, v }) // (A) captures just the last values assigned to k and v from the range
    }

    for _, x := range l {
        k, v := x()
        fmt.Println(k, v)
    }
}

As it is shown in lab1, at the comment // (A) we get just the last values from the range; the output is like printing 9,9 ten times instead of showing expected result like 1,1, 2,2, ... (and of-course maps are not necessarily sorted in Go so we may see 3,3 ten times as the last pair of values; instead of 10,10 ten times as the last pair of values). The same goes for code at comment // (B) at lab2, which was expected because we are trying to capture outer variables inside the inner scope (I put this one too just to try that). In lab3 at code at comment // (C) everything works fine and you will see ten pairs of numbers there like 1,1, 2,2, ....

I was trying to use closure+function as a replacement for tuples in Go.

peterSO · Accepted Answer

Do you want the closure over the variable or the value? For example,

package main

import "fmt"

func VariableLoop() {
    f := make([]func(), 3)
    for i := 0; i < 3; i++ {
        // closure over variable i
        f[i] = func() {
            fmt.Println(i)
        }
    }
    fmt.Println("VariableLoop")
    for _, f := range f {
        f()
    }
}

func ValueLoop() {
    f := make([]func(), 3)
    for i := 0; i < 3; i++ {
        i := i
        // closure over value of i
        f[i] = func() {
            fmt.Println(i)
        }
    }
    fmt.Println("ValueLoop")
    for _, f := range f {
        f()
    }
}

func VariableRange() {
    f := make([]func(), 3)
    for i := range f {
        // closure over variable i
        f[i] = func() {
            fmt.Println(i)
        }
    }
    fmt.Println("VariableRange")
    for _, f := range f {
        f()
    }
}

func ValueRange() {
    f := make([]func(), 3)
    for i := range f {
        i := i
        // closure over value of i
        f[i] = func() {
            fmt.Println(i)
        }
    }
    fmt.Println("ValueRange")
    for _, f := range f {
        f()
    }
}

func main() {
    VariableLoop()
    ValueLoop()
    VariableRange()
    ValueRange()
}

Output:

VariableLoop
3
3
3
ValueLoop
0
1
2
VariableRange
2
2
2
ValueRange
0
1
2

References:

The Go Programming Language Specification

Function literals

Function literals are closures: they may refer to variables defined in a surrounding function. Those variables are then shared between the surrounding function and the function literal, and they survive as long as they are accessible.

Go FAQ: What happens with closures running as goroutines?

To bind the current value of v to each closure as it is launched, one must modify the inner loop to create a new variable each iteration. One way is to pass the variable as an argument to the closure.

Even easier is just to create a new variable, using a declaration style that may seem odd but works fine in Go.

Captured Closure (for Loop Variable) in Go

Answers (2)

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