Does Go Capture For-Loop Variables as Values or Variables?

Captured Closure (for-loop variables) in Go

Are for-loop variables in Go not captured as locally assigned closure variables?

No, captured for-loop variables in Go are by default captured as values, not variables. This behavior differs from languages like C#, which requires extra syntax to capture variables instead of values.

Example: Capturing Loop Variables as Values

package main

import "fmt"

func main() {
    a := 1
    b := func() { fmt.Println(a) } // b captures the value of a, not the variable itself
    a++
    b() // prints 1, not 2
}

In this example, the function b captures the value of a at the time it is defined, rather than retaining a reference to the variable itself. Consequently, any changes made to a after defining b will not be reflected in the value printed by b.

Example: Capturing Loop Variables as Variables

However, it is possible to capture loop variables as variables by using a closure syntax that creates a new scope for each iteration:

package main

import "fmt"

func main() {
    a := 1
    for a < 10 {
        b := func() { fmt.Println(a) } // b captures the variable a in a new scope
        a++
        b() // prints 2, 3, 4, ..., 10
    }
}

In this example, the func keyword is used to create a closure for each iteration of the loop. This technique ensures that each closure captures a unique reference to the loop variable, allowing us to observe the value of a as it changes within the loop.

Conclusion

Go capturaes for-loop variables as values by default, but it also provides a closure syntax that allows for-loop variables to be captured as variables. Understanding this distinction is crucial for avoiding confusion and writing correct and efficient Go code.

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