How Do Go Closures Handle For Loop Variables and Prevent Unexpected Behavior?

Capturing Closure (for Loop Variable) in Go

In Go, for loops are not like foreach loops in C#, where loop variables cannot change within the loop body. In Go, loop variables can be modified, prompting the need to handle captured closure behavior differently.

Differences in Capturing

To illustrate, let's examine three code snippets:

Lab1:

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

Lab2:

l := [](func() (int32, int32)){}
for k, v := range m {
    // Closure captures the outer variables k and v (which are modified in the loop)
    l = append(l, func() (int32, int32) { return k, v })
}

Lab3:

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

Capturing Variable or Value

In Lab1 and Lab2, the closures capture the last values assigned to loop variables within the loop body. This can lead to incorrect results if loop variables are modified within the loop.

Lab3 demonstrates the correct approach. It captures the values assigned to loop variables specifically within each iteration using local variables. This behavior is consistent with closures in Go, where captured variables are by default referentially transparent.

Conclusion

In Go, closures over for loop variables capture values rather than references. To capture values that may change during the loop, it is necessary to create local variables within each iteration that assign the desired values to the closure. This approach ensures the closure behavior meets expectations.

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