使用命名返回类型时不使用变量

Question

Why can I do the following to define an iterator in Go:

func f() func() int {
    i := 1
    return func() int {
        i++
        return i
    }
}

but this will result in a variable not used error (i is not used)?

func f() func() int {
    i := 1
    return func() (i int) {
        i++
        return
    }
}

main function:

func main() {
    iter := f()
    fmt.Println(iter())
    fmt.Println(iter())
    fmt.Println(iter())
    fmt.Println(iter())
}

To me both versions do exactly the same: They use f as an iterator. f makes use of closures (more specifically i). The first version explicitly returns an incremented i and the second implicitly, by referring to it via a named return type.

To me both versions are the same, so why do I get an error?

Answer 1

In the second case, the return value (i int) shadows the outer declaration i := 1. So it's this first i that is unused. This means the second example isn't a closure at all.

From a scoping standpoint, your second example is the same as:

func f() func() int {
    i := 1
    return func(i int) int {
        i++
        return
    }
}

In this case, it should be clear that the inner i is distinct, and shadows the outer one.