重新声明的变量未使用

Question

This code compiles:

func (wc *WordCounter) Write(buf []byte) (int, error) {
    for adv, i := 0, 0; i < len(buf); i += adv {
        adv, _, _ = bufio.ScanWords(buf[i:], true)
        *wc++
    }
    return len(buf), nil
}

But the following does not compile. Notice the short declaration adv, token, _ := .. where I expected that adv would be redeclared (as opposed to be declared as a new var):

func (wc *WordCounter) Write(buf []byte) (int, error) {
    for adv, i := 0, 0; i < len(buf); i += adv {
        // error: adv declared and not used
        adv, token, _ := bufio.ScanWords(buf[i:], true) 
        fmt.Println(string(token))
        *wc++
    }
    return len(buf), nil
}

According to the Go spec:

a short variable declaration may redeclare variables provided they were originally declared earlier in the same block. [...] Redeclaration does not introduce a new variable; it just assigns a new value to the original.

I guess this means that the for statement is a block in and of itself, and that adv therefore is considered to be declared over again (as opposed to being redeclared) in the for body?

Answer 1

This is working as intended. Consider the following:

https://play.golang.org/p/cyJZgM5QYn

package main

import (
    "fmt"
)

func main() {
    for i := 0; i < 10; i++ {
        fmt.Printf("%p", &i)
        i := i
        fmt.Printf(" | %p
", &i)
    }
}

Variables you declare within the for loop header are defined for the entire for loop. The first i printed in the above has the same address on every single iteration. On the other hand, variables declared inside the loop itself are local only to that iteration of the loop! Note that the second i printed on each line has a unique address, as a new variable is being created on each iteration. This makes the lifetime of the two variables, and thus their scopes, different. Since they exist in separate scopes, the inner scope can (usually inadvertently) shadow the outer scope variable via the short-form declaration.