golang defer

The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes
原文

被defer的方法的参数，是在defer声明的时候就准备好的。如果方法带receiver，则receiver也是在defer声明的时候就准备好的。
例1:

for i := 0; i < 5; i++ {
    defer fmt.Printf("%d ", i)
}

output:
4
3
2
1
0

例2:

func trace(s string) string {
    fmt.Println("entering:", s)
    return s
}

func un(s string) {
    fmt.Println("leaving:", s)
}

func a() {
    defer un(trace("a"))
    fmt.Println("in a")
}

func b() {
    defer un(trace("b"))
    fmt.Println("in b")
    a()
}

func main() {
    b()
}

output:
entering: b
in b
entering: a
in a
leaving: a
leaving: b

当声明defer un(trace("b"))的时候，会先把defer un这个方法的参数准备好，也即会执行trace("b")来给被defer的un方法提供参数，所以我们看到entering: b最先打印。

effective_go对此的解释：
it's not block-based but function-based