go - Why add "()" after closure body in Golang?

Question

I'm reading The Go Programming Language Specifications and found myself not truly understand with "()" after closure body:

In Function literals:

func(ch chan int) { ch <- ACK }(replyChan)`

In Defer statements's example:

// f returns 1
func f() (result int) {
    defer func() {
        result++
    }() // why and how?
    return 0
}

I'm not clear about the reason to add & usage of "()" after closure body, hope someone can explain this clearly.

Answer 1

Waitting for answers