做等价代换就清晰了，首先可以看成
func say(g Greeting, name string) {
fmt.Println(g(name))
}

func main() {
say(Greeting(english), "World")
}

然后
func say(g func(name string) string, name string) {
fmt.Println(g(name))
}

func main() {
say(english, "World")
}

接着
func say(name string) {
fmt.Println(english(name))
}

func main() {
say("World")
}

最后

func say(name string) {
fmt.Println("Hello, " + name)
}

func main() {
say("World")
}