25.go

package main

import "fmt"

type Hello struct {}

func (h Hello) String() string {
  return "Hello"
}

type World struct {}

func (w *World) String() string {
  return "world"
}

type Message struct {
  X fmt.Stringer
  Y fmt.Stringer
}

func (v Message) IsGreeting() (ok bool) {
  if _, ok = v.X.(*Hello); !ok {
    _, ok = v.Y.(*Hello)
  }
  return 
}

func main() {
  m := &Message{}
  fmt.Println(m.IsGreeting())
  m.X = Hello{}
  fmt.Println(m.IsGreeting())
  m.X = new(Hello)
  fmt.Println(m.IsGreeting())
  m.X = World{}
}