duck-typing.go

package main

import "fmt"

type Quacker interface {
	Quack()
}

type Duck struct{}

func (d Duck) Quack() {
	fmt.Println("duck: quack quack")
}

type Cat struct{}

func (c Cat) Quack() {
	fmt.Println("cat: quack quack")
}

func main() {
	var q Quacker = new(Duck)
	q.Quack()
	q = new(Cat) // a cat *is* a Quacker
	q.Quack()
}