Duck Typing in Scala

structural-types.scala

class Duck {
  def quack = println("The duck quacks")
  def walk = println("The duck walks")
}

class Dog {
  def quack = println("The dog quacks (barks)")
  def walk = println("The dog walks")
}

def testDuckTyping(animal: { def quack; def walk }) = {
  animal.quack
  animal.walk
}

scala> testDuckTyping(new Duck)
The duck quacks
The duck walks

scala> testDuckTyping(new Dog)
The dog quacks (barks)
The dog walks