Covariance and Contravariance in Scala

CovarianceContravariance.scala

// (c) 2015 Joern Dinkla, see http://www.dinkla.net/en/programming/scala-covariance-contravariance.html

abstract class Animal
class Cat extends Animal {
  def meow() : Unit = println("meow")
}
class Dog extends Animal

def mkPair(a: Animal, b: Animal) = (a,b)
val ps = mkPair(new Cat, new Dog)

val cs = List(new Cat, new Cat)
val ds = List(new Dog, new Dog)
val as = List(new Cat, new Dog)

class Getter[T](val value: T) {
  def get = value
}

val gc = new Getter[Cat](new Cat)
val ga = new Getter[Animal](new Cat)

gc.get 
gc.get : Animal
ga.get 
ga.get : Cat

class Setter[T] { 
  def set(v: T): Unit = { }
}

val sc = new Setter[Cat]
val sa = new Setter[Animal]

sa.set(new Cat : Animal)
sa.set(new Cat)
sc.set(new Cat)
sc.set(new Cat : Animal)