CovarianceContraVariance.scala

  abstract class Animal

  class Dog(name:String) extends Animal

  // if DOG <:Animal does List[Dog]  <: List[Animal]

  val laika = new Dog("Laika")
  val lassie = new Dog("lassie")
  val hachi = new Dog("hachi")

  val myDogs = List(lassie,hachi,laika)
  val myAnimals: List[Animal] = List(lassie, hachi, laika)

  class CovarList[+T]
  val covarDogs:CovarList[Animal] = new CovarList[Dog]

  class InvarList[T]
  val invarAnimals:InvarList[Animal] = new InvarList[Animal]
  val invarDogs:InvarList[Dog] = new InvarList[Dog]

  class ContraList[-T]
  val contraDogs:ContraList[Dog] = new ContraList[Animal]