Type variance in Scala

typeVary.scala

package org.halfcup.scala.learn.vary

class Toy{
	override def toString():String = {
		"Toy"
	}
}

class Ball extends Toy{
	override def toString():String = {
		"Ball"
	}
}
object SmallBall extends Ball{
	override def toString():String = {
		"SmallBall"
	}
}
object RedBall extends Ball{
	override def toString():String = {
		"RedBall"
	}
}
object Stick extends Toy
object Bone extends Toy


class Pet[-A,+T>:A](toy: A) {
	override def toString():String = {
		"Pet bring me "+toy
	}
	def bringMeSome():T = {
		toy
	}
}

class Dog[-A,+T>:A](toy: A) extends Pet[A,T](toy){
	override def toString():String = {
		"Dog bring me "+toy
	}
}


object Test {
  def main(args : Array[String]) : Unit = {
	  val p1 = new Pet[Toy,Toy](RedBall)
	  val p2 = new Dog[Ball,Toy](new Ball)
	  val p3 = new Dog[Toy,Toy](SmallBall)
	  val p5 = new Dog[Ball,Ball](SmallBall)
	  
	  val asdasdsad = new Pet[Ball,Toy](SmallBall)
//	  val asdasdsad2 = new Pet[Toy,Ball](SmallBall) Can't do that
	  
//	  val p4 = new Dog[Ball,Toy](Stick) Can't do that
	  val pets = List(p1,p2,p3,new Pet[Ball,Ball](RedBall))
	  println(pets)
	  pets.map(x => println(x.bringMeSome))
	  
	  var xP = p2//Dog[Ball,Toy]
	  xP = p3//Dog[Toy,Toy]
	  xP = new Dog[Ball,Ball](RedBall)
	  
	  var iP = p3//Dog[Toy,Toy]
//	  iP = p2 Can't do that  Dog[Ball,Toy]
	  
	  var tP = new Dog[Ball,Toy](new Ball) 
	  tP = new Dog[Ball,Ball](SmallBall)
	  
	  var sP = new Dog[Ball,Ball](SmallBall)
//	  sP = new Dog[Ball,Toy](RedBall) Can't do that
	  
	  val toysTrunk = List[Toy](p2.bringMeSome())
	  println(toysTrunk)
	  
  }
}