Why doesn't the example compile, aka how does (co-, contra-, and in-) variance work?

Question

Following on from this question, can someone explain the following in Scala:

class Slot[+T] (var some: T) { 
   //  DOES NOT COMPILE 
   //  \"COVARIANT paramete         


        

Answer 1

@Daniel has explained it very well. But to explain it in short, if it was allowed:

  class Slot[+T](var some: T) {
    def get: T = some   
  }

  val slot: Slot[Dog] = new Slot[Dog](new Dog)   
  val slot2: Slot[Animal] = slot  //because of co-variance 
  slot2.some = new Animal   //legal as some is a var
  slot.get ??

slot.get will then throw an error at runtime as it was unsuccessful in converting an Animal to Dog (duh!).

In general mutability doesn't go well with co-variance and contra-variance. That is the reason why all Java collections are invariant.