Why does a method with type parameter bound >: allow subtypes?

Question

Consider the following simple implementation of a stack in Scala:

abstract class Stack[+A] {
  def top: A
  def pop: Stack[A]
}

case object EmptyStack exten         


        

Answer 1

def push[B >: A](x: B): Stack[B] = ...

...

Why is this allowed? Doesn't the type bound >: mean that only supertypes should be allowed?

Only super-types are allowed as B, in your example Apple. But x: B is in a contravariant (input) position, and as such you can always pass a more specific value as an argument. That has nothing to do with the definition of B. What you will see however is that the inferred type for honeycrisp is Apple not Honeycrisp.

This is indeed perplexing, and I remember having wondered about this once. But if you go through the implications, it really preserves the type soundness. Of course as a consequence, from the point of view of the body of push, x is really Any without specific capabilities it could count on.

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Possibly related:

• Upper and Lower bound on scala type