Scala covariance with lower bound

Question

I am looking at scala in action book and it has this code

sealed abstract class Maybe[+A] {
    def isEmpty: Boolean
    def get: A
    def getOrElse[B >: A](         


        

Answer 1

In Scala you cannot have covariant types in method parameters.

This is because allowing covariant types in method parameters breaks type safety.

In order to have a covariant type you must use a bounded type:

getOrElse[B >: A](default: B): B

This says find some type, B, such that it is a superclass of A and that becomes the method return type.

In your case A is Int and you pass in a String. The only type B which is a superclass of both Int and String is Any.

In this case B becomes Any so the method returns Any.