Scala: abstract comparison method in trait

Question

I have a trait with a size comparison function that I want to implement in a sublcass:

trait A {
    def isLessThan(that: A): Boolean
    ...
}

class SubOfA() extends A {
    ...
    override def isLessThan(that: SubOfA): Boolean = {
        this.size < that.size
    }
    ...
}

However, the method isn't a valid override because the argument type is SubOfA and not A.

---

I've also tried making the argument type this.type, but then when I am calling the method from an abstract setting I can't use an object of type A as the argument:

...
(foo: A, bar: A) => foo.isLessThan(bar)

This would expect type foo.type and not A which are the same but I don't think the compiler knows that yet.

---

Any ideas of how I could get something like this to work? I've looked all over the place to find an answer to this, but can't find anything. Maybe I don't know what is the right question to ask.

Answer 1

You can fix the first method with:

class SubOfA() extends A {

    override def isLessThan(that: A): Boolean = that match {
        case that : subOfA =>  this.size < that.size
        case _ => throw new UnsupportedOperationException("Wrong comparison") //or whatever behaviour deemed suitabe here
    }

}