Question

I’m having trouble extending a base class that extends Ordered[Base]. My derived class can’t extend Ordered[Derived] so can’t be used as a key in a TreeMap. If I create a TreeMap[Base] and then just override compare in Derived that works but it’s not what I want. I would like to be able to have the derived class as a key. Is there a way around this?

case class A(x: Int) extends Ordered[A] {
  def compare(that: A) = x.compare(that.x)
}

// Won't compile
//  case class B(val y : Int) extends A(1) with Ordered[B] {
//    def compare(that: B) = x.compare(that.x) match {
//      case 0 => y.compare(that.y)
//      case res => res
//    }
//  }

// Compiles but can't be used to define a TreeMap key
case class B(y: Int) extends A(1) {
  override def compare(that: A) = that match {
    case b: B => x.compare(b.x) match {
      case 0 => y.compare(b.y)
      case res => res
    }
    case _: A => super.compare(that)
  }
}

def main(args: Array[String]) {
  TreeMap[B, Int]() // Won't compile
}

Edit

This discussion on the scala mailing list seems to be very relevant but it loses me a bit.

Answer 1

You can use a type conversion from B to Ordered[B]:

class OrderedB(me : B) extends Ordered[B]{
    def compare(that: B) = me compare that
}
collection.immutable.TreeMap.empty[B, Int](new OrderedB(_))

I think B has always to be a subtype of A which implies Order[A] whoes type A is invariant. It cannot define a second compare method to implement Order[B] with the same type errasure as the compare method from Ordered[A].

Alternatively you can define an implicit type versions from B to Ordered[B]:

implicit def orderedA2orderedB[B <: A with Ordered[A]](b : B) : Ordered[B] = b.asInstanceOf[Ordered[B]]
collection.immutable.TreeMap[B, Int]()

This should be valid. I’m not aware of a way to express this in the type system without casts.