How to specify that a return type of a method is a subtype of the return type of the superclass method

[Ken McDonald]

It's been a while since I've used Scala, and am wondering if I am making things too difficult. Here's my little bit of code:

abstract class Datum(val model: Model) {

   protected def copy(model: Model): [T <: Datum];

}

class Datum1(val model: Model)

  def copy(model: Model): Datum1 = new Datum1(model)

}

The idea is that there will be many variants of DatumN, each a subtype of Datum, and each will have a "copy" method that returns an object of type DatumN. I want to be able to specify in Datum.copy that all subclasses of Datum have a copy method that returns a subtype of Datum. However, the above fails to parse at [T <: Datum]. Suggestions?

Thanks,

Ken

[Derek Williams]

Just 'Datum' should work. Subclasses can refine the type.

[Ken McDonald]

Yep, just a case me making my life overly complex. Thanks.

Ken

[Josh Suereth]

OR WAS IT?

The example you have was simple enough, don't do anything crazy.  However, when caking or virtual classing, try this out:

trait Foo {

  type T  <: BaseT

  trait BaseT { ..}

  def foo: T

}

object Bar extends Foo {

  type T = MyT

  class MyT extends BaseT { ... }

  def foo = new MyT

}

You only have to go complex if you're doing something complex, like trait-ecosystems and ecosystem inheritance.

- Josh