Minimal framework in Scala for collections with inheriting return type

Question

Suppose one wants to build a novel generic class, Novel[A]. This class will contain lots of useful methods--perhaps it is a type of collection--and therefore you w

Answer 1

I haven't thought this through fully, but it type checks:

object invariant {
  trait Novel[A] {
    type Repr[X] <: Novel[X]

    def reverse: Repr[A]

    def revrev: Repr[A]#Repr[A]
       = reverse.reverse
  }
  class ShortStory[A] extends Novel[A] {
    type Repr[X] = ShortStory[X]

    def reverse = this
  }

  val ss = new ShortStory[String]
  val ss2: ShortStory[String] = ss.revrev
}

object covariant {
  trait Novel[+A] {
    type Repr[X] <: Novel[_ <: X]

    def reverse: Repr[_ <: A]

    def revrev: Repr[_ <: A]#Repr[_ <: A] = reverse.reverse
  }

  class ShortStory[+A] extends Novel[A] {
    type Repr[X] = ShortStory[X]

    def reverse = this
  }

  val ss = new ShortStory[String]
  val ss2: ShortStory[String] = ss.revrev
}

EDIT

The co-variant version can be much nicer:

object covariant2 {
  trait Novel[+A] {
    type Repr[+X] <: Novel[X]

    def reverse: Repr[A]

    def revrev: Repr[A]#Repr[A] = reverse.reverse
  }

  class ShortStory[+A] extends Novel[A] {
    type Repr[+X] = ShortStory[X]

    def reverse = this
  }

  val ss = new ShortStory[String]
  val ss2: ShortStory[String] = ss.revrev
}