How can I override a method with a dependent return type?

Question

I'm having trouble in Scala 2.9.2 implementing a method which declares a dependent return type. The following code

object DependentTypesQuestion {
  def ??? = throw new UnsupportedOperationException
  trait X {
    trait Y
  }
  trait Z {
    def z(x: X): x.Y
  }
  object Z extends Z {
    override def z(x: X): x.Y = ???
  }
}

under 2.9.2 produces the following error message during compilation:

overriding method z in trait Z of type (x: DependentTypesQuestion.X)x.Y;  method z has incompatible type

In 2.10.0-M4 the problem seems to have been fixed, but unfortunately my project is tied to 2.9 for now.

Is it possible to work around this issue in 2.9.2?

(Alternatively, is there any prospect of a 2.9.3 which would include the backported fix from 2.10?)

score 3 · Accepted Answer · answered Aug 30 '12 at 11:49

3

If you're really stuck with 2.9.x then the following might be a workaround for you,

object DependentTypesQuestion {
  def ??? = throw new UnsupportedOperationException
  trait X {
    trait Y
  }
  trait Z[D[_ <: X with Singleton]] {
    def z[T <: X with Singleton](x: T): D[T]
  }

  type Dep[T <: X with Singleton] = T#Y

  object Z extends Z[Dep] {
    override def z[T <: X with Singleton](x: T): x.Y = ???
  }
}

answered Aug 30 '12 at 11:49

Miles Sabin

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I see. Unfortunately in my real case the object Z was an internal implementation, and outside consumers only see the trait Z, so this doesn't achieve the typing I'm really after. – Scott Morrison Aug 30 '12 at 15:36
Time to attempt escaping 2.9, I guess. – Scott Morrison Aug 30 '12 at 15:37

How can I override a method with a dependent return type?

1 Answers1