Covariant Typeparameter in scala needs to be invariant in java interface

Question

hi there

I've got a trait that looks like this (some further information can be found at this related question by myself although I don't think, it's needed for this question)

trait Extractor[-A,+B] {
  def extract(d:A):B
  //lots of other things
}

To use this in an existing java framework I would like this Extractor to either have a function that returns a Comparator[B] (being java.util.Comparator) or even better extend Comparator[A]. Now that poses a problem because Comparators type parameter is ought to be invariant, while A is contravariant and B is covariant.

So I get errors like this:

scala> import java.util.Comparator
import java.util.Comparator

scala> trait Extractor[-A,+B] extends Comparator[A]
<console>:6: error: contravariant type A occurs in invariant position in type [-A,+B]java.lang.Object with java.util.Comparator[A] of trait Extractor
       trait Extractor[-A,+B] extends Comparator[A]
             ^


scala> trait Extractor[-A, +B] {                 
     |   def comp:Comparator[B]
     | }
<console>:7: error: covariant type B occurs in invariant position in type => java.util.Comparator[B] of method comp
         def comp:Comparator[B]
             ^

Do you see any way out of this or is this just one of those cases where "using java generics in scala hurts"?

Answer 1

With help of type-bounds it's possible to do the following:

scala> trait Extractor[-A, +B] {
     | def comp:Comparator[_ <: B]
     | }
defined trait Extractor

Answer 2

You can make Extractor[A,B] extend Comparator[A] by using the @uncheckedVariance annotation.

scala> import scala.annotation.unchecked.uncheckedVariance
import scala.annotation.unchecked.uncheckedVariance

scala> trait Extractor[-A,+B] extends java.util.Comparator[A @uncheckedVariance]
defined trait Extractor

@uncheckedVariance is safe here because Comparator could have been defined as Comparator[-T]. There was a discussion around making Ordering covariant for Scala 2.8 using this annotation.

Edit See this question for more about @uncheckedVariance.