Scala - Generic type does not type check

Question

I am trying to understand my mistake in implementing generics. I have a trait which defines a method that returns a reference with an upper type bound of the trait. How ever a var reference to an implementation of the trait fails to type check with Expression of type Capability[Class] doesn't conform to expected type Capability[Trait]

Here is the code:

trait IAITask {
  def taskTypeReference[T >: IAITask]: Capability[T]
}

object Tasks {
   var Walk: Capability[Walk] = _
}

class Walk extends IAITask {
  override def taskTypeReference[T >: IAITask]: Capability[IAITask] = Tasks.Walk //This line does not type check
}

This is a Java class `public class Capability` @SilvioMayolo — Pieter van Lill, Feb 01 '19 at 22:27
This would compile if `Capability` were covariant on its type parameter, but [Java generics are invariant](https://stackoverflow.com/q/18666710/4993128). — jwvh, Feb 01 '19 at 22:47

score 1 · Answer 1 · answered Feb 02 '19 at 01:49

If it's a Java interface, then you might want to try to imitate Java's use-site variance by wildcards:

trait Capability[A]

trait IAITask {
  def taskTypeReference: Capability[_ <: IAITask]
}

object Tasks {
   var Walk: Capability[Walk] = _
}

class Walk extends IAITask {
  override def taskTypeReference: Capability[_ <: IAITask] = Tasks.Walk
}

Scala - Generic type does not type check

1 Answers1