Type pattern eliminated by erasure in my Akka actor base class

Question

abstract class BaseActor[U: TaskUnit](master: ActorRef) extends Actor with ActorLogging {

    def receive = {
        ..
        ..

        case taskUnit: U => ...

    }

    def performTask(task: U): Future[_]
}

My TaskUnit looks like:

trait TaskUnit {
    def status(status: String): TaskResponse[_] 
}

I am getting 1 error and 1 warning which I need some advice on.

I'm getting an warning saying TaskUnit does not take type parameters (this error is on the abstract class definition line)
Warning saying the abstract type pattern U is unchecked since it is eliminated by erasure (this is for the case taskUnit: U line)

What is the problem with my current design?

L.Lampart L.Lampart · Accepted Answer · 2017-01-27T21:56:04

Well the reason you receive error is quite simple. You are writing:

abstract class X[U: Y]

But you probably meant something like this:

abstract class X[U <: Y]

Difference between those two definitions is quite significant. The second one signals, that type parameter U should be subtype of Y (including Y). But the first one is something different. It is translated to the following:

abstract class X[U](implicit ev: Y[U])

This syntax was introduced in order to make usage of type classes in scala with less boilerplate. I think you can see the source of your error right now. Compiler expects TaskUnit to be Type with exactly one type parameter:

TaskUnit[T] {
  ...
}

But it obviously don't have such shape.

Type pattern eliminated by erasure in my Akka actor base class

1 Answers