Question

I have the following Scala code.

import scala.actors.Actor

object Alice extends Actor {
  this.start
  def act{
    loop{
      react {
        case "Hello" => sender ! "Hi"
        case i:Int => sender ! 0
      }
    }
  }
}
object Test {
  def test = {
    (Alice !? (100, "Hello")) match {
      case i:Some[Int] => println ("Int received "+i)
      case s:Some[String] => println ("String received "+s)
      case _ =>
    }
    (Alice !? (100, 1)) match {
      case i:Some[Int] => println ("Int received "+i)
      case s:Some[String] => println ("String received "+s)
      case _ =>
    }
  }
}

After doing Test.test, I get the output:

scala> Test.test
Int received Some(Hi)
Int received Some(0)

I was expecting the output

String received Some(Hi)
Int received Some(0)

What is the explanation?

As a second question, I get unchecked warnings with the above as follows:

C:\scalac -unchecked a.scala
a.scala:17: warning: non variable type-argument Int in type pattern Some[Int] is unchecked since it is eliminated by erasure
      case i:Some[Int] => println ("Int received "+i)
             ^
a.scala:18: warning: non variable type-argument String in type pattern Some[String] is unchecked since it is eliminated by erasure
      case s:Some[String] => println ("String received "+s)
             ^
a.scala:22: warning: non variable type-argument Int in type pattern Some[Int] is unchecked since it is eliminated by erasure
      case i:Some[Int] => println ("Int received "+i)
             ^
a.scala:23: warning: non variable type-argument String in type pattern Some[String] is unchecked since it is eliminated by erasure
      case s:Some[String] => println ("String received "+s)
             ^
four warnings found

How can I avoid the warnings?

EDIT: Thanks for the suggestions. Daniel’s idea is nice but does not seem to work with generic types, as in the example below

def test[T] = (Alice !? (100, "Hello")) match { 
   case Some(i: Int) => println ("Int received "+i) 
   case Some(t: T) => println ("T received ") 
   case _ =>  
}

The following error warning is encountered: warning: abstract type T in type pattern T is unchecked since it is eliminated by erasure

Answer 1

This is due to type-erasure. The JVM does not know of any type parameter, except on arrays. Because of that, Scala code can’t check whether an Option is an Option[Int] or an Option[String] — that information has been erased.

You could fix your code this way, though:

object Test {
  def test = {
    (Alice !? (100, "Hello")) match {
      case Some(i: Int) => println ("Int received "+i)
      case Some(s: String) => println ("String received "+s)
      case _ =>
    }
    (Alice !? (100, 1)) match {
      case Some(i: Int) => println ("Int received "+i)
      case Some(s: String) => println ("String received "+s)
      case _ =>
    }
  }
}

This way you are not testing what the type of Option is, but what the type of its contents are — assuming there is any content. A None will fall through to the default case.