Scala type eraser

case class Thing[T](value: T)
def processThing(thing: Thing[_]) = {
  thing match {
    case Thing(value: Int) => "Thing of int"
    case Thing(value: String) => "Thing of string"
    case _ => "Thing of something else"
  }
}

println(processThing(Thing(1)))
println(processThing(Thing("hello")))

Above outputs Thing of int and Thing of string. My question is why the type info is still available in runtime if type eraser kicks in?

This is not when type erasure kicks in, if you try this:

def processThing(thing: Thing[_]) = {
  thing match {
    case _: Thing[Int] => "Thing of int"
    case _: Thing[String] => "Thing of string"
    case _ => "Thing of something else"
  }
}

println(processThing(Thing("hello")))

You will get Thing of int. With case Thing(value: Int) you're basically doing a pattern matching with type assertion, I think it would be something like this:

def processThing(thing: Thing[_]) = {
  thing match {
    case Thing(value) if value.isInstanceOf[Int] => "Thing of int"
    case Thing(value) if value.isInstanceOf[String] => "Thing of string"
    case _ => "Thing of something else"
  }
}