Scala 2.10 TypeTag usage

Question

I\'m digging new scala reflection api and can\'t figure out why the following snippet doesn\'t work as expected. Given hierarchy (tried to simplify as much as I can):

Answer 1

So I think the key problem here is that you need to match against the type of msg, but its compile-time type is Any (from the PartialFunction declaration). Essentially, you want a different TypeTag for each element in your List[Any]. But since they all have compile-time type of Any by virtue of all being put into the same list, you're not going to get a TypeTag that's any more specific than that.

I think what you probably want to do is use ClassTag instead of TypeTag:

trait TF[A] {
  implicit def t: ClassTag[A]

  def f: PartialFunction[Any, A] = {
    case msg: A => msg
  }
}

class TFilter[T: ClassTag] extends TF[T] {
  def t = classTag[T]
}

case class Foo(x: Int)

val messages = Seq(1, "hello", Foo(1), List(1), List("a"))
messages collect new TFilter[Foo].f // produces List(Foo(1))

As Ajran points out, just like the Manifest version, you'll have to be aware of all the limitations of runtime types including erasure and boxing issues:

messages collect new TFilter[List[Int]].f // produces List(List(1), List("a"))
messages collect new TFilter[Int].f // produces List()
messages collect new TFilter[java.lang.Integer].f // produces List(1)

There are some suggestions about how to make TypeTag more useful for pattern matching (e.g. SI-6517), but I think those will only help when you're matching against an object with an useful TypeTag, not an object with compile-time type of Any.