Extract class from Option[T] when in None clause - scala

Question

Assuming you have the following code
trait T {
}
case class First(int:Int) extends T
case class Second(int:Int) extends T
val a:Option[T] = Option(First(3))
val b:Option[Second] = None
def test[A](a:Option[A])(implicit manifest:Manifest[Option[A]]) = {
a match {
case Some(z) => println("we have a value here")
case None => a match {
case t:Option[First] => println("instance of first class")
case s:Option[Second] => println("instance of second class")
}
}
}
test(b)
How would you extract what type the enclosing A is if the option happens to be a None. I have attempted to do various combinations of manifests, but none of them seem to work, each time it complains about the types being eliminated with erasure, i.e.
non-variable type argument First in type pattern Option[First] is unchecked since it is eliminated by erasure

Answer 1

You can use a type tag, a modern replacement for Manifest:
import scala.reflect.runtime.universe._
trait T
case class First(int:Int) extends T
case class Second(int:Int) extends T
def test[A: TypeTag](a: Option[A]) = {
a match {
case Some(_) => println("we have a value here")
case None => typeOf[A] match {
case t if t =:= typeOf[First] => println("instance of First")
case t if t =:= typeOf[Second] => println("instance of Second")
case t => println(s"unexpected type $t")
}
}
}
Example
val a = Option(First(3)) // we have a value here
val b: Option[First] = None // instance of First
val c: Option[Second] = None // instance of Second
val d: Option[Int] = None // unexpected type Int
By the way, you are interested in the type of A (which is being eliminated by erasure), so even with manifests you need one on A and not on Option[A].