Unboxed newtype in Scala?

gistfile1.scala

class User
class Checkin
  
type Tagged[U] = { type Tag = U }
type @@[T, U] = T with Tagged[U] // Thanks to @retronym for suggesting this type alias
  
class Tagger[U] {
  def apply[T](t : T) : T @@ U = t.asInstanceOf[T @@ U]
}
def tag[U] = new Tagger[U]

// Manual specialization needed here ... specializing apply above doesn't help
def tag[U](i : Int) : Int @@ U = i.asInstanceOf[Int @@ U]
def tag[U](l : Long) : Long @@ U = l.asInstanceOf[Long @@ U]
def tag[U](d : Double) : Double @@ U = d.asInstanceOf[Double @@ U]

def fetch[A](id: Int @@ A): A = null.asInstanceOf[A]

def tag[U](arr: Array[Int]):Array[Int @@ U] = arr.asInstanceOf[Array[Int @@ U]]
 
tag[User](Array(3, 4, 5)).map(_.toString) // kaboom: java.lang.ClassCastException: [I cannot be cast to [Ljava.lang.Object

There are few problems like this related to varargs and specialization. The front end of the compiler makes some assumptions, like that the only type that will erase to Int is, well, Int.

I had half a fix cobbled together, but can't find the branch right now...

Sure, I figured. It's a pretty cool hack. Just have to be careful with it.

I actually used it to catch a lot of bugs in a refactoring (had to have a lot of indexed ints), but then I had to undo it all once it compiled because of the class cast.

Oh well.

Yeah, I would really love an officially supported version for opaque type aliases. I've got the same problem -- need a bunch of manual indexing in a montecarlo setup...

This seems to be working in 2.10.0, although it might be more through good luck than good management.