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Unboxed newtype in Scala?
// Compile with -optimize to eliminate boxing/unboxing in specialized
// tag methods.
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 main(args: Array[String]): Unit = {
val id = tag[Checkin](10)
fetch[Checkin](id) // Compiles
fetch[User](id) // Does not compile
val ids = tag[User](1) :: tag[User](2) :: tag[User](3) :: Nil
val users : List[(Int @@ User)] = ids // Compiles
val checkins : List[Int @@ Checkin] = ids // Does not compile
}
@adamw
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adamw commented Nov 25, 2015

I need type tagging only (without shapeless or scalaz) in some projects so I published it as a one-class artifact, if anybody would find that useful as well: https://github.com/softwaremill/scala-common

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