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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
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@retronym Yup, I like the look of those.

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soc commented Sep 22, 2011

Brilliant. I'm just researching how units of measurements can be implemented in Scala. Would it be possible to make this also support unit conversions, e. g. Length * Time == Speed and comparable things?

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@soc No I don't think it's up to that. The main problem is that eg. Int @@ T <: Int for any T, so the usual arithmetic operators are applicable (via simple subtype conformance of their arguments) and will have their ordinary result type (ie. Int). So, in particular, tagUser+tagCheckin compiles and has the value 36 : Int ... definitely not what you want for UoM.

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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:

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