scala tagged types

tagged.scala

import com.twitter.bijection.Bijection

package object tagging {
 sealed trait Tagged[+V, +T]
 type @@[+V, +T] = V with Tagged[V, T]

 object Tagged {
   implicit class Untaggable[V, T](val tagged: V @@ T) extends AnyVal {
     @inline def untag: V = tagged
   }
   implicit class UntaggableM[M[+_], V, T](val tagged: M[V @@ T]) extends AnyVal {
     @inline def untagM: M[V] = tagged
   }
 }


 implicit class Taggable[V](val value: V) extends AnyVal {
   @inline def tag[T] = value.asInstanceOf[V @@ T]
   @inline def tagAs[T <: V @@ _] = value.asInstanceOf[T]
 }

 implicit class TaggableM[M[_], V](val value: M[V]) extends AnyVal {
   @inline def tagM[T] = value.asInstanceOf[M[V @@ T]]
   @inline def tagAsM[T <: V @@ _] = value.asInstanceOf[M[T]]
 }
}

object test {
 import tagging._

 trait Foo
 trait Bar

 type Z = Int @@ Foo
 val z: Z = 3.tagAs[Z]

 val t: Long = 42L
 val t2: @@[Long, Foo] = 42L.tag[Foo]
 val x: @@[Long, Foo] = t2
 val x2: @@[Long, Foo] = 2L.tag

 val y: Long = x.untag
 val y2: @@[Long, Bar] = x.untag.tag[Bar]

 val c = Seq(1, 2, 3).tagM[Foo]
 val d = c.untagM

}

object bijectionTest {
 import com.twitter.bijection.Conversion.asMethod
 trait  Foo
 import tagging._
 implicit object xConv extends Bijection[Long, Long @@ Foo] {
   override def apply(a: Long): @@[Long, Foo] = a.tag
   override def invert(b: @@[Long, Foo]): Long = b.untag
 }

 val x = 42L.as[Long @@ Foo]
 val y = x.as[Long]
}