Classifiers in MacWire

gistfile1.scala

/*
A proposition on how classifiers can be implemented in MacWire. Based on @milessabin's https://gist.github.com/milessabin/89c9b47a91017973a35f
*/

// Infrastructure: would come in the library
// note that no changes in the macro are necessary. The qualifiers are purely type-based
type Tag[U] = { type Tag = U }
type @@[T, U] = T with Tag[U]
type Tagged[T, U] = T with Tag[U]
implicit class Tagger[T](t: T) {
  def taggedWith[U]: T @@ U = t.asInstanceOf[T @@ U]
}

// Example usage:
class Berry()
trait Black
trait Blue

class Basket(blueberry: Berry @@ Blue, blackberry: Berry @@ Black) {
  def consume(b: Berry) {}
  consume(blueberry) // can be used as "just" berry
  consume(blackberry)
}
// or
class Basket2(blueberry: Berry Tagged Blue, blackberry: Berry Tagged Black)

lazy val blueberry = wire[Berry].taggedWith[Blue]
lazy val blackberry = wire[Berry].taggedWith[Black]
lazy val basket = wire[Basket]
lazy val basket2 = wire[Basket2]

// Yes/no/would prefer annotations?
// One downside is that when declaring dependencies, you would need to use a type from Macwire (@@)