OpenUnion.scala

object OpenUnion extends App {
  
    trait in[A, B] {
      def inj(a: A): B
    }
    object in {
      implicit def inSelf[A]: (A in A) = (a: A) => a
      implicit def inHead[A, B]: (A in (A Either B)) = (a: A) => Left(a)
      implicit def inRight[A, B, C](implicit r: (A in C)): (A in (B Either C)) =
        (a: A) => Right(r.inj(a))
      implicit def inLeft[A, B, C](implicit r: (A in B)): (A in (B Either C)) =
        (a: A) => Left(r.inj(a))
    }

    def inj[B] = new {
      def apply[A](a: A)(implicit r: (A in B)): B = r.inj(a)
    }

    def handle[A, B, C](a: (A Either B))(f: A => C): Option[B] =
      a.fold(a => Function.const(None)(f(a)), Some(_))

    type :|:[A, B] = Either[A, B]
    type All = String :|: Int :|: Boolean :|: Double

    def handleAll[A](a: A)(implicit r: (A in All)) =
      handle(inj(a))(x => println(x + " : String"))
        .flatMap(handle(_)(x => println(x + " : Int")))
        .flatMap(handle(_)(x => println(x + " : Boolean")))
        .map(x => println(x + " : Double"))

    handleAll("a")
    handleAll(1)
    handleAll(true)
    handleAll(1.0)
}