Widening.scala

case class Id[+A](guid: Long)

sealed trait DeltaIOA[U, A]
case class GetId[U, T <: U]() extends DeltaIOA[U, Id[T]]
case class Put[U, T <: U](create: Id[T] => DeltaIO[U, T]) extends DeltaIOA[U, T]

class Widener[U, V <: U] {
  type DIOU[A] = DeltaIOA[U, A]
  type DIOV[A] = DeltaIOA[V, A]

  val widenUArrow: DIOV ~> DIOU =
    new (DIOV ~> DIOU) {
      def apply[A](v: DIOV[A]): DIOU[A] = v match {
        case GetId() => GetId()
        case Put(create) =>
          val createU = ((a: Id[A]) => a: (Id[V with A])).andThen(create.andThen(dioV => Delta.widenU[U, V, A](dioV.map(vWithA => vWithA : A))))
          Put(createU)
      }
    }
}