User factory

user_id.scala

trait ID

case class SequentialID(id: Int) extends ID
case class OtherID(id: Int) extends ID

case class User(val id: ID, val b: Int, val c: Int)

object UserId {
  def unapply(w: User) = w match {
     case u @ User(SequentialID(id), _, _) => Some(u, id)
     case _ => None
  }
}

val u = User(SequentialID(1),2,3)                 
val y = User(OtherID(1),2,3)                    

u match {
    case UserId(u, id) => id
}                                                

y match {
   case UserId(u, id) => u
   case _ => y
}