TransparentTraits.scala

import scala.util.Random

object TransparentTrait {
  val Sell = "sell"
  val Buy = "buy"

  transparent trait Id {
    def id: Int
  }

  trait Order {
    def quantity: Int
  }

  case class BuyOrder(id: Int, quantity: Int) extends Order, Id

  case class SellOrder(id: Int, quantity: Int) extends Order, Id

  def mkOrder(side: String, qty: Int) =
    if (side.equals(Sell)) SellOrder(mkId, qty) else BuyOrder(mkId, qty)

  def mkId: Int = Random.nextInt()

  def test: Unit = println(mkOrder("sell", 100))
}