stackable_modification.scala

abstract class FuncStackable {
  def f(x: Int): Int
}

class Fidentity extends FuncStackable {
  override def f(x: Int): Int = x
}

trait NInput extends FuncStackable {
  val n: Int
  abstract override def f(x: Int): Int = super.f(x*n)
}

val doublef = new Fidentity with NInput { val n: Int = 10 }

doublef.f(1)

/////////////////////////////////////////
abstract class IntCol {
  def add(x: Int): Unit
  def elements: Seq[Int]
}

class ListWrapper extends IntCol {
  private var l: List[Int] = Nil
  override def add(x: Int): Unit = l = x :: l
  override def elements: Seq[Int] = l
}

trait Duplicate extends IntCol {
  abstract override def add(x: Int): Unit = {
    super.add(x)
    super.add(x)
  }
}




val duplicator = new ListWrapper with Duplicate

duplicator.add(1)
duplicator.elements