Scala: recursive & tail recursive list reversals

gistfile1.scala

/* Not tail recursive. */
  def reverseSlow [X] (xs:List[X]) : List[X] = xs match {
    case Nil => Nil
    case y :: ys => reverseSlow (ys) ::: List (y)
  }

  /* Tail recursive. */
  def reverseFast [X] (xs:List[X]) : List[X] = {
    def aux (xs:List[X], result:List[X]) : List[X] = xs match {
      case Nil => result
      case y :: ys => aux (ys, y :: result)

    }
    aux (xs, Nil)
  }