gistfile1.sbt

sealed trait Heading {
  val turn = Turn(this) _
}
case object N extends Heading
case object S extends Heading
case object E extends Heading
case object W extends Heading

object Turn {
  def apply(h: Heading)(t: Turn) = t(h)
}

sealed trait Turn extends (Heading => Heading)
case object Left extends Turn {
  def apply(h: Heading) = h match {
    case N => W
    case E => N
    case S => E
    case W => S
  }
}
case object Right extends Turn {
  def apply(h: Heading) = h match {
    case N => E
    case E => S
    case S => W
    case W => N
  }
}