datatype direction = North | South | East | West;

fun turnRight North = East
  | turnRight East  = South
  | turnRight South = West
  | turnRight West  = North;

fun turnLeft North = West
  | turnLeft West  = South
  | turnLeft South = East
  | turnLeft East  = North;

fun isVerticalDirection North = true
  | isVerticalDirection South = true
  | isVerticalDirection x     = false; (* i.e. East or West *)