A.hs

data Robot = Robot Bearing Coordinates deriving (Eq, Show)

data Bearing = North | South | East | West deriving (Eq, Show)

left :: Bearing -> Bearing
left North = West
left West = South
left South = East
left East = North

right :: Bearing -> Bearing
right = left . left . left

type Coordinates = (Integer, Integer)

advance :: Bearing -> Coordinates -> Coordinates
advance North (x, y) = (x, y+1)
advance South (x, y) = (x, y-1)
advance East  (x, y) = (x+1, y)
advance West  (x, y) = (x-1, y)
    
move :: Robot -> String -> Robot
move = foldl f
  where
    f (Robot b c) 'A' = Robot b (advance b c)
    f (Robot b c) 'L' = Robot (left b) c
    f (Robot b c) 'R' = Robot (right b) c
    f _ _ = error "move: undefined"