Robot.hs

import Data.List
import Data.Char
import Data.List.Split

type Position = (Int, Int)
data Direction = North | South | East | West
    deriving (Eq, Show, Read)

place :: (Position, Direction) -> (Position, Direction)
place (pos, dir) = (pos, dir)

move :: (Position, Direction) -> (Position, Direction)
move ((x, y), dir)
    | dir == North = (newPos (x, y + 1), dir)
    | dir == South = (newPos (x, y - 1), dir)
    | dir == West = (newPos (x - 1, y), dir)
    | dir == East = (newPos (x + 1, y), dir)
    where newPos pos = if onGrid pos then pos else (x, y)

left :: (Position, Direction) -> (Position, Direction)
left (pos, dir) 
    | dir == North = (pos, West)
    | dir == West = (pos, South)
    | dir == South = (pos, East)
    | dir == East = (pos, North)

right :: (Position, Direction) -> (Position, Direction)
right (pos, dir) 
    | dir == North = (pos, East)
    | dir == East = (pos, South)
    | dir == South = (pos, West)
    | dir == West = (pos, North)

report :: (Position, Direction) -> (Position, Direction)
report x = x

onGrid :: Position -> Bool
onGrid (x, y)
    | x >= 0 && x <= 3 && y >= 0 && y <= 3 = True
    | otherwise = False


robot :: Position -> Direction -> [(Position, Direction) -> (Position, Direction)] -> (Position, Direction)
robot pos dir commands = reduce commands initial
    where
        initial = place (pos, dir)
        reduce fs v = foldl (flip (.)) id fs $ v -- reduce [f, g, h] x = h(g(f(id(x))))

parseToFunc :: String -> ((Position, Direction) -> (Position, Direction))
parseToFunc "MOVE" = move
parseToFunc "LEFT" = left
parseToFunc "RIGHT" = right
parseToFunc "REPORT" = report
parseToFunc str = if isPrefixOf "PLACE" str 
            then place
            else report 

parsePlace :: String -> (Position, Direction)
parsePlace str = ((digitToInt $ head $ (posDirectionSplit !! 0), digitToInt $ head $ (posDirectionSplit !! 1)), (read (posDirectionSplit !! 2) :: Direction) )
    where 
        posDirection = dropWhile (\x -> isSpace x) $ deleteFirstsBy (\x y -> x == y) str "PLACE"
        posDirectionSplit = splitOn "," posDirection

main = do
    line <- getLine
    let (pos, dir) = parsePlace line
    commands <- mainloop []
    putStrLn $ show (robot pos dir commands)

mainloop commands = do
    line <- getLine
    if line == "REPORT" 
        then return commands 
        else do 
            let commands' = commands ++ [parseToFunc line]
            mainloop commands'