pentago.hs

data ValidatedCommand = RegularMove Char Char String Char
                      | WinningMove Char Char
                      | InvalidCommand
              deriving (Show, Eq)

validateCommand :: Command -> ValidatedCommand
validateCommand (row:col:',':rot)
  | length rot >= 2 &&
    row `elem` "abcdef" &&
    col `elem` "123456" &&
    init rot `elem` ["I", "II", "III", "IV"] && 
    last rot `elem` "lr"
    = RegularMove row col (init rot) (last rot)
  | otherwise = InvalidCommand
validateCommand [row, col]
  | row `elem` "abcdef" &&
    col `elem` "123456"
    = WinningMove row col
  | otherwise
    = InvalidCommand
validateCommand _ = InvalidCommand

step :: State -> Command -> (Maybe Message, Maybe State)
step s c = playMove s vc where vc = validateCommand c

playMove :: State -> ValidatedCommand -> (Maybe Message, Maybe State)
playMove s (RegularMove row column quadrant rotation) = undefined

playMove s (WinningMove row column) = undefined

playMove s InvalidCommand = 
  (Just "Invalid command! Must adhere to [a-f][1-6](,[I-IV](r|l))?", Just s)