{------------------------------------------------------------------------------ Control.Monad.Operational Example: An implementation of the game TicTacToe. Each player (human, AI, ...) is implemented in a separate monad which are then intermingled to run the game. This resembles the PoorMansConcurrency.hs example. Many thanks to Yves Pars and Bertram Felgenhauer http://www.haskell.org/pipermail/haskell-cafe/2010-April/076216.html ------------------------------------------------------------------------------} {-# LANGUAGE GADTs, Rank2Types #-} module TicTacToe where import Control.Monad import Control.Monad.Operational import Control.Monad.State import Data.Either import Data.List -- external libraries needed import System.Random {------------------------------------------------------------------------------ The Player monad for implementing players (human, AI, ...) provides two operations readBoard -- read the current board position playMove -- play a move to query the current board position and perform a move, respectively. Moreover, it's actually a monad transformer intended to be used over IO. This way, the players can perform IO computations. ------------------------------------------------------------------------------} data PlayerI a where ReadBoard :: PlayerI Board PlayMove :: Int -> PlayerI Bool type Player m a = ProgramT PlayerI m a readBoard = singleton ReadBoard playMove = singleton . PlayMove -- interpreter runGame :: Player IO () -> Player IO () -> IO () runGame player1 player2 = eval' initialGameState player1 player2 where eval' game p1 p2 = viewT p1 >>= \p1view -> eval game p1view p2 eval :: GameState -> ProgramViewT PlayerI IO () -> Player IO () -> IO () eval game (Return _) _ = return () eval game (ReadBoard :>>= p1) p2 = eval' game (p1 (board game)) p2 eval game (PlayMove mv :>>= p1) p2 = case makeMove mv game of Nothing -> eval' game (p1 False) p2 Just game' | won game' -> let p = activePlayer game in putStrLn $ "Player " ++ show p ++ " has won!" | draw game'-> putStrLn $ "It's a draw." | otherwise -> eval' game' p2 (p1 True) -- example: human vs AI main = do g <- getStdGen runGame playerHuman (playerAI g) {------------------------------------------------------------------------------ TicTacToe Board type and logic The board looks like this: +---+---+---+ some squares already played on | 1 | 2 | 3 | the empty squares are numbered +---+---+---+ | 4 | 5 |OOO| +---+---+---+ | 7 |XXX| 9 | +---+---+---+ ------------------------------------------------------------------------------} data Symbol = X | O deriving (Eq,Show) type Square = Either Int Symbol type Board = [[Square]] data GameState = Game { board :: Board, activePlayer :: Symbol } initialGameState :: GameState initialGameState = Game (map (map Left) [[1,2,3],[4,5,6],[7,8,9]]) X -- list the possible moves to play possibleMoves :: Board -> [Int] possibleMoves board = [k | Left k <- concat board] -- play a stone at a square makeMove :: Int -> GameState -> Maybe GameState makeMove k (Game board player) | not (k `elem` possibleMoves board) = Nothing -- illegal move | otherwise = Just $ Game (map (map replace) board) (switch player) where replace (Left k') | k' == k = Right player replace x = x switch X = O switch O = X -- has somebody won the game? won :: GameState -> Bool won (Game board _) = any full $ diagonals board ++ rows board ++ cols board where full [a,b,c] = a == b && b == c diagonals [[a1,_,b1], [_ ,c,_ ], [b2,_,a2]] = [[a1,c,a2],[b1,c,b2]] rows = id cols = transpose -- is the game a draw? draw :: GameState -> Bool draw (Game board _) = null (possibleMoves board) -- print the board showSquare = either (\n -> " " ++ show n ++ " ") (concat . replicate 3 . show) showBoard :: Board -> String showBoard board = unlines . surround "+---+---+---+" . map (concat . surround "|". map showSquare) $ board where surround x xs = [x] ++ intersperse x xs ++ [x] printBoard = putStr . showBoard {------------------------------------------------------------------------------ Player examples ------------------------------------------------------------------------------} -- a human player on the command line playerHuman :: Player IO () playerHuman = forever $ readBoard >>= liftIO . printBoard >> doMove where -- ask the player where to move doMove :: Player IO () doMove = do liftIO . putStrLn $ "At which number would you like to play?" n <- liftIO getLine b <- playMove (read n) unless b $ do liftIO . putStrLn $ "Position " ++ show n ++ " is already full." doMove -- a random AI, -- also demonstrates how to use a custom StateT on top -- of the Player monad playerAI :: Monad m => StdGen -> Player m () playerAI = evalStateT ai where ai :: Monad m => StateT StdGen (ProgramT PlayerI m) () ai = forever $ do board <- lift $ readBoard n <- uniform (possibleMoves board) -- select a random move lift $ playMove n where -- select one element at random uniform :: Monad m => [a] -> StateT StdGen m a uniform xs = do gen <- get let (n,gen') = randomR (1,length xs) gen put gen' return (xs !! (n-1))