Don's code intrigued me, so I fired up my trusty emacs and ghci, and
turned it into actual code, which type-checks. Well, ok, I kind of
randomly poked at it, while begging for help, which I received in
abundance from #haskell, particularly oerjan, and Don himself. Anyway,
here's the code:
{-# OPTIONS -fglasgow-exts #-}
module Game
where
import Control.Applicative
import Control.Monad.State
import System
newtype Game a = Game (StateT World IO a)
deriving (Functor, Monad, MonadState World, MonadIO)
data Event = Quit | LeftE | RightE | Up | Down
data Board = Board [Int] deriving (Show)
data World = World [Int]
game :: Event -> Game Action
game Quit = liftIO $ exitWith ExitSuccess
game LeftE = return MoveOK
game RightE = return MoveOK
game Up = return MoveOutOfBounds
game Down = return (MoveBadTerrain "Tree")
runGame :: [Event] -> IO [Action]
runGame es = evalStateT s (World [0])
where Game s = mapM game es
data Action = MoveOutOfBounds | MoveBadTerrain String | MoveOK
-- How to display results
instance Show Action where
show MoveOutOfBounds = "Sorry you can't move in that direction."
show (MoveBadTerrain a) = case a of
"Wall" -> "You walk into a wall."
"Tree" -> "There is a tree in the way."
otherwise -> "You can't move there."
show MoveOK = "Good move."
main = do
events <- map processInput <$> getContents
mapM_ print =<< runGame events
processInput :: Char -> Event
processInput = undefined
On Dec 3, 2007 10:28 PM, Don Stewart <[EMAIL PROTECTED]> wrote:
> stefanor:
> > On Mon, Dec 03, 2007 at 08:47:48PM -0600, David McBride wrote:
> > > I am still in the early stages learning haskell, which is my first foray
> > > into functional programming. Well there's no better way to learn than to
> > > write something, so I started writing a game.
> > >
> > > Mostly the thing looks good so far, far better than the C version did.
> > > However, my problem is that code like the following is showing up more
> > > often and it is becoming unwieldy.
> > >
> > > gameLoop :: World -> IO ()
> > > gameLoop w = do
> > > drawScreen w
> > >
> > > action <- processInput
> > >
> > > let (result, w') = processAction action w
> > >
> > > case result of
> > > MoveOutOfBounds -> putStrLn "Sorry you can't move in that direction."
> > > MoveBadTerrain a -> case a of
> > > Wall -> putStrLn "You walk into a wall."
> > > Tree -> putStrLn "There is a tree in the way."
> > > otherwise -> putStrLn "You can't move there."
> > > otherwise -> return ()
> > >
> > > let w'' = w' { window = updateWindowLocation (window w') (location $
> > > player w')}
> > >
> > > unless (action == Quit) (gameLoop w'')
> > >
> > > Where world contains the entire game's state and so I end up with w's with
> > > multiple apostrophes at the end. But at the same time I can't really
> > > break
> > > these functions apart easily. This is error prone and seems pointless.
> > >
> > > I have been reading about control.monad.state and I have seen that I could
> > > run execstate over this and use modify but only if each function took a
> > > world and returned a world. That seems really limiting. I'm not even
> > > sure
> > > if this is what I should be looking at.
> > >
> > > I am probably just stuck in an imperative mindset, but I have no idea what
> > > to try to get rid of the mess and it is only going to get worse over time.
> > > Any suggestions on what I can do about it?
> >
> > I'd recommend using StateT World IO. You can always run other functions
> > using 'lift'; for instance lift can be :: IO () -> StateT World IO ().
>
> The fact your passing state explicitly, which is error prone, pretty much
> demands a State monad., And the IO in the main loop seems needless -- the game
> is really just a function from :: World -> [Event] -> [(World',Action)]
>
> So strongly consider lifting the IO out of the main loop, and just have your
> game be a function from input events, to output game states, Which you draw as
> they're received.
>
> The game would run in an environment something like:
>
> newtype Game a = Game (StateT World IO) a
> deriving (Functor, Monad, MonadState World)
>
> The inner loop would be something like:
>
> game :: Event -> Game Action
> game Quit = exitWith ExitSuccess
> game Left = ... >> return MoveOK
> game Right = ... >> return MoveOK
> game Up = return MoveOutOfBounds
> game Down = return (MoveBadTerrain Tree)
>
> Running the game over the input events, producing a sequence of screens
> to print:
>
> runGame :: [Event] -> [(Board,Action)]
> runGame es = evalState (mapM game es) 0
>
> Use show for the result action, to avoid ugly print statements,
>
> data Action
> = MoveOutOfBounds
> | MoveBadTerrain Object
> | MoveOK
>
> -- How to display results
> instance Show Action where
> show MoveOutOfBounds = "Sorry you can't move in that direction."
> show (MoveBadTerrain a) = case a of
> Wall -> "You walk into a wall."
> Tree -> "There is a tree in the way."
> otherwise -> "You can't move there."
> show MoveOk = "Good move."
>
> And at the top level,
>
> main = do
> events <- map processInput <$> getContents
> mapM_ print (runGame events)
>
> This isn't real code, just a sketch.
>
> -- Don
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
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