[Haskell-cafe] Advice for clean code.

[Don Stewart]

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