BrickBreaker Spinoff in Haskell

Question

I have an implementation of a BrickBreaker-like game where instead of pieces being just removed from the ceiling, each impact results in a new ball being released, gradually building up to a pretty chaotic game.

The SDL related code that actually draws the game is not included here.

This is my second "big" project using Haskell, so I'd appreciate some critique, specifically concerning the approach and collisionBlock functions. First in approach I zip Data.Map.lookup over a list of keys, and then use msum to get the first successful lookup. Then in collisionBlock I use Data.Map.updateLookupWithKey again on the key I already know is successful and had already performed a lookup with. I'd like to be able to eliminate this extra lookup or otherwise improve the approach function.

{-# LANGUAGE BangPatterns #-}

module BrickBreaker where

import Control.Monad
import Data.List
import qualified Data.Map as M
import System.Random

width  = 640 :: Int
height = 420 :: Int
blockW = width
blockH = height `quot` 3

data Particle        = Particle { partX, partY, partDX, partDY :: !Int }
data Paddle          = Paddle { paddleX, paddleW, paddleH :: !Int }
data GameState       = GS ![Particle] !Block
data CollisionResult = Miss | Hit Particle Block
type Block           = M.Map Pos Particle
type Pos             = (Int, Int)

getPos, getSpeed :: Particle -> Pos
getPos pt   = (partX pt, partY pt)
getSpeed pt = (partDX pt, partDY pt)

genBlock :: Block
genBlock = mkMap [Particle w h 0 0 | w <- [1..blockW], h <- [1..blockH]]
    where mkMap = M.fromList . map (\pt -> (getPos pt, pt))

approach :: Particle -> Block -> Maybe Particle
approach pt bs = msum $ zipWith ((flip M.lookup bs .) . addSpeed) (enum dx) (enum dy)
    where (x, y) = getPos pt; (dx, dy) = getSpeed pt
          addSpeed dx dy = (x + dx, y + dy)
          enum 0 = repeat 0
          enum n = let i = if n < 0 then (-1) else 1 in enumFromThenTo i (i+i) n

collisionBlock :: Particle -> Block -> CollisionResult
collisionBlock pt bs
    | dy > 0 && y > blockH      = Miss
    | dy < 0 && y > blockH - dy = Miss
    | otherwise =
        case approach pt bs of
             Just pt -> let ~(Just pt', bs') = searchRemove pt in Hit pt' bs'
             Nothing -> Miss
    where y = partY pt; dy = partDY pt
          searchRemove = flip (M.updateLookupWithKey (\_ _ -> Nothing)) bs . getPos

collisionPaddle :: Paddle -> Particle -> Bool
collisionPaddle pd pt =
    y >= height - paddleH pd && x `between` (padX, padX + paddleW pd) && dy > 0
    where n `between` (a, b) = n >= a && n <= b
          (x, y) = getPos pt; dy = partDY pt
          padX   = paddleX pd

checkCollisions :: Paddle -> GameState -> GameState
checkCollisions pd (GS ps bs) = foldl go (GS [] bs) ps
    where go (GS ps bs) pt
            | collisionPaddle pd pt = GS (bar:ps) bs
            | otherwise =
                case collisionBlock pt bs of
                     Hit pt bs' -> GS (blk:(randomParticle pt:ps)) bs'
                     Miss       -> GS (pt:ps) bs
            where (x, y) = getPos pt; (dx, dy) = getSpeed pt
                  bar = Particle x (min y height) dx (-dy)
                  blk = Particle x y dx (abs dy)

randomParticle :: Particle -> Particle
randomParticle pt = Particle x y (ceiling $ dx * 10) (ceiling $ dy * 9 + 1)
    where (x, y)  = getPos pt
          (dx, g) = randomR ((-1.0), 1.0) (mkStdGen $ x + y) :: (Double, StdGen)
          (dy, _) = randomR (0.1, 1.0) g :: (Double, StdGen)

updateParticle :: Particle -> Particle
updateParticle pt = Particle (x + dx') (y + dy') dx' dy'
    where (x, y) = getPos pt; (dx, dy) = getSpeed pt
          dx' = if (x > width && dx > 0) || (x < 0 && dx < 0) then (-dx) else dx
          dy' = if y < 0 && dy < 0 then (-dy) else dy

updateGame :: Paddle -> GameState -> GameState
updateGame pd gs = GS (map updateParticle $ filter inBounds ps) bs
    where GS ps bs = checkCollisions pd gs
          inBounds = (<= height) . partY

Here is the main loop in the SDL related code that calls the drawing functions and updates the GameState:

updateWorld :: Surface -> Paddle -> GameState -> IO ()
updateWorld screen pd gs = do
    ticks       <- getTicks
    quit        <- whileEvents
    drawGame screen pd gs
    (x, _, _)   <- getMouseState
    ticks'      <- getTicks
    let pd'   = Paddle x (paddleW pd) (paddleH pd)
        delta = ticks' - ticks
    when (delta < (fromIntegral secsPerFrame)) $
        delay $ fromIntegral secsPerFrame - delta
    unless (quit || gameOver gs) (updateWorld screen pd' $ updateGame pd' gs)
    where whileEvents = do
              event <- pollEvent
              case event of
                   KeyDown (Keysym key _ _) ->
                       case key of
                            SDLK_q  -> return True
                            _       -> return False                                                            
                   _ -> return False

Answer 1

One comment on your modelling...

Consider making the paddle position part of your GameState. Regardless of how you are going to control the paddle, conceptually it is part of the state. In particular, it is required in order to draw the game screen. Your game loop will look something like:

gameLoop :: GameState -> IO ()
gameLoop s = if stillPlaying s
                then do drawScreen s
                        e <- getEvent
                        gameLoop $ nextState s e
                else return ()

drawScreen :: GameState -> IO ()
...

getEvent :: IO Event
...

stillPlaying has the signature GameState -> Bool and returns false when the game is over.

nextState has the signature GameState -> Event -> GameState and creates the next state by applying the effects of an event.

One of the events could be "move the paddle left" or "move the paddle right" which would affect the position of the paddle.

Also, don't forget to put a random number generator into your GameState - I'm sure you will want to have some randomness in your game eventually.

Update: It's a good idea to think in alternative use cases when defining the roles and responsibilities of your functions.

For instance, one possible definition of drawScreen is to simply putStrLn $ show s - assuming that you've derived a Show instance for GameState. And getEvent could simply read a number from stdin and create the Event value. Then you can test your game code without using SDL.

Answer 2

You can use let i = signum n in enum instead of if.