Tuesday, September 18, 2012


Hi, welcome to the 5th article of this blog.

I am very exited! In the last Post we wrote a Pong like ... well ... let's call it javascript application. In this post I have expanded it by the following properties:

  • The game does not start until you hit enter.
  • There are blocks that can be hit by the ball and disappear.
  • There are even blocks with 2 lives (dark blue) that turn into normal blocks on the first hit.
  • The game stops when the ball leaves the canvas downward.

This means, there is a goal and there is a game over situation. So at this point one could actually call it a game. And it even has a start screen ... did I mention that I am exitied? /,anma But, now, as always, here is a preview. As always you have to click the canvas to get input focus. If you are not viewing this blog article on blogspot and the application does not work, try the original article page.

Note: This currently only works if you are viewing this article only (not in the flow of the complete blog). I am working on the problem ...

But let me tell you how I did it :).

By the way, this post assumes that you have read the last Post.

More Coroutine helpers

There are two aspects of this game (yes, game!)

  • The blocks are a dynamic set of objects, that disappear as the game progresses
  • There are different "game states" (the start screen and the actual game)

So this has been added to Coroutine.hs

-- manages a set of coroutines which are deletet when returning Nothing
manager :: [Coroutine a (Maybe b)] -> Coroutine [a] [b]
manager cos = Coroutine $ \is ->
  let res  = map (\(co, i) -> runC co i) $ zip cos is
      res' = filter (isJust . fst) res
      (result, cos') = unzip res'
  in (catMaybes result, manager cos')

-- switcher, starts with a specific coroutine and switches whenever a new coroutine is send via an event
switch :: Coroutine a b -> Coroutine (Event (Coroutine a b), a) b
switch init = Coroutine $ \(e,i) ->
  let init' = last $ init : e --the last coroutine sent through
      (o, init'') = runC init' i
  in  (o, switch init'')

-- replace the contents of an event
(<$) :: Event a -> b -> Event b      
(<$) events content = map (\_ -> content) events

manager: The manger is for managing the blocks. Every blocks state is produced by a coroutine, and in the beginning there is a set of blocks in the game (first parameter to manager). The manager distributes its input to all its coroutines. So the input list should have the same length. The output if each coroutines are collected in a list which is the output of the manager.

Every block Coroutine returns "Nothing" when the block is destroyed, the manager than removes the block from the set.

Note that at present there is no way of inserting new blocks in the manager, it is not needed in this game.

switch: Switch allows us to switch between different game states, which all are described by coroutines of the same type.

Initially switch behaves as the init Coroutine (its first parameter) with an extra parameter holding events with other Coroutines. Whenever one of these events occurs, switch switches to the coroutine carried in the event.

<$: This is a operator. When applied to an event it replaces the contents of the event with the second parameter. We need this to replace the content of the KeyDown event with the main Coroutine when the start key is pressed. You will see!

From Pong to Breakout

All very exiting, but the real excitement start now. The main source file Breakout.hs is based on the last posts Pong.hs. Here I will go over the differences.


The game state needs to reflect the blocks and the start screen. It has changed to:

data PlayerState = PlayerState {xPos :: Double}
data BallState = BallState {ballPos :: Vector}
data BlockState = BlockState {blockPos :: Vector, blockLives :: Int}

data GameState = GameState {player :: PlayerState,
                            ball :: BallState,
                            blocks :: [BlockState]}
                 | StartScreen

data BallCollision = LeftBounce | RightBounce | UpBounce | DownBounce
data BlockCollision = BlockCollision
data Rect = Rect { x::Double, y::Double, width ::Double, height::Double}

The BlockState has been added, which contains the block position and the number of lives (1 or 2) of the block. The GameState has been expanded by a list of BlockStates AND can be just the start screen (when the game has not started).

BlockCollision is a type for creating Events where the block collides with the ball. A type synonym to () would also work, but I choose this more verbose way.

blockWidth = 60.0
blockHeight = 20.0
blockColor1live = "blue"
blockColor2live = "darkblue"

initBlockStates = [BlockState (x,y) lives | x <- [20.0, 140.0, 240.0, 340.0, 440.0, 520.0], (y, lives) <- [(60.0,2), (100.0,1), (140.0,2), (180.0,1), (220.0,1), (260.0,1)]]

restartKeyCode = 32
canvasName = "canvas3"

The color of the blocks depend if they have 1 or 2 lives. initBlockStates describes the blocks as the game starts. They are evenly spaced, 6 in x and 6 in y directions. 2 of the y rows have 2 lives, the rest 1.

The restartKeyCode is the key code of the enter bar and the canvasName is the name of the canvas in the html code of this blog.


draw :: GameState -> IO ()
draw StartScreen = do
  ctx <- getContext2d canvasName
  clear ctx
  -- draw the text
  setFillColor ctx "black"
  fillText ctx "Press Enter to start --- (click the canvas for input focus)" (screenWidth/2.0 - 100.0) (screenHeight/2.0)

draw (GameState playerState ballState blockStates) = do
  ctx <- getContext2d canvasName
  clear ctx
  -- draw player
  setFillColor ctx playerColor
  let pRect = playerRect playerState
  fillRect ctx (x pRect) (y pRect) (width pRect) (height pRect)
  --draw blocks
  mapM_ (drawBlock ctx) $ blockStates
  --draw ball
  setFillColor ctx ballColor
  let (x,y) = ballPos ballState
  fillCircle ctx x y ballRadius

drawBlock :: Context2D -> BlockState -> IO ()
drawBlock ctx bs = do
  setFillColor ctx (if blockLives bs == 1 then blockColor1live else blockColor2live)
  let r = blockRect bs
  fillRect ctx (x r) (y r) (width r) (height r)

draw pattern matches its argument, to test if it is the start screen. If so, a short message telling the player to press enter is displayed (see fillText in some javascript documentation).


gameOver :: GameState -> Bool
gameOver (GameState _ (BallState (_, by)) _) = by > screenHeight
gameOver _ = False

blockRect :: BlockState -> Rect
blockRect (BlockState (bx,by) _) = Rect bx by blockWidth blockHeight

gameOver is a little helper function to test if the ball has left the canvas. It returns False on the start screen.

blockRect returns the rectangle occupied by a block.

Main coroutine

mainCoroutine :: MainCoroutineType
mainCoroutine = proc inEvents -> do
    let startEvent = filter (\ke -> ke == KeyUp restartKeyCode) inEvents <$ mainGameCoroutine
        stopEvent  = if gameOver oldState then [mainStartScreenCoroutine] else []
    state <- switch mainStartScreenCoroutine -< (startEvent ++ stopEvent, inEvents)
    oldState <- delay StartScreen -< state
  returnA -< state

mainStartScreenCoroutine :: MainCoroutineType
mainStartScreenCoroutine = arr $ const StartScreen

mainGameCoroutine :: MainCoroutineType
mainGameCoroutine = proc inEvents -> do
  plState <- playerState -< inEvents
    let (ballBlockColls, blockColls) = ballBlocksCollisions oldBallState oldBlockStates
    let colls = (ballWallCollisions oldBallState) ++ (ballPlayerCollisions plState oldBallState) ++ ballBlockColls
    currBallState   <- ballState            -< colls --long names ...
    currBlockStates <- blockStates          -< blockColls
    oldBallState    <- delay initBallState  -< currBallState
    oldBlockStates  <- delay initBlockStates-< currBlockStates
  returnA -< GameState plState currBallState currBlockStates

The original main coroutine has been renamed to mainGameCoroutine. There is a new "main coroutine" mainStartScreenCoroutine which is used while in the start screen. The new mainCoroutine switches between these two coroutine when the player pressed enter, or the game is over.

Remember, the <$ operator replaces the contents of an event with its second parameter (here the mainGameCoroutine) and switch receives events containing coroutines to which it switches.

mainGameCoroutine has been extended by the blocks. ballBlocksCollisions, as we will see later, returns a tuple with the ballCollisions events due to collisions with the blocks, and a list of BlockCollision events. This list has the same length as the list of blocks (in oldBlockStates). The n-th element of this list are the collisions with the n-th block.

The block collisions are than passed to the blockStates arrow while the ballCollisions are added to the collisions passed to ballState.

I dislike the long names like "currBallState" here. I would have called it ballState, but there is already an arrow with the same name. I wonder if there is a less clumsy way of doing this ...

Ball-Block collisions

ballBlocksCollisions :: BallState -> [BlockState] -> (Event BallCollision, [Event BlockCollision])
ballBlocksCollisions ballState blockStates =
  let ballR = ballRect ballState
      foldStep (ballC, blockC) blockState =
        if rectOverlap ballR (blockRect blockState) then
          (ballRectCollisions ballState (blockRect blockState) ++ ballC, blockC ++ [[BlockCollision]])
          (ballC, blockC ++ [[]])
  in foldl' foldStep ([],[]) blockStates

In my opinion, this is the most complicated function. It takes the ball state and the block states (as a list) and produces ball collisions events, and a list of block collision events, which has the same length as the input block state list.

The foldStep function takes the next block, tests it for collision and updates the list of ball and block collisions. Here the ball collision events are only expanded when a collision happens. The list of block collision events is always expanded. By an empty event (empty list) when no collision happens, and by a BlockCollision event in case of collision. This is because the position in this list reflects the block that will receive it.

Updating the block state

blockState :: BlockState -> Coroutine (Event BlockCollision) (Maybe BlockState)
blockState initState = scanE update (Just initState)
  update :: Maybe BlockState -> BlockCollision -> Maybe BlockState
  update Nothing   _ = Nothing
  update (Just bs) _ = if (blockLives bs == 1) then Nothing else Just $ bs{blockLives=1}

blockStates :: Coroutine ([Event BlockCollision]) ([BlockState])
blockStates = manager $ map blockState initBlockStates

Every block has its own coroutine, which receives block collision events. In case of such an event, the number of lives is reduced or the block is removed (if there are no lives left). The coroutines return a Maybe data type, because they are inserted into the manager. Nothing is returned if the block should be deleted.

blockStates uses the manager to manage all "living" blocks.


The compilation is the same as for Pong int the last post.


For haste make sure the newest version is installed. Because we use vector-space we need to install it for haste.

vector space is needed, see the last post.

Now put Breakout.hs, Coroutine.hs, the haste version of JavaScript.hs and the javascript helper functions helpers.js in a directory and compile with

hastec Breakout.hs --start=asap --with-js=helpers.js

You should receive a file "Breakout.js" which can be included in a html file, like this one: haste html


With UHC it is a little bit more work. UHC does not support arrow syntax, so we must translate the haskell file with arrowp:

cabal install arrowp
arrowp Breakout.hs > BreakoutNA.hs

I choose the name BreakoutNA.hs for "Breakout no arrows". For some reason I also can not get vector space to compile with UHC. Luckily we have not used much of vector space, only the + operator. So edit PongNA.hs and replace the line

import Data.VectorSpace


(^+^) :: Num a => (a,a) -> (a,a) -> (a,a)
(^+^) (a1,a2) (b1,b2) = (a1+b1, a2+b2)

Now copy Coroutine.hs and JavaScript.hs (the UHC version) into the directory and compile with

uhc -tjs BreakoutNA.hs -iuhc 

The canvas needs to be added to the generated html file, so add

<canvas height="400" id="canvas3" style="background-color: white;" width="600" tabindex="1"></canvas>

Since we do not need any additional javascript functions, the generated html page should work!


Well that is it. At places I find it a bit clumpsy and I wonder if another FRP library like Reactive Banana or elerea would help. I will look into these!

Creative Commons License
Writing JavaScript games in Haskell by Nathan Hüsken is licensed under a Creative Commons Attribution 3.0 Germany License.


  1. Thank you, this series seems very helpful to cut my teeth on Haskell.

  2. Good day! Do you use Twitter? I'd like to follow you if that would be okay. I'm undoubtedly enjoying your blog and look forward to new updates.
    Feel free to surf my blog post ... pickel und akne

    1. I am not really an active twitter user, but I just connected this blog via twitterfeed to my twitter account. It is here: https://twitter.com/RudolfKrugstein

      So in the future new blog posts should be tweeted there (I hope).

  3. Great series, waiting to hear more! What is your experience, is UHC stable enough? (no compiler crashes, etc). Thanks!

    1. I never had stability issues with UHC. But it has some limitations as it is not jet supporting type families, GADTs and other things.

    2. I tried to compile some small pieces of code, and with haste-inst the required cabal packages (mtl, transformers, dlist) were pulled in just fine. I couldn't get dlist used by UHC, and didn't try the others afterwards. Did you have such problem?

    3. I a not sure what you mean. I never needed dlist in UHC. Did you try
      cabal --uhc install dlist?

    4. I didn't know about the --uhc option, thank you. Dlist installed fine, but transformers failed with syntax error. Will improve in future I guess.

    5. Unfortantly I can not say more than that it does not work here neither. You could ask at irc://irc.freenode.net/uhcjs, create an issue at github (https://github.com/UU-ComputerScience/uhc-js) or ask on the UHC mailing list (ttp://lists.science.uu.nl/mailman/listinfo/uhc-users).

  4. Thanks for sharing this useful info..
    Frvi 4