Panda Canvas 2 prototyping...

canvas.hs

import Control.Monad.Writer
import Data.IORef
import Data.List.Zipper (Zipper(..))
import qualified Data.List.Zipper as Zipper
import Graphics.Rendering.OpenGL hiding (flush)
import Graphics.UI.GLUT hiding (flush)

data CanvasAction = PointAction GLfloat GLfloat | FlushAction
type Canvas = Writer [CanvasAction] ()

pixel :: GLfloat -> GLfloat -> Canvas
pixel x y = writer ((), [PointAction x y])

flush :: Canvas
flush = writer ((), [FlushAction])

drawActions :: Zipper CanvasAction -> [(GLfloat, GLfloat)]
drawActions (Zip actions _) =
    foldr drawable [] $ reverse actions
  where
    drawable (PointAction x y) acc = (x, y):acc
    drawable FlushAction acc = acc

flushActions :: Zipper CanvasAction -> Zipper CanvasAction
flushActions z = if Zipper.endp z then z
    else case Zipper.cursor z of
        PointAction x y -> flushActions $ Zipper.right z
        FlushAction -> Zipper.right z

draw :: String -> Canvas -> IO ()
draw title canvas = do
    getArgsAndInitialize
    initialWindowSize $= Size 640 480
    initialDisplayMode $= [DoubleBuffered, WithAlphaComponent]
    createWindow title
    actions <- newIORef $ Zipper.fromList points
    displayCallback $= display actions
    reshapeCallback $= Just reshape
    mainLoop
  where
    ((), points) = runWriter canvas

display :: IORef (Zipper CanvasAction) -> IO ()
display actionsRef = do
    clear [ColorBuffer]
    actions <- get actionsRef
    let flushedActions = flushActions actions
    renderPrimitive Points $ mapM_ pointToVertex (drawActions flushedActions)
    swapBuffers
    actionsRef $= flushedActions
  where
    pointToVertex (x, y) = vertex $ Vertex3 x y 0

reshape :: Size -> IO ()
reshape size = do
    viewport $= (Position 0 0, size)
    postRedisplay Nothing

main :: IO ()
main = draw "Hello World" $ do
    pixel 0 0
    pixel 0 0.25
    flush
    pixel 0 0.5