free-gameで円を描く！

@fumieval さんのお作りになった free-game(http://hackage.haskell.org/package/free-game) というGUIを扱うパッケージの(骨組み)サンプルです。

画面に円を描きます。
(円を描く函数renderCircleはfree-game 3.0では未実装なのでここでは自前で実装しています)

sample.hs

{-# LANGUAGE TemplateHaskell #-}
import Graphics.FreeGame
import Data.Word
import Data.Array.Repa
import Control.Monad

import Control.Lens

data World = World{
  _object :: Picture,
  _pos :: (Float, Float)
  }

$(makeLenses ''World)

initWorld :: Picture ->  World
initWorld p = World { _object = p, _pos = (100,300) }

renderCircle :: Int -> (Word8, Word8, Word8, Word8) -> Bitmap
renderCircle size (r,g,b,a) = toStableBitmap $ computeS $ fromFunction (Z :. size :. size :. 4) render where
    center = fromIntegral size / 2
    render (Z:.y:.x:.0)
        | s < 0 = a
        | s >= 1 = 0
        | otherwise = floor ((1 - s) * 256)
        where
            r = sqrt $ (fromIntegral y - center) ^ 2 + (fromIntegral x - center) ^ 2
            s = r - fromIntegral size / 2
    render (Z:._:._:.c) = [undefined,b,g,r] !! c

draw :: World -> Game ()
draw w = let (x,y) = (w ^. pos) in drawPicture . Translate (Vec2 x y) $ (w ^. object)

main :: IO (Maybe ())
main = runGame defaultGameParam $ do
  p <- loadPicture $ renderCircle 128 (128, 216, 128, 255)
  run $ initWorld p

  where
    run :: World -> Game ()
    run w = do
      isQuit <- askInput KeyEsc

      draw w

      tick
      unless isQuit $ run w

簡単で強力なfree-gameをあなたもぜひ！