readPixels.hs

myReadPixels:: Int -> Int -> Int -> Int -> IO [Pixel]
myReadPixels x y w h = do
    let arraySize = w * h * 4
    array <- mallocForeignPtrArray arraySize :: IO (ForeignPtr Word8)
    let pos = Position (fromIntegral x) (fromIntegral y)
    withForeignPtr array $ \ptr -> do
        -- fromIntegral is needed because Position and Size store GLints not Ints
        readPixels pos
                   (Size (fromIntegral w) (fromIntegral h))
                   (PixelData RGBA UnsignedByte ptr)
        peekArray arraySize ptr >>= return . pixelGroups
  where
    pixelGroups (a:b:c:d:xs) = (Pixel a b c d) : pixelGroups xs
    pixelGroups [] = []