How to create a gradually faded image via sdl2?

[zhengqigao]

System : MacOS, ghc with sdl2 installed.

As stated in the title, how can I create a gradually faded image via sdl2? (Note that the figure is given by a .bmp file located somewhere in the PC.)

I have already written the code below. myFaded is actually the function I need. However, currently haskell will complain that there is no setSurfaceAlphamod.

{-# LANGUAGE OverloadedStrings #-}
module Main (main) where

import Control.Concurrent (threadDelay)
import Foreign.C.Types
import SDL.Vect
import SDL.Raw.Video
import qualified SDL

screenWidth, screenHeight :: CInt
(screenWidth, screenHeight) = (960, 720)

fadedtime, fadednum :: Int
(fadedtime, fadednum) = (2000000, 10)

getDataFileName :: FilePath -> IO FilePath
getDataFileName = return



myFaded :: Int -> Int -> SDL.Surface -> SDL.Surface -> SDL.Window -> IO ()
myFaded fadedtime fadednum surface screenSurface window
      | fadednum <= 0 = return ()
      | otherwise = do 
          SDL.surfaceBlit surface Nothing screenSurface Nothing
          SDL.updateWindowSurface window
          threadDelay holdtime
          newsurface <- SDL.setSurfaceAlphaMod surface alpha
          myFaded (fadedtime - holdtime) (fadednum - 1)  newsurface screenSurface window
          where alpha = 2
                holdtime = round $ fromIntegral $ fadedtime `div` fadednum




main :: IO ()
main = do
  SDL.initialize [SDL.InitVideo]
  window <- SDL.createWindow "our super mario" SDL.defaultWindow { SDL.windowInitialSize = V2 screenWidth screenHeight }
  SDL.showWindow window
  screenSurface <- SDL.getWindowSurface window

  helloWorld <- getDataFileName "figs/Entry.bmp" >>= SDL.loadBMP
  --SDL.surfaceBlit helloWorld Nothing screenSurface Nothing
  --SDL.updateWindowSurface window
  myFaded fadedtime fadednum helloWorld screenSurface window


  --SDL.updateWindowSurface window

  --threadDelay 2000000

  SDL.destroyWindow window
  SDL.freeSurface helloWorld
  SDL.quit