Haskell无法将类型`[]'与`IO'匹配

Question

我正在尝试将绘图函数映射到列表中的每个元素.函数本身,drawMap似乎没问题但是当我使用它时,我收到以下错误:

Couldn't match type `[]' with `IO'
Expected type: IO (IO Bool)
  Actual type: [IO Bool]
In the return type of a call of `drawMap'
In a stmt of a 'do' block: drawMap testMap 32 img screen
In the second argument of `($)', namely
  `do { screen <- SDL.setVideoMode 640 480 32 [SDL.SWSurface];
        img <- SDL.loadBMP "grass.bmp";
        drawMap testMap 32 img screen;
        SDL.flip screen;
        .... }'

从阅读本文开始,我现在明白它与函数返回的内容有关,但我现在还不知道如何修复它.

我的代码看起来像这样:

testMap = [Tile 0 0 1, Tile 0 1 1, Tile 0 2 1, Tile 0 3 1]

drawTile :: Int -> SDL.Surface -> SDL.Surface -> Tile -> IO Bool
drawTile tilesize img dst tile = applySurface (tileX tile * tilesize) (tileY tile * tilesize) img dst

drawMap :: [Tile] -> Int -> SDL.Surface -> SDL.Surface -> [IO Bool]
drawMap tiles tilesize img dst =
  map (drawTile tilesize img dst) tiles

main :: IO ()
main = SDL.withInit [SDL.InitEverything] $ do

  screen <- SDL.setVideoMode 640 480 32 [SDL.SWSurface]

  img <- SDL.loadBMP "grass.bmp"

  --drawTile 32 img screen (testMap !! 1)
  drawMap testMap 32 img screen

  SDL.flip screen

  mainLoop

Answer 1

我想你想要:

drawMap tiles tilesize img dst =
  mapM (drawTile tilesize img dst) tiles

mapM是来自Control.Monad.类型drawMap变为:

drawMap :: [Tile] -> Int -> SDL.Surface -> SDL.Surface -> IO [Bool]

即它是一个IO动作返回Bool列表,而不是IO动作列表.