當我想加載一個 bmp 檔案時，我這樣做：
loadBMP :: String -> IO Picture
如果不確定給定路徑是否存在，可以執行以下操作：
saveLoad :: String -> IO Picture
saveLoad str = do
b <- doesFileExist str
if b then loadBMP str
else return Blank
另一種方式也可以是；使用單子變壓器：
saveLoadM :: String -> MaybeT IO Picture
saveLoadM s = do
b <- lift $ doesFileExist s
if b then do
p <- lift$ loadBMP s
return p
else MaybeT $ return Nothing
但是您將如何處理檔案串列？
test ::[String] -> ListT IO Picture
test [] = ListT $ return []
test (x:xs) = do
b <- lift $ doesFileExist x
if b then do
p <- lift$ loadBMP x
return p -- would yield a simple [p]
-- therefore:
p : test xs -- wrong -> not working
else test xs
uj5u.com熱心網友回復：
你只需要fmap.
test :: [String] -> IO [Picture]
-- this bit as before...
if b then do
p <- loadBMP x
(p:) <$> test xs
else test xs
但是你不應該使用doesFileExist這種方式。相反，加載位圖并在檔案不存在時捕獲例外；這可以防止競爭條件。
test (x:xs) = do
imgE <- try (loadBMP x)
case imgE of
Right img -> (img:) <$> test xs
Left err | isDoesNotExistError err -> test xs
| otherwise -> throwIO err -- reraise other exceptions
如果你對擴展沒問題，我覺得匿名案例很適合這里。
test (x:xs) = try (loadBMP x) >>= \case
Right img -> ...
Left err | ...
| ...
uj5u.com熱心網友回復：
您可以使用package witherable，如下所示：
import Witherable
test :: [String] -> IO [Picture]
test = wither $ \x -> do
b <- doesFileExist x
if b then do
p <- loadBMP x
return $ Just p
else return Nothing
為了直觀地理解什么wither意思，想象一下如果它被命名mapMaybeM了。
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標籤：哈斯克尔