在Haskell中，有没有办法在函数防护中进行IO？

发布于 2024-08-19 04:50:38 字数 217 浏览 15 评论 0原文

例如：

newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
          | doesFileExist x == True = return False
          | otherwise = return True

这可以工作吗？

原文

For example:

newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
          | doesFileExist x == True = return False
          | otherwise = return True

Can this be made to work?

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望喜 2024-08-26 04:50:38

您已经在 IO monad 中，那么为什么不使用以下内容呢？

newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
          | otherwise = do exists <- doesFileExist x
                           return $ not exists

对于应用性的好处：

import Control.Applicative

newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
          | otherwise = not <gt; doesFileExist x

正如您所看到的，应用性路线比您想在问题中使用的防护更加简洁！

You're already in the IO monad, so why not use the following?

newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
          | otherwise = do exists <- doesFileExist x
                           return $ not exists

For applicative goodness:

import Control.Applicative

newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
          | otherwise = not <gt; doesFileExist x

As you can see, the applicative route is even more concise than the guards you'd like to use in your question!

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蓝眸 2024-08-26 04:50:38

不，没有办法做到这一点（缺少不安全的技巧，这在这里是完全不合适的）。

顺便说一句，如果可能的话，doesFileExist x == True 最好写成 doesFileExist x 。

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|煩躁 2024-08-26 04:50:38

这有效并完成了所需的操作：

newfile :: FilePath -> IO Bool
newfile fn = do 
    x <- runErrorT $ do
        when ((length fn) <= 0) (throwError "Empty filename")
        dfe <- liftIO $ doesFileExist fn
        when (dfe) (throwError "File already exists")
        return True
    return $ either (\_ -> False) id x

This works and does what's needed:

newfile :: FilePath -> IO Bool
newfile fn = do 
    x <- runErrorT $ do
        when ((length fn) <= 0) (throwError "Empty filename")
        dfe <- liftIO $ doesFileExist fn
        when (dfe) (throwError "File already exists")
        return True
    return $ either (\_ -> False) id x

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