Haskell - 检查文件句柄的有效性

发布于 2024-11-04 18:34:58 字数 214 浏览 9 评论 0原文

好吧，伙计们，超级简单的问题（谷歌没有帮助我解决这个问题似乎很奇怪）：

import IO

--.... yadda, yadda, yadda

  file <- openFile "/some/path" ReadMode

如何检查从 openFile 获得的句柄是否是有效的句柄，即文件存在并且打开成功？

原文

Ok, guys, super easy question (it seems weird that Google didn't help me with this one):

import IO

--.... yadda, yadda, yadda

  file <- openFile "/some/path" ReadMode

How do I check if the handle that I got from openFile is a valid handle, i.e. that the file exists and was opened successfully?

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澜川若宁 2024-11-11 18:34:58

如果文件不存在，或者发生其他错误，对 openFile 的调用将失败并出现异常。

例如：

import System.IO

main = do
   openFile "/some/path" ReadMode

失败原因：

A.hs: /some/path: openFile: does not exist (No such file or directory)

openFile 可能抛出的异常类型此处列出，并包括：

isAlreadyInUseError 如果文件已打开且无法重新打开；
isDoesNotExistError 如果文件不存在；则返回
如果用户没有打开文件的权限，

isPermissionError 。您可以使用 Control.Exception< 捕获这些错误/a>，像这样：

{-# LANGUAGE ScopedTypeVariables #-}

import System.IO
import Control.Exception

main = do
    handle (\(e :: IOException) -> print e >> return Nothing) $ do
      h <- openFile "/some/path" ReadMode
      return (Just h)

If the file doesn't exist, or some other error occurs, the call to openFile will fail with an exception.

For example:

import System.IO

main = do
   openFile "/some/path" ReadMode

Fails with:

A.hs: /some/path: openFile: does not exist (No such file or directory)

The types of exception that may be thrown by openFile are listed here, and include:

isAlreadyInUseError if the file is already open and cannot be reopened;
isDoesNotExistError if the file does not exist; or
isPermissionError if the user does not have permission to open the file.

You can catch these errors using Control.Exception, like so:

{-# LANGUAGE ScopedTypeVariables #-}

import System.IO
import Control.Exception

main = do
    handle (\(e :: IOException) -> print e >> return Nothing) $ do
      h <- openFile "/some/path" ReadMode
      return (Just h)

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