处理readFile和writeFile异常的正确方法

Question

处理readFile和writeFile异常的正确方法

8

我正在使用Haskell编写一个应用程序，如果readFile或writeFile失败，我希望向用户显示有意义的错误消息。我目前使用Control.Exception.tryJust捕获IOError并将其转换为人类可读的文本。

然而，我不知道应该捕获哪些错误以及如何从中提取信息。例如，假设“/bin”是一个目录，“/bin/ls”是一个文件，readFile "/bin"和readFile "/bin/ls/asdf"都会给出“不适当的类型”，但我认为它们是不同的错误。在第一种情况下，我可以通过处理目录中的每个文件来恢复，而第二种情况更像是一种“不存在”的错误类型。

与前面的例子相关，似乎没有一种便携式的方法来捕获“不适当的类型”错误。查看GHC.IO.Exception，InappropriateType被标记为仅限于 GHC ，因此我不能在ioeGetErrorType上进行模式匹配。我可以在ioeGetErrorString上进行模式匹配，但我不确定这些字符串是否在不同的平台、编译器和区域设置中总是相同。

总之，我的问题如下：

1. 我应该捕获哪些异常以处理readFile/writeFile？ 2. 一旦我有了一个异常，我应该如何提取其中的信息？ 3. 是否存在一种便携式的方法来捕获仅适用于GHC的异常，例如InappropriateType？

更新：

根据@ErikR的回答，我正在查看GHC.IO.Exception.IOException的字段，使用以下Haskell程序：

import Control.Exception (try)
import GHC.IO.Exception (IOException(..))
import qualified Data.ByteString as B


main :: IO ()
main = do
    try (readFile "/nonexistent") >>= printException
    try (writeFile "/dev/full" " ") >>= printException
    try (readFile "/root") >>= printException
    try (readFile "/bin") >>= printException
    try (writeFile "/bin" "") >>= printException
    try (readFile "/bin/ls/asdf") >>= printException
    try (writeFile "/bin/ls/asdf" "") >>= printException
    try (B.readFile "/dev/null") >>= printException

    -- I have /media/backups mounted as read-only. Substitute your own read-only
    -- filesystem for this one
    try (writeFile "/media/backups/asdf" "") >>= printException

printException :: Either IOError a -> IO ()
printException (Right _) = putStrLn "No exception caught"
printException (Left e) = putStrLn $ concat [ "ioe_filename = "
                                            , show $ ioe_filename e
                                            , ", ioe_description = "
                                            , show $ ioe_description e
                                            , ", ioe_errno = "
                                            , show $ ioe_errno e
                                            ]

在Debian Sid GNU/Linux上，使用GHC 7.10.3的输出结果如下：

ioe_filename = Just "/nonexistent", ioe_description = "No such file or directory", ioe_errno = Just 2
ioe_filename = Just "/dev/full", ioe_description = "No space left on device", ioe_errno = Just 28
ioe_filename = Just "/root", ioe_description = "Permission denied", ioe_errno = Just 13
ioe_filename = Just "/bin", ioe_description = "is a directory", ioe_errno = Nothing
ioe_filename = Just "/bin", ioe_description = "Is a directory", ioe_errno = Just 21
ioe_filename = Just "/bin/ls/asdf", ioe_description = "Not a directory", ioe_errno = Just 20
ioe_filename = Just "/bin/ls/asdf", ioe_description = "Not a directory", ioe_errno = Just 20
ioe_filename = Just "/dev/null", ioe_description = "not a regular file", ioe_errno = Nothing
ioe_filename = Just "/media/backups/asdf", ioe_description = "Read-only file system", ioe_errno = Just 30

- Matthew

1

在当前的Haskell基础设施状态下，“可移植”异常并不乐观。我们已经停滞在单编译器模式中多年了。希望这种情况很快会改变。 - dfeuer

1个回答

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- ErikR · Accepted Answer

我应该为readFile/writeFile捕获哪些异常？

在OS X下，如果你使用openFile后跟hGetContents而不是readFile，那么你将会得到你提到的不同异常。

openFile "/bin/ls/asdf" ...会抛出“没有这个文件或目录”异常，而openFile "/bin" ...则会抛出“不适当的类型”异常。

在Linux下，两个open调用都会抛出“不适当的类型”异常。然而，你可以通过ioe_errno和ioe_description字段区分它们：

import System.IO
import GHC.IO.Exception
import Control.Exception

foo path = do
  h <- openFile path ReadMode
  hClose h

show_ioe :: IOException -> IO ()
show_ioe e = do
  putStrLn $ "errno: " ++ show (ioe_errno e)
  putStrLn $ "description: " ++ ioe_description e

bar path = foo path `catch` show_ioe

示例ghci会话：

*Main> bar "/bin"
errno: Nothing
description: is a directory
*Main> bar "/bin/ls/asd"
errno: Just 20
description: Not a directory

我一旦有异常，该如何从中提取信息？

每个异常都有自己的结构。IOException的定义可以在这里找到。

要将字段访问器引入作用域，您需要导入GHC.IO.Exception。

是否有一种便携式的方法来捕获仅限于GHC的异常，例如InappropriateType？

正如@dfeuer所说，实际上GHC是目前唯一的Haskell实现。

更新

运行程序后的结果。我没有包含最后一个结果，因为我周围没有只读文件系统进行测试，但我确定错误是相同的。

ioe_filename = Just "/nonexistent", ioe_description = "No such file or directory", ioe_errno = Just 2
ioe_filename = Just "/dev/full", ioe_description = "Permission denied", ioe_errno = Just 13
ioe_filename = Just "/root", ioe_description = "is a directory", ioe_errno = Nothing
ioe_filename = Just "/bin", ioe_description = "is a directory", ioe_errno = Nothing
ioe_filename = Just "/bin", ioe_description = "Is a directory", ioe_errno = Just 21
ioe_filename = Just "/bin/ls/asdf", ioe_description = "Not a directory", ioe_errno = Just 20
ioe_filename = Just "/bin/ls/asdf", ioe_description = "Not a directory", ioe_errno = Just 20
ioe_filename = Just "/dev/null", ioe_description = "not a regular file", ioe_errno = Nothing