haskell - 处理 readFile 和 writeFile 异常的正确方法

Question

我正在用 Haskell 编写一个应用程序，如果 readFile，我想向用户显示一条有意义的错误消息。或 writeFile失败。我目前正在抓 IOError与 Control.Exception.tryJust并将它们转换为人类可读的文本。

但是，我无法确定应该捕获哪些错误以及如何从中提取信息。例如，假设“/bin”是目录，“/bin/ls”是文件，readFile "/bin"和 readFile "/bin/ls/asdf"两者都给出“不适当的类型”，但(在我看来)它们是不同的错误。在第一个的情况下，我可以通过处理目录中的每个文件来恢复，而第二个更像是“不存在”类型的错误。

关于前面的示例，似乎没有一种可移植的方法来捕获“不适当的类型”错误。看GHC.IO.Exception , InappropriateType被标记为 GHC-only 所以我不能只在 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在不同的平台、编译器、语言环境等中是否总是相同的。

总之，我的问题是:

我应该为 readFile 捕获哪些异常？/writeFile ?

一旦出现异常，我应该如何从中提取信息？

是否有一种可移植的方式来捕获仅 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
                                            ]

使用 GHC 7.10.3 的 Debian Sid GNU/Linux 上的输出是:

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

Answer 1

1. Which exceptions should I be catching for readFile/writeFile?

在 OS X 下，如果您使用 openFile后跟 hGetContents而不是 readFile那么对于您提到的情况，您将获得不同的异常(exception)情况。
openFile "/bin/ls/asdf" ...将抛出“没有这样的文件或目录”异常，而 openFile "/bin" ...会抛出“不合适的类型”。

在 Linux 下，两个 open 调用都会抛出“不适当的类型”异常。但是，您可以通过 ioe_errno 区分两者。和 ioe_description领域:

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 session :

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

2. Once I have an exception, how should I go about extracting information from it?

每个异常都有自己的结构。 IOException 的定义见 here .

要将字段访问器带入范围，您需要导入 GHC.IO.Exception .

3. Is there a portable way of catching the GHC-only exceptions such as InappropriateType?

正如@dfeuer 所说，出于所有实际目的，GHC 是目前唯一的 Haskell 实现。

更新

运行程序的结果。我没有包括最后一个结果，因为我没有一个只读文件系统来测试它，但我确信错误是一样的。

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