纠正读取文件和写入文件异常的方法

Question

我在Haskell中编写应用程序，并且如果readFile或writeFile失败，想向用户显示有意义的错误消息。我目前正在使用Control.Exception.tryJust来捕捉IOError，并将它们转换为人类可读的文本。

但是，我很难弄清楚我应该捕捉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"都给出了“不恰当的类型”，但是（在我看来）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，而第二种更像是“不存在”类型的错误。

与前面的例子相比，似乎没有捕获“不恰当类型”错误的便携方式。看着GHC.IO.Exception，InappropriateType被标记为GHC-only，所以我不能只在ioeGetErrorType模式匹配。我能模式匹配上ioeGetErrorString，但我不知道如果这些字符串总是在不同的平台，编译器，语言环境是相同的，等

总之，我的问题是：

1. 哪些异常应该我赶上readFile/writeFile？
2. 一旦我有一个例外，我应该如何去从中提取信息？
3. 是否有一种便携的方式来捕获GHC唯一的例外，如InappropriateType？

更新：

基于@ ErikR的回答我看的GHC.IO.Exception.IOException具有以下的Haskell程序中的字段：

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希德GNU/Linux的输出与GHC 7.10.3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. 我应该捕捉readFile/writeFile的哪些异常？

在OS X中，如果使用openFile其次是hGetContents代替readFile，那么你会得到你所提到的情况下，不同的异常。

openFile "/bin/ls/asdf" ...将抛出“没有这样的文件或目录”异常，而openFile "/bin" ...将抛出“不适当的类型”。

在Linux下，这两个打开的调用都会抛出“不恰当的类型”异常。但是，可以通过ioe_errno和ioe_description字段区分两者：

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 

样品ghci的会话：

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

1. 曾经有一个例外，我应该如何去从中提取信息？

开始=>

每个异常都有自己的结构。可以发现IOException的定义here。

要将字段访问器纳入范围，您需要导入GHC.IO.Exception。

1. 是否有一种可移植的方式来捕获仅限GHC的异常，如ValidityType？

正如@dfeuer说，所有的实际目的GHC是此时唯一的Haskell的实现。

更新运行您的程序

结果。我没有包含最后的结果，因为我没有一个只读的文件系统来测试它，但我相信错误会是一样的。

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