糾正讀取文件和寫入文件異常的方法

Question

我在Haskell中編寫應用程序，並且如果readFile或writeFile失敗，想向用戶顯示有意義的錯誤消息。我目前正在使用Control.Exception.tryJust來捕捉IOError，並將它們轉換爲人類可讀的文本。

但是，我很難弄清楚我應該捕捉哪些錯誤以及如何從中提取信息。例如，假設「/ bin」是一個目錄，「/ bin/ls」是一個文件，readFile "/bin"和readFile "/bin/ls/asdf"都給出了「不恰當的類型」，但是（在我看來）它們是不同的錯誤。在第一種情況下，我可以通過處理目錄中的每個文件來恢復，而第二種更像是「不存在」類型的錯誤。

與前面的例子相比，似乎沒有捕獲「不恰當類型」錯誤的便攜方式。看着GHC.IO.Exception，InappropriateType被標記爲GHC-only，所以我不能只在ioeGetErrorType模式匹配。我能模式匹配上ioeGetErrorString，但我不知道如果這些字符串總是在不同的平臺，編譯器，語言環境是相同的，等

總之，我的問題是：

1. 哪些異常應該我趕上readFile/writeFile？
2. 一旦我有一個例外，我應該如何去從中提取信息？
3. 是否有一種便攜的方式來捕獲GHC唯一的例外，如InappropriateType？

更新：

基於@ 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的輸出與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. 我應該捕捉readFile/writeFile的哪些異常？

在OS X中，如果使用openFile其次是hGetContents代替readFile，那麼你會得到你所提到的情況下，不同的異常。

openFile "/bin/ls/asdf" ...將拋出「沒有這樣的文件或目錄」異常，而openFile "/bin" ...將拋出「不適當的類型」。

在Linux下，這兩個打開的調用都會拋出「不恰當的類型」異常。但是，可以通過ioe_errno和ioe_description字段區分兩者：

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 

樣品ghci的會話：

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

曾經有一個例外，我應該如何去從中提取信息？

每個異常都有自己的結構。可以發現IOException的定義here。

要將字段訪問器納入範圍，您需要導入GHC.IO.Exception。

1. 是否有一種可移植的方式來捕獲僅限GHC的異常，如ValidityType？

正如@dfeuer說，所有的實際目的GHC是此時唯一的Haskell的實現。

更新運行您的程序

結果。我沒有包含最後的結果，因爲我沒有一個只讀的文件系統來測試它，但我相信錯誤會是一樣的。

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