Question

我正在Haskell中编写一个应用程序，并希望在readFile或writeFile失败时向用户显示有意义的错误消息。我目前正在使用IOError抓取Control.Exception.tryJust并将其转换为人类可读的文字。

但是，我无法弄清楚应该捕获哪些错误以及如何从中提取信息。例如，假设“/ bin”是一个目录而“/ bin / ls”是一个文件，readFile "/bin"和readFile "/bin/ls/asdf"都给出“不合适的类型”，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。

与前面的例子相比，似乎没有一种可移植的方式来捕获“不适当类型”错误。查看GHC.IO.Exception，InappropriateType仅标记为GHC，因此我无法在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平台，编译器，语言环境等之间是否总是相同。

总之，我的问题是：

我应该抓住readFile / writeFile？
一旦出现异常，我该如何从中提取信息？
是否有一种可移植的方式来捕获仅限GHC的例外，例如InappropriateType？

更新

根据@ ErikR的回答，我正在查看GHC.IO.Exception.IOException的字段，其中包含以下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

我应该为readFile / writeFile捕获哪些异常？

在OS X下，如果您使用openFile后跟hGetContents而不是readFile，那么您提到的案例会有不同的例外情况。

openFile "/bin/ls/asdf" ...会抛出一个＆＃34;没有这样的文件或目录＆＃34;异常而openFile "/bin" ...将抛出＆＃34;不合适的类型＆＃34;。

在Linux下，两个打开的电话都会抛出一个＆＃34;不合适的类型＆＃34;例外。但是，您可以通过ioe_errno和ioe_description字段区分这两者：

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

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

一旦出现异常，我该如何从中提取信息？

每个例外都有自己的结构。可以找到IOException的定义here。

要将字段访问者纳入范围，您需要导入GHC.IO.Exception。

是否有一种可移植的方式来捕获仅限GHC的异常，例如InappropriateType？

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

<强>更新

运行程序的结果。我没有包含最后一个结果，因为我没有只读文件系统来测试它，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

正确处理readFile和writeFile异常的方法

1 个答案: