IO操作期间的Haskell IO错误处理

2023年3月3日 299次阅读

在System.Directory库中,getPermissions函数可能返回IO错误.

文档说它可能会因isPermissionError或isDoesNotExistError而失败.

如何在调用getPermissions期间处理IO错误？

尝试：

input <- try (do 
        permissions <- getPermissions filepath 
        print permissions)
case input of
        Left e  -> print "a"
        Right e -> print "b"

错误：

No instance for (Exception e0) arising from a use of ‘try’
The type variable ‘e0’ is ambiguous
Note: there are several potential instances:
  instance Exception NestedAtomically
    -- Defined in ‘Control.Exception.Base’
  instance Exception NoMethodError
    -- Defined in ‘Control.Exception.Base’
  instance Exception NonTermination
    -- Defined in ‘Control.Exception.Base’
  ...plus 7 others
In a stmt of a 'do' block:
  input <- try
             (do { permissions <- getPermissions filepath;
                   print permissions })
In the expression:
  do { input <- try
                  (do { permissions <- getPermissions filepath;
                        print permissions });
       case input of {
         Left e -> print "a"
         Right e -> print "b" } }
In an equation for ‘checkwritefilepermissions’:
    checkwritefilepermissions filepath
      = do { input <- try
                        (do { permissions <- getPermissions filepath;
                              print permissions });
             case input of {
               Left e -> print "a"
               Right e -> print "b" } }

最佳答案 错误消息说,无法确定要捕获的异常类型(即,Exception的实例).一种可能的解决方案是提供一个类型注释来指定它,如：

case (input :: Either IOError String) of
    Left e -> print "a"
    Right r -> print "b"

或者,如果在System.IO.Error中使用isDoesNotExistError和friends来区分错误情况,则异常类型将被推断为IOError,而不需要额外的注释.

有关基本异常捕获实践的相关讨论可在the Control.Exception documentation中找到.

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