IO 操作期间的 Haskell IO 错误处理

Question

在 System.Directory图书馆，getPermissions函数可能会返回 IO 错误。
文档说它可能会失败 isPermissionError或 isDoesNotExistError .
如何在调用过程中处理 IO 错误 getPermissions ?

试图:

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" } }

Answer 1

该错误消息表示无法确定您想要捕获的异常类型(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，如下所示:

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

或者，如果您使用 isDoesNotExistError和 friend 在 System.IO.Error 为了区分错误情况，异常类型将被推断为 IOError不需要额外的注释。

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