Haskell IO error handling during IO operation

Question

In the System.Directory library, getPermissions function could return IO errors. The documentation says it could fail with isPermissionError or isDoesNotExistError. How do I handle IO errors during calling getPermissions?

Attempt:

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

Error:

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

The error message says it was impossible to figure out the type of exception (that is, the instance of Exception) you want to catch. One possible solution is providing a type annotation which specifies it, as in:

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

Alternatively, if you use isDoesNotExistError and friends in System.IO.Error to distinguish the error cases the exception type will be inferred as IOError without requiring extra annotations.

A relevant discussion of basic exception catching practices can be found in the Control.Exception documentation.

Answer 2

There is an easy fix (not a best fix) to this problem, replace

Left e

by

Left (e :: SomeException)

---

By the way, your code could be simplified like this

input <- try $ getPermissions filepath
case input of
    Left (e :: SomeException) -> print "a"
    Right e -> print "b"

And you really should improve those names in your code.