Well, my problems have now multiplied beyong belief! Allow me to summerise: 1. There exists an I/O exception called "invalid parameter", which is not detectable with any of the 8 predicates. (Isn't this what "illegal operation" is supposed to be for? A general catch-all?) 2. Attempting to open an invalid filename yields the undetectable "invalid parameter" exception (rather than, say, illegal operation). 3. Attempting to read a directory with an invalid name does the same. 3. Attempting to open a *file* you don't have permission for yields "permission error", as expected. However, attempting to read a *directory* yields... invalid parameter again. (Thus, even if this exception were detectable, there's no way to tell the difference between a permission error and a malformed filename.) 4. Not actually related to exceptions, but doesDirectoryExist "C:\" yields False. (Even though getDirectoryContents "C:\" yields the correct result.) So I can't even use that to filter out invalid directory names. Aaaarrrgh! >_< Clearly, making my program give the user meaningful error messages is going to be very hard - unless I resort to trying to *parse* the output of the Show instance for IOError... Does anybody have any helpful hints for me at this point?