I've just been through the process of converting some code from using Control.Exception to using an Error monad, and I would recommend that as a more straightforward and manageable alternative, unless you need Control.Exception for other reasons. I started by changing my potentially-failing functions to return Either Exception a, where Exception is my own user-defined error type, but you could use String, and a is the type of the "real" return value. I initially used explicit Left and Right to construct appropriate values. My testing code is hand-written, and explicitly matched against Left e and Right x to decode the return value. I ended up with functions returning MonadError Exception m => m a, using throwError and return to construct appropriate values. My testing code uses ErrorT Exception (StateT s IO) a to manage a combination of error-handling, state (an error count) and IO, with the option of using Either Exception to test expected errors for individual cases. -- Iain Alexander ia@stryx.demon.co.uk