Hi Ron Generally you would want a monad transformer for combining the effects of of two monads - one monad remains as it is, the other is modified slightly to make it a "transformer" rather than a regular monad. It looks like Attoparsec doesn't supply a transformer instance so that obliges you to make CBC a transformer instead - prosaically this means making CBC an instance of monad transformer class Trans and extending the type of CBC to be something like "CbcT m a" rather than "CBC a" so the monad it transforms can be supplied as a parameter "m". As Attoparsec isn't a transformer you will have to "lift" Attoparsec operations into the CbcT monad so func will look something like this - note the change on the second line: func = do x <- lift $ word128 let r = do y <- decryptCBC x if (predicate y) then return bar else -- somehow read more from the parser? return r Best wishes Stephen