Hi, when thinking about this SO question http://stackoverflow.com/q/18583416/1333025, I couldn't find a combinator that allows a parser to /optionally/ fail without consuming input, or consume input and return its value. So I'm suggesting such a function: |-- | @emptyIf p@ parses @p@ and if its return value is @Nothing@, pretends -- that an error has occured with no input consumed. -- -- If @p@ fails and consumes some input, so does @emptyIf p@. Combine with -- 'try' if this is undesirable. emptyIf :: (Stream s m t) =>ParsecT s u m (Maybe a) ->ParsecT s u m a emptyIf p =ParsecT $ \s cok cerr eok eerr -> let cok' (Just x) s e = cok x s e cok'Nothing _ e = eerr e eok' (Just x) s e = eok x s e eok'Nothing _ e = eerr e in unParser p s cok' cerr eok' eerr| With this function, the answer to the SO question becomes really easy: |rcomb :: (Stream s m t) =>ParsecT s u m a ->ParsecT s u m b ->ParsecT s u m b rcomb p q = emptyIf $ runMaybeT (opt p *> opt q) where opt =MaybeT . optional-- optional from Control.Applicative!| Whenever |p| or |q| fails without consuming input, then |rcomb p q| fails without consuming input. Unfortunately |ParsecT| constructor isn't exported so I'm not able to implement it outside /parsec/. (Perhaps it would make sense to export |ParsecT| in some module such as |Text.Parsec.Internal|?) Therefore I'm suggesting to add such a function to /parsec/ (darcs patch included). Perhaps change the name, I couldn't think of anything better. Best regards, Petr