I often come across the problem to insert into a collection which might not exist yet, or delete from it and want the collection to be deleted if it is empty afterwards. I always end up with these two functions: deleteM :: (Ord a) => a -> Maybe (Set a) -> Maybe (Set a) deleteM e s = liftM (S.delete e) s >>= \s' -> if S.null s' then return s' else Nothing insertM :: (Ord a) => a -> Maybe (Set a) -> Maybe (Set a) insertM e s = case s of (Just s') -> return $ S.insert e s' Nothing -> return $ S.insert S.empty Is there a way to express each in a (polymorphic) point-free one-liner? If not, why isn't there such a function for the standard collection, because IMHO this is what you need when using 'alter'. Thanks, Martin