Martin Hofmann wrote:
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'.
Yes, there is a way. :) deleteM e = (\s -> if S.null s then Just s else Nothing) . S.delete e insertM e = Just . S.insert e . maybe S.empty id The maybe function is from Data.Maybe . I did wonder why you want to delete the empty set, but now I see that you need it for alter . Regards, apfelmus -- http://apfelmus.nfshost.com