I don't remember why right now, but it's moderately well-known that there is no possible Monad instance compatible with the Applicative instance for ZipList. See the answers to https://stackoverflow.com/questions/6463058/help-on-writing-the-colist-monad-exercise-from-an-idioms-intro-paper by pigworker (Conor McBride) and C. A. McCann.

On Thu, Jun 4, 2020, 2:53 AM Dannyu NDos <ndospark320@gmail.com> wrote:

instance Monad ZipList where
    ZipList [] >>= _ = ZipList []
    ZipList (x:xs) >>= f = ZipList $ do
        let ZipList y' = f x
        guard (not (null y'))
        let ZipList ys = ZipList xs >>= ZipList . join . maybeToList . fmap snd . uncons . getZipList . f
        head y' : ys

instance MonadFail ZipList where
    fail _ = empty

instance MonadPlus ZipList

_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries