Add instance Monad ZipList

[David Feuer]

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 at 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
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