Add instance Monad ZipList

[Simon Jakobi]

This sounds worth documenting though.

Dannyu, would you be interested in updating the ZipList docs to say
why it doesn't have a Monad instance?

Cheers,
Simon

Am Do., 4. Juni 2020 um 17:21 Uhr schrieb chessai . <chessai1996 at gmail.com>:
>
> David is right. This can't happen, unfortunately
>
> On Thu, Jun 4, 2020, 12:48 AM David Feuer <david.feuer at gmail.com> wrote:
>>
>> 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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