Add instance Monad ZipList

[chessai .]

Agreed, that would be a great addition.

On Thu, Jun 4, 2020, 8:31 AM Simon Jakobi <simon.jakobi at googlemail.com>
wrote:

> 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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