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<div dir="auto">I know of counterexamples that show why the “obvious” instances don’t work. One nice one is here:<br />
<br />
<a href="https://www.reddit.com/r/haskell/comments/nfyvy/instance_monad_ziplist_where/c38x9q9?utm_source=share&utm_medium=web2x" target="_blank">https://www.reddit.com/r/haskell/comments/nfyvy/instance_monad_ziplist_where/c38x9q9?utm_source=share&utm_medium=web2x</a><br />
<br />
But I don’t know of a simple full proof of why no such instance is possible. My guess would be that we should consider the action of the operations on lengths of lists, and show that the requirements for monad and ziplist conflict.<br />
<br />
-g</div>
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<div name="messageReplySection">On Jun 4, 2020, 11:57 AM -0400, David Feuer <david.feuer@gmail.com>, wrote:<br />
<blockquote type="cite" style="border-left-color: grey; border-left-width: thin; border-left-style: solid; margin: 5px 5px;padding-left: 10px;">
<div dir="auto">To add documentation, we need an explanation of *why* it's impossible.</div>
<br />
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<div dir="ltr" class="gmail_attr">On Thu, Jun 4, 2020, 11:35 AM chessai . <<a href="mailto:chessai1996@gmail.com">chessai1996@gmail.com</a>> wrote:<br /></div>
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<div dir="auto">Agreed, that would be a great addition.</div>
<br />
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<div dir="ltr" class="gmail_attr">On Thu, Jun 4, 2020, 8:31 AM Simon Jakobi <<a href="mailto:simon.jakobi@googlemail.com" target="_blank" rel="noreferrer">simon.jakobi@googlemail.com</a>> wrote:<br /></div>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">This sounds worth documenting though.<br />
<br />
Dannyu, would you be interested in updating the ZipList docs to say<br />
why it doesn't have a Monad instance?<br />
<br />
Cheers,<br />
Simon<br />
<br />
Am Do., 4. Juni 2020 um 17:21 Uhr schrieb chessai . <<a href="mailto:chessai1996@gmail.com" rel="noreferrer noreferrer" target="_blank">chessai1996@gmail.com</a>>:<br />
><br />
> David is right. This can't happen, unfortunately<br />
><br />
> On Thu, Jun 4, 2020, 12:48 AM David Feuer <<a href="mailto:david.feuer@gmail.com" rel="noreferrer noreferrer" target="_blank">david.feuer@gmail.com</a>> wrote:<br />
>><br />
>> 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 <a href="https://stackoverflow.com/questions/6463058/help-on-writing-the-colist-monad-exercise-from-an-idioms-intro-paper" rel="noreferrer noreferrer noreferrer" target="_blank">https://stackoverflow.com/questions/6463058/help-on-writing-the-colist-monad-exercise-from-an-idioms-intro-paper</a> by pigworker (Conor McBride) and C. A. McCann.<br />
>><br />
>> On Thu, Jun 4, 2020, 2:53 AM Dannyu NDos <<a href="mailto:ndospark320@gmail.com" rel="noreferrer noreferrer" target="_blank">ndospark320@gmail.com</a>> wrote:<br />
>>><br />
>>> instance Monad ZipList where<br />
>>> ZipList [] >>= _ = ZipList []<br />
>>> ZipList (x:xs) >>= f = ZipList $ do<br />
>>> let ZipList y' = f x<br />
>>> guard (not (null y'))<br />
>>> let ZipList ys = ZipList xs >>= ZipList . join . maybeToList . fmap snd . uncons . getZipList . f<br />
>>> head y' : ys<br />
>>><br />
>>> instance MonadFail ZipList where<br />
>>> fail _ = empty<br />
>>><br />
>>> instance MonadPlus ZipList<br />
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