Re: Add instance Monad ZipList

I know of counterexamples that show why the “obvious” instances don’t work. One nice one is here: https://www.reddit.com/r/haskell/comments/nfyvy/instance_monad_ziplist_where/c38x9q9?utm_source=share&utm_medium=web2x 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. -g On Jun 4, 2020, 11:57 AM -0400, David Feuer , wrote:

To add documentation, we need an explanation of *why* it's impossible.

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On Thu, Jun 4, 2020, 11:35 AM chessai . wrote:

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Agreed, that would be a great addition.

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On Thu, Jun 4, 2020, 8:31 AM Simon Jakobi wrote:

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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 . :

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David is right. This can't happen, unfortunately

On Thu, Jun 4, 2020, 12:48 AM David Feuer 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... by pigworker (Conor McBride) and C. A. McCann. > > On Thu, Jun 4, 2020, 2:53 AM Dannyu NDos 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

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