Re: Add instance Monad ZipList

4 Jun 2020

      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 . :
...
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
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Re: Add instance Monad ZipList

Simon Jakobi