Re: Add instance Monad ZipList

4 Jun 2020

      Here's my thoughts on why it's impossible:

ZipList is basically ReaderT Natural Maybe with the requirement that
  f m === Nothing -> forall n > m. f n === Nothing

The Applicative and Alternative instances are derived straightforwardly in
this manner and satisfy the requirements, but there is no way to verify the
requirement for the Monad instance. I'm pretty sure that this:
  ZipList [0..] >>= (\n -> if even n then pure (div n 2) else empty)
would produce nonsense no matter the definition of (>>=).

On Thu, Jun 4, 2020, 10:57 David Feuer  wrote:
...
To add documentation, we need an explanation of *why* it's impossible.
On Thu, Jun 4, 2020, 11:35 AM chessai .  wrote:
...
Agreed, that would be a great addition.
On Thu, Jun 4, 2020, 8:31 AM Simon Jakobi 
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@gmail.com>:
...
David is right. This can't happen, unfortunately
On Thu, Jun 4, 2020, 12:48 AM David Feuer 
...
...
I don't remember why right now, but it's moderately well-known that
wrote:
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

Zemyla