[Haskell-cafe] Re: Re: Data.Ring -- Pre-announce

On Mon, 2010-01-04 at 07:17 -0700, Luke Palmer wrote:

On Mon, Jan 4, 2010 at 6:51 AM, Maciej Piechotka wrote:

...

About comonad - not exactly as every comonad is copointed and the only possible way is extract Empty = _|_

I think this module could be cleaned up by disallowing empty lists. You have this nice semantic property that "every clist has a focus", but when you add empty you have to add "unless it's empty". focus returns a Maybe, isEmpty is necessary.

I mean, it could be that your use case requires empty clists and would be uglier without empty, but think about it. I find in Haskell that simplicity breeds simplicity; i.e. I'm willing to wager that whatever algorithm you are using clist for will actually be cleaner if you got rid of empty and modify the algorithm accordingly. We shall see though...

Luke

However then we lost the monoid (ok. I haven't send patch but please accept[1]) along with alternative/monad plus - which is much more popular, standard and useful then Copointed/Comonad. Additionally it would introduce: fromList [] = _|_ Is is somehow similar to 0 \in N - sometimes it is better to include it sometimes to not include it. Regards [1]

instance Monoid CList where mempty = Empry mappend = mplus