On Thu, Dec 23, 2010 at 5:25 PM, Stephen Tetley <stephen.tetley@gmail.com> wrote:
On 23 December 2010 21:43, Mario Blažević <mblazevic@stilo.com> wrote:
> Why are Cofunctor and Comonad classes not a part of the base library?
[SNIP]
> Later on I found that this question has been raised before by Conal Elliott,
> nearly four years ago.
>
> http://www.haskell.org/pipermail/libraries/2007-January/006740.html


From a somewhat "philistine" persepective, that Conal's question went
unanswered says something:

"Does anyone have useful functionality to go into a Cofunctor module
(beyond the class declaration)?"

Successful post-H98 additions to Base (Applicative, Arrows, ...)
brought a compelling programming style with them. For Comonads,
Category-extras does define some extra combinators but otherwise they
have perhaps seemed uncompelling.


There are plenty of potential Cofunctor instances on Hackage, as I've pointed out. The other side of the proof of the utility of the class would be to find existing libraries that could be parameterized by an arbitrary functor: in other words, some examples in Hackage of

> class Cofunctor c => ...
> instance Cofunctor c => ...
> f :: Cofunctor c => ...

This would be rather difficult to prove - such signatures cannot be declared today, and deciding if existing declarations could be generalized in this way would require a pretty deep analysis. The only thing I can say is "build it and they will come".

To turn the proof obligation around, what could possibly be the downside of adding a puny Cofunctor class to the base library?