+1 for adding a Contrafunctor/ContraFunctor to base somewhere. But I agree completely with Tony, please call it contramap. ;) Otherwise people will wonder why comonads are not cofunctors -- a matter which can be cleared up by avoiding sloppy terminology.
+1 for adding Comonads. As an aside, since Haskell doesn't have (nor could it have) coexponential objects, there is no 'missing' Coapplicative concept that goes with it, so there can be no objection on the grounds of lack of symmetry even if the Functor => Applicative => Monad proposal goes through.
I have been meaning to split off a 'comonads' package from category-extras for a while, in a way that avoids requiring tons of crazy machinery. I have a candidate that I just need to polish up a bit and throw on hackage -- perhaps that could serve as a straw man proposal?
-Edward
On Fri, Dec 24, 2010 at 4:51 AM, Stephen Tetley
<stephen.tetley@gmail.com> wrote:
On 24 December 2010 02:16, Mario Blažević <
mblazevic@stilo.com> wrote:
> To turn the proof obligation around, what could possibly be the downside of
> adding a puny Cofunctor class to the base library?
For the record I'm personally neutral on Cofunctor and on balance
would like to see Comonad added to Base.
My reservation is really at the "meta-level" - I suspect there are a
lot of candidates for adding to Base if you want to Base to be
systematic about "modeling structures". At the moment and possibly by
accident rather than explicit intention, the structures in Base
(Monoid, Applicative, Monad, Arrow) add good sets of operational
combinators as well as modeling structures (in Monoid's case it only
adds one operational combinator but it is the basis for Foldable, the
Writer Monad and more).
For Comonad, Cofunctor (Bifunctor, Semigroup...) not having the
visibility of being in Base certainly means there is less motivation
to discover valuable operations that use them, but should they go into
Base without an initial strong operational value, instead maybe
something between Base and Hackage is needed?
Certainly, Hackage isn't great for developing "Base candidates". The
bike shedding on the Libraries list, whilst frustrating for a
proposer, is valuable for teasing out more regular designs than single
authored packages often manage, and having lots of small packages for
Base-like things is a dependency burden that hinders adoption.
Best wishes
Stephen