[Haskell-cafe] (Co/Contra)Functor and Comonad

wren ng thornton wren at freegeek.org
Sat Dec 25 01:29:00 CET 2010

On 12/24/10 12:26 AM, C. McCann wrote:
> As far as I understand (which may not actually be all that far),
> contravariant functors just go to or from an opposite category, a
> distinction that is purely a matter of definition, not anything
> intrinsic.


> On the other hand, Applicative and Monad are based on
> endofunctors specifically, i.e. functors from a category to itself,
> which would seem to necessarily exclude functors from a category to
> its opposite.

Except for when the category is self-dual (i.e., C = C^op). But 
self-duality brings all sorts of other fun things into play as well.

Live well,

More information about the Haskell-Cafe mailing list