Arrow Classes

[Ashley Yakeley]

In article <20030710133324.GA2318@soi.city.ac.uk>,
 Ross Paterson <ross@soi.city.ac.uk> wrote:

> As it happens, I would like yet another intermediate class:
> 
> 	class BiFunctor a where
> 		bimap :: (b' -> b) -> (c -> c') -> a b c -> a b' c'

This can be decomposed into:

  fmap :: (c -> c') -> a b c -> a b c'

  cofmap2 :: (b' -> b) -> a b c -> a b' c

  bimap :: (Cofunctor2 a,Functor (a b)) =>
   (b' -> b) -> (c -> c') -> a b c -> a b' c'
  bimap bb cc = (cofmap2 bb) . (fmap cc)

It would be nice to be able to write this:

  class (Cofunctor2 a,forall b. Functor (a b)) =>
   BiFunctor a
  instance (Cofunctor2 a,forall b. Functor (a b)) =>
   BiFunctor a

Unfortunately, foralls are not allowed in class or instance contexts...

-- 
Ashley Yakeley, Seattle WA