[Hs-Generics] type constructor genericity with regular

[Jan Christiansen]

On 15.09.2010, at 13:41, José Pedro Magalhães wrote:

> Ah, I now see what you mean. Yes, you are right: regular does not  
> abstract over type parameters, hence you can't do much to them. We  
> have a similar library which does abstract over one parameter,  
> though. It's called generic-deriving on Hackage, and used by the  
> Utrecht Haskell Compiler [1, 2].

This looks quite interesting, thanks.

> However, in your application of shape to Tree, I see that you want  
> to replace not only the parameters but also the recursive occurrences.

I don't see what you mean here.

> For curiosity: what do you want this shape function for?

This is probably a quite odd "application". I have proved some  
properties about functions f :: [a] -> [a] by representing them by a  
combination of (!!) and a function shape. That is, we have

   f xs == map (!!xs) (shape xs)

where shape has the type [a] -> [Int] and simply numbers the elements  
of the list. I want to generalize these statements to functions f :: F  
a -> G a where F and G are arbitrary functors by providing  
generalizations of shape and (!!) for arbitrary functors.

Cheers, Jan