[Haskell-cafe] A generics question

[Stefan Holdermans]

Henry,

Jason pointed out:

> You'd get fromEnum and toEnum.  Which I think, would give you the  
> int mapping that you are after.
>
> fromEnum :: Enum a => a -> Int
> toEnum :: Enum a => Int -> a

To me, this would indeed seem the way to go for your particular example.

Moreover, as for generic producer functions in general, the pattern  
suggested by the Prelude would be to have

   c :: Color
   c = undefined

   convert :: Data a => Int -> a
   convert i x =
     let c = dataTypeConstrs (dataTypeOf x) !! (i-1)
     in  fromConstr c

and then use it as in

   convert 1 `asTypeOf` c

You'll find out that in most cases the (pseudo) "type annotation"  
isn't really needed and the type of the value to produce can be  
determined automatically by the context.

Cheers,

   Stefan