instance Functor Set, was: Re: Export lists in modules

[Johannes Waldmann]

Malcolm Wallace wrote:

> But if contexts-on-datatypes worked correctly,
> 
>     data Set a = Ord a => ....
> 
> then even the "real" map from Data.Set:
> 
>     map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b
> 
> could be an instance method of Functor. 

I'd love that. But I don't quite understand:
do you think this is/should be possible with:
current Haskell? Haskell-Prime? Current ghc (what extensions)?
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