Superclass equalities

[Edward Z. Yang]

Yay! This is very exciting :-)

Edward

Excerpts from Simon Peyton-Jones's message of Wed Jun 22 12:57:28 -0400 2011:
> Friends
> 
> I have long advertised a plan to allow so-called superclass equalities.  I've just pushed patches to implement them. So now you can write
> 
>     class (F a ~ b) => C a b where  { ... }
> 
> This email is just to encourage you to try them out.  
> 
> Currently this is just in the HEAD git repository.  It'll appear in GHC 7.2, a release we are now actively preparing.  But the feature isn't heavily tested (since it didn't exist until today), so I'd appreciate people trying it out.
> 
> Thanks
> 
> Simon
>