[Haskell-cafe] Extending constraints

[Andres Löh]

What you want seems a bit tricky.

> p :: ProxyWrapper Num
> p = ProxyWrapper (Proxy :: Proxy Int)

At this point, all you know about p is that it is a Num. You don't
know that it is Typeable, because you choose to forget about that. You
could give p the type 'ProxyWrapper (Ext Typeable Num)' and it would
work.

> then the following would give a type error:
>
> oops :: TypeRep
> oops = typeOfInnerProxy p

Yes, and correctly so. Because Typeable isn't even a superclass of
Num. So there's no way to know that p actually contains a Typeable
proxy.

Cheers,
  Andres

-- 
Andres Löh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com