fundeps help

[Ganesh Sittampalam]

On Mon, 3 Dec 2007, Simon Peyton-Jones wrote:

> No, you didn't miss anything.  I wouldn't expect anyone to write these 
> types directly.  But it can happen:

>        class C a b | a->b
>        instance C Int Bool
>
>        class D x where
>          op :: forall y. C x y => x -> y
>
>        instance D Int where
>          op = ...
>
> In the (D Int) instance, what's the type of op?  Well, it must be
>        op :: forall y. C Int y => Int -> y
>
> And here we know that y=Bool; yet since we don't write the type sig 
> directly we can't say it.  So GHC's implementation of fundeps rejects 
> this program; again it can't be translated into System F.

Conveniently, this is a good example of my other problem with fundeps :-) 
I can work around the problem from my first email with an unsafeCoerce, 
but is there any way I can get around the issue above at the moment in 
either 6.8 or the HEAD? I actually plan to recast the code in question 
using associated type synomyms once they're working properly, but would 
like to be able to make some progress now.

Cheers,

Ganesh