fundeps help

Ganesh Sittampalam ganesh at
Mon Dec 3 17:18:16 EST 2007

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.



More information about the Glasgow-haskell-users mailing list