Chris Okasaki wrote:
> I'm using the November 99 Hugs on an x86 running Linux.
> It seems to be hanging when it tries to access a dictionary
> through a superclass. Here's the simplest program
> I can come up with that exhibits the bug.
>
> > class Eq a => Super c a where
> > empty :: c a
> > class Super c a => Sub c a where
> > f :: c a -> Bool
> > g :: c a -> Bool
> >
> > instance Eq a => Super [] a
> > instance Eq a => Sub [] a where
> > f [x,y] = x == y
> > g [x,y] = True
> >
> > p :: [Int]
> > p = [1,2]
>
> Now, when I say
> g p
> it immediately returns True as expected. But when I say
> f p
> it hangs.
The problem is that hugs fools itself into thinking that the `Sub'
instance doesn't really depend on `Eq a' (if you comment out the `Eq a',
in the `Subst' instance, hugs is happy), and conjures an `Eq' dictionary
from out of the ether, which in this case means bottom.
I have no idea how easy this is to fix.
--Jeff
