Anomalous Class Fundep Inference

[Ashley Yakeley]

At 2001-05-03 01:42, Simon Peyton-Jones wrote:

>| In Hugs 98 Feb2001 (as hugs -98), this piece of code gives an error:
>| 
>| class X a b | a -> b where x :: a;
>| class (X a b) => Y a b | a -> b where y :: a;
>| instance (X a b) => Y a b where y = x;
>| instance (Num a) => X a a where x = 0; -- line A
>
>Quite right too!  The final instance declaration says that the two
>arguments of X must always be the same:

Hmm...

>	* the class says a->b

Yes.

>	* the instance matches any first arg

I was rather hoping that the instance would match only (Num a => a) 
rather than (a).

>   and says that the second must be equal.

Yes.


-- 
Ashley Yakeley, Seattle WA