RE: Anomalous Class Fundep Inference

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