At 2001-08-12 16:55, Hal Daume III wrote:

but supposing you wanted a sort of relaxed equality wherein, if you chould check for equality on the second element, you would, otherwise you would just check the first element. i would want to say something like:

instance (Eq a, Not (Eq b)) => Eq (MyPair a b) where (MyPair a _) == (MyPair a' _) = a == a'

But from what you're telling me there's no way to have both of these instances, correct?

There's no 'Not', so you can't have the second one. If you had a 'Not', it would allow some ugliness: -- data PQ = P | Q equalPQ = (MyPair 'a' P) == (MyPair 'a' Q) -- Note that equalPQ is True. But if someone else introduces an "instance Eq PQ" in another module, equalPQ could become False. I think this sort of thing is supposed to be prohibited: code that is correct cannot change its meaning due to modules it does not depend on. -- Ashley Yakeley, Seattle WA