At 2002-04-08 04:14, Simon Peyton-Jones wrote:

I don't know how to make existentials "know" about functional dependencies.

Existentials are not the only things that don't "know" about functional dependencies. This for instance does not compile: class C a b | a -> b g :: (C a b,C a b') => b -> b' g x = x I submit that it should, even if it's bad style to write the type of 'g' like that. This may seem a pointless case, given that one can easily rewrite the type to tell GHC what it can't figure out for itself (that b and b' are the same), but I suspect that it's the same principle as the existential datatype case below, where there doesn't appear to be a workaround.

In the type system that GHC implements, your example fails, and I don't see any meaningful way to make it succeed. But maybe someone else does.

Does anyone even know of a workaround? Given this, find an implementation of 'f' that retrieves the contents of its 'D' argument: class C a b | a -> b data D a = forall b. (C a b) => MkD b f :: (C a b) => D a -> b -- f (MkD b) = b won't compile It's very annoying if it can't be done. -- Ashley Yakeley, Seattle WA