Superclass equalities

Friends I have long advertised a plan to allow so-called superclass equalities. I've just pushed patches to implement them. So now you can write class (F a ~ b) => C a b where { ... } This email is just to encourage you to try them out. Currently this is just in the HEAD git repository. It'll appear in GHC 7.2, a release we are now actively preparing. But the feature isn't heavily tested (since it didn't exist until today), so I'd appreciate people trying it out. Thanks Simon