Semi-applied datatypes in instance declarations

I apologise if this has been raised before, but the code I am writing now would look rather nicer if "partially applied type constructors" were permitted in instances. For example: class Event e where sync :: e a -> IO a data Event extraData a = blah blah . . . instance (context on extradata) => Event extradata where blah blah . . . Any chance of this? Or are there reasons why this would be wholly ridiculous?