I would like to be able to write something like this: > data Foo c = forall a . c a => Foo a Unfortunately, this isn't allowed (apparently) because it's an illegal class assertion (or at least that's what ghc tells me). ... so I have two questions: 1) Is there any particular reason whey we can't do this (it would break some type safety things or whatever) 2) Any ways to get around it? Thanks! - Hal Hmm. Interesting. I wrote a paper a couple of years ago, on "Restricted Data Types", which discusses among other things how to simulate abstraction over a class by abstraction over the corresponding dictionary type. You might find the technique useful. It's at http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps John Hughes