I have written a first attempt at a fold function for the heterogenious list: class RFold i r where rFold :: (forall a . a -> i -> i) -> i -> r -> i instance RFold i RNil where rFold f i RNil = i instance RFold i r => RFold i (a `RCons` r) where rFold f i (x `RCons` xs) = f x (rFold f i xs) This works providing the folded 'op' has the type: forall a . a -> i -> i which means it does not work for functions like show :: forall a . Show a => a -> i -> i as the types are different. I have not figured out a way to make it accept a constraint like Show for example. Here is an example: length = rFold (\_ -> (+1)) 0 relation The use of such a function seems limited, if constraints like Show cannot be used, as most useful applications of fold would require some kind of class membership for example: string = rFold shows "" relation This fails to compile because shows has type: shows :: forall a . Show a => a -> i -> i but fold expects op :: forall a . a -> i -> i Regards, Keean Schupke.