| instance Eval (a -> b) where | .... You could say the same of instance Num Int, because Int is a primitive type. But yes, seq on functions is not lambda-definable, and that is indeed a qualitivative difference between seq and deepSeq I agree. It's not worth making a meal of this. All I'm saying is that there are two distinct decisions 1. Add seqFun :: (a->b) -> c -> c 2. Given (1), one could provide seq via a type-class, or without. H98 chooses the latter For deepSeq, (1) doesn't arise, but the same choice arises for (2), and with the same arguments for and against. See in particular John Hughes's impassioned pleas for not changing type signatures "all the way up" when adding a 'seq'. I don't think we disagree here. But equally I don't there's a clear "right" answer. Simon