[Haskell-cafe] Puzzled by error with forall quantifier.

Can anyone explain the following error message from Hug98 (with extensions enabled)? I have

class Space t class Space t => HasY v t where assignY :: v -> t -> t getY :: t -> v varY :: forall w . Space w => v -> (t->t) -> (w->w)

I get an error on the last definition "quantifier does not mention type variable v" (and if I quantify v, then it complains that t is not quantified). The idea I'm trying to express is that for any instance of HasY v t there should be a function varY with type v -> (t->t) -> (w->w) for some type w of class Space. Any help much appreciated. Is there any kind of tutorial introduction to "forall" out there? Cheers, Theo Norvell