Hi, I think I may have found a way to get zip & friends to fuse with *both* of their input lists. I am not a heavy ghc hacker, though, so I may be missing something important that makes this unworkable. I have no idea what kind of code this would actually end up creating. Anyway, here's my attempt; it ties in with the current foldr2 scheme. I eagerly any await comments or questions, especially from the foldr/build gurus. -- foldr2_both.lhs: Attempting to fuse zip with both input lists. We seem to be forced to use a recursive datatype to accomplish this. We're using newtype, so there should be no overhead from construction/deconstruction of this type, right? \begin{code} newtype BuildZip a b = BZ ((a -> (BuildZip a b) -> b) -> b) bz :: (forall b. (a->b->b)->b->b) -> b -> BuildZip a b bz f n = f (\x xs -> BZ (\c -> c x xs)) (BZ (\_ -> n)) {-# INLINE bz #-} foldr2_both :: (a->b->c->c) -> BuildZip a c -> BuildZip b c -> c foldr2_both k (BZ xs) (BZ ys) = xs (\x xs' -> ys (\y ys' -> k x y (foldr2_both k xs' ys') ) ) {-# RULES "foldr2/both" forall k n (f::forall z.(a->z->z)->z->z) (g::forall z.(b->z->z)->z->z) . foldr2 k n (build f) (build g) = foldr2_both k (bz f n) (bz g n) #-} \end{code} -- END foldr2_both.lhs Best regards, Matt Harden