In #haskell on freenode we had a discussion about isPrefixOf, which is probably implemented roughly as so: isPrefixOf [] _ = True isPrefixOf _ [] = False isPrefixOf (x:xs) (y:ys) = x == y && isPrefixOf xs ys Well, this is basically just a zip with a special base case. But you can't just write it with zipWith because zipWith stops when it exausts either list. How about we define zipWith'' like this: zipWith'' _ [] _ l _ = [l] zipWith'' _ _ [] _ r = [r] zipWith'' f (x:xs) (y:ys) l r = f x y : zipWith'' f xs ys l r Then we can write: isPrefixOf xs ys = and (zipWith'' (==) xs ys True False) A point free reduction might look like the following and probably isn't worth it: isPrefixOf = (and .) . flip flip False . flip flip True . zipWith'' (==) Are there lots of other places where this zipWith'' would come in handy? It seems like I've found lots of times when I needed to manually code the recursion because of the way zip behaves when it exhausts one of its parameter lists. Jason