zip in terms of foldr

zip.hs

import Prelude hiding (zip)

-- `zip` accepts the first list and lazily returns a
-- function that accepts the second list and lazily
-- returns the list of pairs, so this implementation
-- is lazy in both arguments.
zip :: [a] -> [b] -> [(a, b)]
zip =
  foldr
    zipxs
    -- Base case which discards any
    -- remaining ys and returns []
    (const [])
  where
    zipxs x zipxs' ys = case ys of
      -- Base case which discards any
      -- remaining xs and returns []
      [] -> []
      (y : ys) -> (x, y) : zipxs' ys

main :: IO ()
main = print $ take 4 $ zip [1, 3 ..] [2, 4 ..]