A modified zip function in OCaml that sums up the two elements of the input lists in the second item of the output tuple.

zip_sum.ml

(* Define the unfold function.*)
let rec unfold p g b1 b2 =
  if p b1 b2 then [] else
    (match g b1 b2 with (a, (b1prime, b2prime)) ->
       a :: unfold p g b1prime b2prime)
;;

(* Define the zip_sum function.*)
let zip_sum = unfold
  (* Define p.*)
  (fun x y -> ((List.length x = 0) || (List.length y = 0)))
  (* Define g.*)
  (fun x y -> ( (List.hd x, (List.hd x + List.hd y)), (List.tl x, List.tl y)))
;;

See my blog post.