Implementation of a return construct in OCaml with exceptions and references.

return.rkt

let with_return f x =
  let r = ref None in
  try
    f (fun v -> r := (Some v); raise Not_found) x
  with _ ->
    match !r with
    | None -> raise Not_found
    | Some x -> x

let til_first_even =
  with_return (fun return xs ->
    List.iter (fun x ->
      if x mod 2 = 0 then
        return x
      else
        print_int x; print_string ",") xs;
    -1)

let _ =
  print_int (til_first_even [1; 3; 5; 8; 17; 21]);
  print_string "!\n"