https://qiita.com/kmizu/items/26f4ca063f00579de47f

countier.ml

let run_continuer : type a. ((unit -> (unit -> a)) -> a) -> a =
  fun f ->
  let module M = struct
    effect Save : unit -> (unit -> a);;
    let save () = perform @@ Save ()
  end
  in
  let open M in
  match f save with
  | effect (Save ()) k ->
    let rec c k =
      let k' = Obj.clone_continuation k in
      continue k @@ fun () -> c k'
    in c k
  | x -> x

print_int @@ run_continuer @@ fun save ->
  let i = ref 0 in
  let c = save () in
  if !i < 10 then
    let () = incr i in
    c ()
  else
    !i