effekt.ml

open Effect
open Effect.Shallow

type 'a t += Recur : 'a -> 'b t

let recur a = perform (Recur a)

let loop : 'a -> ('a -> 'b) -> 'b =
 fun init fn ->
  let rec run : ('a, 'b) continuation -> 'a -> 'b =
   fun k input ->
    continue_with k input
      {
        retc = (fun x -> x);
        exnc = (fun e -> raise e);
        effc =
          (fun (type b) (eff : b t) ->
            match eff with
            | Recur value ->
                Some
                  (fun (_ : (b, _) continuation) ->
                    run (fiber fn) (Obj.magic value))
            | _ -> None);
      }
  in
  run (fiber fn) init

let fact i =
  loop (i, 1) (fun (iter, acc) ->
      if iter > 0 then recur (iter - 1, iter * acc, 1) else acc)

let () = print_int (fact 10)