fix.ml

let fix f x =
  match f x with
  | `Ok -> Gen.singleton x
  | `Gen gen ->
    let s = Stack.create () in
    Stack.push gen s;
    let rec aux () =
      if Stack.is_empty s then None
      else begin
        let g = Stack.top s in
        match g () with
        | Some y -> y
        | None ->
          let _ = Stack.pop s in
          aux ()
      end
    in
  aux