modrec.ml

let print_res = function
  | Ok n -> Printf.printf "Ok %d\n" n;
  | Error exn -> Printf.printf "Error (%s)\n" (Printexc.to_string exn)

let run f =
  match f () with
  | n -> Ok n
  | exception e -> Error e

module rec Bad : sig
  val f : unit -> int
  val a : (int, exn) result Lazy.t
  val b : (int, exn) result Lazy.t
  val c : (int, exn) result Lazy.t
  val d : (int, exn) result Lazy.t
end = struct
  let f () = 42
  let a = lazy (run f)
  let b = lazy (run Bad.f)
  let c = lazy (run f)
  let d = lazy (run Bad.f)

  let _ = Lazy.force c
  let _ = Lazy.force d
end

open Bad
let e = run f

let () = List.iter print_res [Lazy.force a; Lazy.force b; Lazy.force c; Lazy.force d; e]