timewarp.sml

(* Inspired by: https://www.reddit.com/r/ProgrammingLanguages/comments/1golfwz/emit_a_time_travelling_programming_language/ *)

structure Timewarp =
  struct
    structure Cont = SMLofNJ.Cont;

    type t = unit Cont.cont option ref

    fun point () =
      let
        val pt : t = ref NONE
      in
        Cont.callcc(fn k => pt := SOME k);
        pt
      end

    fun travelOnce pt =
      case !pt of
      | NONE   => ()
      | SOME k => pt := NONE before Cont.throw k ()
  end


fun main () =
  let
    open Timewarp

    val x = ref 10
    val p = point ()
  in
    print ("x=" ^ Int.toString (!x) ^ "\n");
    x := 20;
    travelOnce p
  end


(*
 * Needs SML/NJ with Successor ML features. Run with:
 *
 * $ sml -Cparser.succ-ml=true < timewarp.sml
 *)
do main ();