copied from js_of_ocaml/toplevel/examples/lwt_toplevel

effects_flags.ml

@@ -0,0 +1,15 @@
+let enable b n =
+  let f = if b then "--enable" else "--disable" in
+  [ f; n ]
+
+let () =
+  let major = String.split_on_char '.' Sys.ocaml_version |> List.hd |> int_of_string in
+  let has_effect = major >= 5 in
+  let l = enable has_effect "effects" in
+  match Sys.argv |> Array.to_list |> List.tl with
+  | "txt" :: [] -> List.iter print_endline l
+  | "sexp" :: [] ->
+      print_endline "(";
+      List.iter print_endline l;
+      print_endline ")"
+  | _ -> assert false