hash module https://reasonml.github.io/en/try?rrjsx=false&reason=LYewJgrgNgpgBAFwJ4Ad4BECiA1AkgYUzgF44BvAKDjlgTjACcBDAdwC44BnBBgSwDsA5iQB8cCP14IA3BQC+s2vRgA3XgGMYnDgAkmnABYIARlAB0CABTc+QgDRxQkWHCx5CAShJw9hk+fUGGCYEGEsARgB2D1knaHgAZWwAcW9KaiVGVm8bAWFiMQApTjMoEEEAJksAIk4VYTjYaodcoRj5WV8jUzMmMDBLMFUNLQdqpOTmuEtGxJSPDhnweNccAkwPdtm4AAV0ADE0qhoYOiyWH…

HashModule.re

module type DEVICE = {
  let draw: string => unit;
};
let devices: Hashtbl.t(string, module DEVICE) = Hashtbl.create(17);
module SVG = {
  let draw = string => Js.log2("svg module", string);
};
Hashtbl.add(devices, "SVG", (module SVG): (module DEVICE));
module PDF = {
  let draw = string => Js.log2("pdf module", string);
};
Hashtbl.add(devices, "PDF", (module PDF): (module DEVICE));
exception E;
module Device = (
  val try (Hashtbl.find(devices, "PDF")) {
      | Not_found =>
        Js.log("Unknown device %s\n");
        raise(E);
      }:
    DEVICE
);

let draw_using_device = (device_name, picture) => {
  module Device = (val Hashtbl.find(devices, device_name): DEVICE);

  Device.draw(picture);
};

Js.log(devices);
draw_using_device("PDF", "+_+")