{ fetchurl
, mruby
, mrbgems
, writeShellScriptBin
, lib
, callPackage

# Configuration
, withSimulator ? false      # Builds lvgui with simulator.
}:

let
  mruby-lvgui-native-fragment = callPackage ./mruby-lvgui-native-fragment {
    inherit withSimulator;
  };
in
# We need a reference to this package for the passthru `wrap` helper.
let loader = mruby.builder {
  pname = "mobile-nixos-script-loader";
  version = "0.2.0";

  src = ./.;

  # `main.rb` is where the magic happens.
  buildPhase = ''
    makeBin loader main.rb
  '';

  # This script loader handles all "applets" and scripts that will run during
  # stage-1.
  gems = with mrbgems; [
    { core = "mruby-exit"; }
    { core = "mruby-io"; }
    { core = "mruby-sleep"; }
    { core = "mruby-time"; }
    mruby-dir
    mruby-dir-glob
    mruby-env
    mruby-file-stat
    mruby-json
    mruby-logger
    mruby-open3
    mruby-process-clock_gettime
    mruby-regexp-pcre
    mruby-singleton
    mruby-time-strftime
    mruby-zmq

    # Glue that serves to link the proper dependencies into the project.
    mruby-lvgui-native-fragment

    # Though this needs to be the real last gem, as it has
    # special significance during the build.
    mruby-require
  ];

  passthru = {
    # Wraps an `mrb` applet into a runner script that uses this loader.
    # Note that this is not used in stage-1.
    # NOTE: `env` escapes values using `builtins.toJSON`. This escapes some pitfalls, while keeping variables expandable.
    wrap = {name, applet, env ? {}}: (writeShellScriptBin name ''
      ${let
        varList = lib.attrsets.mapAttrsToList (name: value: "${name}=${builtins.toJSON value}") env;
        scriptEnv = lib.strings.concatStringsSep "\n" varList;
      in scriptEnv}
      exec ${loader}/bin/loader ${applet} "$@"
    '').overrideAttrs(old: {
      passthru = {
        inherit applet;
      };
    });
  };
};
in loader