sclass FunctionsLoader implements AutoCloseable {
  Set<S> functions;
  O blob;
  Throwable exception;
  bool loading;
  L<Runnable> onLoaded = syncList(); // also called on fail
  
  *(Iterable<S> functions) { functions = asSet(functions); }

  synchronized void loadInBackground {
    if (!loading) {
      loading = true;
      startThread(r load);
    }
  }
  
  void load {
    try {
      blob = loadFunctions(functions);
    } catch print e {
      exception = e;
    }
    pcallFAll(onLoaded);
  }
  
  bool loaded() { ret blob != null || exception != null; }
  
  public void close {
    dispose blob;
  }
}