sclass WatchVariable<A> extends Var<A> {
  IF0<A> f;
  ReliableSingleThread rst = new(r update_impl);
  
  event change;
  
  *(IF0<A> *f) {
    update();
  }
  
  // only run in rst
  void update_impl() {
    A value = f?!;
    if (!eq(value, get())) {
      set(value);
      change();
    }
  }
  
  void update() {
    rst!;
  }
}