sclass Cache<A> {
  O maker; // func -> A
  A value;
  long loaded;
  sbool debug;
  
  *() {}
  *(O *maker) {}

  synchronized A get() {
    if (loaded == 0) {
      value = (A) callF(maker);
      loaded = sysNow();
    }
    ret value;
  }

  synchronized void clear {
    if (debug && loaded != 0)
      print("Clearing cache");
    value = null;
    loaded = 0;
  }

  // clear if older than x seconds
  synchronized void clear(double seconds) {
    if (loaded != 0 && sysNow() >= loaded+seconds*1000)
      clear();
  }
}