sclass Cache<A> {
  O maker; // func -> A
  A value;
  long loaded;
  sbool debug;
  long changeCount;
  Lock lock = lock();
  
  *() {}
  *(O *maker) {}
  *(IF0<A> *maker) {}

  A get() {
    if (hasLock(lock)) ret value; // Must be called from within maker
    lock lock;
    if (loaded == 0) {
      value = make();
      changeCount++;
      loaded = sysNow();
    }
    ret value;
  }

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

  // clear if older than x seconds
  // 0 does not do anything
  void clear(double seconds) {
    lock lock;
    if (seconds != 0 && loaded != 0 && sysNow() >= loaded+seconds*1000)
      clear();
  }
  
  // override
  void set(A a) {
    lock lock;
    value = a;
    ++changeCount;
    loaded = sysNow();
  }
  
  A make() {
    ret (A) callF(maker);
  }
}