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);

  }

}