// f : func -> A
// onChange: voidfunc(A) or Runnable
static java.util.Timer onValueChange(int interval, fO f, fO onChange) {
  ret doEvery(interval, new Runnable {
    O lastValue = pcallF(f);
    
    public void run {
      O value = pcallF(f);
      if (neq(value, lastValue)) {
        lastValue = value;
        callF(onChange, value);
      }
    }
  });
}