sclass DeferredComputation {
  new Flag done;
  O value;
  Throwable error;
  Runnable continuation, onError;

  void setContinuation(Runnable r) {
    synchronized(done) {
      if (!done.isUp()) ret with continuation = r;
    }
    if (error == null)
      callF(r);
    else
      if (onError != null)
        callF(onError);
      else
        printStackTrace(error);
  }

  void setValue(O value) {
    synchronized(done) {
      if (done.isUp()) fail("setValue/setError called twice");
      this.value = value;
      done.raise();
    }
    callF(continuation);
  }

  void setError(Throwable error) {
    synchronized(done) {
      if (done.isUp()) fail("setValue/setError called twice");
      this.error = error;
      done.raise();
    }
    if (onError != null)
      callF(onError);
    else
      printStackTrace(error);
  }

  O getValue() {
    assertTrue(done.isUp());
    ret value;
  }
}