static <A> A androidEvalInUIThread(final F0<A> f) {
  final new Var<A> v;
  final new Var<Throwable> error;
  androidUI_wait(r {
    try {
      v.set(callF(f));
    } catch e {
      error.set(e);
    }
  });
  if (error.has()) throw rethrow(error!); else ret v!;
}