main.dart

Future<int> add(int a, int b) async {
  return Future.delayed(Duration(seconds: 2)).then((f) {
    return a + b;
  });
}

Future<int> test(Future<int> Function(int a, int b) func) async {
  return await func(3, 2);
}

void main() {
  test(add)
    .then(print); /// will return result of 3+2 after 2 seconds
}