Example problem needing FutureOr<> in Dart

main.dart

import 'dart:async';

import 'package:http/http.dart' as Http;

String callbackOne() => "hello";

Future<String> callbackTwo() async => (await Future.delayed(Duration(seconds: 1),() => "This is a sentence"));

Future<int> getLengthOfResult2(String Function() callback) async {
  return callback().length;
}

main() async {
  getLengthOfResult2(callbackOne);
  getLengthOfResult2(callbackTwo);
}

// For solution, see: https://dartpad.dev/?id=be619b223f20656ad5833402f93487ad