Levels of abstraction of computation in Dart — see http://worrydream.com/#!2/LadderOfAbstraction

levels.dart

  String level0() {
    return "asd";
  }
  Future<String> level1Async() async {
    await Future.delayed(Duration(seconds: 1), () {});
    return "asd";
  }
  Iterable<String> level1Iteration() sync* {
    yield "a";
    yield "s";
    yield "d";
  }
  Stream<String> level2IterationAndAsync() async* {
    yield "a";
    await Future.delayed(Duration(seconds: 1), () {});
    yield "s";
    yield "d";
  }