Comment by mraleph

[disclaimer: I work on Dart, though not on the language team]

With primary constructors this will become something along the lines of:

    sealed class Message();

    class IncrementBy(final int value) extends Message;

    class DecrementBy(final int value) extends Message;

    class Set(final int value) extends Message;

Which is considerably less repetitive, though `extends Message` is still there. I am fairly optimistic that the next step would be to eliminate that[1], though I think we would need to gather a bit more feedback from users as people are getting more and more reps in with Dart 3.0 features. I personally would prefer something along the lines of:

    sealed class Message() {
      case IncrementBy(final int value);
      case DecrementBy(final int value);
      case Set(final int value);
    }

Current syntax is not all that bad if you are going to do OO and add various helper methods on `Message` and its subclasses, but if you just want to define your data and no behavior / helpers - then it is exceedingly verbose.

[1]: https://github.com/dart-lang/language/issues/3021