Dart: sealed-класс и switch с деструктуризацией

sealed class Shape {}

class Circle extends Shape {
  final double r;
  Circle(this.r);
}

class Rect extends Shape {
  final double w, h;
  Rect(this.w, this.h);
}

String describe(Shape s) => switch (s) {
      Circle(r: 0) => 'точка',
      Circle(:final r) => 'круг $r',
      Rect(:final w, :final h) when w == h => 'квадрат $w',
      Rect(:final w, :final h) => 'прямоугольник ${w * h}',
    };

void main() {
  for (final s in [Circle(0), Circle(2), Rect(3, 3), Rect(2, 5)]) {
    print(describe(s));
  }
}