Embedded Swift on the Raspberry Pi Pico(forums.swift.org)

    sealed class MySomething<T> {
    }

    final class One extends MySomething<Never> {
    }

    final class Two extends MySomething<Never> {
      final Child child; 
      
      Two(this.child);
    }

    final class Three<T> extends MySomething<T> {
      final T value;

      Three(this.value);
    }

    final class Four extends MySomething<Never> {
      final int Function() callback;

      Four(this.callback);
    }

    int foo(MySomething<String> foo) {
      return switch (foo) {
        One() => 1,
        Two(child: Child(:final value)) => value,
        Three(:final value) => int.parse(value),
        Four(:final callback) => callback(),
      };
    }