Flutter/Dart pattern matching

pattern_matching.dart

void main() {
  Greeting message = Hi();

  final result = match(message, {
    on((Hi hi) => "hello, ${hi.name}"),
    on((Bye bye) => "bye, see you at ${bye.seeYouAgain}"),
    otherwise(() => throw Error()),
  });

  print(result);
}

abstract class Greeting {}

class Hi implements Greeting {
  String name;
}

class Bye implements Greeting {
  DateTime seeYouAgain;
}

pattern_matching_2.dart

void main2() {
  final future = ObservableFuture.value(10);

  final futureResult = match(future.status, {
    when(FutureStatus.fulfilled, () => Text("done ${future.value}")),
    when(FutureStatus.pending, () => CircularProgressIndicator()),
    when(FutureStatus.rejected, () => Text("error ${future.error})),
  });

  runApp(futureResult);
}

z_pattern_matching.dart

abstract class MatchingActions<T, R> {}

class Otherwise<T, R> implements MatchingActions<T, R> {
  final R Function() onMatched;
  Otherwise(this.onMatched);
}

class On<T, R> implements MatchingActions<T, R> {
  final R Function(T type) onMatched;
  Type matchesType;
  On(this.onMatched) : matchesType = T;
}

class OnValue<T, R> implements MatchingActions<T, R> {
  final R Function() onMatched;
  final T value;
  OnValue(this.value, this.onMatched);
}

MatchingActions<T, R> on<T, R>(R Function(T type) onMatched) {
  return On<T, R>(onMatched);
}

MatchingActions<T, R> when<T, R>(T value, R Function() onMatched) {
  return OnValue<T, R>(value, onMatched);
}

MatchingActions<Null, R> otherwise<R>(R Function() onMatched) {
  return Otherwise<Null, R>(onMatched);
}

R match<T, R, U extends T>(T value, Set<MatchingActions<U, R>> matchers) {
  Otherwise _otherwise;
  for (final matcher in matchers) {
    if (matcher is On && value.runtimeType == (matcher as On).matchesType) {
      return (matcher as On).onMatched(value);
    }
    if (matcher is OnValue && (matcher as OnValue).value == value) {
      return (matcher as OnValue).onMatched();
    }
  }
  return (_otherwise.onMatched());
}