Recoded Sum Types example

domain_model_1.dart

// Root
import 'package:meta/meta.dart';
import 'package:sealed_unions/sealed_unions.dart';

/// ======================================================================
/// Everything above this line is a Doublet & Triplet Sealed Union thing
/// ======================================================================

enum UsState { florida }
enum CaProvince { ontario }

class PostalInfo {
  String street;
  StateOrProvince stateOrProvince;
  PostalInfo({
    @required this.street,
    @required this.stateOrProvince,
  })  : assert(street != null),
        assert(stateOrProvince != null);

  @override
  String toString() => 'PostalInfo street: $street, stateOrProvince: $stateOrProvince';
}

class StateOrProvince extends Union2Impl {
  StateOrProvince(Union2 union) : super(union);
  // PRIVATE low-level factory
  // Used for instantiating individual "subclasses"
  static final Doublet<UsState, CaProvince> _factory = const Doublet<UsState, CaProvince>();

  // PRIVATE constructor which takes in the individual weather states
  StateOrProvince._(Union2<UsState, CaProvince> union) : super(union);

  // PUBLIC factories which hide the complexity from outside classes
  factory StateOrProvince.usState(UsState state) => StateOrProvince._(_factory.first(state));

  factory StateOrProvince.caProvince(CaProvince province) => StateOrProvince._(_factory.second(province));

  @override
  String toString() => join(
        (state) => '$state',
        (province) => '$province',
      );
}

void main() {
  // Correct
  final postalInfoUS = PostalInfo(
    street: 'Some US Street',
    stateOrProvince: StateOrProvince.usState(UsState.florida),
  );
  print(postalInfoUS);

  // Correct
  final postalInfoCA = PostalInfo(
    street: 'Some Canadian Street',
    stateOrProvince: StateOrProvince.caProvince(CaProvince.ontario),
  );
  print(postalInfoCA);

  sendLetter(postalInfoUS);
  sendLetter(postalInfoCA);
}

void sendLetter(PostalInfo postalInfo) {
  postalInfo.stateOrProvince.join(
    (usState) {
      printLetter(postalInfo);
      shipLetterUSPS();
    },
    (caProvince) {
      printLetter(postalInfo);
      shipLetterCanadaPost();
    },
  );
}

void printLetter(PostalInfo postalInfo) {
  // Imagine this prints a physical letter...
  print(
    '''
*******
${postalInfo.street}
${postalInfo.stateOrProvince.toString()}
Yours truly,
The Chief
*******
''',
  );
}

void shipLetterUSPS() {
  print('Shipping to United States\n\n');
}

void shipLetterCanadaPost() {
  print('Shipping to Canada\n\n');
}