Pattern matching

Let's take this Username modddel again as an example :

The 'form' validationStep contains two validations : 'length' validation and 'characters' validation. The 'availability' validationStep contains only one validation : 'reserved' validation.

There are multiple pattern matching methods generated by the modddels generator.

/// Use the [map] method to do pattern matching between all the concrete
/// union-cases : the valid union-case + the invalid-step union-cases
///
String message1 = username.map(
    valid: (ValidUsername validUsername) => 'The username is valid',
    invalidForm: (InvalidUsernameForm invalidUsernameForm) => 
        'The username has an invalid form',
    invalidAvailability: (InvalidUsernameAvailability invalidUsernameAvailability) =>
        'The username is not available');

/// Use the [maybeMap] method when you only want to handle a few cases. It
/// returns the result of [orElse] for unhandled cases.
///
bool isAvailable = username.maybeMap(
  invalidAvailability: (InvalidUsernameAvailability invalidUsernameAvailability) => false,
  orElse: () => true,
);

/// Use the [mapOrNull] method when you only want to handle a few cases. It
/// returns null for unhandled cases.
///
String? message2 = username.mapOrNull(
  invalidAvailability: (InvalidUsernameAvailability invalidUsernameAvailability) =>
      'The username is not available',
);

/// Use the [mapValidity] method to do pattern matching between the valid
/// union-case and the abstract invalid union-case
///
username.mapValidity(
  valid: (ValidUsername validUsername) {
    print('Valid username');
  },
  invalid: (InvalidUsername invalidUsername) {
    print('Invalid username');
  },
);

/// The [maybeMapValidity] method is similar to the [maybeMap] method,
/// but :
/// - The [valid] callback parameter is required
/// - The [orElse] callback parameter is required too and has [invalid] as a
/// parameter.
///
String message3 = username.maybeMapValidity(
  valid: (ValidUsername validUsername) => 'Valid',
  invalidAvailability: (InvalidUsernameAvailability invalidUsernameAvailability) => 
      'Not available',
  orElse: (InvalidUsername invalidUsername) => 'Invalid for some other reason',
);

For the abstract invalid union-case (in this example it's InvalidUsername), there are other pattern matching methods you can use to map between the different invalid-step union-cases.

username.mapValidity(
  valid: (ValidUsername validUsername) {},
  invalid: (InvalidUsername invalidUsername) {
    /// Use the [mapInvalid] method to do pattern matching between all the
    /// invalid-step union-cases.
    ///
    String message1 = invalidUsername.mapInvalid(
        invalidForm: (InvalidUsernameForm invalidUsernameForm) => 
            'The username has an invalid form',
        invalidAvailability: (InvalidUsernameAvailability invalidUsernameAvailability) =>
            'The username is not available');

    /// Use the [maybeMapInvalid] method when you don't want to handle all
    /// invalid cases separately. The [orElse] callback parameter is required.
    ///
    invalidUsername.maybeMapInvalid(
      invalidAvailability: (InvalidUsernameAvailability invalidUsernameAvailability) => 
          print('Unavailable'),
      orElse: () => print('Invalid'),
    );

    /// The [mapOrNullInvalid] method is similar to [maybeMapInvalid], but it
    /// returns null for unhandled cases.
    ///
    String? message2 = invalidUsername.mapOrNullInvalid(
      invalidAvailability: (InvalidUsernameAvailability invalidUsernameAvailability) => 
          'Unavailable',
    );

    /// [whenInvalid], [maybeWhenInvalid], and [whenOrNullInvalid] give you a
    /// direct access to the failures of each invalid-step union-case.
    ///
    invalidUsername.whenInvalid(
      formFailures: (lengthFailure, charactersFailure) {},
      availabilityFailures: (reservedFailure) {},
    );

    invalidUsername.maybeWhenInvalid(
      availabilityFailures: (reservedFailure) {},
      orElse: () {},
    );

    invalidUsername.whenOrNullInvalid(
      availabilityFailures: (reservedFailure) {},
    );
  },
);