Reading the failures

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.

You can easily access the failure(s) of an invalid-step union-case :

username.mapOrNull(
  invalidAvailability: (invalidUsernameAvailability) {
    /// InvalidUsernameAvailability has only one validation, and thus only one
    /// failure that it can hold.
    ///
    /// For this reason, the failure is not nullable.
    ///
    UsernameReservedFailure failure = invalidUsernameAvailability.reservedFailure;
  },
  invalidForm: (invalidUsernameForm) {
    /// InvalidUsernameForm has two validations, and thus can hold one or two
    /// failures.
    ///
    /// For this reason, the failures are nullable.
    ///
    UsernameCharactersFailure? failure1 = invalidUsernameForm.charactersFailure;
    UsernameLengthFailure? failure2 = invalidUsernameForm.lengthFailure;

    /// You can use the "hasFailure" getters to check if the invalid-step
    /// union-case has a particular failure.
    ///
    if (invalidUsernameForm.hasCharactersFailure) {
      // At this point, the charactersFailure is not null.
      final failure = invalidUsernameForm.charactersFailure!;
    }
    if (invalidUsernameForm.hasLengthFailure) {
      // At this point, the lengthFailure is not null.
      final failure = invalidUsernameForm.lengthFailure!;
    }
  },
);

You can also access a list of all the failures of an invalid union-case, using the failures getter :

username.mapValidity(
  valid: (validUsername) {},
  invalid: (invalidUsername) {
    /// We can access the list of failures from the abstract invalid union-case
    ///
    final failures = invalidUsername.failures;

    /// We can also access it from each invalid-step union-case
    ///
    invalidUsername.mapOrNull(
      invalidForm: (invalidUsernameForm) {
        final failures = invalidUsernameForm.failures;
      },
    );
  },
);