Custom error messages

Give users a clear next step when a check fails. Pass message to a validator for fixed text, or use messageBuilder where supported to include the configured constraint.

Set messages on a schema

import 'package:acanthis/acanthis.dart';

void main() {
  final account = object({
    'name': string().notEmpty(message: 'Enter your name'),
    'age': integer().gte(
      18,
      messageBuilder: (minimum) => 'You must be at least $minimum years old',
    ),
    'email': string().email(message: 'Enter a valid email address'),
  });

  final result = account.tryParse({
    'name': '',
    'age': 16,
    'email': 'invalid-email',
  });

  for (final issue in result.issues) {
    print('${issue.jsonPointer}: ${issue.message}');
  }
}

/name: Enter your name
/age: You must be at least 18 years old
/email: Enter a valid email address

notEmpty() checks a supplied string. A missing object key is a separate required-field condition; see objects.

Group messages by field

Use result.issues.formatFields() to get messages grouped by JSON pointer. A field can have more than one message. Use formatTree() when your UI needs the nested structure.

Localize at presentation time

Keep schemas reusable across languages by resolving messages from each issue’s code and parameters. A resolver returns null to keep the schema’s fallback message.

See presentation and localization for a complete resolver example and the available output formats.