livr

Fast, zero-dependency Dart validator for the LIVR specification (Language Independent Validation Rules, v2.0). Rules are plain data: store them in JSON, send them over the wire and validate the same way, with the same error codes, in Dart, JavaScript, Perl, Python, Go and other languages.

• Returns only the fields that have rules, everything else is dropped.
• Reports errors for all fields at once, as codes shaped like the data.
• Cleans the data on the way: trims strings, converts "25" to 25, sets defaults.
• Passes the whole LIVR test suite, on the Dart VM, AOT and the web.

Install

dart pub add livr

Quick start

A validator needs the schema and the type of its output. Describe the type by hand, for example as an extension type over the output map: it costs nothing at run time, the output is not copied.

import 'package:livr/livr.dart';

extension type SignUp(Map<String, Object?> _) {
  String get name => _['name'] as String;
  String get email => _['email'] as String;
  String? get gender => _['gender'] as String?;
  int get age => _['age'] as int;
  String get password => _['password'] as String;
}

final signUpValidator = Validator({
  'name': 'required',
  'email': ['required', 'email'],
  'gender': {'one_of': ['male', 'female']},
  'age': ['required', 'positiveInteger', {'minNumber': 18}],
  'password': ['required', {'min_length': 10}],
  'password2': {'equal_to_field': 'password'},
}, SignUp.new);

switch (signUpValidator.validate(input)) {
  case ValidationSuccess(:final output):
    save(output.email, output.age); // String, int
  case ValidationFailure(:final errors):
    respond422(errors); // {'email': 'WRONG_EMAIL', 'age': 'TOO_LOW'}
}

• Validator(schema, convert) compiles the schema once. An invalid schema (an unknown rule, an invalid regular expression) throws LivrSchemaException right away. A validator is immutable: create it once and validate any number of inputs.
• The type of the output comes from convert: SignUp.new of an extension type, or User.fromJson of a model class (json_serializable, freezed).
• validate() never changes its input. It accepts any Map, for example the output of jsonDecode(); data that is not a map is a ValidationFailure('FORMAT_ERROR').
• validateOrThrow() returns the output or throws LivrValidationException.
• Rule names work as in the specification (min_length) and in camelCase (minLength). Field names are data and are never converted.
• autoTrim: true (or Validator.defaultAutoTrim = true) trims all strings of the input before validation.

Types of the output

Nothing checks that the type matches the schema, so write the types the rules return:

Rules	Type
integer, positive_integer	int
decimal, positive_decimal	double
min_number, max_number, number_between	num
string rules, email, url, iso_date, modifiers	String
nested_object, variable_object	Map<String, Object?>, or an extension type over it
list_of, list_of_objects, list_of_different_objects	List<Object?>

A field without required or default can be missing, so its type is nullable. A mismatch is a TypeError when the field is read, or in fromJson.

Schemas without a type

When the schema is not known at compile time (loaded from a database, different for every customer) or a service only checks data, Validator.untyped() returns the output as a Map<String, Object?>:

final validator = Validator.untyped(rulesFromDatabase);

result.map(User.fromJson) converts the output of one result.

Custom rules

Everything is a rule: the built-in rules in lib/src/rules/ are written exactly like your own, one rule per file, each with its own error codes. A rule builder gets the arguments of the rule from the schema and returns the rule. The rule returns a RuleResult: RuleResult.valid, a new value with RuleResult.value(v), or an error with RuleResult.error('CODE'). The compiler checks that every path returns one.

import 'package:livr/livr.dart';
import 'package:livr/util.dart';

LivrRule strongPassword(List<Object?> args, RuleContext context) {
  final minLength = numberArg(args, 0, 'strong_password'); // {strong_password: 8}

  return (value, allValues) {
    if (isNoValue(value)) return RuleResult.valid; // leave empty values to 'required'
    if (value is! String) return RuleResult.error('FORMAT_ERROR');
    if (value.length < minLength) return RuleResult.error('WEAK_PASSWORD');
    return RuleResult.valid;
  };
}

extension type Account(Map<String, Object?> _) {
  String get password => _['password'] as String;
}

final accountValidator = Validator(
  {'password': ['required', {'strong_password': 8}]},
  Account.new,
  rules: {'strong_password': strongPassword}, // also found as strongPassword
);

• Validator.registerDefaultRules({...}) registers rules for all validators created afterwards.
• Do the expensive work in the builder (parse arguments, compile regular expressions), so the rule only checks values.
• allValues is the object the value belongs to, for rules that compare fields.
• package:livr/util.dart has the helpers the built-in rules use: isNoValue, isPrimitive, livrToString, parseNumber, argument readers.
• Metarules compile nested rules through the RuleContext: context.compileField(rules) and context.compileObject(schema) return rules that validate a value or an object. All built-in metarules are written this way.

Aliases

An alias names a set of rules, with an optional error code, without code:

extension type Person(Map<String, Object?> _) {
  int get age => _['age'] as int;
}

final personValidator = Validator(
  {'age': ['required', 'adult_age']},
  Person.new,
  aliases: [
    const LivrAlias(
      name: 'adult_age',
      rules: ['positive_integer', {'min_number': 18}],
      error: 'WRONG_AGE',
    ),
  ],
);

LivrAlias.fromJson() reads the JSON form, and Validator.registerAliasedDefaultRule() registers an alias for all validators.

Rules

Group	Rules
Common	required, not_empty, not_empty_list, any_object
String	string, eq, one_of, min_length, max_length, length_equal, length_between, like
Numeric	integer, positive_integer, decimal, positive_decimal, min_number, max_number, number_between
Special	email, url, iso_date, equal_to_field
Modifiers	trim, to_lc, to_uc, remove, leave_only, default
Metarules	nested_object, variable_object, list_of, list_of_objects, list_of_different_objects, or

See livr-spec.org for every rule and its error codes.

Dart specifics

The specification leaves some details to the language. This implementation chooses:

• Output types follow the rule, see Types of the output.
• Numbers as strings are formatted like in JavaScript, so values compare the same way in all implementations: 10.0 is "10".
• Booleans are not numbers: numeric rules return their error for true.
• Strings of numbers are decimal numbers only, so no hexadecimal, Infinity or NaN.
• String lengths are counted in characters (Unicode code points): an emoji is one character.
• Empty values: a missing field, null and '' are all empty. A field with null stays in the output as null; a missing field stays missing.
• like uses Dart regular expressions, which have the syntax of JavaScript ones. Only the i flag is supported, as in the specification.

Performance

Schemas are compiled into closures once, never into source code. Validation is a loop over the compiled rules that allocates little: the output map is laid out by the schema, so it is filled by index without hashing.

The scenarios of the LIVR benchmarks (a five-field form; an object with two levels of nesting and a list), with a validator created once (static) and a new validator per validation (dynamic), on one core:

Scenario	Dart AOT	Dart JIT
Simple, static	5.6 M ops/sec	7.0 M ops/sec
Simple, dynamic	2.0 M ops/sec	2.6 M ops/sec
Nested, static	3.3 M ops/sec	3.2 M ops/sec
Nested, dynamic	1.2 M ops/sec	1.3 M ops/sec

On the same machine, the JavaScript implementation (Node.js 26) does 4.4, 1.9, 2.5 and 0.95 M ops/sec. Run the benchmark yourself:

dart run benchmark/benchmark.dart
dart compile exe benchmark/benchmark.dart -o build/benchmark && build/benchmark

Development

dart test              # unit tests and the LIVR test suite
dart test -p node      # the tests on the web (dart2js), needs Node.js
dart analyze --fatal-infos && dart format --set-exit-if-changed .

test/test_suite is a copy of the LIVR test suite; the page has the command that replaces it with the current one.