dars (dart result) #

English | 日本語

A Dart-idiomatic Result type package with first-class developer experience.

⚠️ Currently under development. Breaking changes may occur.

Features #

• Dart-idiomatic Result type - Intuitive API designed for Dart developers
• Pattern matching support - Leverage Dart 3's powerful pattern matching
• Early return with $ syntax - Inspired by rust package - thank you!
• Full async support - FutureResult with all methods available
• Nullable-to-Result conversion - Easy T?.toResult() extension
• Type-safe API - Compile-time safety for error handling
• Testing support - Dedicated dars_test package with smart Result matchers

Installation #

dependencies:
  dars:

Or run:

dart pub add dars

Quick Start #

import 'package:dars/dars.dart';

Result<int, String> divide(int a, int b) {
  if (b == 0) return Err('Division by zero');
  return Ok(a ~/ b);
}

void main() {
  final result = divide(10, 2);

  // Pattern matching
  switch (result) {
    case Ok(:final value): print('Result: $value');
    case Err(:final error): print('Error: $error');
  }
}

Core Concepts #

Result Type #

Result<T, E> is a sealed class that represents either success (Ok<T, E>) or failure (Err<T, E>).

const ok = Ok<int, String>(42);
const err = Err<int, String>('Something went wrong');

print(ok.isOk);   // true
print(err.isErr); // true

Pattern Matching #

Use Dart 3's pattern matching for clean and exhaustive handling:

final message = switch (result) {
  Ok(:final value) => 'Got value: $value',
  Err(:final error) => 'Got error: $error',
};

Early Return with $ Syntax #

Simplify error handling with the $ syntax for early returns:

Result<int, String> calculate(int a, int b, int c) {
  return Result(($) {
    final ab = divide(a, b)[$];  // Returns early if Err
    final cd = divide(c, 1)[$];
    return Ok(ab + cd);
  }, onCatch: (e, s) => Err('Unexpected: $e'));
}

Async Support #

Full async support with Result.async and FutureResult:

Future<Result<String, String>> fetchUser(int id) {
  return Result.async(($) async {
    if (id <= 0) return Err('Invalid ID');

    final name = await fetchName(id)[$]; // Early return if fetchName returns Err
    return Ok('User: $name');
  }, onCatch: (e, s) => Err('Unexpected: $e'));
}

API Reference #

Method Comparison (Rust / Swift) #

Type Aliases #

typedef FutureResult<T, E> = Future<Result<T, E>>;
typedef VoidResult<E> = Result<void, E>;

Nullable Extension #

final String? name = getUserName();
final result = name.toResult(orElse: () => 'Name not found');

Complete Example #

See example/example.dart for a comprehensive demonstration.

Additional Information #

• 📋 Design Document (日本語)

License #

BSD 3-Clause License - see LICENSE for details.