Effective Dart Skill

This skill defines how to write idiomatic, high-quality Dart and Flutter code following Effective Dart guidelines.

1. Naming Conventions

• Capitalize acronyms and abbreviations longer than two letters like words: HttpRequest, not HTTPRequest.
• Avoid abbreviations unless the abbreviation is more common than the full term.
• Prefer putting the most descriptive noun last in names.
• Use terms consistently throughout your code.
• Follow mnemonic conventions for type parameters: E (element), K/V (key/value), T/S/U (generic types).
• Consider making code read like a sentence when designing APIs.
• Prefer a noun phrase for non-boolean properties or variables.
• Prefer a non-imperative verb phrase for boolean properties or variables; prefer the positive form.
• Consider omitting the verb for named boolean parameters.
• Avoid starting a function or method name with get; prefer removing get and using a getter when the API conceptually exposes a property.

2. Types and Functions

• Use class modifiers (final, sealed, interface, base, mixin) to control whether a class can be extended or implemented.
• Type annotate variables without initializers.
• Type annotate fields and top-level variables if the type isn't obvious.
• Annotate return types on function declarations.
• Annotate parameter types on function declarations.
• Write type arguments on generic invocations that aren't inferred.
• Annotate with dynamic instead of letting inference fail.
• Use Future<void> as the return type of async members that do not produce values.
• Use getters for operations that conceptually access properties.
• Use setters for operations that conceptually change properties.
• Use a function declaration to bind a function to a name.
• Use inclusive start and exclusive end parameters to accept a range.
• Don't annotate a return type on a setter — including the []= operator, which behaves like a setter.

// Prefer: explicit class modifier
final class AppConfig {
  final String apiUrl;
  final int timeout;
  const AppConfig({required this.apiUrl, required this.timeout});
}

// Prefer: sealed for exhaustive pattern matching
sealed class Result<T> {}
class Success<T> extends Result<T> { final T value; Success(this.value); }
class Failure<T> extends Result<T> { final Exception error; Failure(this.error); }

3. Style

dart format .

• Format code with dart format — don't manually format.
• Use curly braces for all flow control statements.
• Prefer final over var when variable values won't change.
• Use const for compile-time constants.
• Prefer lines 80 characters or fewer for readability.

4. Imports and Files

• Don't import libraries inside the src directory of another package.
• Don't allow import paths to reach into or out of lib.
• Prefer relative import paths within a package.
• Don't use /lib/ or ../ in import paths.
• Consider writing a library-level doc comment for library files.

5. Structure

• Keep files focused on a single responsibility.
• Limit file length to maintain readability.
• Group related functionality together.
• Prefer making fields and top-level variables final.
• Consider making constructors const if the class supports it.
• Prefer making declarations private — only expose what's necessary.

6. Usage Patterns

// Adjacent string concatenation (not +)
final greeting = 'Hello, '
    'world!';

// Collection literals
final list = [1, 2, 3];
final map = {'key': 'value'};

// Initializing formals
class Point {
  final double x, y;
  Point(this.x, this.y);
}

// Empty constructor body
class Empty {
  Empty();  // not Empty() {}
}

// rethrow to preserve stack trace
try {
  doSomething();
} catch (e) {
  log(e);
  rethrow;
}

• Use whereType<T>() to filter a collection by type.
• Follow a consistent rule for var and final on local variables.
• Initialize fields at their declaration when possible.
• Override hashCode if you override ==; ensure == obeys mathematical equality rules.
• Prefer specific exception handling: use on SomeException catch (e) instead of broad catch (e) or .catchError handlers.

7. Documentation

/// Returns the sum of [a] and [b].
///
/// Throws [ArgumentError] if either value is negative.
int add(int a, int b) { ... }

• Format comments like sentences (capitalize, end with period).
• Use /// doc comments — not /* */ block comments — for types and members.
• Prefer writing doc comments for public APIs; consider them for private APIs too.
• Start doc comments with a single-sentence summary, separated into its own paragraph.
• Avoid redundancy with the surrounding context.
• Start function/method comments with a third-person verb if the main purpose is a side effect.
• Start with a noun or non-imperative verb phrase if returning a value is the primary purpose.
• Start boolean variable/property comments with "Whether" followed by a noun or gerund phrase.
• Use [identifier] in doc comments to refer to in-scope identifiers.
• Use prose to explain parameters, return values, and exceptions (e.g., "The [param]", "Returns", "Throws" sections).
• Put doc comments before metadata annotations.
• Document why code exists or how it should be used, not just what it does.

8. Testing Patterns

• Write unit tests for business logic, using group and descriptive test names:

import 'package:test/test.dart';

void main() {
  group('CartService', () {
    late CartService cart;

    setUp(() => cart = CartService());

    test('addItem increases item count', () {
      cart.addItem(Product(id: '1', name: 'Widget', price: 9.99));
      expect(cart.items, hasLength(1));
    });

    test('removeItem decreases total price', () {
      final product = Product(id: '1', name: 'Widget', price: 9.99);
      cart.addItem(product);
      cart.removeItem(product.id);
      expect(cart.totalPrice, equals(0.0));
    });
  });
}

• Write widget tests using testWidgets and WidgetTester:

import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('LoginButton shows loading indicator when tapped',
      (WidgetTester tester) async {
    await tester.pumpWidget(const MaterialApp(home: LoginScreen()));
    await tester.tap(find.byType(ElevatedButton));
    await tester.pump();
    expect(find.byType(CircularProgressIndicator), findsOneWidget);
  });
}

9. Code Review Workflow

When reviewing Dart code for Effective Dart compliance, the agent should check:

1. Naming — verify all identifiers follow the conventions in Section 1.
2. Type annotations — confirm public API parameters, return types, and uninitialized variables are annotated.
3. Class modifiers — verify final, sealed, or interface is used where appropriate.
4. Documentation — confirm all public members have /// doc comments with a single-sentence summary.
5. Style — run dart format --output=none --set-exit-if-changed . to verify formatting.
6. Analysis — run dart analyze and confirm zero issues.

References

• Effective Dart
• Dart Site WWW GitHub Repository