dart-modern-features

Dart Modern Features

1. When to use this skill

Use this skill when:

• Writing or reviewing Dart code targeting Dart 3.0 or later.
• Refactoring legacy Dart code to use modern, concise, and zero-cost features
  such as records, extension types, class modifiers, null-aware collection
  elements, dot shorthands, and digit separators.
• Looking for idiomatic ways to handle multiple return values or zero-cost
  domain wrappers.

For pattern matching, switch expressions, and list/map/split destructuring, use
dart-use-pattern-matching. For converting closed class hierarchies into
exhaustive sealed types, use dart-seal-type-hierarchies.

When NOT to use (Abstention Guardrails)

Do NOT apply modern features or refactor code when:

• SDK Constraint < 3.0.0: The package's pubspec.yaml specifies an SDK
  constraint that supports Dart 2.x (e.g., sdk: '>=2.19.0 <4.0.0').
  Refactoring to Dart 3 features will introduce syntax errors for Dart 2 users.
• Public API Records with > 3 Fields: Records work well for 2–3 return
  values or internal tuples, but complex public API payloads are clearer and
  more extensible as dedicated classes or extension types.

Discovery

To find candidates for modernization:

Null-Aware Elements

Search for collection if statements checking for null:

• Regex: if\s*\(\w+\s*!=\s*null\)\s*\w+

Digit Separators

Search for long numbers without separators:

• Regex: \b\d{6,}\b (Matches numbers with 6 or more digits).

2. Features

Records

Use records as anonymous, immutable, aggregate structures to bundle multiple
objects without defining a custom class. Prefer them for returning multiple
values from a function or grouping related data temporarily.

Avoid: Creating a dedicated class for simple multiple-value returns.

class UserResult {
  final String name;
  final int age;
  UserResult(this.name, this.age);
}

UserResult fetchUser() {
  return UserResult('Alice', 42);
}

Prefer: Using records to bundle types seamlessly on the fly.

(String, int) fetchUser() {
  return ('Alice', 42);
}

void main() {
  var user = fetchUser();
  print(user.$1); // Alice
}

Class Modifiers

Use class modifiers (final, base, interface, sealed) to restrict how
classes can be subtyped outside their defining library:

• interface class: External libraries may implement, but cannot extend.
• base class: External libraries may extend, but cannot implement
  (preserving private implementation invariants).
• final class: External libraries can neither extend nor implement.
• sealed class: Closed family of subtypes within the same library enabling
  exhaustive switching (see dart-seal-type-hierarchies).

Avoid: Leaving internal implementation classes open to arbitrary external
subclassing or interface implementation when invariants must be enforced.

class TokenStore {
  void save(String token) {}
}

Prefer: Declaring explicit subtyping capabilities with class modifiers.

final class TokenStore {
  void save(String token) {}
}

Extension Types

Use extension types for a zero-cost wrapper around an existing type. Use them to
restrict operations or add custom behavior without runtime overhead.

Avoid: Allocating new wrapper objects just for domain-specific logic or type
safety.

class Id {
  final int value;
  Id(this.value);
  bool get isValid => value > 0;
}

Prefer: Using extension types which compile down to the underlying type at
runtime.

extension type Id(int value) {
  bool get isValid => value > 0;
}

Digit Separators

Use underscores (_) in number literals strictly to improve visual readability
of large numeric values.

Avoid: Long number literals that are difficult to read at a glance.

const int oneMillion = 1000000;

Prefer: Using underscores to separate thousands or other groupings.

const int oneMillion = 1_000_000;

Wildcard Variables

Use wildcards (_) as non-binding variables or parameters to explicitly signal
that a value is intentionally unused.

Avoid: Inventing clunky, distinct variable names to avoid "unused variable"
warnings.

void handleEvent(String ignoredName, int status) {
  print('Status: $status');
}

Prefer: Explicitly dropping the binding with an underscore.

void handleEvent(String _, int status) {
  print('Status: $status');
}

Null-Aware Elements

Use null-aware elements (?) inside collection literals to conditionally
include items only if they evaluate to a non-null value.

Avoid: Using collection if statements for simple null checks.

var names = [
  'Alice',
  if (optionalName != null) optionalName,
  'Charlie'
];

Prefer: Using the ? prefix inline.

var names = ['Alice', ?optionalName, 'Charlie'];

Dot Shorthands

Use dot shorthands to omit the explicit type name when it can be confidently
inferred from context, such as with enums or static fields.

Avoid: Fully qualifying type names when the type is obvious from the
context.

LogLevel currentLevel = LogLevel.info;

Prefer: Reducing visual noise with inferred shorthand.

LogLevel currentLevel = .info;

Related Skills

• dart-use-pattern-matching: Authoritative guide for Dart 3 pattern
  matching, switch expressions, and list/map/String.split() destructuring.
• dart-seal-type-hierarchies: Converting closed class hierarchies into
  sealed types for exhaustive switching.
• dart-best-practices: General code style and foundational Dart idioms
  that predate or complement the modern syntax features.