dart-language

name	dart-language
description	Dart 3.x language feature standards: null safety, records, sealed classes, switch pattern matching, extensions, and async/await. Use when using !, ?., ??, late, sealed classes, record types, switch expressions, or async patterns — and before introducing any new Dart 3.x construct to confirm the modern idiomatic approach.
metadata	{"triggers":{"files":["**/*.dart"],"keywords":["sealed","record","switch","pattern","!","late","async","extension"]}}

Dart Language Patterns

Priority: P0 (CRITICAL)

Implementation Guidelines

• Null Safety: Avoid !. Use ?., ??, or short-circuiting. Use late only if necessary.
• Immutability: Use final for all variables. Use @freezed for data classes.
• Pattern Matching (3.x): Use switch (value) with patterns and destructuring.
• Records: Use a Record (e.g., (String, int)) for multiple values; use named fields when meaning matters. Destructure at call sites: final (error, data) = fetchUser();.
• Sealed Classes: Use sealed class for exhaustive state handling in domain logic.
• Extensions: Use extension to add utility methods to third-party types.
• Wildcards (3.7+): Use _ for unused variables in declarations and patterns.
• Tear-offs: Prefer using tear-offs (e.g., list.forEach(print)) over anonymous lambdas (e.g., list.forEach((e) => print(e))).
• Asynchrony: Prefer async/await over raw Future.then. Use unawaited for fire-and-forget logic if necessary.
• Encapsulation: Use _ prefix for library-private members. Prefer final properties.
• Collections: Use collection-if, collection-for, and spread operators ....
• Safe List Access: Prefer .firstOrNull, .lastOrNull, or .elementAtOrNull(i).
• No dynamic: Use Object or generics instead of dynamic.
• Type Aliases: Use typedef for complex IDs or callbacks.

Anti-Patterns

• No ! Operator: not use bang operator ! unless you can prove value non-null via if or assert.
• No var for members: not use var for class members; use final or explicit types.
• No logic in constructors: not perform complex calculations or async work inside constructors.
• No zero-arg methods for pure computations: Use a getter. int get invoiceType => not int toInvoiceType().
• No generic conversion names: Name value-object converters for their target context: get apiFilterType not get invoiceType.

Code

// Sealed class and Switch expression
sealed class Result {}
class Success extends Result { final String data; Success(this.data); }
class Failure extends Result {}

String message(Result r) => switch (r) {
  Success(data: var d) => "Got $d",
  Failure() => "Error",
};

References

• feature-based-clean-architecture | tooling

Null-safety checklist

• Replace ! with ?-aware access, explicit narrowing, or a guard clause; do not silence nullability with a force unwrap.

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• Replace ! with ?
• destructure,destructuring,final (error,result.error