Name: dart-modern-features
Author: kevmoo

Guidelines for using modern Dart features (v3.0 - v3.10) such as Records, Pattern Matching, Switch Expressions, Extension Types, Class Modifiers, Wildcards, Null-Aware Elements, and Dot Shorthands.

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. Prefer: Using records to bundle types seamlessly on the fly. 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. Prefer: Declaring explicit subtyping capabilities with class modifiers. 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. Prefer: Using extension types which compile down to the underlying type at runtime. 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. Prefer: Using underscores to separate thousands or other groupings. 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. Prefer: Explicitly dropping the binding with an underscore. 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. Prefer: Using the ? prefix inline. 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. Prefer: Reducing visual noise with inferred shorthand. 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. [dart use pattern matching]: https://github.com/dart lang/skills/blob/main/skills/dart use pattern matching/SKILL.md [dart seal type hierarchies]: https://github.com/kevmoo/dash skills/blob/main/skills/dart seal type hierarchies/SKILL.md [dart best practices]: https://github.com/kevmoo/dash skills/blob/main/skills/dart best practices/SKILL.md