just-works: Skill for Claude Code

Swift Coding

Project conventions override these defaults. Read 2-3 nearby files first, and check the project's own target: Package.swift or the .xcodeproj for Swift version, language mode, platform targets, and the concurrency build settings (strict concurrency level, default actor isolation). Do not assume a concurrency default the project has not set.

Never rules

• Never force unwrap (!) or try! outside tests, @IBOutlet, and provable programmer errors (a hard-coded URL literal) with an explaining comment. Use guard let, if let, ??, do-catch, or try?.
• Never use a bare catch { } that swallows the error. Catch specific types, or log and rethrow.
• Never keep mutable global or static state. Use actors, @MainActor, or dependency injection. Swift 6 language mode reports these as errors.
• Never block @MainActor. No Thread.sleep(), synchronous network calls, or heavy computation on the main actor; use async/await or a background context.
• Never print() in production code. Use os.Logger: it is filterable, structured, and searchable in Console.app.
• Never generate key material by hand. Use CryptoKit (SymmetricKey(size:)) or SecRandomCopyBytes for keys, tokens, and session IDs. SystemRandomNumberGenerator and Int.random(in:) are fine for general use.
• Never use Any or AnyObject where a protocol or generic works. Prefer generics, some, or any with a specific protocol.
• Default to struct. Use class only for reference semantics, identity, inheritance, or Objective-C interop.

Closure capture and memory

Use [weak self] (or [unowned self] when the closure cannot outlive self) only when a retain cycle is possible: self stores the closure (directly or through an object it owns) and the closure uses self. Typical cases are closures kept in properties, long-lived callbacks and observers, and Task loops that never finish. Do not add it to non-escaping closures, one-shot Task { } bodies, TaskGroup child tasks, or closures self does not retain; it only adds optional unwrapping. After a weak capture, unwrap with guard let self else { return } before doing work. A capture list such as [currentCount] snapshots a value at closure creation. Delegates are weak var with an AnyObject protocol. (Source: The Swift Programming Language, "Automatic Reference Counting", strong reference cycles for closures.)