Swift Idioms

Swift Idioms and Patterns

Swift rewards value types, optionals, and protocol-oriented design. Idiomatic Swift = safe, expressive, Swifty.

Scope: Swift coding idioms. Test naming: GEMINI.md § Testing Strategy.

Value Types and Optionals

1. Prefer structs over classes — value semantics by default. Classes only for identity, inheritance, or reference counting.
2. Optionals — never force-unwrap (!) in production:

   // ✅ Guard let for early exit
   guard let task = storage.findById(id) else {
       throw TaskError.notFound(id)
   }
   
   // ✅ Optional chaining
   let title = task?.title ?? "Untitled"
   
   // ❌ Force unwrap — crash risk
   let task = storage.findById(id)!
   

Error Handling

1. Typed throws (Swift 6) or Error protocol:

   enum TaskError: Error {
       case notFound(String)
       case validationFailed(field: String, message: String)
   }
   
   func getTask(id: String) throws(TaskError) -> Task { ... }
   

2. Result type for async callbacks:

   func fetchTask(id: String) async -> Result<Task, TaskError> { ... }
   

Concurrency

1. Structured concurrency with async/await:

   func loadDashboard() async throws -> Dashboard {
       async let user = fetchUser(id)
       async let tasks = fetchTasks(userId: id)
       return Dashboard(user: try await user, tasks: try await tasks)
   }
   

2. @Sendable for closures crossing concurrency domains.

3. Actors for thread-safe mutable state.

Naming (Swift API Design Guidelines)

1. camelCase for functions, properties, variables.
2. PascalCase for types, protocols, enums.
3. Omit needless words — remove(at:) not removeItem(atIndex:).
4. Protocols for capabilities use -able/-ible: Codable, Identifiable.

Testing

XCTest or Swift Testing (6.0+). Mock via protocols.

Formatting and Static Analysis

Related

• Code Idioms and Conventions GEMINI.md § Code Idioms and Conventions
• Testing Strategy GEMINI.md § Testing Strategy
• Error Handling Principles GEMINI.md § Error Handling Principles