Swift Development (6+)

Modern Swift best practices for writing safe, expressive, and idiomatic code. Targets Swift 6+ — language and standard library only, no platform frameworks.

Applicable to all Swift targets: Apple platforms, server-side (Vapor, Hummingbird), CLI tools, cross-platform (Linux, Windows).

Reference Files

• references/type-system.md — Generics, protocols with associated types, opaque types (some), existentials (any), metatypes, @dynamicMemberLookup, @dynamicCallable
• references/concurrency.md — Actors, task groups, async sequences, Sendable, isolation, GCD migration, Combine interop
• references/patterns.md — Property wrappers, result builders, key paths, Codable, extensions, copy-on-write, DSL design
• references/project-structure.md — Swift Package Manager setup, multi-target packages, build configurations, plugins, testing setup
• references/static-analysis.md — SwiftLint (configuration, rules, custom rules, auto-correct), SwiftFormat (configuration, formatting rules), combined setup, CI/CD integration, pre-commit hooks

Code Style

• Short functions — target under 20 lines. Extract well-named helpers when a block needs a comment.
• Imports at the top — group by module. No @_exported unless building a module facade.
• Access control — default to private or internal. Use public only at module boundaries.

Naming Conventions

Follow the Swift API Design Guidelines:

• Name functions and methods according to their side effects: noun for non-mutating (distance(to:)), verb for mutating (sort()), -ed/-ing for non-mutating variant (sorted(), removing()).
• Prefer clarity over brevity. A name should read naturally at the call site.
• Label closure parameters and tuple members.

Value Types vs Reference Types

struct User {                          // Value type — prefer by default
    let id: UUID
    var name: String
}

class UserStore {                      // Reference type — when identity matters
    private var users: [User] = []
}

Use struct by default. Use class when you need identity, inheritance, or reference semantics. Actors when you need thread-safe mutable state.

Optionals

let name: String? = user?.name         // safe chaining
let displayName = name ?? "Anonymous"  // nil coalescing

// NEVER force-unwrap unless you have a provable invariant with a comment
guard let user = fetchUser(id) else { return }  // early exit — preferred

Safe patterns

// Shorthand optional binding (Swift 5.7+) — use when binding name matches the variable
if let name {
    print(name)
}

guard let user else { return }

// Explicit binding — only when intentionally binding to a different name
if let email = user.email {
    sendConfirmation(to: email)
}

// map / flatMap
let uppercased = name.map { $0.uppercased() }           // String?
let nested = fetchUser(id).flatMap { $0.address }        // Address?

// Coalescing with throwing
let user = try optionalUser ?? { throw AppError.notFound }()

Always use shorthand optional binding when the binding name matches the original variable name. Write if let value { } instead of if let value = value { }. This applies to all optional binding contexts: if let, guard let, while let, and multi-condition bindings (e.g., guard let foo, let bar else { }). Only use the explicit if let renamed = original form when intentionally binding to a different name.

NEVER use ! to silence the compiler. Acceptable only with a provable invariant and a comment explaining why.

Enums with Associated Values

enum LoadingState<T> {
    case idle
    case loading
    case loaded(T)
    case failed(Error)
}

Prefer enums over boolean flags for state modeling. Enables exhaustive switch.

Protocols and Extensions

protocol Repository {
    associatedtype Entity
    func fetch(id: UUID) async throws -> Entity
    func save(_ entity: Entity) async throws
}

extension Array where Element: Identifiable {
    func element(withID id: Element.ID) -> Element? {
        first { $0.id == id }
    }
}

Rules:

• Prefer protocol composition (Fetchable & Cacheable) over large monolithic protocols.
• Use extensions to organize conformances — one extension per protocol conformance.
• Default implementations in protocol extensions for shared behavior.

For generics, opaque types, existentials, and advanced protocol patterns see references/type-system.md.

Error Handling

// Define domain errors
enum AppError: LocalizedError {
    case networkUnavailable
    case unauthorized
    case notFound(resource: String)

    var errorDescription: String? {
        switch self {
        case .networkUnavailable: "Network is unavailable"
        case .unauthorized: "Authentication required"
        case .notFound(let resource): "\(resource) not found"
        }
    }
}

// Use typed throws (Swift 6+)
func fetchUser(id: UUID) async throws(AppError) -> User { ... }

// Use Result for expected outcomes in callbacks
func validate(_ input: String) -> Result<ValidatedInput, ValidationError> { ... }

Rules:

• Use guard for preconditions with early exit.
• Catch the narrowest error type. Never catch Error broadly unless at a top-level boundary.
• Prefer async throws over Result for async operations.
• Use LocalizedError for user-facing error messages.
• Typed throws (Swift 6+) for functions with a single known error type.

Concurrency Essentials

// async/await
func fetchUser(id: UUID) async throws -> User {
    let data = try await networkClient.get("users/\(id)")
    return try decode(data)
}

// Structured concurrency — parallel execution
func loadDashboard() async throws -> Dashboard {
    async let profile = fetchProfile()
    async let notifications = fetchNotifications()
    return Dashboard(profile: try await profile, notifications: try await notifications)
}

// Actor — thread-safe mutable state
actor Cache<Key: Hashable, Value> {
    private var storage: [Key: Value] = [:]

    func get(_ key: Key) -> Value? { storage[key] }
    func set(_ key: Key, value: Value) { storage[key] = value }
}

Rules:

• Use structured concurrency — async let, TaskGroup. Avoid Task.detached unless truly needed.
• Never block the calling thread. Offload heavy work with Task or actors.
• Sendable compliance — Swift 6 strict concurrency requires types crossing isolation boundaries to be Sendable.
• Use Task.isCancelled or try Task.checkCancellation() to respond to cancellation.
• Mark nonisolated explicitly when actor methods don't need isolation.

For actors, task groups, async sequences, Sendable patterns, GCD migration, and Combine interop see references/concurrency.md.

Collections

// Prefer higher-order functions over manual loops
let activeUsers = users.filter { $0.isActive }
let names = users.map(\.name)
let totalAge = users.reduce(0) { $0 + $1.age }

// Lazy for large collections
let firstMatch = users.lazy.filter { $0.isActive }.first

// Dictionary grouping
let byDepartment = Dictionary(grouping: employees, by: \.department)

Closures

// Trailing closure syntax
let sorted = users.sorted { $0.name < $1.name }

// Multi-line closures — use named parameters
let transformed = items.map { item in
    Item(
        id: item.id,
        name: item.name.uppercased()
    )
}

// @escaping — when closure outlives the function
func fetch(completion: @escaping (Result<User, Error>) -> Void) { ... }

// @Sendable — when closure crosses concurrency boundaries
func execute(_ work: @Sendable () async -> Void) { ... }

Rules:

• Use $0, $1 only for short, single-expression closures.
• Name parameters explicitly when the closure body is multi-line.
• Prefer async functions over completion handler closures in new code.

Project Structure

MyPackage/
├── Package.swift
├── Sources/
│   ├── MyLibrary/
│   │   ├── Models/
│   │   ├── Services/
│   │   └── Extensions/
│   └── MyExecutable/
│       └── main.swift
├── Tests/
│   └── MyLibraryTests/
└── Plugins/

Rules:

• Use Swift Package Manager for dependency management.
• Organize by domain, not by technical role.
• One type per file, file name matches type name.
• Separate targets for libraries and executables.

For SPM setup, multi-target packages, build configurations, plugins, and testing see references/project-structure.md.

Testable Design

• Dependency injection — inject protocols, not concrete types. Use init injection.
• Pure logic — keep business logic free of framework and I/O dependencies.
• Fakes over mocks — write simple in-memory protocol conformances.
• Test naming — test_methodName_whenCondition_expectedResult.

protocol UserRepository {
    func fetch(id: UUID) async throws -> User
}

// Production
struct RemoteUserRepository: UserRepository {
    let client: HTTPClient
    func fetch(id: UUID) async throws -> User { ... }
}

// Test
struct FakeUserRepository: UserRepository {
    var stubbedUser: User?
    func fetch(id: UUID) async throws -> User {
        guard let user = stubbedUser else { throw AppError.notFound(resource: "User") }
        return user
    }
}

Swift Testing (Swift 6+)

import Testing

@Test("Fetches user by ID")
func fetchUser() async throws {
    let repo = FakeUserRepository(stubbedUser: User(id: testID, name: "Alice"))
    let user = try await repo.fetch(id: testID)
    #expect(user.name == "Alice")
}

@Test("Throws when user not found")
func fetchUserNotFound() async {
    let repo = FakeUserRepository(stubbedUser: nil)
    await #expect(throws: AppError.self) {
        try await repo.fetch(id: testID)
    }
}

Prefer Swift Testing (@Test, #expect) over XCTest for new code. Use XCTest when the project requires it or for UI/integration tests on Apple platforms.

Quick Reference: Common Mistakes