Swift Patterns

Core language patterns for production Swift (5.9+).

When to Activate

• Writing Swift types and business logic
• Designing protocols and generics
• Handling errors and optionals safely
• Serializing/deserializing data with Codable
• Reviewing Swift code for force unwraps, bare catch blocks, or public mutable state that should be private
• Choosing between struct and class for a new model or service type
• Deciding between existentials (any Protocol) and generics (<T: Protocol>) for a function signature

Value Types vs Reference Types

Swift's type system distinguishes between value types (copied) and reference types (shared):

// Value type: struct — prefer for data, no shared mutable state
struct Point {
    var x: Double
    var y: Double

    func distance(to other: Point) -> Double {
        sqrt(pow(x - other.x, 2) + pow(y - other.y, 2))
    }
}

// Value type: enum — use for modeling states or choices
enum Direction {
    case north, south, east, west

    var opposite: Direction {
        switch self {
        case .north: return .south
        case .south: return .north
        case .east:  return .west
        case .west:  return .east
        }
    }
}

// Reference type: class — use for shared mutable state, identity, ObjC interop
final class UserSession {
    private(set) var currentUser: User?
    private(set) var isAuthenticated = false

    func login(user: User) {
        currentUser = user
        isAuthenticated = true
    }
}

// Rule of thumb:
// Struct: data, models, DTOs, view models (no identity)
// Class:  services, managers, delegates, ObjC types (identity matters)

Protocols

Defining and Conforming

// Define behavior contracts
protocol Repository {
    associatedtype Entity
    associatedtype ID

    func findById(_ id: ID) async throws -> Entity?
    func save(_ entity: Entity) async throws -> Entity
    func delete(id: ID) async throws
}

// Protocol with default implementation
protocol Loggable {
    var logDescription: String { get }
}

extension Loggable {
    func log() {
        print("[\(type(of: self))] \(logDescription)")
    }
}

// Conformance
struct User: Loggable {
    let id: UUID
    var name: String
    var email: String

    var logDescription: String {
        "User(id: \(id), name: \(name))"
    }
}

Protocol Composition

// Compose multiple protocols with &
typealias Identifiable = Hashable & CustomStringConvertible

protocol Persistable: Codable, Identifiable {}

// Protocol as parameter type
func logAll(_ items: [any Loggable]) {
    items.forEach { $0.log() }
}

Existentials vs Generics

// Generic (preferred — static dispatch, better performance)
func processAll<T: Persistable>(_ items: [T]) {
    for item in items { save(item) }
}

// Existential (any) — use when type varies at runtime
func processHeterogeneous(_ items: [any Persistable]) {
    for item in items { save(item) }
}

// some — opaque return type (hides concrete type, keeps static dispatch)
func makeValidator() -> some Validator {
    EmailValidator()  // Caller knows it's Validator, not EmailValidator
}

Generics

// Generic function
func first<T>(_ array: [T], where predicate: (T) -> Bool) -> T? {
    array.first(where: predicate)
}

// Generic type with constraint
struct Stack<Element> {
    private var storage: [Element] = []

    mutating func push(_ element: Element) {
        storage.append(element)
    }

    mutating func pop() -> Element? {
        storage.popLast()
    }

    var top: Element? { storage.last }
    var isEmpty: Bool { storage.isEmpty }
}

// Where clauses
extension Stack where Element: Equatable {
    func contains(_ element: Element) -> Bool {
        storage.contains(element)
    }
}

// Generic with multiple constraints
func merge<T: Hashable & Comparable>(_ a: Set<T>, _ b: Set<T>) -> [T] {
    a.union(b).sorted()
}

Optionals

// NEVER use force unwrap (!) in production code
// Use these safe alternatives instead:

// 1. Optional binding
if let user = findUser(id: userId) {
    greet(user)
}

// 2. Guard — early exit pattern
func processOrder(userId: String) throws {
    guard let user = findUser(id: userId) else {
        throw AppError.userNotFound(userId)
    }
    guard user.isActive else {
        throw AppError.accountInactive
    }
    // user is non-optional here
}

// 3. nil coalescing — provide default
let displayName = user.nickname ?? user.name

// 4. Optional chaining — propagate nil
let city = user.address?.city?.uppercased()

// 5. compactMap — filter and unwrap in collections
let validEmails = users.compactMap { $0.email }

// 6. map on Optional — transform if present
let uppercased: String? = optionalString.map { $0.uppercased() }

Error Handling

// Typed errors with enum
enum NetworkError: Error {
    case noConnection
    case timeout(after: TimeInterval)
    case httpError(statusCode: Int, body: Data?)
    case decodingFailed(reason: String)
}

// Throwing function
func fetchUser(id: UUID) async throws -> User {
    let url = URL(string: "/api/users/\(id)")!
    let (data, response) = try await URLSession.shared.data(from: url)

    guard let http = response as? HTTPURLResponse else {
        throw NetworkError.noConnection
    }
    guard http.statusCode == 200 else {
        throw NetworkError.httpError(statusCode: http.statusCode, body: data)
    }

    do {
        return try JSONDecoder().decode(User.self, from: data)
    } catch {
        throw NetworkError.decodingFailed(reason: error.localizedDescription)
    }
}

// Consuming errors
func loadUser(id: UUID) async {
    do {
        let user = try await fetchUser(id: id)
        display(user)
    } catch NetworkError.noConnection {
        showOfflineMessage()
    } catch NetworkError.httpError(let code, _) where code == 404 {
        showNotFound()
    } catch {
        showGenericError(error)
    }
}

Result Type

Use Result<Success, Failure> for synchronous operations where error is expected:

// Return Result instead of throwing for non-async code
func parseEmail(_ raw: String) -> Result<Email, ValidationError> {
    guard raw.contains("@") else {
        return .failure(.invalidFormat("Missing @ symbol"))
    }
    guard !raw.hasPrefix("@") else {
        return .failure(.invalidFormat("Missing local part"))
    }
    return .success(Email(raw: raw))
}

// Use Result combinators
let result = parseEmail(input)
    .map { email in email.normalized }          // transform success
    .mapError { err in UserFacingError(err) }  // transform failure

// Pattern match
switch result {
case .success(let email): sendWelcome(to: email)
case .failure(let error): showError(error)
}

// get() throws on failure
let email = try parseEmail(input).get()

Codable

// Automatic synthesis — property names match JSON keys
struct Product: Codable {
    let id: UUID
    let name: String
    let price: Decimal
    let isAvailable: Bool
}

// Custom keys with CodingKeys
struct APIUser: Codable {
    let id: Int
    let fullName: String
    let emailAddress: String

    enum CodingKeys: String, CodingKey {
        case id
        case fullName    = "full_name"
        case emailAddress = "email"
    }
}

// Custom date decoding
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
decoder.keyDecodingStrategy  = .convertFromSnakeCase  // snake_case → camelCase

let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .iso8601
encoder.keyEncodingStrategy  = .convertToSnakeCase

// Decode
let user = try decoder.decode(APIUser.self, from: data)

// Encode
let json = try encoder.encode(user)

Module Organization

Sources/
  MyApp/
    Domain/          # Pure types, no frameworks
      User.swift
      Order.swift
      Errors.swift
    Application/     # Use cases — depends only on Domain
      UserService.swift
      OrderService.swift
    Infrastructure/  # Adapters (URLSession, CoreData, etc.)
      NetworkClient.swift
      UserRepository.swift
    Presentation/    # ViewModels, UI adapters
      UserViewModel.swift

Tests/
  MyAppTests/
    Domain/
      UserTests.swift
    Application/
      UserServiceTests.swift

Access Control

// Minimal visibility — expose only what callers need
public struct UserService {
    private let repository: any UserRepository  // hidden
    private let mailer: any Mailer

    // Public initializer
    public init(repository: any UserRepository, mailer: any Mailer) {
        self.repository = repository
        self.mailer = mailer
    }

    // Public methods
    public func register(name: String, email: String) async throws -> User { ... }

    // Internal helper
    func validateEmail(_ email: String) throws { ... }
}

// private(set) — readable externally, writable internally
public struct Order {
    public private(set) var status: OrderStatus = .pending

    public mutating func confirm() throws {
        guard status == .pending else { throw OrderError.invalidTransition }
        status = .confirmed
    }
}

Quick Reference

Feature	Use case
struct	Data, models, DTOs — value semantics
class	Shared state, identity, ObjC interop
final class	Class that won't be subclassed (preferred)
protocol	Define behavior contracts
some Protocol	Opaque return type (hides concrete type)
any Protocol	Existential (heterogeneous collections)
guard let	Early exit on nil/error
??	Nil coalescing default
Result<T, E>	Sync operations with expected errors
throws	Propagate unexpected errors
Codable	JSON encode/decode
private(set)	Read-only outside, writable inside

Anti-Patterns

Force Unwrapping Optionals in Production Code

Wrong:

func displayUser(id: String) {
    let user = findUser(id: id)!  // crashes at runtime if nil
    label.text = user.name
}

Correct:

func displayUser(id: String) {
    guard let user = findUser(id: id) else {
        showError("User not found")
        return
    }
    label.text = user.name
}

Why: Force unwrapping silently propagates assumptions; guard let makes the nil case explicit and recoverable.

Using Class When Struct Suffices

Wrong:

class Point {          // heap allocation, reference semantics, thread-unsafe by default
    var x: Double
    var y: Double
    init(x: Double, y: Double) { self.x = x; self.y = y }
}

Correct:

struct Point {         // stack allocation, value semantics, naturally thread-safe
    var x: Double
    var y: Double
}

Why: Classes carry reference semantics and shared-mutability risks; use structs for data without identity.

Catching All Errors with a Bare catch

Wrong:

func loadProfile() async {
    do {
        let profile = try await fetchProfile()
        display(profile)
    } catch {
        // swallows everything — NetworkError, DecodingError, CancellationError all look the same
        showGenericError()
    }
}

Correct:

func loadProfile() async {
    do {
        let profile = try await fetchProfile()
        display(profile)
    } catch NetworkError.noConnection {
        showOfflineBanner()
    } catch NetworkError.httpError(let code, _) where code == 401 {
        redirectToLogin()
    } catch {
        showGenericError(error)
    }
}

Why: A bare catch collapses typed error information and makes distinct failure modes indistinguishable to callers.

Existential any Protocol Where Generic <T: Protocol> Is Better

Wrong:

func processItems(_ items: [any Persistable]) {
    // dynamic dispatch on every call, box allocation per element
    for item in items { save(item) }
}

Correct:

func processItems<T: Persistable>(_ items: [T]) {
    // static dispatch, no boxing, inlineable
    for item in items { save(item) }
}

Why: Existentials (any) incur heap allocation and dynamic dispatch; prefer generics when the concrete type is uniform at the call site.

Exposing Mutable State Publicly

Wrong:

public struct Order {
    public var status: OrderStatus = .pending  // callers can set any status directly
}

Correct:

public struct Order {
    public private(set) var status: OrderStatus = .pending

    public mutating func confirm() throws {
        guard status == .pending else { throw OrderError.invalidTransition }
        status = .confirmed
    }
}

Why: Public mutable properties let callers bypass business rules; private(set) with mutating methods enforces invariants at compile time.

For advanced Swift — property wrappers, result builders, Combine, opaque/existential types, advanced protocol patterns, and performance optimization — see skill: swift-patterns-advanced.