language-swiftlisted

# Swift ## Optionals - `?` makes absence explicit in the type — `String?` vs `String`. - `if let` / `guard let` unwrap safely. `guard` exits scope immediately — prefer it for preconditions. - `??` provides a default for `nil`. **Never force-unwrap (`!`) in production** unless the nil case is a programmer error that should crash loudly. ```swift guard let email = user.email else { return } let display = nickname ?? username ``` ## Value vs reference types - Default to `struct` — value semantics, copy-on-write, no shared-mutation bugs. - Use `class` only when identity matters (reference equality, inheritance), or when Objective-C interop requires it. - Use `actor` to encapsulate mutable state that must be safe to access from multiple async tasks. ```swift actor Counter { private var count = 0 func increment() { count += 1 } func value() -> Int { count } } ``` ## Protocols and protocol-oriented design - Model capabilities as protocols, not class hierarchies. `Hashable`, `Codable`, `Identifiable` are the idiom. - Extension methods add behavior without subclassing. - `some Protocol` (opaque return type) keeps the concrete type private while preserving type identity. - `any Protocol` (existential) erases the type — use only when you need a heterogeneous collection. ```swift protocol Fetchable { associatedtype Item; func fetch(id: String) async throws -> Item } func loadFirst<F: Fetchable>(from source: F) async throws -> F.Item { try await source.fetch(id: "1") } `