Generics and Protocol Constraints

// Protocol with associated type
protocol Repository {
    associatedtype Entity
    func findAll() -> [Entity]
    func findById(_ id: Int) -> Entity?
    func save(_ entity: Entity)
}

struct UserRepository: Repository {
    private var users: [Int: User] = [:]

    func findAll() -> [User] { Array(users.values) }
    func findById(_ id: Int) -> User? { users[id] }
    mutating func save(_ user: User) { users[user.id] = user }
}

// some (opaque return type) - concrete type hidden but fixed
func makeRepository() -> some Repository {
    return UserRepository()
}

// Generic function constrained to protocol
func printAll<R: Repository>(_ repo: R) where R.Entity: CustomStringConvertible {
    repo.findAll().forEach { print($0) }
}

// Primary associated type (Swift 5.7+)
protocol Collection<Element> { }

// any for type-erased storage
var repos: [any Repository] = []