@swift-5.8.1 @swift-5.9.2 @swift-main
protocol Middleware<Input, Output, NextInput, NextOutput> { associatedtype Input associatedtype Output associatedtype NextInput associatedtype NextOutput } protocol Routes<Input, Output> { associatedtype Input associatedtype Output } extension Routes { func compose<M>(_ middleware: M) -> any Routes<M.NextInput, M.NextOutput> where M: Middleware, M.Input == Input, M.Output == Output { fatalError() } func mapInput<NewInput>( _ transform: @escaping (Input) -> NewInput ) -> any Routes<NewInput, Output> { fatalError() } } struct Request {} struct Response {} struct User {} struct AuthMiddleware<Input, Output>: Middleware { typealias NextInput = (Input, login: User) typealias NextOutput = Output } func main(routes: any Routes<Request, Response>) { let routes = routes.compose(AuthMiddleware()) .mapInput { (request: $0.0, login: $0.login) } }