enum Valid {} enum Invalid {} struct Configurable { let configuration: Configuration<Valid, Valid> } struct Configuration<FlagA, FlagB> { private var a: Int? func setFlagA(a: Int) -> Configuration<Valid, FlagB> { return Configuration<Valid, FlagB>(a: a, b: b) } private var b: String? func setFlagB(b: String) -> Configuration<FlagA, Valid> { return Configuration<FlagA, Valid>(a: a, b: b) } } func noConfiguration() -> Configuration<Invalid, Invalid> { return Configuration() } let configuration = noConfiguration() .setFlagA(a: 1) .setFlagB(b: "b") Configurable(configuration: configuration)