The document discusses using phantom types to represent state in a class. A HogeOperation class is defined with a generic State parameter constrained to an OperationState protocol. Extensions are used to provide state-specific functionality based on the State. This allows initializing an operation in an Init state, transitioning it to a Prepared state where additional methods become available, and avoiding errors from calling methods associated with invalid states. The phantom types encode state without runtime representation, catching errors at compile-time.

3. #kbkz_tech

9. class HogeOperation {
private(set) var isPrepared = false
func prepare() {
guard !isPrepared else {
fatalError("prepare を複数回呼ばないこと")
}
isPrepared = true
}
}

10. // Init 状態の HogeOperation を生成
let operation = HogeOperation<Init>()
// 最初は prepared を呼べて準備完了したものを取得可能
let preparedOp = operation.prepared()
// 準備が終われば prepared はビルドエラーで実行不可
preparedOp.prepared()

12. // これで "操作状態を表現する型" を表現
protocol OperationState {}
// 操作状態ごとにクラスを定義
class Init: OperationState {}
class Prepared: OperationState {}

13. // 型パラメーターで型に状態を付与
class HogeOperation<State: OperationState> {
/*
今回は内部で、
型パラメーターを使わないのがポイント
*/
}

14. extension HogeOperation where State: Init {
// 準備を実行し、準備完了状態のインスタンスを返す
func prepared() -> HogeOperation<Prepared> {…}
}
extension HogeOperation where State: Prepared {
// 目的の操作を実行する
func execute() {…}
}

16. let operation = HogeOperation<Init>()
// Init 状態では、まだ execute は存在しない
operation.execute()
// prepared を呼ぶことで Prepared 状態のものを取得
let preparedOp = operation.prepared()
// Prepared 状態には、もう prepared は存在しない
preparedOp.prepared()

22. let operation = HogeOperation<Init>()
// HogeOperation<Init> 型だから prepared が呼べる
let preparedOp: HogeOperation<Prepared>()
= operation.prepared()
// HogeOperation<Prepared> 型だから execute が呼べる
preparedOp.execute()
// Init クラスや Prepared クラスを、実行時には使わない
// ビルドの段階で、もう役目が済んでいる

24. let operation = HogeOperation<Init>()
let preparedOp = operation.prepared()
let type1 = type(of: operation)
let type2 = type(of: preparedOp)
type1 == type2 // false

25. // 型パラメーターで型に状態を付与
class HogeOperation<State: OperationState> {
/*
内部では、型パラメーターを使っていない
型の在り方を説明するためだけに使っている
*/

27. /// Phantom Type に出逢う前の認識
// Array はテンプレート的なもので…
struct Array<Element> {
}
// 型パラメーターによって、異なる型になる
let values: Array<Int> = Array<String>()

29. // protocol OperationState {}
// class Init: OperationState {}
// class Prepared: OperationState {}

30. class Operation {
fileprivate var data: OperationData
fileprivate init(data: OperationData) {
self.data = data
}
func mob() {}
}

31. class OperationInit: Operation {
convenience init() {…}
func prepared() -> OperationPrepared {…}
}
class OperationPrepared: Operation {
func execute() {…}
}

32. let operation = OperationInit()
let preparedOp = operation.prepared()
preparedOp.execute()

34. struct OperationInit {
fileprivate var data: OperationData
fileprivate init(data: OperationData) {…}
// 共通機能
func mob() {}
// 固有の機能
init() {…}
func prepared() -> OperationPrepared {…}
}

35. struct OperationPrepared {
fileprivate var data: OperationData
fileprivate init(data: OperationData) {…}
// 共通機能
func mob() {}
// 固有の機能
func execute() {…}
}

36. let operation = OperationInit()
let preparedOp = operation.prepared()
preparedOp.execute()

37. protocol Operation {}
struct OperationInit: Operation {
…
}
struct OperationPrepared: Operation {
…
}

39. struct Operation {
struct Init {
…
}
struct Prepared {
…
}
}

40. let operation = Operation.Init()
let preparedOp = operation.prepared()
preparedOp.execute()

42. class Operation {
fileprivate var data: OperationData
fileprivate init(data: OperationData) {
self.data = data
}
func mob() {}
}

43. extension Operation {
class Init: Operation {
…
}
class Prepared: Operation {
…
}
}

44. let operation = Operation.Init()
let preparedOp = operation.prepared()
preparedOp.execute()

49. class HogeOperation<State: OperationState> {
func mob() {…}
}
extension HogeOperation where State: Init {
func prepared() -> HogeOperation<Prepared> {…}
}
extension HogeOperation where State: Prepared {
func execute() {…}
}

50. let operation = HogeOperation<Init>()
operation.execute()

51. let operation = OperationInit()
operation.execute()

53. // 準備前と準備後を、自分自身や同じ変数に書き戻せない
var operation = HogeOperation<Init>()
operation = operation.prepared()

54. protocol Operation {}
class HogeOperation<State: OperationState>: Operation {
}
var op: Operation
op = HogeOperation<Init>()
op = (op as! HogeOperation<Init>).prepared()
op = (op as! HogeOperation<Prepared>).execute()

56. class Driver {
init(channel: Int? = nil, volume: Int? = nil,
pan: Int? = nil, format: Format? = nil,
route: Route? = nil) {
}
}
// 設定項目に何があるかや、設定順番に気を使う
let driver = Driver(volume: 10,
format: ulaw, route: .speaker)

57. // 本体のクラスを Phantom Type で定義して…
class Driver<State> where State: AudioState {
}
// 準備が整ったときの機能を実装し…
extension Driver where State: Ready {
func start() { … }
}

58. // 初期化中にできることを規定すると…
extension Driver where State: Setup {
func prepared() -> Driver<Ready> { … }
func set(channel: Int) -> Driver { return self }
func set(volume: Int) -> Driver { return self }
func set(pan: Int) -> Driver { return self }
func set(format: Format) -> Driver { return self }
func set(route: Route) -> Driver { return self }
}

59. // 初期設定では、順番を気にせず設定できる・補完が効く
let driver = Driver<Setup>()
.set(format: ulaw)
.set(volume: 10)
.set(route: .speaker)
.prepared() // ここで Driver<Ready>() を返す
// 設定完了を宣言 (prepared) して、使い始める
driver.start()

60. let driver = Driver<Setup>()
.format // → Driver<FormatSetup>
.set(sampleRate: 44100)
.set(channelsPerFrame: 2)
.general // → Driver<GeneralSetup>
.set(volume: 10)
.set(route: .speaker)
.prepared() // → Driver<Ready>

61. let driver = Driver.setup() // → DriverSetup
.format // → FormatSetup
.set(sampleRate: 44100)
.set(channelsPerFrame: 2)
.general // → AudioSetup
.set(volume: 10)
.set(route: .speaker)
.prepared() // → Driver

63. // 設定項目を、初期値を持った構造体で用意して…
struct Description {
var channel: Int = default
var volume: Int = default
var pan: Int = default
var format: Format = default
var route: Route = default
}

64. let description = Description()
description.format.sampleRate = 44100
description.format.channelsPerFrame = 2
description.volume = 10
description.route = .speaker

65. // Driver は Description で初期化するようにして…
class Driver {
init(description: Description) { … }
}
// 設定項目を渡して、初期化を完成する
let driver = Driver(description: description)

68. class Controller {
var environment: Environment<Neutral>
}

69. struct Environment<State> {
fileprivate(set) var platform: Platform
fileprivate(set) var version: Double
func startEditing() -> Environment<Editing> {
return Environment<Editing>(
platform: platform, version: version)
}
}

70. extension Environment where State: Editing {
mutating func set(platform: Platform) { … }
mutating func set(version: Double) { … }
func endEditing() -> Environment<Neutral> {
return Environment<Neutral>(
platform: platform, version: version)
}
}

71. func update() {
// 編集状態で取り出さないと、書き込めない
var environment = self.environment.startEditing()
environment.set(platform: .macOS)
environment.set(version: Platform.macOS.latest)
// ローカルで編集を終了したら、書き戻す
self.environment = environment.endEditing()
}

73. extension Environment where State: Editing {
var platform: Platform
var version: Double
// 同じ内容の、別インスタンスを作り直している
func endEditing() -> Environment<Neutral> {
return Environment<Neutral>(
platform: platform, version: version)
}

74. extension Environment where State: Editing {
// 内容を原始的にコピーしないといけないとき
func endEditing() -> Environment<Neutral> {
var result = Environment<Neutral>()
result.platform = platform
result.version = version
return result
}

76. struct Environment<State> {
fileprivate(set) var platform: Platform
fileprivate(set) var version: Double
}
extension Environment {
fileprivate init<S>(takeover: Environment<S>) {
platform = takeover.platform
version = takeover.version
}
}

77. extension Environment where State: Editing {
func endEditing() -> Environment<Neutral> {
return Environment<Neutral>(takeover: self)
}
}

79. struct Environment<State> {
// データをここで集中管理する
fileprivate struct Context {
var platform: Platform
var version: Double
}
// これだけを引き継げば済む状況を作る
fileprivate var _context: Context
}

80. extension Environment {
fileprivate init(context: Context) {
_context = context
}
}

81. extension Environment where State: Editing {
func endEditing() -> Environment<Neutral> {
return Environment<Neutral>(context: _context)
}
}

82. extension Environment {
var platform: Platform {
get { _context.platform }
set { _context.platform = newValue }
}
var version: Double {
get { _context.version }
set { _context.version = newValue }
}

84. extension Environment where State: Editing {
func endEditing() -> Environment<Neutral> {
// 準備不要で、いきなりビットキャスト可能
return unsafeBitCast(self,
to: Environment<Neutral>.self)
}
}

85. extension Environment {
fileprivate init<S>(takeover: Environment<S>) {
self = unsafeBitCast(takeover,
to: Environment.self)
}
}

86. let sub: Base = Sub()
let base: Base = Base()
// 実体が Sub なので、全ての機能が使える
let obj: Sub = unsafeBitCast(sub, to: Sub.self)
// 実体が Base なので、Sub の機能を使うとクラッシュする
let obj: Sub = unsafeBitCast(base, to: Sub.self)

89. class Test: OperationState {}
extension HogeOperation where State: Test {
func testSomething() {…}
}
extension HogeOperation where State: Init {
func testing() -> HogeOperation<Test> {…}
}

91. // Test 状態を Prepared から継承させれば…
class Test: Prepared {}
// Test には Prepared の機能も備わる
extension HogeOperation where State: Prepared {
func execute() {…}
}
extension HogeOperation where State: Test {
func testSomething() {…}
}