A deep dive into the result type — Appdevcon 2019

In this presentation—given at Appdevcon march 2019—we take a closer look at Swift 5’s result type and how this functional programming paradigm compares to “regular” imperative error handling.

Transcript

1. A DEEP DIVE INTO THE RESULT TYPE
2. Tjeerd in ’t Veen @tjeerdintveen
 www.swiftindepth.com www.manning.com/books/swift-in-depth
3. WHAT WE’LL COVER ▸ Swift’s error handling ▸ Result’s purpose ▸ Dynamic errors handling + Result ▸ Mixing Result with throwing functions ▸ Functional Combinators ▸ Downsides
4. EVOLUTION OF SWIFT’S ERROR HANDLING
5. SWIFT LOOKS AT FUNCTIONAL PROGRAMMING LANGUAGES ▸ Higher-order functions ▸ Optional (Haskell’s Maybe type) ▸ Algebraic data types (structs, tuples, and enums)
6. SWIFT 2 INTRODUCES ERROR HANDLING
7. HASKELL’S EITHER TYPE?
8. HASKELL’S EITHER TYPE? RUST’S RESULT TYPE?
9. HASKELL’S EITHER TYPE? RUST’S RESULT TYPE?
10. JAVA-LIKE + OBJ-C BRIDGING HYBRID
11. enum ImpendingDoomError: ErrorType { case alienAttack case explosion case volcanicEruption }
12. enum ImpendingDoomError: Error { case alienAttack case explosion case volcanicEruption }
13. defer { findShelter() } do { try pressButton() } catch ImpendingDoomError.explosion { // handle specific error } catch { // handle anything else }
14. let error: NSError = ImpendingDoomError.explosion as NSError print(error) // Error Domain=com.impendingdoom Code=500 "(null)" UserInfo={Oh no=We are doomed}
15. enum ImpendingDoomError: CustomNSError { case alienAttack case explosion case volcanicEruption static var errorDomain: String { return "com.impendingdoom" } /// The error code within the given domain. var errorCode: Int { return 500 } /// The user-info dictionary. var errorUserInfo: [String : Any] { return ["Oh no" : "We are doomed"] } }
16. SOME TRADE-OFFS
17. func pressButton() throws { try warmupMachine() try startChemicalReaction() }
18. defer { findShelter() } do { try pressButton() } catch ImpendingDoomError.explosion { // handle specific error } catch { // handle anything else }
19. ASYNC ERROR HANDLING?
20. let url = URL(string: “https://itunes.apple.com/search?term=harry%20potter")! let task = URLSession.shared.dataTask(with: url) { (data, response, error) -> Void in if let data = data, error == nil { store(data: data) } else if let error = error { handle(error: error) } else { // both data and error are nil, what now? fatalError("Shouldn't come here ") } } task.resume()
21. let url = URL(string: “https://itunes.apple.com/search?term=harry%20potter")! let task = URLSession.shared.dataTask(with: url) { (data, response, error) -> Void in if let data = data, error == nil { store(data: data) } else if let error = error { handle(error: error) } else { // both data and error are nil, what now? fatalError("Shouldn't come here ") } } task.resume()
22. let url = URL(string: “https://itunes.apple.com/search?term=harry%20potter")! let task = URLSession.shared.dataTask(with: url) { (data, response, error) -> Void in if let data = data, error == nil { store(data: data) } else if let error = error { handle(error: error) } else { // both data and error are nil, what now? fatalError("Shouldn't come here ") } } task.resume()
23. let url = URL(string: “https://itunes.apple.com/search?term=harry%20potter")! let task = URLSession.shared.dataTask(with: url) { (data, response, error) -> Void in if let data = data, error == nil { store(data: data) } else if let error = error { handle(error: error) } else { // both data and error are nil, what now? fatalError("Shouldn't come here ") } } task.resume()
24. SWIFT LANGUAGE DESIGNERS: “WAIT UNTIL ASYNC/AWAIT”
26. enum Result<Value, Failure: Error> { case success(Value) case failure(Failure) } enum Result<Value, Failure> { case success(Value) case failure(Failure) } enum Result<Value> { case success(Value) case error(NSError) case canceled } enum Result<Value> { case success(Value) case failure(Error) }
28. RAW STRINGS DELIMITER SE-0200
29. #"We can easily use \"quotes"."# We can easily use \"quotes".
30. CODE: EXAMPLE
31. OPAQUE RESULT TYPES SE-0244
32. CODE: EXAMPLE
33. RESULT IS INTRODUCED SE-0235 AVAILABLE IN SWIFT 5
34. WHAT IS RESULT?
35. public enum Result<Success, Failure> where Failure : Error { /// A success, storing a `Success` value. case success(Success) /// A failure, storing a `Failure` value. case failure(Failure) // ... rest omitted } public enum Optional<Wrapped> { case none case some(Wrapped) }
36. WHY IS RESULT?
37. PREVENTING IMPOSSIBLE PATHS BENEFIT #1
38. let url = URL(string: “https://itunes.apple.com/search?term=harry%20potter")! let task = URLSession.shared.dataTask(with: url) { (data, response, error) -> Void in if let data = data, error == nil { store(data: data) } else if let error = error { handle(error: error) } else { // both data and error are nil, what now? fatalError("Shouldn't come here ") } } task.resume()
39. let url = URL(string: “https://itunes.apple.com/search?term=harry%20potter")! let task = URLSession.shared.dataTask(with: url) { (result: Result<Data, Error>) in switch result { case .success(let data): store(data: data) case .failure(let error): handle(error: error) } } task.resume()
40. extension URLSession { func dataTask(with url: URL, completionHandler: @escaping (Result<Data, Error>) -> Void) -> URLSessionDataTask { return dataTask(with: url) { (data, response, error) in switch (data, error) { case let (data?, nil): completionHandler(Result.success(data)) case let (nil, error?): completionHandler(Result.failure(error)) default: fatalError("Shouldn't come here ") } } } }
41. COMPILE-TIME KNOWLEDGE BENEFIT #2
42. let pancake: Pancake = try bakePancake(temperature: 110) /// - Throws: BakingError func bakePancake(temperature: Int) throws -> Pancake { if temperature < 100 { throw BakingError.uncookedPancake } else if temperature > 300 { throw BakingError.burntPancake } return Pancake() }
43. let pancake: Result<Pancake, BakingError> = bakePancake(temperature: 110) func bakePancake(temperature: Int) -> Result<Pancake, BakingError> { if temperature < 100 { return Result.failure(BakingError.rawPancake) } else if temperature > 300 { return Result.failure(BakingError.burntPancake) } let pancake = Pancake() return Result.success(pancake) }
44. let pancake: Result<Pancake, Error> = bakePancake(temperature: 110) func bakePancake(temperature: Int) -> Result<Pancake, Error> { if temperature < 100 { return Result.failure(BakingError.rawPancake) } else if temperature > 300 { return Result.failure(BakingError.burntPancake) } else if temperature > 500 { return Result.failure(CookingError.kitchenFire) } let pancake = Pancake() return Result.success(pancake) }
45. typealias GenericResult<T> = Result<T, Error> func bakePancake(temperature: Int) -> GenericResult<Pancake> { if temperature < 100 { return Result.failure(BakingError.rawPancake) } else if temperature > 300 { return Result.failure(BakingError.burntPancake) } else if temperature > 500 { return Result.failure(CookingError.kitchenFire) } let pancake = Pancake() return Result.success(pancake) }
46. HAPPY PATH PROGRAMMING BENEFIT #3
47. FUNCTIONAL COMBINATORS
48. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { // … }
49. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } }
50. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } }
51. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } }
52. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } do { let data = try Data(contentsOf: fileURL) return Result.success(data) } catch { return Result.failure(error) }
53. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } do { let data = try Data(contentsOf: fileURL) return Result.success(data) } catch { return Result.failure(error) }
54. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } do { let data = try Data(contentsOf: fileURL) return Result.success(data) } catch { return Result.failure(error) }
55. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } do { let data = try Data(contentsOf: fileURL) return Result.success(data) } catch { return Result.failure(error) }
56. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } return Result(catching: { try Data(contentsOf: fileURL) })
57. func loadFile(resource: String, type: String) -> Result<Data, Error> { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } return Result(catching: { try Data(contentsOf: fileURL) })
58. func loadWeatherReport() -> Result<WeatherReport, Error> { let result: Result<Data, Error> = loadFile(resource: "weather", type: “”) switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } }
59. func loadWeatherReport() -> Result<WeatherReport, Error> { let result: Result<Data, Error> = loadFile(resource: "weather", type: “json”) switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } }
60. func loadWeatherReport() -> Result<WeatherReport, Error> { let result: Result<Data, Error> = loadFile(resource: "weather", type: “json”) switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } }
61. func loadWeatherReport() -> Result<WeatherReport, Error> { let result: Result<Data, Error> = loadFile(resource: "weather", type: “json”) switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } }
62. func loadWeatherReport() -> Result<WeatherReport, Error> { let result: Result<Data, Error> = loadFile(resource: "weather", type: “json”) switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } }
63. func loadWeatherReport() -> Result<WeatherReport, Error> { let result: Result<Data, Error> = loadFile(resource: "weather", type: “json”) switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } }
64. Success Failure Data WeatherReport
65. Result<WeatherReport, Error> Result<Data, Error>
66. MAP
67. func loadWeatherReport() -> Result<WeatherReport, Error> { let result: Result<Data, Error> = loadFile(resource: "weather", type: “json”) switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } }
68. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result
69. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result
70. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result
71. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result
72. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result
73. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result
74. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result
75. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(WeatherReport.init) return result
76. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result
77. func loadWeatherReport() -> Result<WeatherReport, Error> { let result = loadFile(resource: "weather", type: "") switch result { case .success(let data): let report = WeatherReport(data: data) return Result.success(report) case .failure(let error): return Result.failure(error) } } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result
78. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result
79. Success Failure Data WeatherReport WeatherError
80. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result
81. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
82. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
83. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
84. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
85. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
86. Result<Data, Error> Result<Result<WeatherReport, Error>, Error>
87. Result<WeatherReport, Error> Result<Data, Error>
88. FLATMAP
89. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
90. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
91. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
92. CONCRETE ERRORS
93. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { // … } func loadWeatherReport() -> Result<WeatherReport, Error> { // … }
94. Result<WeatherReport, Error> Result<Data, Error> map flatMap
95. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
96. func loadWeatherReport() -> Result<WeatherReport, Error> { } let result: Result<WeatherReport, Error> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, WeatherError> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }
98. Success Failure Error WeatherError
99. Result<WeatherReport, WeatherError> Result<WeatherReport, Error>
100. MAPERROR
101. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { } let result: Result<WeatherReport, WeatherError> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad }
102. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { } let result: Result<WeatherReport, WeatherError> = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad }
103. ERROR RECOVERY
104. Success Failure Error (Transformed) Error Recovered Value
105. FLATMAPERROR
106. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { } let result: Result<WeatherReport, WeatherError> = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad } .flatMapError { _error -> Result<Data, Error> in loadFile(resource: "weather_backup", type: "json") }
107. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { } let result: Result<WeatherReport, WeatherError> = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad } .flatMapError { _error -> Result<Data, Error> in loadFile(resource: "weather_backup", type: "json") }
108. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { } let result: Result<WeatherReport, WeatherError> = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad } .flatMapError { _error -> Result<Data, Error> in loadFile(resource: "weather_backup", type: "json") }
109. Result<WeatherReport, WeatherError> Result<Data, Error> mapError flatMapError map flatMap
110. func loadWeatherReport() -> Result<WeatherReport, WeatherError> { } let result: Result<WeatherReport, WeatherError> = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result<WeatherReport, Error> in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad } .flatMapError { _error -> Result<Data, Error> in loadFile(resource: "weather_backup", type: "json") }
111. let report: Result<WeatherReport, WeatherError> = loadWeatherReport()
112. DOWNSIDES OF RESULT
113. NO ASYNC CHAINING SUPPORT
114. DEBUGGING PIPELINES IS FINICKY
115. NO OBJECTIVE-C BRIDGING ☹
116. NO SYNTACTIC SUGAR
117. MORE STYLE DISCUSSIONS (TWO IDIOMS)
118. @tjeerdintveen
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