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A DEEP DIVE INTO THE RESULT TYPE

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Tjeerd in ’t Veen @tjeerdintveen
 www.swiftindepth.com www.manning.com/books/swift-in-depth

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WHAT WE’LL COVER ▸ Swift’s error handling ▸ Result’s purpose ▸ Dynamic errors handling + Result ▸ Mixing Result with throwing functions ▸ Functional Combinators ▸ Downsides

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EVOLUTION OF SWIFT’S ERROR HANDLING

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SWIFT LOOKS AT FUNCTIONAL PROGRAMMING LANGUAGES ▸ Higher-order functions ▸ Optional (Haskell’s Maybe type) ▸ Algebraic data types (structs, tuples, and enums)

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SWIFT 2 INTRODUCES ERROR HANDLING

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HASKELL’S EITHER TYPE?

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HASKELL’S EITHER TYPE? RUST’S RESULT TYPE?

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HASKELL’S EITHER TYPE? RUST’S RESULT TYPE?

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JAVA-LIKE + OBJ-C BRIDGING HYBRID

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enum ImpendingDoomError: ErrorType { case alienAttack case explosion case volcanicEruption }

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enum ImpendingDoomError: Error { case alienAttack case explosion case volcanicEruption }

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defer { findShelter() } do { try pressButton() } catch ImpendingDoomError.explosion { // handle specific error } catch { // handle anything else }

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let error: NSError = ImpendingDoomError.explosion as NSError print(error) // Error Domain=com.impendingdoom Code=500 "(null)" UserInfo={Oh no=We are doomed}

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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"] } }

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SOME TRADE-OFFS

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func pressButton() throws { try warmupMachine() try startChemicalReaction() }

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defer { findShelter() } do { try pressButton() } catch ImpendingDoomError.explosion { // handle specific error } catch { // handle anything else }

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ASYNC ERROR HANDLING?

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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()

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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()

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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()

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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()

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SWIFT LANGUAGE DESIGNERS: “WAIT UNTIL ASYNC/AWAIT”

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enum Result { case success(Value) case failure(Failure) } enum Result { case success(Value) case failure(Failure) } enum Result { case success(Value) case error(NSError) case canceled } enum Result { case success(Value) case failure(Error) }

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RAW STRINGS DELIMITER SE-0200

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#"We can easily use \"quotes"."# We can easily use \"quotes".

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CODE: EXAMPLE

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OPAQUE RESULT TYPES SE-0244

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CODE: EXAMPLE

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RESULT IS INTRODUCED SE-0235 AVAILABLE IN SWIFT 5

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WHAT IS RESULT?

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public enum Result 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 { case none case some(Wrapped) }

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WHY IS RESULT?

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PREVENTING IMPOSSIBLE PATHS BENEFIT #1

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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()

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let url = URL(string: “https://itunes.apple.com/search?term=harry%20potter")! let task = URLSession.shared.dataTask(with: url) { (result: Result) in switch result { case .success(let data): store(data: data) case .failure(let error): handle(error: error) } } task.resume()

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extension URLSession { func dataTask(with url: URL, completionHandler: @escaping (Result) -> 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 ") } } } }

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COMPILE-TIME KNOWLEDGE BENEFIT #2

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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() }

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let pancake: Result = bakePancake(temperature: 110) func bakePancake(temperature: Int) -> Result { if temperature < 100 { return Result.failure(BakingError.rawPancake) } else if temperature > 300 { return Result.failure(BakingError.burntPancake) } let pancake = Pancake() return Result.success(pancake) }

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let pancake: Result = bakePancake(temperature: 110) func bakePancake(temperature: Int) -> Result { 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) }

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typealias GenericResult = Result func bakePancake(temperature: Int) -> GenericResult { 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) }

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HAPPY PATH PROGRAMMING BENEFIT #3

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FUNCTIONAL COMBINATORS

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func loadWeatherReport() -> Result { // … }

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func loadFile(resource: String, type: String) -> Result { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } }

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func loadFile(resource: String, type: String) -> Result { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } }

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func loadFile(resource: String, type: String) -> Result { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } }

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func loadFile(resource: String, type: String) -> Result { 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) }

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func loadFile(resource: String, type: String) -> Result { 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) }

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func loadFile(resource: String, type: String) -> Result { 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) }

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func loadFile(resource: String, type: String) -> Result { 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) }

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func loadFile(resource: String, type: String) -> Result { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } return Result(catching: { try Data(contentsOf: fileURL) })

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func loadFile(resource: String, type: String) -> Result { if let fileURL = Bundle.main.url(forResource: resource, withExtension: type) { } else { return Result.failure(FileError.couldNotFindFile) } } return Result(catching: { try Data(contentsOf: fileURL) })

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func loadWeatherReport() -> Result { let result: 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) } }

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func loadWeatherReport() -> Result { let result: Result = 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) } }

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func loadWeatherReport() -> Result { let result: Result = 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) } }

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func loadWeatherReport() -> Result { let result: Result = 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) } }

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func loadWeatherReport() -> Result { let result: Result = 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) } }

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func loadWeatherReport() -> Result { let result: Result = 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) } }

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Success Failure Data WeatherReport

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Result Result

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MAP

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func loadWeatherReport() -> Result { let result: Result = 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) } }

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map({ (data: Data) -> WeatherReport in WeatherReport(data: data) }) return result

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(WeatherReport.init) return result

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result

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func loadWeatherReport() -> Result { 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 = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result

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Success Failure Data WeatherReport WeatherError

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) .map(WeatherReport.init) return result

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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Result Result, Error>

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Result Result

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FLATMAP

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .map { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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CONCRETE ERRORS

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func loadWeatherReport() -> Result { // … } func loadWeatherReport() -> Result { // … }

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Result Result map flatMap

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } }

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Success Failure Error WeatherError

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Result Result

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MAPERROR

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: "json") .map(cleanData) return result .flatMap { (data: Data) -> Result in if let report = WeatherReport(data: data) { return Result.success(report) } else { return Result.failure(WeatherReport.failedToLoad) } } .mapError { _error in WeatherError.failedToLoad }

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ERROR RECOVERY

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Success Failure Error (Transformed) Error Recovered Value

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FLATMAPERROR

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result 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 in loadFile(resource: "weather_backup", type: "json") }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result 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 in loadFile(resource: "weather_backup", type: "json") }

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result 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 in loadFile(resource: "weather_backup", type: "json") }

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Result Result mapError flatMapError map flatMap

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func loadWeatherReport() -> Result { } let result: Result = loadFile(resource: "weather", type: “json”) .map(cleanData) return result .flatMap { (data: Data) -> Result 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 in loadFile(resource: "weather_backup", type: "json") }

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let report: Result = loadWeatherReport()

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DOWNSIDES OF RESULT

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NO ASYNC CHAINING SUPPORT

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DEBUGGING PIPELINES IS FINICKY

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NO OBJECTIVE-C BRIDGING ☹

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NO SYNTACTIC SUGAR

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MORE STYLE DISCUSSIONS (TWO IDIOMS)

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@tjeerdintveen
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