Result Driven Development

Functional Swift Conferencer 2015

Transcript

1. Result Driven Development BRIAN PARTRIDGE Square Register Team
2. Agenda • What is Result? • Result with HTTP Requests • Result Handling Pipelines • Parsing Using Generics
3. What is Result?
4. enum Optional<T> { case None case Some(T) init() init(_ some: T) }
5. protocol ResultType { typealias Value init(value: Value) init(error: ErrorType) var value: Value? { get } var error: ErrorType? { get } }
6. enum Result<T>: ResultType { typealias Value = T case Success(T) case Failure(ErrorType) }
7. Why?
8. try?
9. (Widget?, NSError?) -> ()
10. (Result<Widget>) -> ()
11. enum Optional<T> { … func map<U>(f: (Wrapped) -> U) -> U? func flatMap<U>(f: (Wrapped) -> U?) -> U? }
12. protocol ResultType { … func map<U>(f: (Value) -> U) -> Result<U> func flatMap<U>(f: (Value) -> Result<U>) -> Result<U> } enum Optional<T> { … func map<U>(f: (Wrapped) -> U) -> U? func flatMap<U>(f: (Wrapped) -> U?) -> U? }
13. Result with HTTP Requests
14. func executeRequest(request: …, session: …, completion: (NSData?, NSError?)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in }).resume() }
15. func executeRequest(request: …, session: …, completion: (NSData?, NSError?)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in if let error = error { completion(nil, error) return } }).resume() }
16. func executeRequest(request: …, session: …, completion: (NSData?, NSError?)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in if let error = error { completion(nil, error) return } guard let httpResponse = response as? NSHTTPURLResponse else { fatalError(…) } if httpResponse.statusCode != 200 { completion(nil, HTTPError()) return } }).resume() }
17. func executeRequest(request: …, session: …, completion: (NSData?, NSError?)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in if let error = error { completion(nil, error) return } guard let httpResponse = response as? NSHTTPURLResponse else { fatalError(…) } if httpResponse.statusCode != 200 { completion(nil, HTTPError()) return } guard let data = data else { completion(nil, BadDataError()) return } }).resume() }
18. func executeRequest(request: …, session: …, completion: (NSData?, NSError?)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in if let error = error { completion(nil, error) return } guard let httpResponse = response as? NSHTTPURLResponse else { fatalError(…) } if httpResponse.statusCode != 200 { completion(nil, HTTPError()) return } guard let data = data else { completion(nil, BadDataError()) return } completion(data, nil) }).resume() }
19. func executeRequest(request: …, session: …, completion: (NSData?, NSError?)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in if let error = error { completion(nil, error) return } guard let httpResponse = response as? NSHTTPURLResponse else { fatalError(…) } if httpResponse.statusCode != 200 { completion(nil, HTTPError()) return } guard let data = data else { completion(nil, BadDataError()) return } completion(data, nil) }).resume() }
20. func executeRequest(request: …, session: …, completion: (NSData?, NSError?)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in if let error = error { completion(nil, error) return } guard let httpResponse = response as? NSHTTPURLResponse else { fatalError(…) } if httpResponse.statusCode != 200 { completion(nil, HTTPError()) return } guard let data = data else { completion(nil, BadDataError()) return } completion(data, nil) }).resume() }
21. func executeRequest(request: …, session: …, completion: (Result<NSData>)->()) { session.dataTaskWithRequest(request, completionHandler: { (data, response, error) in if let error = error { completion(.Failure(error: error)) return } guard let httpResponse = response as? NSHTTPURLResponse else { fatalError(…) } if httpResponse.statusCode != 200 { completion(.Failure(error: HTTPError())) return } guard let data = data else { completion(.Failure(error:BadDataError())) return } completion(.Success(value: data)) }).resume() }
22. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { result in } }
23. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { result in switch result { case let .Failure(error): completion(.Failure(error)) case let .Success(data): let widget = Widget(data: result.value) completion(.Success(widget)) } } }
24. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let widgetResult = httpResult.map { Widget(data: $0) } completion(widgetResult) } }
25. Result Handling Pipelines
26. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in switch result { case let .Failure(error): completion(.Failure(error)) case let .Success(data): } } }
27. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in switch result { case let .Failure(error): completion(.Failure(error)) case let .Success(data): let json = // Parse JSON } } }
28. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in switch result { case let .Failure(error): completion(.Failure(error)) case let .Success(data): let json = // Parse JSON let widget = // Instantiate Widget } } }
29. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in switch result { case let .Failure(error): completion(.Failure(error)) case let .Success(data): let json = // Parse JSON let widget = // Instantiate Widget completion(.Success(widget)) } } }
30. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in switch result { case let .Failure(error): completion(.Failure(error)) case let .Success(data): let json = // Parse JSON // Handle failures let widget = // Instantiate Widget // Handle failures completion(.Success(widget)) } } }
31. func parseJSONData(data: NSData) -> Result<NSDictionary> func parseWidgetJSON(json: NSDictionary) -> Result<Widget>
32. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let jsonResult = httpResult.flatMap(parseJSONData) let widgetResult = jsonResult.flatMap(parseWidgetJSON) completion(widgetResult) } }
33. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let result = httpResult .flatMap(parseJSONData) .flatMap(parseWidgetJSON) completion(result) } }
34. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let result = httpResult // HTTP 500 .flatMap(parseJSONData) .flatMap(parseWidgetJSON) completion(result) // HTTP 500 } }
35. func fetchLatestWidget(session: …, completion: (Result<Widget>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let result = httpResult .flatMap(parseJSONData) // JSON Error .flatMap(parseWidgetJSON) completion(result) // JSON Error } }
36. Pipeline Benefits • Localized Data Transformation • Smaller Functions • More Testable
37. Parsing Using Generics
38. protocol PBMessageType { init(data: NSData) } class CompleteBillRequest: PBMessageType { let billID: String let completionDate: NSDate } class CompleteBillResponse: PBMessageType { let status: Status let bill: Bill } class Status: PBMessageType { let success: Bool let localizedDescription: String? }
39. protocol PBMessageType { init(data: NSData) } class CompleteBillRequest: PBMessageType { let billID: String let completionDate: NSDate } class CompleteBillResponse: PBMessageType { let status: Status let bill: Bill } class Status: PBMessageType { let success: Bool let localizedDescription: String? }
40. protocol PBMessageType { init(data: NSData) } class CompleteBillRequest: PBMessageType { let billID: String let completionDate: NSDate } class CompleteBillResponse: PBMessageType { let status: Status let bill: Bill } class Status: PBMessageType { let success: Bool let localizedDescription: String? }
41. func parseProtoResponseData<T>(data: NSData) -> Result<T> { }
42. func parseProtoResponseData<T>(data: NSData) -> Result<T> { let object = T(data: data) }
43. func parseProtoResponseData<T>(data: NSData) -> Result<T> { let object = T(data: data) if object.status.success { return .Success(value: object) } else { return .Failure(StatusError()) } }
44. func parseProtoResponseData<T>(data: NSData) -> Result<T> { let object = T(data: data) // 'T' cannot be constructed if object.status.success { return .Success(value: object) } else { return .Failure(StatusError()) } }
45. func parseProtoResponseData<T:PBMessageType>(data: NSData) -> Result<T> { let object = T(data: data) if object.status.success { return .Success(value: object) } else { return .Failure(StatusError()) } }
46. func parseProtoResponseData<T:PBMessageType>(data: NSData) -> Result<T> { let object = T(data: data) if object.status.success { // Value of type 'T' has no member 'status' return .Success(value: object) } else { return .Failure(StatusError()) } }
47. protocol PBResponseMessageType { let status: Status } extension CompleteBillResponse: PBResponseMessageType {} func parseProtoResponseData<T: PBResponseMessageType>(data: NSData) -> Result<T> { … }
48. protocol PBResponseMessageType { let status: Status } extension CompleteBillResponse: PBResponseMessageType {} func parseProtoResponseData<T: PBResponseMessageType>(data: NSData) -> Result<T> { … }
49. protocol PBResponseMessageType { let status: Status } extension CompleteBillResponse: PBResponseMessageType {} func parseProtoResponseData<T: PBResponseMessageType>(data: NSData) -> Result<T> { … }
50. let parsedResult = parseProtoResponseData(data) // Generic parameter 'T' could not be inferred.
51. let parsedResult: Result<CompleteBillResponse> = parseProtoResponseData(data)
52. let parsedResult: Result<CompleteBillResponse> = parseProtoResponseData(data) // T is CompleteBillResponse
53. func completeBill(session: …, completion: (Result<Bill>)->()) { let request = ... executeRequest(request, session: session) { httpResult in } }
54. func completeBill(session: …, completion: (Result<Bill>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let completeBillResult: Result<CompleteBillResponse> } }
55. func completeBill(session: …, completion: (Result<Bill>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let completeBillResult: Result<CompleteBillResponse> completeBillResult = httpResult.flatMap(parseProtoResponseData) } }
56. func completeBill(session: …, completion: (Result<Bill>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let completeBillResult: Result<CompleteBillResponse> completeBillResult = httpResult.flatMap(parseProtoResponseData) let billResult = completeBillResult.map { $0.bill } } }
57. func completeBill(session: …, completion: (Result<Bill>)->()) { let request = ... executeRequest(request, session: session) { httpResult in let completeBillResult: Result<CompleteBillResponse> completeBillResult = httpResult.flatMap(parseProtoResponseData) let billResult = completeBillResult.map { $0.bill } completion(billResult) } }
58. Summary • Clearer Semantics • Pipelining • Generics For Parsing
59. References Networking With Monads By John Gallagher @ Functional Swift Conf 2014 http://2014.funswiftconf.com/speakers/john.html antitypical/Result https://github.com/antitypical/Result
60. Q&A
61. bp@squareup.com @brianpartridge