Intermediate Combine

Apr 13 2021 · Swift 5.3, macOS 11.1, Xcode 12.2

Part 1: Intermediate Combine

04. Mapping Errors

Episode complete

Play next episode

Next
About this episode
Leave a rating/review
See forum comments
Cinema mode Mark complete Download course materials
Previous episode: 03. Managing Backpressure Next episode: 05. Retrying and Catching Errors

Get immediate access to this and 4,000+ other videos and books.

Take your career further with a Kodeco Personal Plan. With unlimited access to over 40+ books and 4,000+ professional videos in a single subscription, it's simply the best investment you can make in your development career.

Learn more Already a subscriber? Sign in.

Heads up... You’re accessing parts of this content for free, with some sections shown as obfuscated text.

Heads up... You’re accessing parts of this content for free, with some sections shown as obfuscated text.

Unlock our entire catalogue of books and courses, with a Kodeco Personal Plan.

Unlock now

The differences between map and tryMap goes beyond the fact that tryMap allows throwing errors. map carries over any existing failure types, but tryMap does not - it instead erases the error to a plain Swift Error type. So what do you do if you want to keep some knowledge of that error type around? That’s where mapError comes into play.

example(of: "map vs tryMap") {
  // 1
  enum NameError: Error {
    case tooShort(String)
    case unknown
  }
  // 2
  Just("Hello")
    .setFailureType(to: NameError.self) // 3
    .map { $0 + " World!" } // 4
    .sink(
      receiveCompletion: { completion in
        // 5
        switch completion {
        case .finished:
          print("Done!")
        case .failure(.tooShort(let name)):
          print("\(name) is too short!")
        case .failure(.unknown):
          print("An unknown name error occurred")
        }
      },
      receiveValue: { print("Got value \($0)") }
    )
    .store(in: &subscriptions)
}

.mapError { $0 as? NameError ?? .unknown }
.tryMap { throw NameError.tooShort($0) }