Use Cases

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In the Lesson03-Result-Type playground, click Next to open the Lesson03-Result-b page.

Convert Throwing Expression to Result

Result has a nifty initializer init(catching:) to convert any throwing expression to a Result. You can use this to collect results while looping over a collection.

let bakery = Bakery()
var results = [Result<Int, Error>]()  // 1

for (key, value) in bakery.itemsForSale {
  let result = Result {  // 2 
    try bakery.orderPastry(
      item: key,
      amountRequested: 1,
      flavor: value.flavor)
  }
  results.append(result)  // 3
}

let itemsForSale = [
  "Cookie": Pastry(flavor: "ChocolateChip", numberOnHand: 20),
  "PopTart": Pastry(flavor: "WildBerry", numberOnHand: 0),  // guarantee error
  "Donut" : Pastry(flavor: "Sprinkles", numberOnHand: 24),
  "HandPie": Pastry(flavor: "Cherry", numberOnHand: 6)
]
Results array
Rutugpv omtig

Completion Handlers

Click Next to open the Lesson03-Result-c page.

enum DataFetchError: Error {
  case networkFailure
  case dataCorrupted
  case unauthorized
}

// Define a Result type for fetching data
typealias FetchResult = Result<Data, DataFetchError>

func fetchData(
  from urlString: String,
  completion: @escaping @Sendable (FetchResult) -> Void) {  // 1
  guard let url = URL(string: urlString) else {
    completion(.failure(.networkFailure))  // 2
    return
  }

  // Simulate fetching data
  URLSession.shared.dataTask(with: url) { data, response, error in  // 3
    guard let data = data else {
      completion(.failure(.dataCorrupted))  // 4
      return
    }
    completion(.success(data))  // 5
  }.resume()
}

fetchData(from: "https://example.com") { result in
  switch result {
  case .success(let data):
    print("Successfully fetched \(data.count) bytes.")
  case .failure(let error):
    switch error {
    case .networkFailure:
      print("Network failure - Please check your connection.")
    case .dataCorrupted:
      print("Data corrupted - Unable to process the data received.")
    case .unauthorized:
      print("Unauthorized - Access denied.")
    }
  }
}
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