Result Type
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Open the Lesson03-Result-Type playground. Start on the Lesson03-Result-a page.
Basic Use of Result Type
The easiest way to see how to use Result is to use it as the return value of a function. Code that calls this function can check whether its return value is .success or .failure, then take appropriate action.
enum EvenNumberError: Error {
case emptyArray
}
// Function returns [Int], throws EvenNumberError
func evenNumbersThrow(in collection: [Int]) throws(EvenNumberError) -> [Int] {
guard !collection.isEmpty else { throw .emptyArray }
let evenNumbers = collection.filter { number in number % 2 == 0 }
if evenNumbers.isEmpty {
throw .emptyArray
} else {
return evenNumbers
}
}
func evenNumbers(in collection: [Int]) -> Result<[Int], EvenNumberError> { // 1
guard !collection.isEmpty else {
return .failure(.emptyArray) // 2
}
let evenNumbers = collection.filter { number in number % 2 == 0 }
if evenNumbers.isEmpty {
return .failure(.emptyArray)
} else {
return .success(evenNumbers) // 3
}
}
let emptyArray = [Int]()
let numbers: [Int] = [2,3,6,8,10]
let oddNumbers: [Int] = [1,3,5]
let result = evenNumbers(in: emptyArray)
switch result {
case .success(let array):
print(array)
case .failure(let error):
print("Array is empty")
}
do {
let array = try result.get()
print(array)
} catch {
print(error)
}
Task.result
Task has a result property whose type is Result. Here’s an example from Hacking With Swift’s Understanding Swift’s Result Type
func fetchReadings() async {
let fetchTask = Task {
let url = URL(string: "https://hws.dev/readings.json")!
let (data, _) = try await URLSession.shared.data(from: url)
let readings = try JSONDecoder().decode([Double].self, from: data)
return "Found \(readings.count) readings"
}
let result = await fetchTask.result // Note: this doesn't throw, so you don't try
}
do {
let output = try result.get()
} catch {
let output = "Error: \(error.localizedDescription)"
}
switch result {
case .success(let str):
let output = str
case .failure(let error):
let output = "Error: \(error.localizedDescription)"
}