# Writing Failable Asynchronous APIs Vend results as part of an API when you can’t return errors synchronously. ## Overview When writing a function, method, or other API that might fail, you use the `throws` keyword on the declaration to indicate that the API call can throw an error. However, you can’t use the `throws` keyword to model APIs that return asynchronously. Instead, use the [`Result`](/documentation/Swift/Result) enumeration to capture information about whether an asychronous call succeeds or fails, and use the associated values for the [`Result.success(_:)`](/documentation/Swift/Result/success(_:)) and [`Result.failure(_:)`](/documentation/Swift/Result/failure(_:)) cases to carry information about the result of the call. ### Return Result Instances Asynchronously The following example models an asynchronous source of random numbers. The `fetchRemoteRandomNumber(completion:)` method returns `Void` synchronously, and asynchronously calls a completion handler with a `Result` instance that contains either a random result or information about the failure. ```swift let queue = DispatchQueue(label: "com.example.queue") enum EntropyError: Error { case entropyDepleted } struct AsyncRandomGenerator { static let entropyLimit = 5 var count = 0 mutating func fetchRemoteRandomNumber( completion: @escaping (Result) -> Void ) { let result: Result if count < AsyncRandomGenerator.entropyLimit { // Produce numbers until reaching the entropy limit. result = .success(Int.random(in: 1...100)) } else { // Supply a failure reason when the caller hits the limit. result = .failure(.entropyDepleted) } count += 1 // Delay to simulate an asynchronous source of entropy. queue.asyncAfter(deadline: .now() + 2) { completion(result) } } } ``` Users of your remote random number generator can decide how to handle both the success and failure cases: ```swift var generator = AsyncRandomGenerator() // Request one more number than the limit to trigger a failure. (0..