# init(unfolding:) Constructs an asynchronous throwing stream from a given element-producing closure. ``` @preconcurrency init(unfolding produce: @escaping @Sendable () async throws -> Element?) where Failure == any Error ``` ## Parameters `produce` A closure that asynchronously produces elements for the stream. ## Discussion Use this convenience initializer when you have an asynchronous function that can produce elements for the stream, and don’t want to invoke a continuation manually. This initializer “unfolds” your closure into a full-blown asynchronous stream. The created stream handles adherence to the `AsyncSequence` protocol automatically. To terminate the stream with an error, throw the error from your closure. The following example shows an `AsyncThrowingStream` created with this initializer that produces random numbers on a one-second interval. If the random number is divisible by 5 with no remainder, the stream throws a `MyRandomNumberError`. ``` let stream = AsyncThrowingStream { await Task.sleep(1 * 1_000_000_000) let random = Int.random(in: 1...10) if random % 5 == 0 { throw MyRandomNumberError() } return random } // Call point: do { for try await random in stream { print(random) } } catch { print(error) } ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)