# prefix(while:) Returns an asynchronous sequence, containing the initial, consecutive elements of the base sequence that satisfy the given error-throwing predicate. ``` @preconcurrency func prefix(while predicate: @escaping @Sendable (Self.Element) async throws -> Bool) rethrows -> AsyncThrowingPrefixWhileSequence ``` ## Parameters `predicate` A error-throwing closure that takes an element of the asynchronous sequence as its argument and returns a Boolean value that indicates whether to include the element in the modified sequence. ## Return Value An asynchronous sequence that contains, in order, the elements of the base sequence that satisfy the given predicate. If the predicate throws an error, the sequence contains only values produced prior to the error. ## Discussion Use `prefix(while:)` to produce values while elements from the base sequence meet a condition you specify. The modified sequence ends when the predicate closure returns `false` or throws an error. In this example, an asynchronous sequence called `Counter` produces `Int` values from `1` to `10`. The `prefix(_:)` method causes the modified sequence to pass through values less than `8`, but throws an error when it receives a value that’s divisible by `5`: ``` do { let stream = try Counter(howHigh: 10) .prefix { if $0 % 5 == 0 { throw MyError() } return $0 < 8 } for try await number in stream { print(number, terminator: " ") } } catch { print("Error: \(error)") } // Prints "1 2 3 4 Error: MyError() " ``` --- 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)