# prefix(while:) Returns an asynchronous sequence, containing the initial, consecutive elements of the base sequence that satisfy the given predicate. ``` @preconcurrency func prefix(while predicate: @escaping @Sendable (Self.Element) async -> Bool) rethrows -> AsyncPrefixWhileSequence ``` ## Parameters `predicate` A closure that takes an element as a parameter and returns a Boolean value indicating whether the element should be included in the modified sequence. ## Return Value An asynchronous sequence of the initial, consecutive elements that satisfy `predicate`. ## 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`. In this example, an asynchronous sequence called `Counter` produces `Int` values from `1` to `10`. The `prefix(while:)` method causes the modified sequence to pass along values so long as they aren’t divisible by `2` and `3`. Upon reaching `6`, the sequence ends: ``` let stream = Counter(howHigh: 10) .prefix { $0 % 2 != 0 || $0 % 3 != 0 } for try await number in stream { print(number, terminator: " ") } // Prints "1 2 3 4 5 " ``` --- 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)