# tryPrefix(while:) Republishes elements while an error-throwing predicate closure indicates publishing should continue. ``` func tryPrefix(while predicate: @escaping (Self.Output) throws -> Bool) -> Publishers.TryPrefixWhile ``` ## Parameters `predicate` A closure that takes an element as its parameter and returns a Boolean value indicating whether publishing should continue. ## Return Value A publisher that passes through elements until the predicate throws or indicates publishing should finish. ## Discussion Use [`tryPrefix(while:)`](/documentation/Combine/Publisher/tryPrefix(while:)) to emit values from the upstream publisher that meet a condition you specify in an error-throwing closure. The publisher finishes when the closure returns `false`. If the closure throws an error, the publisher fails with that error. ``` struct OutOfRangeError: Error {} let numbers = (0...10).reversed() cancellable = numbers.publisher .tryPrefix { guard $0 != 0 else {throw OutOfRangeError()} return $0 <= numbers.max()! } .sink( receiveCompletion: { print ("completion: \($0)", terminator: " ") }, receiveValue: { print ("\($0)", terminator: " ") } ) // Prints: "10 9 8 7 6 5 4 3 2 1 completion: failure(OutOfRangeError()) " ``` --- 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)