# assertNoFailure(_:file:line:) Raises a fatal error when its upstream publisher fails, and otherwise republishes all received input. ``` func assertNoFailure(_ prefix: String = "", file: StaticString = #file, line: UInt = #line) -> Publishers.AssertNoFailure ``` ## Parameters `prefix` A string used at the beginning of the fatal error message. `file` A filename used in the error message. This defaults to `#file`. `line` A line number used in the error message. This defaults to `#line`. ## Return Value A publisher that raises a fatal error when its upstream publisher fails. ## Discussion Use `assertNoFailure()` for internal integrity checks that are active during testing. However, it is important to note that, like its Swift counterpart `fatalError(_:)`, the `assertNoFailure()` operator asserts a fatal exception when triggered during development and testing, *and* in shipping versions of code. In the example below, a `CurrentValueSubject` publishes the initial and second values successfully. The third value, containing a `genericSubjectError`, causes the `assertNoFailure()` operator to assert a fatal exception stopping the process: ``` public enum SubjectError: Error { case genericSubjectError } let subject = CurrentValueSubject("initial value") subject .assertNoFailure() .sink(receiveCompletion: { print ("completion: \($0)") }, receiveValue: { print ("value: \($0).") } ) subject.send("second value") subject.send(completion: Subscribers.Completion.failure(SubjectError.genericSubjectError)) // Prints: // value: initial value. // value: second value. // The process then terminates in the debugger as the assertNoFailure operator catches the genericSubjectError. ``` --- 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)