---
title: "assertNoFailure(_:file:line:)"
framework: combine
role: symbol
role_heading: Instance Method
path: "combine/publisher/assertnofailure(_:file:line:)"
---

# assertNoFailure(_:file:line:)

Raises a fatal error when its upstream publisher fails, and otherwise republishes all received input.

## Declaration

```swift
func assertNoFailure(_ prefix: String = "", file: StaticString = #file, line: UInt = #line) -> Publishers.AssertNoFailure<Self>
```

## 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

Return Value A publisher that raises a fatal error when its upstream publisher fails.

## Discussion

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<String, Error>("initial value") subject     .assertNoFailure()     .sink(receiveCompletion: { print ("completion: \($0)") },           receiveValue: { print ("value: \($0).") }     )

subject.send("second value") subject.send(completion: Subscribers.Completion<Error>.failure(SubjectError.genericSubjectError))

// Prints: //  value: initial value. //  value: second value. //  The process then terminates in the debugger as the assertNoFailure operator catches the genericSubjectError.

## See Also

### Handling errors

- [catch(_:)](combine/publisher/catch(_:).md)
- [tryCatch(_:)](combine/publisher/trycatch(_:).md)
- [retry(_:)](combine/publisher/retry(_:).md)