---
title: "mapError(_:)"
framework: swift
role: symbol
role_heading: Instance Method
path: "swift/result/maperror(_:)"
---

# mapError(_:)

Returns a new result, mapping any failure value using the given transformation.

## Declaration

```swift
consuming func mapError<NewFailure>(_ transform: (Failure) -> NewFailure) -> Result<Success, NewFailure> where NewFailure : Error
```

## Parameters

- `transform`: A closure that takes the failure value of the instance.

## Return Value

Return Value A Result instance with the result of evaluating transform as the new failure value if this instance represents a failure.

## Discussion

Discussion Use this method when you need to transform the value of a Result instance when it represents a failure. The following example transforms the error value of a result by wrapping it in a custom Error type: struct DatedError: Error {     var error: Error     var date: Date

init(_ error: Error) {         self.error = error         self.date = Date()     } }

let result: Result<Int, Error> = // ... // result == .failure(<error value>) let resultWithDatedError = result.mapError { DatedError($0) } // result == .failure(DatedError(error: <error value>, date: <date>))

## See Also

### Transforming a Result

- [map(_:)](swift/result/map(_:).md)
- [flatMap(_:)](swift/result/flatmap(_:).md)
- [flatMapError(_:)](swift/result/flatmaperror(_:).md)