# mapError(_:) Returns a new result, mapping any failure value using the given transformation. ``` consuming func mapError(_ transform: (Failure) -> NewFailure) -> Result where NewFailure : Error ``` ## Parameters `transform` A closure that takes the failure value of the instance. ## Return Value A `Result` instance with the result of evaluating `transform` as the new failure value if this instance represents a failure. ## 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 = // ... // result == .failure() let resultWithDatedError = result.mapError { DatedError($0) } // result == .failure(DatedError(error: , date: )) ``` --- 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)