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

# mapError(_:)

Converts any failure from the upstream publisher into a new error.

## Declaration

```swift
func mapError<E>(_ transform: @escaping (Self.Failure) -> E) -> Publishers.MapError<Self, E> where E : Error
```

## Parameters

- `transform`: A closure that takes the upstream failure as a parameter and returns a new error for the publisher to terminate with.

## Return Value

Return Value A publisher that replaces any upstream failure with a new error produced by the transform closure.

## Discussion

Discussion Use the mapError(_:) operator when you need to replace one error type with another, or where a downstream operator needs the error types of its inputs to match. The following example uses a tryMap(_:) operator to divide 1 by each element produced by a sequence publisher. When the publisher produces a 0, the tryMap(_:) fails with a DivisionByZeroError. The mapError(_:) operator converts this into a MyGenericError. struct DivisionByZeroError: Error {} struct MyGenericError: Error { var wrappedError: Error }

func myDivide(_ dividend: Double, _ divisor: Double) throws -> Double {        guard divisor != 0 else { throw DivisionByZeroError() }        return dividend / divisor    }

let divisors: [Double] = [5, 4, 3, 2, 1, 0] divisors.publisher     .tryMap { try myDivide(1, $0) }     .mapError { MyGenericError(wrappedError: $0) }     .sink(         receiveCompletion: { print ("completion: \($0)") ,         receiveValue: { print ("value: \($0)", terminator: " ") }      )

// Prints: "0.2 0.25 0.3333333333333333 0.5 1.0 completion: failure(MyGenericError(wrappedError: DivisionByZeroError()))"

## See Also

### Mapping elements

- [map(_:)](combine/publisher/map(_:)-99evh.md)
- [tryMap(_:)](combine/publisher/trymap(_:).md)
- [replaceNil(with:)](combine/publisher/replacenil(with:).md)
- [scan(_:_:)](combine/publisher/scan(_:_:).md)
- [tryScan(_:_:)](combine/publisher/tryscan(_:_:).md)
- [setFailureType(to:)](combine/publisher/setfailuretype(to:).md)