> For the complete documentation index, see [llms.txt](https://demystifyfp.gitbook.io/fstoolkit-errorhandling/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://demystifyfp.gitbook.io/fstoolkit-errorhandling/fstoolkit.errorhandling.icedtasks/index-1/maperrors.md).

# mapErrors

### CancellableTaskValidation.mapErrors

Namespace: `FsToolkit.ErrorHandling`

Function Signature:

```fsharp
('errorInput list -> 'errorOutput list)
  -> CancellableTaskValidation<'ok, 'errorInput>
  -> CancellableTaskValidation<'ok, 'errorOutput>
```

Applies a mapping function to the **entire error list** inside a `CancellableTaskValidation`. If the computation is `Ok`, the mapping function is not called and the value is propagated unchanged.

Note: `mapErrors` operates on the whole list at once. To map over individual errors, use [`mapError`](/fstoolkit-errorhandling/fstoolkit.errorhandling.icedtasks/index-1/maperror.md).

### Examples

#### Example 1

Deduplicating errors before returning them:

```fsharp
let validateInput (input: FormInput) : CancellableTaskValidation<ValidatedForm, string> =
    runValidation input
    |> CancellableTaskValidation.mapErrors List.distinct
```

#### Example 2

Sorting and limiting the number of errors returned to a caller:

```fsharp
let validateOrder (order: Order) : CancellableTaskValidation<ValidatedOrder, string> =
    runOrderValidation order
    |> CancellableTaskValidation.mapErrors (fun errs -> errs |> List.sort |> List.truncate 5)
```

#### Example 3

Converting a list of typed errors to a list of user-facing messages:

```fsharp
let processRequest (req: Request) : CancellableTaskValidation<Response, string> =
    validateRequest req
    |> CancellableTaskValidation.mapErrors (List.map (fun (e: DomainError) -> e.UserMessage))
```