mapErrorToResult<NewFailure extends Object> method

@optionalTypeArgs Result<Success, NewFailure> mapErrorToResult<NewFailure extends Object>(Result<Success, NewFailure> transform(Failure error))

When this Result is a success, simply return the value as-is. When this Result is a failure, transform the error into a new Result.

After the transformation, isSuccess may be different between the new and old Results—the transform IS allowed to change the nature of the outcome.

A Result.success will always transform to a Result.success.

A Result.failure may transform to either a Result.success or a Result.failure.

Useful to recover from some errors by returning a success value when a workaround exists. The value must match the success type, but the error type can change.

Here, mapErrorToResult is used to ignore errors which can be resolved by a cache lookup. Both unrecoverable DioErrors and internal errors accessing the cache are expressed in the more generic FetchError of the final output.

final Result<Person, DioError> raw = await dioGetApiPerson(id);
final Result<Person, FetchError> output = raw.mapErrorToResult((error) => _getPersonCache(id));

Result<Person, FetchError> _getPersonCache(int id) {
  // ...
}

Here, an initial failure is transformed into a success whenever the required value is available in the local cache.

Implementation

@optionalTypeArgs
Result<Success, NewFailure> mapErrorToResult<NewFailure extends Object>(
  Result<Success, NewFailure> Function(Failure error) transform,
) =>
    when(
      success: (value) => Result.success(value),
      failure: (error) => transform(error),
    );