Help converting error types

[berbsd]

I’m struggling to see how I can convert from one error type to another.

I have a function that returns a TaskEither<ApiError, Client>. I want to use the resulting client in another function that returns a TaskEither<ServiceError, String> . The two error types are not sharing a common base class.

I tried to chain the calls but could not make it work. The code is something along these lines. Also tried to add a mapLeft but did not work either. I have been going in circles and any help would be greatly appreciated.

The library is awesome, but would love to see more examples for common use-cases.

TaskEither<ServiceError, String> apiCall(URI uri) {
  
  return getClient()
      .mapLeft((error) => ServiceError.mapError(error))
      .flatMap((client) => TaskEither.tryCatch(
                () async => client.get(uri),
                ServiceError.mapError
              ).chainEither( // …. Remaining code to parse response

}

// the signature for getClient is as follow

TaskEither<ApiError, String> getClient() {
 …
}

[SandroMaglione]

Hi @berbsd

How is ServiceError.mapError defined?

mapLeft((error) => ServiceError.mapError(error)) should convert ApiError to ServiceError for this to work, for example:

class ServiceError {
  ServiceError();
  factory ServiceError.mapError(ApiError error) => ServiceError();
}

The second ServiceError.mapError inside tryCatch cannot have the same signature, since the error in tryCatch gives you Object + StackTrace instead of ApiError.

If for example you refactor like below it compiles:

typedef URI = String;

class ServiceError {
  ServiceError();
  factory ServiceError.mapError(ApiError error) => ServiceError();
}

class ApiError {}

TaskEither<ApiError, String> getClient = TaskEither.of("");
TaskEither<ServiceError, String> apiCall(URI uri) {
  return getClient
      .mapLeft(
        (error) => ServiceError.mapError(error),
      )
      .flatMap(
        (client) => TaskEither.tryCatch(
          () async => client + "",
          (_, __) => ServiceError(),
        ),
      );
}

Let me know if you have any question