How to extract Left or Right easily from Either type in Dart (Dartz)

Question

I am looking to extract a value easily from a method that return a type Either<Exception, Object> .

I am doing some tests but unable to test easily the return of my methods.

For example:

final Either<ServerException, TokenModel> result = await repository.getToken(...);

To test I am able to do that

expect(result, equals(Right(tokenModelExpected))); // => OK

Now how can I retrieve the result directly?

final TokenModel modelRetrieved = Left(result); ==> Not working..

I found that I have to cast like that:

final TokenModel modelRetrieved = (result as Left).value; ==> But I have some linter complain, that telling me that I shouldn't do as to cast on object...

Also I would like to test the exception but it's not working, for example:

expect(result, equals(Left(ServerException()))); // => KO

So I tried this

expect(Left(ServerException()), equals(Left(ServerException()))); // => KO as well, because it says that the instances are different.

Answer 1

Ok here the solutions of my problems:

To extract/retrieve the data

final Either<ServerException, TokenModel> result = await repository.getToken(...);
result.fold(
 (exception) => DoWhatYouWantWithException, 
 (tokenModel) => DoWhatYouWantWithModel
);

//Other way to 'extract' the data
if (result.isRight()) {
  final TokenModel tokenModel = result.getOrElse(null);
}

To test the exception

//You can extract it from below, or test it directly with the type
expect(() => result, throwsA(isInstanceOf<ServerException>()));

Answer 2

Another way to extract the value is simply to convert to Option , then to a dart nullable:

final Either<Exception, String> myEither = Right("value");

final String? myValue = myEither.toOption().toNullable();

If you like you can define a simple extension to shortcut this:

extension EitherHelpers<L, R> on Either<L, R> {
  R? unwrapRight() {
    return toOption().toNullable();
  }
}

Answer 3

I can't post a comment... But maybe you could look at this post . It's not the same language, but looks like it's the same behaviour.

Good luck.

Answer 4

Friends,

just create dartz_x.dart like this one.

import 'package:dartz/dartz.dart';

extension EitherX<L, R> on Either<L, R> {
  R asRight() => (this as Right).value; //
  L asLeft() => (this as Left).value;
}

And use like that.

import 'dartz_x.dart';

void foo(Either<Error, String> either) {
  if (either.isLeft()) {
    final Error error = either.asLeft();
    // some code
  } else {
    final String text = either.asRight();
    // some code
  }
}

Answer 5

  Future<Either<Failure, FactsBase>> call(Params params) async {
final resulting = await repository.facts();
return resulting.fold(
  (failure) {
    return Left(failure);
  },
  (factsbase) {
    DateTime cfend = sl<EndDateSetting>().finish;        
    List<CashAction> actions = factsbase.transfers.process(facts: factsbase, startDate: repository.today, finishDate: cfend); // process all the transfers in one line using extensions
    actions.addAll(factsbase.transactions.process(facts: factsbase, startDate: repository.today, finishDate: cfend));
    for(var action in actions) action.account.cashActions.add(action); // copy all the CashActions to the Account.
    for(var account in factsbase.accounts) account.process(start: repository.today);
    return Right(factsbase);
  },
);

}

Answer 6

extension EitherExtension<L, R> on Either<L, R> {
  R? getRight() => fold<R?>((_) => null, (r) => r);
  L? getLeft() => fold<L?>((l) => l, (_) => null);
}

Answer 7

I return fold result on Either type

  @override
  Future<Either<Failure, bool>> call(UpdateReceiptParams params) async {
    Either<Failure, ProcurementStore> procurementStore =
        await _getProcurementStore();
    return procurementStore.fold((_) => Left(ServerFailure()),
        (procurementlStore) async {
      List<ReceiptItem> receiptItems =
          _getReceiptItem(params.shipmentStatus, params.quantityAmounts);
      final Receipt receipt = Receiplt(
          procurementStore: procurementStore,
          costCenter: const CostCenter(),
          receiptItems: receiptItems,
          shipmentId: params.shipmentStatus.Id,
          receiptType: params.shipmentStatus.ShipmentType,
          description: params.description,
          operationPeriodId: 0,
          receiptDate: params.date,
          receiptTime: params.time);
      return await shipmentsRepository.updateReceipt(receipt);
    });
  }

Answer 8

To extract left or right:

Left:

print(result.fold((l) => l, (r) => null))

Right:

print(result.fold((l) => null, (r) => r))

Answer 9

You can cast it but make sure u use isLeft / isRight first

final workflowUnload = await someEither();
 if (workflowUnload.isLeft()) {
  emit(state.copyWith.call(
     status: JobOrderStatus.error,
     failure: (workflowUnload as Left).value,
   ));
  }