好了，这里的解决方案我的问题：

提取/检索数据

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);
}

测试异常

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

朋友们，
只需像下面这样创建dartz_x. dart。

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;
}

像这样使用。

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
  }
}

我不能发表评论...但也许你可以看看这个post。它不是同一种语言，但看起来像是同一种行为。
祝你好运。

提取值的另一种方法是简单地转换为Option，然后转换为dart可空值：

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

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

如果愿意，您可以定义一个简单的扩展名来简化此操作：

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

向左或向右提取：
左侧：

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

右：

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

我返回任一类型的折叠结果

@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);
    });
  }

使用Either给出的fold方法，并将不想为null的部分Map到自身：

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

然后导入包含此扩展名的文件，您可以在任一示例上调用.getRight（）或.getLeft（）。

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);
  },
);

}