如何从 Dart (Dartz) 中的 Either 类型中轻松提取 Left 或 Right

Question

我希望从 return 类型 Either<Exception, Object> 的方法中轻松提取值。

我正在做一些测试，但无法轻松测试我的方法 return。

例如：

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

为了测试我能做到这一点

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

现在如何直接检索结果？

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

我发现我必须这样投:

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...

我也想测试异常，但它不起作用，例如：

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

所以我尝试了这个

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

Answer 1

我无法 post 发表评论...但也许您可以看看这个 post。这不是同一种语言，但看起来是相同的行为。

祝你好运。

Answer 2

好的，这里是我的问题的解决方案：

到extract/retrieve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>()));

Answer 3

  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 4

提取值的另一种方法是简单地转换为 Option，然后转换为 dart 可为空：

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

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

如果你愿意，你可以定义一个简单的扩展来简化这个：

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

Answer 5

朋友，

像这样创建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;
}

然后像那样使用。

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
  }
}

Answer 6

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

如何从 Dart (Dartz) 中的 Either 类型中轻松提取 Left 或 Right

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

unit-testing

flutter

dart

either