英文:
Unexpected return type of vavr's Either in reactor
问题
以下是您要翻译的内容：
有两种使用vavr的Either的简单方法。
public Either<String, Integer> testEither(int s) {
if (s == 0)
return Either.left("Wrong");
return Either.right(s);
}
public Mono<Either<String, Integer>> testReactorEither(int s) {
return Mono.just(s).filter(x -> x == 0).map(Either::right)
.switchIfEmpty(Mono.just(Either.left("ERROR")));
}
testEither方法正常工作，但与此同时，testReactorEither方法引发了"不兼容类型"的编译错误，错误消息说提供的返回类型"reactor.core.publisher.Mono<io.vavr.control.Either<java.lang.Object,java.lang.Integer>>"与所需返回类型"reactor.core.publisher.Mono<io.vavr.control.Either<java.lang.String,java.lang.Integer>>"不兼容。
我担心问题只是因为"map(Either::right)"方法只定义了"Integer"的"Right"类型，但没有定义"Left"类型，然后方法的返回类型是"Either<?, Integer>"。那么问题是，在这种情况下，我如何获得预期的返回类型呢？
[已更新]
正如Hinse在他的评论中提到的，这个问题与Java类型推断的限制有关，我找到的一些与该问题相关的链接如下：
https://bugs.eclipse.org/bugs/show_bug.cgi?id=511252
https://e.printstacktrace.blog/java-type-inference-generic-methods-chain-call/
https://openjdk.java.net/jeps/101
英文:
There are two simple methods using vavr's Either.
public Either<String, Integer> testEither(int s) {
if (s == 0)
return Either.left("Wrong");
return Either.right(s);
}
public Mono<Either<String, Integer>> testReactorEither(int s) {
return Mono.just(s).filter(x -> x == 0).map(Either::right)
.switchIfEmpty(Mono.just(Either.left("ERROR")));
}
The testEither works normally, but meanwhile, the testReactorEither raises a compile error of "incompatible types" which says the provided return type of reactor.core.publisher.Mono<io.vavr.control.Either<java.lang.Object,java.lang.Integer>> is incompatible with required return of reactor.core.publisher.Mono<io.vavr.control.Either<java.lang.String,java.lang.Integer>>.
I'm afraid the problem is just because the method of map(Either::right) just defines the Right type of Integer but not do the Left type, and then the return type of the method is Either<?, Integer>. Then the question is how I can get the expected return type in this case?
[UPDATED]
As Hinse mentioned in his comment, the issue is related to the limitation of Java type inference, and some links I found for the problem is listed as follows:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=511252
https://e.printstacktrace.blog/java-type-inference-generic-methods-chain-call/
答案1
得分: 1
第二个示例中，在应用map(Either::right)后，左侧似乎丢失了类型信息。
在map方法调用中添加一些类型提示应该可以解决问题。因此，testReactorEither将如下所示：
public Mono<Either<String, Integer>> testReactorEither(int s) {
return Mono.just(s)
.filter(x -> x == 0)
.<Either<String, Integer>>map(Either::right)
.switchIfEmpty(Mono.just(Either.left("ERROR")));
}
英文:
The second example seems to 'lose' the type information for the left side when the map(Either::right) is applied.
Adding some type 'hints' to the map method call should do the trick. So the testReactorEither will look like this:
public Mono<Either<String, Integer>> testReactorEither(int s) {
return Mono.just(s)
.filter(x -> x == 0)
.<Either<String, Integer>>map(Either::right)
.switchIfEmpty(Mono.just(Either.left("ERROR")));
}
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