如何根据选项值获得 Either left/right

Question

我正在尝试根据选项值 return Either 值。如果选项存在，我的目标是 return Either.right() 否则，代码应该 return Either.left()。我使用 Java 8 和 vavr 0.9.2

我想避免条件重叠

public Either<String, Integer> doSomething() {
    Optional<Integer> optionalInteger = Optional.of(Integer.MIN_VALUE);
    Option<Integer> integerOption = Option.ofOptional(optionalInteger);

    return integerOption.map(value -> {
      //some other actions here
      return Either.right(value);
    }).orElse(() -> {
      //some other checks her also 
      return Either.left("Error message");
    });
}

编译器失败并显示此消息

Error:(58, 7) java: no suitable method found for orElse(()->Either[...]age"))
    method io.vavr.control.Option.orElse(io.vavr.control.Option<? extends io.vavr.control.Either<java.lang.Object,java.lang.Integer>>) is not applicable
      (argument mismatch; io.vavr.control.Option is not a functional interface
          multiple non-overriding abstract methods found in interface io.vavr.control.Option)
    method io.vavr.control.Option.orElse(java.util.function.Supplier<? extends io.vavr.control.Option<? extends io.vavr.control.Either<java.lang.Object,java.lang.Integer>>>) is not applicable
      (argument mismatch; bad return type in lambda expression
          no instance(s) of type variable(s) L,R exist so that io.vavr.control.Either<L,R> conforms to io.vavr.control.Option<? extends io.vavr.control.Either<java.lang.Object,java.lang.Integer>>)

Answer 1

orElse returns Option<T> 而 doSomething return 类型需要 Either<String, Integer>.

相反，尝试使用 getOrElse which returns T:

public Either<String, Integer> doSomething() {
    // ...
    return integerOption.map(
        Either::<String, Integer>right).getOrElse(
            () -> Either.left("Error message"));
}

Answer 2

因为你想 return Either<...> 而不是 Option<Either<...>> 你必须使用 getOrElse

Answer 3

我认为@oleksandr-pyrohov 的回答中的冗余泛型和格式让事情变得有点复杂。我只是这样写：

public Either<String, Integer> doSomething() {
    // ...
    return integerOption
        .map(Either::right)
        .orElseGet(() -> Either.left("Error message"));
}

更新：虽然它似乎并不总是可悲地自行推断泛型。

Answer 4

对我有用

public Either<String, Integer> doSomething() {
// ...
return integerOption
    .<Either<String, Integer>>map(Either::right)
    .orElseGet(() -> Either.left("Error message"));
}

如何根据选项值获得 Either left/right

How to get Either left/right depending of Option value

java

either

vavr