How to get Either left/right depending of Option value

Question

I'm trying to return Either value depending on option value. My goal is to return Either.right() if the option is present otherwise, the code should return Either.left(). I use Java 8 and vavr 0.9.2

I want to avoid conditional imbrication

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");
    });
}

the compiler fail with this message

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>>)

Oleksandr Pyrohov · Accepted Answer · 2019-06-08T22:26:31.940

8

orElse returns Option<T> while doSomething return type requires Either<String, Integer>.

Instead, try to use getOrElse which returns T:

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

edited Jun 08 '19 at 22:26

answered Jun 08 '19 at 21:34

Oleksandr Pyrohov

14,685
6
61
90

Thanks for your answer but with getOrElse, there is no way to infer Either it always infers Either – EFOE Jun 08 '19 at 21:52
2

Yes, that's why you need to provide an explicit type argument: `Either::right`, no matter it is `getOrElse` or `orElse`. – Oleksandr Pyrohov Jun 08 '19 at 22:17
I believe it's called `orElseGet`, not `getOrElse` in java ;) – Klesun Jun 04 '20 at 14:43
1

@Klesun No, as it's a [vavr api method](https://www.javadoc.io/doc/io.vavr/vavr/0.9.2/io/vavr/control/Option.html#getOrElse-T-). – Oleksandr Pyrohov Nov 27 '20 at 15:14

score 0 · Answer 2 · answered Jun 08 '19 at 21:45

0

Since you want to return Either<...> not an Option<Either<...>> you have to use getOrElse

answered Jun 08 '19 at 21:45

adil.hilmi

197
5

Klesun · Answer 3 · 2020-06-26T18:38:23.830

0

I think redundant generics and formatting in @oleksandr-pyrohov's answer overcomplicate things a bit. I'd just write it this way instead:

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

Upd.: though it does not seem to always infer generics on it's own sadly.

edited Jun 26 '20 at 18:38

answered Jun 04 '20 at 14:45

Klesun

12,280
5
59
52

score 0 · Answer 4 · answered Mar 15 '22 at 21:14

0

It works for me

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

answered Mar 15 '22 at 21:14

luprogrammer

155
1
3
12

How to get Either left/right depending of Option value

4 Answers4