Good day. I have a function which returns an Eithe...

# arrow

[Johan Basson]

Good day. I have a function which returns an Either<DomainError, Id> which i need to invoke in a loop for a few times. What would be the best way to implement this loop taking into account that when a Either.Left<DomainError> is returned then the loop should stop?

[Satyam Agarwal]

You can do parSequence and use takeWhile to check if it is left or right. If you are using arrow-fix-coroutines

[than_]

either{
    someData.map{ yourFuncThatReturnsEither(it).bind() }
}

[Scott Christopher]

There is also Either.tailRecM , which has a bit of a confusing signature (particularly when dealing with Either). https://arrow-kt.io/docs/apidocs/arrow-core-data/arrow.core/-either/tail-rec-m.html

If the type you're working with is Either<DomainError, Id> , the type signature is something like:

<A> Either.tailRecM(repeatedValue: A, (A) -> Either<DomainError, Either<A, Id>>) : Either<DomainError, Id>

[Scott Christopher]

The idea being that you provide the initial value of type A (which can be any type), and it is repeatedly given to the provided function, which is expected to return either a Left<DomainError> in the case of an error, Right<Left<A>> if you need to loop again with a new A, or a Right<Right<Id>> when you're done.

[Scott Christopher]

The benefit of tailRecM is that it should be stack safe.

[pakoito]

you’ve been given all 3 options: sequence/parSequence, a tailrec suspend fun where you use either { } , or tailrecM

[julian]

@Satyam Agarwal Can you show an example of using parSequence and takeWhile as you recommend? Thanks.

[Satyam Agarwal]

listOf(1,2,3,5)
            .map { element ->
                suspend { doSomethingWithElement(element) }
            }
            .parSequenceN(semaphoreLimit)
            .takeWhile { p -> p.isRight() }

its raw but I basically changed real use case to this

[Satyam Agarwal]

@julian

[julian]

Thank you! @Satyam Agarwal

[Johan Basson]

Thanks for the feedback