Arrow-kt: how to turn an Either>> into Either>

Question

I receive an Either<E, List<A>> from a function call and need to transform the List<A> into a List. The transformation of each A returns an Either<E,B>, so that my result is an Either<E, List<Either<E,B>>>. How can I turn the Either<E, List<Either<E,B>>> into an Either<E, List>,

if all transformation succeeded (Should result in Either.Left, if a single transformation fails)
create an Either<E,List> containing all Bs, for which the transformation succeeded and ignoring failed AtoB transformations

A little code snippet below:

fun getListOfA(): Either<Exception, List<A>> {
    TODO()
}

fun A.transformAtoB(): Either<Exception, B> {
    TODO()
}


fun getListB(): Either<Exception, List<B>> {
    return getListOfA().map {

        // this is now Either<Exception, List<Either<Exception, B>>>
        listOfA -> listOfA.map { it.transformAtoB() }

        // ????? => Either<E, List<B>>
    }
}

nomisRev · Accepted Answer · 2021-09-15T17:28:44.847

2

There is a function called traverseEither that allows you to do these kind of operations.

public inline fun <E, A, B> Iterable<A>.traverseEither(f: (A) -> Either<E, B>): Either<E, List>

For every value of A in the Iterable it will call f, and if all the results are Either.Right then it will result in Either.Right<List> and otherwise it will result in the first Either.Left<E> it encounters.

So we can rewrite your snippet:

fun getListOfA(): Either<Exception, List<A>> = TODO()

fun A.transformAtoB(): Either<Exception, B> = TODO()

fun getListB(): Either<Exception, List<B>> =
    getListOfA().flatMap { listOfA ->

        // this is now Either<Exception, List<Either<Exception, B>>>
        listOfA.traverseEither { it.transformAtoB() }
    }

It also exists for Validated, Option etc https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core/kotlin.collections.-iterable/index.html#extensions-for-kotlincollectionsiterable

And you can also find parallel variants inside Arrow Fx Coroutines

edited Sep 15 '21 at 17:28

answered Sep 15 '21 at 14:20

nomisRev

1,881
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15

1

.map{} needs to be changed to .flatMap{} to get Either>. But otherwise this is the solution I was looking for! – macnixde Sep 15 '21 at 14:47
If the transformation would return an Option instead of an Either, such as in A.transformToB():Option, how could this be traversed? – macnixde Sep 15 '21 at 14:49
There is also `traverseOption`, and `traverseValidated`, etc. You can find them all in the link shared in the post. – nomisRev Sep 15 '21 at 17:29
I am sorry, but I do not see any traverseOption(). I read the linked page before, but there is also no traverseOption in the description. There is a traverse(), traverseEither() and traverseValidated(). On a List I can only do traverseEither/traverseValidated. Am I missing to include another library here? I can apply filterOption() to a List – macnixde Sep 18 '21 at 08:06
My apologies, `traverseOption` is only added since `0.13.3` & `1.0.0` which is to be released September 2021 (later this week of writing). – nomisRev Sep 20 '21 at 07:37

Arrow-kt: how to turn an Either>> into Either>

1 Answers1