How can i convert a `List<Either<A,B>>` TO `Either<List<A>,List&...

# arrow

[Shabinder Singh]

How can i convert a

List<Either<A,B>>

TO

Either<List<A>,List<B>>
OR
ValidaterNel<A,List<B>>

basically it iterates over either and collect both errors and success,
• .sequence shortCircuits which isnt wanted here*,*
• .traverse {} is there but I thought there would a extension function for above as its a common useCase I guess, is there an extension for above ?

[Shabinder Singh]

have builtSomething like this:

fun <A, B> Iterable<Either<A, B>>.toValidatedNel(): ValidatedNel<A, List<B>> {
    return traverse(semigroup = Semigroup.nonEmptyList()) { it.toValidatedNel() }
}

[simon.vergauwen]

This is your best bet, there is nothing out of the box for this in Arrow atm. We are unifying Either and Validated in Arrow 2.x.x, so this would be covered by that.

[raulraja]

still on going but work is happening in this branch https://github.com/arrow-kt/arrow/pull/2778

[Shabinder Singh]

have come up with below:

Suggestions are welcome.

[simon.vergauwen]

Hey @Shabinder Singh, I don’t think your observation is correct.

It starts with an acc = Valid(emptyList()) and then it runs f with the first element of the Iterable. If val res = f(a) returns Invalid it will check the state of acc and combine the results when needed.

So for example listOf(1).traverse { Invalid(it) } it will start with acc = Valid(emptyList<B>()) and then run val res = Invalid(1).

Since res is Invalid and acc = Valid it will return the Invalid case.

Here are a property based test that verify this, https://github.com/arrow-kt/arrow/blob/96c774a08b1c86f369f075b4b8dd81a8904121d1/ar[…]ore/arrow-core/src/commonTest/kotlin/arrow/core/IterableTest.kt