Heyo, Is there a built in method to iterate over the list and collect all failures from all items of...

# arrow

[Masrur Mirboboev]

Heyo, Is there a built in method to iterate over the list and collect all failures from all items of the iterable? traverse seems to short circuit on the first failure.

[Masrur Mirboboev]

I created below helper function, but wondering if I could achieve it with built it capabilities:

fun <A, B> Iterable<B>.traverseAll(f: (B) -> Either<A, B>): Either<List<A>, List<B>> {
    val lefts: MutableList<A> = mutableListOf()
    val rights: MutableList<B> = mutableListOf()

    forEach { f(it).fold(lefts::add, rights::add) }

    return conditionally(
        test = lefts.isEmpty(),
        ifTrue = { rights },
        ifFalse = { lefts }
    )
}

[qohatpp]

If you have a List<B> and one function of type (B) -> Either<A, B> and you want to build Either<A, List<B>> accumulating the errors, You should think about Semigroup, So I think you can take a look at traverse(Semigroup, B -> Validated<A, B>) or parTraverseValidated() , Here a really good example https://www.47deg.com/blog/functional-domain-modeling-part-2

[simon.vergauwen]

You can also use Validated<NonEmptyList<E>, A> and it will return Validated<NonEmptyList<E>, List<A>> from traverse and there will be no need to pass Semigroup.

It uses Semigroup for NonEmptyList by default which is { listA, listB -> listA + listB } so concatenation of the errors lists.

[Masrur Mirboboev]

Sounds good. Wondering if is semantically correct or not when you process list of items which might have side effects.

[stojan]

typically in arrow we use suspend for functions with side effects.... so you can use Validated + suspend functions