I’m finding myself doing a bunch of validations on an unknown number of items and I end up with a `L...

# arrow

[Steven Sherry]

I’m finding myself doing a bunch of validations on an unknown number of items and I end up with a List<ValidatedNel<E, A>> and I’m converting that to a ValidatedNel<E, List<A>>. Is there a more idiomatic way of doing that? The actual flow of data is more like this: List<A> -> List<ValidatedNel<E, B>> -> ValidatedNel<E, List<B>> . It works, but it feels like there is probably something more “native” to arrow that would handle this kind of flow.

[aballano]

that’s traverse! (sorry, classic FP joke 😄)

[aballano]

but yes, traverse should do what you need, also sequence if you don’t need to map your items

[Steven Sherry]

awesome, I knew there had to be a particular operation that corresponded to the “shape” of that flow

[Steven Sherry]

I’ll take a look at that

[simon.vergauwen]

Yes, you want.

val aas: List<A> = listOf(...)

val validated: List<ValidatedNel<E, B>> = aas.map(::validate)

val res: ValidatedNel<E, List<B>> = validated.sequence(Validated.applicative(NonEmptyList.applicative()).map { it.fix() }

OR

val aas: List<A> = listOf(...)

aas.traverse(Validated.applicative(NonEmptyList.applicative()) { a ->
  validate(a)
}.map { it.fix() }

[Steven Sherry]

Traverse did the trick and now I get to delete a bunch of code! Thanks everyone