Sorry for disturbing the discussion but I have que...

# arrow

[Stian N]

Sorry for disturbing the discussion but I have question regarding the use of a group of Eithers.

Is List<Either<a,b>> an anti-pattern? If not, how do you handle the left cases in a good way? Let's say that if one of the Eithers is left I want to stop and bubble the Left?

[pakoito]

list.sequence(Either.applicative()) may be what you're after

[pakoito]

there's also the Monoid version of Either, which AFAIK should do the same: https://github.com/arrow-kt/arrow/blob/master/modules/core/arrow-core-data/src/main/kotlin/arrow/typeclasses/Monoid.kt#L19

[pakoito]

oh, it actually doesn't short on errors, it just aggregates them together 😄

[pakoito]

https://github.com/arrow-kt/arrow/blob/master/modules/core/arrow-core/src/main/kotlin/arrow/core/extensions/either.kt#L73

[pakoito]

so yeah, sequence is what you want

[pakoito]

it'll stop on the first Either error tho

[pakoito]

if what you want is aggregate errors, map to ValidatedNel first

[pakoito]

Validated never shorts, and it aggregates errors in the NonEmptyList (nel) on the left side

[Stian N]

list.asSequence(Either.applicative<T>()) ?

[pakoito]

no

[pakoito]

sequence, the extension function. It should be declared for List, otherwise list.k().sequence(...)

[Stian N]

Sorry for asking dumb questions but is this the right sequence? https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/-sequence/index.html

[Bob Glamm]

No, it should be the sequence defined on ListK in Arrow

[pakoito]

No such thing as dumb questions Stian. You have to make sure to have the right import, in this case the extension function sequence defined in the interface Traverse

[pakoito]

https://github.com/arrow-kt/arrow/blob/1a88e688b8edb5bb50c4fd518f436641dc0d80b5/modules/core/arrow-core-data/src/main/kotlin/arrow/typeclasses/Traverse.kt#L25

[pakoito]

import arrow.core.extensions.list.traverse.*

[pakoito]

https://arrow-kt.io/docs/arrow/data/listk/

[Stian N]

Thank you 🙂