What’s the best way to transform a `List<Either<Error, Id>>` to a `Either<Error, List...

# arrow

[jean]

What’s the best way to transform a List<Either<Error, Id>> to a Either<Error, List<Id>> ? I want a Either.Left return as soon as possible, not a list of all result eventually containing errors. Either an error immediately when something fails or a list of valid result

[simon.vergauwen]

Hey @jean,

You can use sequenceEither() to turn List<Either<Error, Id>> into Either<Error, List,<Id>> whilst returning as soon as a first Either.Left is found.

Alternatively, you can also use traverseEither to apply (A) -> Either<E, B> to List<A> and get back Either<E, List<B>>.

[simon.vergauwen]

This function also exists for Option, Validated, and more.

And there are also parallel version of this called parTraverseXXX & parSeauenceXXX inside Arrow Fx Coroutines

[jean]

are those methods located in arrow.core? IntelliJ can resolve Either but not the functions you listed in this thread

[simon.vergauwen]

Yes, they’re available in arrow.core.*

[simon.vergauwen]

https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core/traverse-either.html

[simon.vergauwen]

https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core/sequence-either.html

[jean]

oh I get it now, it should be applied as an operator, not something like sequenceEither { … }

[jean]

thanks, it solved my issue 🙂