Hi! Given I have 2 `Either` of type `<List<Error>, Success>>`, how could I collect bo...

# arrow

[Philipp Mayer]

Hi!
Given I have 2 Either of type <List<Error>, Success>>, how could I collect both of their lefts (if both are left).

My setup looks like this:

import arrow.core.Either

data class Error(val message: String)
data class Success(val data: Int)
data class SuccessSum(val success1: Success?, val success2: Success?)

fun myFunction(): Either<List<Error>, SuccessSum> {
    val responseA = someNetworkCall()
    val responseB = someNetworkCall()

    return TODO()
}


fun someNetworkCall(aGivenParameter: Int = 1): Either<List<Error>, Success> {
    TODO()
}

and I have the following possibilities regarding my return type:
it can either be:
• a list of errors originating from the first response
• a list of errors originating from the second response
• a list of errors originating from both responses
• SuccessSum , my happy path
Is there any way to check if both eithers are null and if they are, merge their left values together into one list?

My happy path would then look like that:

fun myFunction(): Either<List<Error>, SuccessSum> {
    val responseA = someNetworkCall()
    val responseB = someNetworkCall()

    /* Guard Clause checking for lefts here */

    return SuccessSum(
        success1 = responseA.orNull(),
        success2 = responseB.orNull()
    )
}

That’s at least my initial idea. I guess Validated could be a better option but I didn’t look into it yet.
Thanks in advance! 🙂

[raulraja]

Hi @Philipp Mayer if you have two network calls that return Either you can turn them toValidatedNel and zip them together as shown in the https://arrow-kt.io/docs/patterns/error_handling/ in order to accumulate errors. Alternatively you may have your network calls return ValidatedNel instead of Either and then you can also use the bind in either blocks to treat the Validated values as if they where Either values.

[Philipp Mayer]

Thanks a lot for your input @raulraja.
I worked through the example and am now close to a solution (that’s what I hope at least 😄 )
My solution now looks like that:

import arrow.core.Either
import arrow.core.ValidatedNel
import arrow.core.zip

data class Error(val message: String)
data class Success(val data: Int)
data class SuccessSum(val success1: Success?, val success2: Success?)

//my network call is now returning ValidatedNel<List<Error>, Success>
//this function merges both responses into an either
infix fun ValidatedNel<List<Error>, Success>.mergeWith(
    leaseResponse: ValidatedNel<List<Error>, Success>,
): Either<List<Error>, SuccessSum> =
    zip(leaseResponse) { firstSuccess, secondSuccess -> SuccessSum(firstSuccess, secondSuccess) }
        .mapLeft { it.flatten() }
        .toEither()

The thing that I’d like to remove is the mapLeft + ultimately returning an Nel<Error> instead of a List<Error>. Any hints on that?

[raulraja]

You can then simplify it to not use a List which is not correct since you can’t never have an empty List of Errors, you either have errors or you don’t and it could be encoded like:

[raulraja]

validated_mergeWith_to_Either.cpp

[raulraja]

ValidatedNel already implies the left is a NonEmptyList so ValidatedNel<List… is redundant and can be avoided. Your network services should just return ValidatedNel<Error, ..>

[raulraja]

or Validated<NonEmptyList<Error>, ...> which is the same

[Philipp Mayer]

Oh it’s so obvious. I knew I wanted a NEL but wasn’t sure how to form one from the List. Well, ofc simply create one in the network call. Sometimes it’s the easiest things..

[Philipp Mayer]

Thanks a lot @raulraja!

[raulraja]

no problem, Nel also has unsafeFromList for cases where the list is handed to you and you are sure it has something

[Philipp Mayer]

uhhh, good to know!

[raulraja]

but if you are in control definetly use the Nel from the beginning