Hi, can you show me an example of how to use zipOrAccumulate method using coroutines? Just basic acc...

# arrow

[Michal Patejko]

Hi, can you show me an example of how to use zipOrAccumulate method using coroutines? Just basic accumulation of 2/3 eithers and returning lefts (or all lefts merged into single object) if there's any.

[Alejandro Serrano Mena]

let’s suppose you have the following scenario:

data class Person(val name: String, val age: Int)

sealed interface Problem {
  object NameProblem
  object AgeProblem
}

fun validName(name: String): Either<NameProblem, String> = TODO()
fun validAge(age: Int): Either<AgeProblem, Int> = TODO()

[Alejandro Serrano Mena]

then you can run both validations and accumulate any possible error by using

[Alejandro Serrano Mena]

fun validPerson(name: String, age: Int): Either<Nel<Problem>, Person> = 
  either {
    zipOrAccumulate(
      { validName(name).bind() },
      { validAge(age).bind() }
    ) { n, a -> Person(n, a) }
  }

[simon.vergauwen]

Here validName and validAge can also be suspend, and Either.zipOrAccumulate(validName(age), validAge(age)) { n, a -> Person(n, a) } also exists.

The benefit of the DSL version that Alejandro showed is that it works for both Either<NonEmptyList<E>, A> and Either<E, A> in a single DSL. Whilst with the Either.zipOrAccumulate version you can only combine Either of the same type and need to manually call toEitherNel() to turn Either<E, A> into Either<NonEmptyList<E>, A>.

[Michal Patejko]

Thank you! So easy!

[Erik Dreyer]

I'm a little late to the game here. Does this mechanism chain to subobjects? For instance, extending the example above...

data class Pet(val name: String)
data class Person(val name: String, val age: Int, val pet: Pet)

fun createPerson(val name: String, val age: Int, pet: EitherNel<Problem, Pet>) = either {
  zipOrAccumulate(
    { validName(name).bind() },
    { validAge(age).bind() },
    { pet.bind() }
  ) { n, a, p -> Person(n, a, p)
}

Or, put another way, what's the best way to fully validate nested objects when constructing both of them?

[simon.vergauwen]

I would use the technique you propose here @Erik Dreyer, that is a great way to split up validation into smaller pieces! Splitting into seperate pieces, and functions is always a great option IMO to simplify code

[Erik Dreyer]

@simon.vergauwen Could you take a look at this snippet? Either I'm doing something wrong, or there's a possible bug in zipOrAccumulate
You should be able to run this as a scratch file in IntelliJ. This is using Arrow 1.2.1

I have validated that both NonEmptyString and FutureDate both individually return Either.Left with the provided inputs, but when combined into a wrapper type (MissingValidations) only the FutureDate validation is returned

bug in zipOrAccumulate?.kt

[simon.vergauwen]

Hey @Erik Dreyer,

It's an issue we currently have with @BuilderInference, you need to use bindNel for FutureDate.of. I hope that helps

fun of (string: String, date: LocalDate): EitherNel<DomainError, MissingValidations> =
    either {
        zipOrAccumulate(
            { NonEmptyString.of(string).bind() },
            { FutureDate.of(date).bindNel() }
        ) { s, d -> MissingValidations(s, d) }
    }

[Erik Dreyer]

ahh, nice. Didn't know about that. Thank you.