I'm trying to integrate Arrow-kt's Either into an application that needs to parse and validate filter input. This input is a comma separated list of criteria. Either sound like a good candidate for that job because it does both jobs at the same time.


I've got a parser that takes care of the first step:

interface Parse<out T> {
    operator fun invoke(value: String): Either<String, T>

object ParseList : Parse<List<String>> {
    override fun invoke(value: String): Either<String, List<String>> {
        val values =
                .map { it.trim() }
                .filter { it.isNotEmpty() }
        return if (values.any()) Either.Right(values)
        else Either.Left("Value is not a list.")

and other parsers that take care of an individual value like a date:

class ParseDate : Parse<LocalDate> {
    var formatter: DateTimeFormatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT)
    override fun invoke(value: String) =
        runCatching { formatter.parse(value) }
            .map { LocalDate.from(it) }
            .map { Either.Right(it) }
            .getOrElse { Either.Left("Value is not a date.") }

Combining these to components results in:

List<Either<String, T>> 

so it needs to be converted into this:

Either<String, List<T>> 


In orther to convert a List of Eithers into an Either of Lists i wrote the merge extension that processes each item and stops at the first Left, but keeps running on Right and collecting their values into a new list that is then return as a new Either:

fun <A, B> List<Either<A, B>>.merge(): Either<A, List<B>> {
    val result = mutableListOf<B>()
    for (x in this) {
        when (x) {
            is Either.Right -> result.add(x.value)
            is Either.Left -> return Either.Left(x.value)
    return Either.Right(result)

class MergeTest : FunSpec({
    test("Can stop on left.") {
        val list = listOf(
        list.merge() shouldBe "bar".left()
    test("Can merge right.") {
        val list = listOf(
        list.merge() shouldBe listOf("foo", "bar").right()

Is it necessary to do this on your own or is there a more clever way?


1 Answer 1


I think there is not simple built-in way to do this.

That said, your approach can be simplified a little bit:

fun <A, B> List<Either<A, B>>.merge(): Either<A, List<B>> =
    Either.Right(map {
        when (it) {
            is Either.Right -> it.value
            is Either.Left -> return it

It can be even reduced to a single line:

fun <A, B> List<Either<A, B>>.merge(): Either<A, List<B>> =
    Either.Right(map { (it as? Either.Right)?.value ?: return it as Either.Left })

Although this one is significantly less readable.

  • \$\begingroup\$ The usage of the return inside the map with the implicit outer return is crazy! I love it. I didn't know this was possible. \$\endgroup\$
    – t3chb0t
    Jan 16 at 16:51

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