Hello people, is there a way to “merge” 2 validated? E.g. given two `Validated<A, B>` , if the...

# arrow

[Davide Giuseppe Farella]

Hello people, is there a way to “merge” 2 validated?
E.g. given two Validated<A, B> , if they were Either I would do, for example

either {
  val a = eitherA.bind()
  val b = eitherB.bind()
  Combine(a, b)
}

What’s the right way to do it with Validated? Either with the either block or any other solution 🙂

[phldavies]

You would normally use the zip extension for this but you'd need to provide some way of merging A in case both were invalid. Usually this is covered by using ValidatedNel<A, B> (a typealias for Validated<NonEmptyList<A>, B>)

[Davide Giuseppe Farella]

Thank you for the answer.
Could you please provide a real example, considering two Validated<String, Int> ? 🙂

[Davide Giuseppe Farella]

I’m not very familiar with different types than Either , so I’m struggling to understand how to use it

[phldavies]

How would you want to merge the two error Strings?

[Davide Giuseppe Farella]

Let’s concatenate them

[Davide Giuseppe Farella]

Or just provide an example with the ValidateddNel , I just need an example of how to use zip , then I’ll take it from there 🙂

[phldavies]

Regarding that, with Arrow 1.2.x onwards, Validated is going to be deprecated, and removed from Arrow 2.0.0 to simplify this.

[Davide Giuseppe Farella]

Oh, I see! So I’ll just use Either?

[phldavies]

For ValidatedNel it's relatively simple to use .validNel() and .invalidNel() to construct your values much like you would .valid() and .invalid(). Once you have two values you can join them using a.zip(b) { validA, validB -> doSomethingWith(validA, validB) }

[phldavies]

With 1.2.x and above there'll be an zipOrAccumulate set of functions that will provide the same logic (combining one or more lefts, or providing all rights to a function)

[Davide Giuseppe Farella]

Ok. Thank you so much!

[Davide Giuseppe Farella]

With ValidatedNel I felt less stupid and everything worked straight away 🙂

[phldavies]

With Validated<A, B> you need to provide an additional argument to zip of a Semigroup<A> (which is effectively just a function A.(A) -> A like String::plus )

With a custom combine to comma-delimit errors you can have

lateinit var a: Validated<String, Int>
lateinit var b: Validated<String, Int>

fun String.combine(other: String) = "$this, $other"

a.zip(String::combine, b) { validA, validB -> validA + validB }

[Davide Giuseppe Farella]

I see! Thanks again!