What's the best way to combine errors in List<V...

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[Petr Makagon]

What's the best way to combine errors in List<ValidatedNel<MyError, Unit>> to ValidatedNel<MyError, Unit> ?

[albertosh]

val SE = Nel.semigroup<MyError>()
val ap = ValidatedNel.applicative(SE)
val validated = list.sequence(ap).fix()

?

[Scott Christopher]

Wouldn't that result in a ValidatedNel<MyError, List<Unit>>?

[Scott Christopher]

I guess List<Unit> is probably as useful as a Unit though.

[Scott Christopher]

Otherwise I think you should be able to do

list.reduce { a, b ->
    a.combineK(Nel.semigroup(), b)
}

[Scott Christopher]

Although I think combineK won't collect validation errors, if either of them are successful.

[Scott Christopher]

I suspect you'd need to define a semigroup instance for Unit and use combine rather than combineK.

[Scott Christopher]

Like:

object MonoidUnit : Monoid<Unit> {
    override fun Unit.combine(b: Unit) = this
    override fun empty() = Unit
}

list.reduce { a, b ->
    a.combine(Nel.semigroup(), MonoidUnit, b)
}

[albertosh]

Wouldn’t that result in a ValidatedNel<MyError, List<Unit>> ?

It will, yes. If you need a Unit result you could go with the Unit monoid or just add a .map { Unit }

[Petr Makagon]

Thank you!