I have a question about Optics How to get a focus on an item in a list, that's a property of an optional (sealed subtype) In this code

private val lWeatherListState = WeatherScreenModel.weatherList.state
private val lEveryWeatherItem = lWeatherListState compose Prism<Lce<String, List<WeatherItemModel>>, Lce.Content<List<WeatherItemModel>>>(
        getOption = { (it as? Lce.Content<List<WeatherItemModel>>).toOption() },
        reverseGet = { it }
    ).value().every(Every.list())

Here I want to further focus.. on items after `every`` But all I found was an index function.. is there a way? Or is there something missing? I've tried the docs but it didn't cover that part.

You can use

index

(in arrow.optics.dsl). You can find examples in the arrow-kt.io website, under Immutable Data > Optionals > Indexed collections

Thanks! @Alejandro Serrano.Mena what if I want to focus on an element matching a certain criteria, perhaps a certain ID? Can index do that? For lists I can only focus on index using

index

one could get a

Fold

in that way (getting them is easy, not sure what "setting" means in this case)

Is there an example somewhere for fold, sir?

even better, it turns out we already have it in the library! https://apidocs.arrow-kt.io/arrow-optics/arrow.optics/-fold/-companion/select.html

That's amazing! Thank you! What happens if the predicate returns true for two or more elements in

Every

?

that's ok, because a

Fold

may focus on more than one element

That's very convenient 🙏 Is there a good resource for finding examples covering the multiple use cases of Arrow-Optics? Because i found no hint of Fold in the documentation at https://arrow-kt.io/learn

Traversals are the "get + set" version of Fold https://arrow-kt.io/learn/immutable-data/traversal/

Oh, okay, nice read That means when using arrow v2 i'll have to refactor my code that's using lenses and prisms?

you'll only need to refactor if you use fold, getter, or setter. The rest (lens, optional, prism, traversal) stay Everything we'll remove is marked as Deprecated in version 1.2.x. So if you use that version and your code compiles without warnings, you're good to go

I'm on v1.2.1 And fold usage has no warnings

mm 🤔 I'll try to have a look at the reason during the week

Thanks a lot for your time 🙏 btw can you tell me if i'm using the prism correctly here? Or there is an easier way to access sealed class nested properties?

you are, but I recommend you to define the prism

lceContent

separately,

val lceContent: Prism<...> = Prism(
  getOption = ...
  reverseGet = ...
)

and then define

val <A> Optional<A, Lce<String, List<WeatherItemModel>>.lceContent: Optional<A, Lce.Content<List<WeatherItemModel>>> = this compose lceContent

so you can write

lWeatherListState.lceContent.value().every(Every.list())...

That's much better. Thanks!