I have a question about Optics How to get a focus...

# arrow

[Gemy]

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.

[Alejandro Serrano.Mena]

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

[Gemy]

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

[Gemy]

@simon.vergauwen What do you think, sir.

[Alejandro Serrano.Mena]

one could get a Fold in that way (getting them is easy, not sure what "setting" means in this case)

[Gemy]

Is there an example somewhere for fold, sir?

[Alejandro Serrano.Mena]

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

[Gemy]

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

[Alejandro Serrano.Mena]

that's ok, because a Fold may focus on more than one element

[Gemy]

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

[Gemy]

and the API docs got no examples for each type

[Alejandro Serrano.Mena]

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

[Alejandro Serrano.Mena]

as described at the end of https://arrow-kt.io/learn/immutable-data/intro/#many-optics-to-rule-them-all, we plan to remove half of the optics in 2.0, that's why some are not so well documented

[Gemy]

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

[Gemy]

Is there a way to try arrow v2 right now?

[Alejandro Serrano.Mena]

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

[Gemy]

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

[Alejandro Serrano.Mena]

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

[Gemy]

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?

[Alejandro Serrano.Mena]

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())...

[Gemy]

That's much better. Thanks!