:wave: do we have anything similar to this in optics? ```public fun <A> listPredicate(p: (A)...

# arrow

[thanh]

👋 do we have anything similar to this in optics?

public fun <A> listPredicate(p: (A) -> Boolean): Optional<List<A>, A> = Optional(
      getOption = { it.firstOrNull(p).toOption() },
      set = { list, newHead ->
        val index = list.indexOfFirst(p)

        if (index != -1) list.take(index) + newHead + list.drop(index + 1) else list
      }
    )

[simon.vergauwen]

Hey @thanh,

Yes, there is something like this in Arrow Optics.
https://arrow-kt.io/docs/optics/filterindex/

The docs seem.. a bit lacking 😅

[simon.vergauwen]

Here is some usage.

https://github.com/47degrees/helios/blob/master/helios-optics/src/main/kotlin/helios/optics/JsonTraversalPath.kt#L127

[simon.vergauwen]

Optics could really use some love…

[thanh]

Thanks @simon.vergauwen! I think what I need is a bit different than the example with FilterIndex I want to filter with the value of the item in the list, not with the index. Maybe I missed something here?

[simon.vergauwen]

Oh, I see. It’s only updating the first encountered in the list?

[thanh]

yes, it is, and I think it should. Maybe we can make another version of it with type Lens<List<A>, List<A>> which can get/set/modify all satisfied items.

[simon.vergauwen]

That wouldn’t work in a composition chain, right?

[thanh]

yeah, that probably wouldn't work. Just poped out on my head 😄

[thanh]

@simon.vergauwen the code above seems to works really well. Should I create a PR to add it to optics? Maybe I can do some research how Haskell Lens deals with this problem first.

[simon.vergauwen]

Hey @Alejandro Serrano Mena, do you know if something like this exists in Optics? I’ve used indexed traversals before, but just like FilterIndex they’re defined over indices of the list.

[simon.vergauwen]

The easiest way to figure out if it composes correctly with the rest of the library would be to follow the set of OptionalLaws from arrow-optics-test. Have you tried running those against this instance?

[thanh]

I haven't tried to test it with Laws yet, but I can try to tests with the Laws tomorrow. I used it in a project and did some tests and it composes well with other optics functions.

[thanh]

Hi @simon.vergauwen, I'm struggling a bit on how to use the OptionalLaws and how to run the tests. Maybe I'll just create a PR and you can correct me there.

[simon.vergauwen]

Hey Thanh, if that works for you that’d be easiest I think.

[thanh]

In the other note, I check Scala Monocle, it has this function: https://github.com/optics-dev/Monocle/blob/master/core/shared/src/main/scala/monocle/Optional.scala#L226 which solves a problem when we want to access to all items that satisfied a predicate in a list.

[thanh]

I don't see it in Arrow's optics now, I can create another PR for this as well

[thanh]

I'll submit a PR in the next few days.

[simon.vergauwen]

I see, thanks @thanh. Yes, a PR would be amazing! I’m excited to add new functionality in Arrow Optics and to clean it up, but I don’t have the time.

[simon.vergauwen]

I see it also exists in Haskell and is called filtered in Haskell

[thanh]

yeah, it also says that in the scala docs. also agree that optics can definitely use some loves.