I'm working with `@optics` to modify a `data class`. Let's say I have a `data class Foo(val list: Li...

# arrow

[Luke]

I'm working with @optics to modify a data class. Let's say I have a data class Foo(val list: List<Bar>), is there an operator to modify the only element in the list that matches a predicate?

[Luke]

For a more detailed example, let's say I have:

@optics
data class Foo(
    val list: List<Bar>,
) {
    companion object
}

@optics
data class Bar(
    val string: String,
) {
    companion object
}

I'm looking for something like this:

Foo.list./*find element*/.string.modify(foo, someTransformation)

[simon.vergauwen]

Yes, there is filterIndex to which you can pass a predicate

[simon.vergauwen]

https://arrow-kt.io/docs/optics/filterindex/

I see it’s missing support for the DSL. It should have a file like this: https://github.com/arrow-kt/arrow/blob/main/arrow-libs/optics/arrow-optics/src/commonMain/kotlin/arrow/optics/dsl/index.kt

And then you can use it like this:
https://arrow-kt.io/docs/optics/dsl#index

[Luke]

This would allow me to filter by receiving the index in the predicate, right? Is there a way to receive the bar instance instead? I guess I was unclear in what kind of predicate I wanted sorry. In the example I gave, I could write something like:
Foo.list.first { it.startsWith('H') }.string.modify(foo, someTransformation)
Otherwise, I think I can use the links you shared to implement something custom.