Is an `Index` the idiomatic way of focusing down on a specific item in a list based on a criteria (e...

# arrow

[sindrenm]

Is an Index the idiomatic way of focusing down on a specific item in a list based on a criteria (e.g. “find by ID”), or is there something I haven't realized yet? We find ourselves needing to do this quite often, and we've resorted into essentially this pattern:

@optics data class Lesson(val id: String, val name: String) {
    companion object
}

@optics data class Lessons(val items: List<Lesson>) {
    companion object
}

fun main() {
    val lessons = Lessons(listOf(Lesson("1", "Lesson One"), Lesson("2", "Lesson Two")))

    Lessons.items.index(atLessonId, "1").name.set(lessons, "New Lesson One Name")
}

private val atLessonId = Index<List<Lesson>, String, Lesson> { id ->
    Optional(
        getOption = { lesson: List<Lesson> ->
            lesson.firstOrNone { it.id == id }
        },
        set = { lessons: List<Lesson>, lesson: Lesson ->
            TODO("Not implemented for brevity")
        },
    )
}

[Alejandro Serrano.Mena]

you can use filter for that https://apidocs.arrow-kt.io/arrow-optics/arrow.optics.dsl/filter.html

[Alejandro Serrano.Mena]

Lessons.items.filter { it.id == id }.name.set(...)

[sindrenm]

Right, I guess I did actually check filter, but I noticed it was part of a “delicate optics API”, so I got a little scared.

Also, I guess you'd need an every in there?

Lessons.items.every
  .filter { it.id == "1" }
  .name.set(lessons, "New Lesson One Name")

[sindrenm]

But thanks, this does indeed seem to be what I'm looking for! What's the reason for the @DelicateOptic annotation on it, though?

[sindrenm]

Oh, I guess it's this:

⚠️ Warning: when using modify with this optic, the transformation should not alter the values that are taken into account by the predicate. For example, it is fine to filter by name and then increase the age, but not to filter by name and then capitalize the name.

[sindrenm]

Okay, that's perfect, thanks a bunch!

[Alejandro Serrano.Mena]

What's the reason for the @DelicateOptic annotation on it, though?

exactly what you mention. I've been bitten a couple of times by this myself, so I think a remainder for "please read the docs and think twice" is good here.

[sindrenm]

I'm curious to know why or when it's a problem, though. All of these tests seem to yield the expected result:

fun main() {
  val lessons = Lessons(listOf(Lesson("1", "Lesson One"), Lesson("2", "Lesson Two")))
  val filterId1 = Lessons.items.every.filter { it.id == "1" }

  // Overwriting the entire lesson
  println(filterId1.set(lessons, Lesson("3", "Lesson One")))
    // => Lessons(items=[Lesson(id=3, name=Lesson One), Lesson(id=2, name=Lesson Two)])

  // Setting the filtered value using `set`
  println(filterId1.id.set(lessons, "3"))
    // => Lessons(items=[Lesson(id=3, name=Lesson One), Lesson(id=2, name=Lesson Two)])

  // Setting the filtered value using `modify`
  println(filterId1.id.modify(lessons) { "3" })
    // => Lessons(items=[Lesson(id=3, name=Lesson One), Lesson(id=2, name=Lesson Two)])
}

Unless I'm misunderstanding the warning?

[Alejandro Serrano.Mena]

the problem here is that once you start chaining modifications, some of them may be "lost" since the filter no longer applies to them

[sindrenm]

I wasn't aware you could chain modifications, tbh. Aren't calls to set, modify or the like “terminal”? As in, they won't give you back a Lens, Optional, Traversal or anything Optics-specific?

[Alejandro Serrano.Mena]

I mean using several modify or set in a row

[sindrenm]

You mean something like this:

var lessons = Lessons(listOf(Lesson("1", "Lesson One"), Lesson("2", "Lesson Two")))
val filterId1 = Lessons.items.every.filter { it.id == "1" }

lessons = filterId1.id.set("3")
lessons = filterId1.id.set("4")

I can definitely see why that wouldn't work, but perhaps in more contrived setups it's less obvious.