Hello guys. I’m new to Arrow and functional progra...

# arrow

[ubu]

Hello guys. I’m new to Arrow and functional programming. Trying to find a way to create a copy of sealed class instance with some modification avoiding the following `when`s (in this case, I would like to update abstract val property):

@optics
sealed class TestArrow {
    abstract val isSelected : Boolean

    companion object {}

    @optics
    data class Test1(override val isSelected: Boolean, val fooBar1: String) : TestArrow() {
        companion object {}
    }

    @optics
    data class Test2(override val isSelected: Boolean, val fooBar1: String) : TestArrow() {
        companion object {}
    }
    @optics
    data class Test3(override val isSelected: Boolean,  val fooBar1: String) : TestArrow() {
        companion object {}
    }
}

fun TestArrow.updateSelected(isSelected: Boolean) : TestArrow {
    return when(this) {
        is TestArrow.Test1 -> this.copy(isSelected = isSelected)
        is TestArrow.Test2 -> this.copy(isSelected = isSelected)
        is TestArrow.Test3 -> this.copy(isSelected = isSelected)
    }
}

Any help will be very appreciated

[simon.vergauwen]

Hey ubu,

That's definitely a great use-case to write a Lens to update a common value in a sealed class but it's not yet supported by the @optics generator.

There is a ticket for it, which has been open for quite some time, but we're waiting until we can replace kapt by a MPP meta solution. Which is expected soon-ish.

You can however easily manually write a optic for it:

val isSelectedLens: Lens<TestArrow, Boolean> =
  Lens(
    get = { it.isSelected },
    set = {
      when(it) {
         is TestArrow.Test1 -> this.copy(isSelected = isSelected)
         is TestArrow.Test2 -> this.copy(isSelected = isSelected)
         is TestArrow.Test3 -> this.copy(isSelected = isSelected)
      }
    } 
  )

[simon.vergauwen]

You can also make it work with the DSL if needed 🙂 I'd be glad to help!

[ubu]

@simon.vergauwen, I’m starting to appreciate Arrow. what if I would like to update two fields, which is not present by all children classes? how would you do it?

[ubu]

From your example I didn’t quite understand how to apply the modification on TestArrow class. Could you elaborate on that?

[simon.vergauwen]

Hey Ubu,
Sorry for the delay in reply.

Modifying TestArrow can be done with isSelectedLens in the following manners.

val value: TestArrow = ...
isSelectedLenes.modify(value) { boolean -> !boolean } // There is also `get`, `set`, etc

Or when nested in other types
val values: List<TestArrow> = ...

(Every.list<TestArrow>() compose isSelectedLens).modify(values) { bool -> !bool } // Modifies *all* TestArrow in the list

[simon.vergauwen]

Updating multiple values at once is not possible with Optics out of the box, in the sense that @optics always generates optics for all values independenly.

You could however create a: Lens<TestArrow, Pair<Boolean, String>> which also updates the foobar properties even though it's not a shared value in TestArrow.

val isSelectedLens: Lens<TestArrow, Boolean> =
  Lens(
    get = { 
      when(it) {
         is TestArrow.Test1 -> Pair(isSelected, foobar1)
         is TestArrow.Test2 -> Pair(isSelected, foobar1)
         is TestArrow.Test3 -> Pair(isSelected, foobar1)
      }    },
    set = { value, (isSelected, foobar1) ->
      when(value) {
         is TestArrow.Test1 -> value.copy(isSelected = isSelected, foobar1= foobar1)
         is TestArrow.Test2 -> value.copy(isSelected = isSelected, foobar1= foobar1)
         is TestArrow.Test3 -> value.copy(isSelected = isSelected, foobar1= foobar1)
      }
    } 
  )

Which then could be used in the same manner as the other Lens above.

[simon.vergauwen]

More info can be found here: https://arrow-kt.io/docs/optics/lens/