What’s the best way to modify a nested property in...

# getting-started

[Daniele B]

What’s the best way to modify a nested property in an immutable data class object?

data class AppState (
    val masterState : MasterState = MasterState(),
)
data class MasterState (
    val isLoading : Boolean = false,
)

is there a more concise way than this?

val myState = AppState()
val newState = myState.copy(masterState = myState.masterState.copy(isLoading = true))

[ephemient]

https://arrow-kt.io/docs/optics/lens/ is a more concise way (although also more magical)

[Daniele B]

I wanted to create a very simple function like updateSubstate , where I could perform the substate copy inside the block, and the root state copy inside the function:

// this is the specific function
fun MyModel.anExtensionFunction() {
	updateSubstate {
	     it.masterState.copy(isLoading = true)
	}
}

but I don’t know how to deal with the reflection:

class MyModel {
    var myState = AppState()

    // this is the generic function
    fun updateSubstate(block: (AppState) -> Any) {
        val output = block(myState)
	...
        myState = myState.copy(PROPERTY_NAME_TO_GET_FROM_BLOCK_BETWEEN_IT_AND_COPY = output)
    }

}

[ephemient]

val copyFunction = AppState::copy
val thisParameter = copyFunction.parameters.find { it.kind == INSTANCE }!!
val propertyParameter = copyFunction.parameters.find { it.name == parameterName }!!
myState = copyFunction.callBy(mapOf(thisParameter to myState, propertyParameter to output))

but (as all things reflection) there is no safety here. do you really need to do this?

[Daniele B]

thanks!
does this just work on JVM or on all platforms?

[Daniele B]

I am running it in CommonMain and the IDE can’t find many properties. Do I need to import Reflect as a dependency too?

[Daniele B]

I think I found a good solution, even without reflection, cleaner than before:

[Daniele B]

updateMasterState {
    it.copy(isLoading = true)
}

even if I have to create a function for each substate:

fun updateMasterState(block: (MasterState) -> MasterState) {
    myState = myState.copy(masterState = block(myState.masterState))
}