```data class Form( .. val fields: Fields ) { data class Fields( val personalNum...

# getting-started

[ursus]

data class Form(
    ..
    val fields: Fields
) {
    data class Fields(
        val personalNumber: InputField,
        ...
    )
}

data class InputField(
    val value: String,
    ..
}

this is my model

I want to update value

obviously its a copy of a copy of a copy, which is ugly, I want to stuff into a extension

private fun <T> Form.updatePersonalNumberValue(value: String): Form {
    val field = fields.personalNumber.copy(value = value)
    copy(fields = fields.copy(personalNumber = field)
}

works but obviously doesn't scale as I have 20 fields, is there a way to make this generic somehow?

private fun Form.updateFieldValue(selector: (Form.Fields) -> InputField, value: String) {

I was thinking something like this, a selector lambda
val field = selector(fields)
however how do I assign it back to the Form? So seems like a dead end. It seems I can only do Fields -> Fields lambda at best

Is this really the best you can do in kotlin?

form.updateFields {
    copy(
    	personalNumber = personalNumber.copy(
    		value = "new personal number"
		)
	)
}

TLDR; is there a way to generically select a copy parameter?

[Youssef Shoaib [MOD]]

The pattern you're looking for is called Lens . I would recommend using something like Arrow Optics to generate those lenses for you either at compile-time or runtime.

[ursus]

I really don't want a library for what essentially is just a form.fields.personaNumber.value = x

[ursus]

I could live with a

updateFields(
	selector = { it.personalNumber },
	updater = { it.copy(value = "x")}
)

[Youssef Shoaib [MOD]]

You can roll your own if you'd like. https://github.com/dmcg/gilded-rose-tdd/blob/main/src/main/java/com/gildedrose/foundation/lenses.kt This is a very good one-file implementation

[ursus]

so what would be the final syntax? am I not just trading copy for update?

[Youssef Shoaib [MOD]]

val personalNumberValueLens = Form::fields.asLens() andThen Fields::personalNumber.asLens() andThen InputField::value.asLens()
val form2 = form.with(personalNumberValueLens, "x")

[Youssef Shoaib [MOD]]

You can define your own andThen that uses properties directly

[ursus]

oh boy, guess Im gonna tolerate 2 copy calls 😕

[ursus]

isnt value semantics coming to kotlin? atleast the sugar? dont reall care about the jvm values which are 10 years in making

[Youssef Shoaib [MOD]]

There's only design notes available, but it's something that should be focused on more after K2 is out

[ursus]

any idea on the ETA of the K2?

[CLOVIS]

"Sometimes this year"

I wouldn't be surprised if it happened around KotlinConf (May–June)

[Klitos Kyriacou]

Is this a violation of the Law of Demeter? (Probably not, as these are just plain data classes, but worth thinking about.)

[CLOVIS]

Value semantics come from FP and have very different maintainability rules than OOP. Especially because stuff is immutable.