Hi :wave: Arrow newbie here. I'm trying to modify deeply nested immutable data classes which sounds ...

# arrow

[mbonnin]

Hi 👋 Arrow newbie here. I'm trying to modify deeply nested immutable data classes which sounds exactly like what Optics are for but I'm having a hard time wrapping my head around it. Is there a sample of how to do this somewhere? Ideally something that does the equivalent of the (simple) example below?

// How do I do this with Optics?
val user = user.copy(
  address = user.address.copy(
    city = newCity
  )
)

[Ties]

val addressLens: Lens<User, Adress> = Lens(
	get = { it.address},
	set = { s : User, v : Adress> -> s.copy(address= v) }
) 

val cityLens: Lens<Adress, City> = Lens(
	get = { it.city},
	set = { s : Adress, v : City> -> s.copy(city= v) }
) 

val adressCity = addressLens.compose(cityLens)
adressCity.modify(user) { newCity }

[mbonnin]

Yikes, that's more code 😅 .

[Ties]

yes 😛 but arrow can generate this for you!

[mbonnin]

Ah!

[Ties]

you can do this with your dataclasses:

@optics data Person(...) { companion object }

so add an @optics annotation, and add a companion object and make sure you have the optics maven/gradle plugin configured

[mbonnin]

Thanks !

[Ties]

after generating it will be:
User.adress.city.modify(user){ newCity }