Is there some annotation magic I can sprinkle on a...

# announcements

[joelpedraza]

Is there some annotation magic I can sprinkle on a val property to signal to the compiler that it’s guaranteed to be immutable (as apposed to only read only).

Use case: cross module nullability smart casts

[Stephan Schroeder]

const val isn’t an option? (only works with compile-time constants) Apart from that I’m not aware of anything.

[joelpedraza]

@Stephan Schroeder No it’s not an option, these are runtime values. Dumb data classes where I’d like to make guarantees about the immutability of fields

[joelpedraza]

data class Person(val name: String?)

I want to guarantee name will always return the same value, was hoping there would be a contract or annotation for it

[joelpedraza]

See https://discuss.kotlinlang.org/t/2201

[Stephan Schroeder]

then you’re out of luck, as far as I know.

[joelpedraza]

As an aside, why doesn’t @JvmField on a val work this way

[Stephan Schroeder]

I’m a bit out of my depth but that just results in no getter being created, but field level access in the byte-code as far as I know. so nothing in regards to immutability.

[Stephan Schroeder]

I watched the closing panel of the Kotlin conf 2018, and on the question along the lines of “which features would you like to see in Kotlin” one participant said “more immutability support”

https://www.youtube.com/watch?v=heqjfkS4z2I▾

[joelpedraza]

A final field is referentially immutable, a val can be a getter

class Foo { val bar get() = Random.nextInt() }

[Stephan Schroeder]

i know

[joelpedraza]

Oh, of course. Because multiplatform.

[Matej Drobnič]

yeah cross module immutability issues are pretty PITA. It sucks that there is no way to tell the compiler "I know what I'm doing I will not suddenly change these without recompilation"