Wouldn’t it be great if I could easily create an i...

# language-proposals

[Michael Böiers]

Wouldn’t it be great if I could easily create an immutable version of an instance of a data class with mutable properties? I wonder how that could be done. It’s not easy, but it would be really cool if I could define something like a MutablePerson, and it would then automatically have a method toPerson(): Person.

[Michael Böiers]

Ok, generating the Person class would be too much to ask from the compiler. How about something like this:

data class MutablePerson(var name: String? = null, var age: Int? = null) {
    immutable derived data class Person(val name: String, val age: Int)
}

val p: Person = buildPerson {
    // receiver is a new MutablePerson instance
    name = "Fred"
    age = 42 
}

[Michael Böiers]

Maybe it would also make sense to turn it around - define the immutable data class and have a mutable version generated.

[jimn]

interface MyPair<A,B>{val a:A;val b:B};class MyPair2(override var a:A,override var b:B):MyPair{};

[Paul Griffith]

The KEEP for value classes talks about a possible future enhanced syntax for immutable classes to make their modification easier
https://github.com/Kotlin/KEEP/blob/master/notes/value-classes.md#updating-immutable-classes

[Paul Griffith]

Sort of the inverse of what you're proposing - define the immutable class yourself, and just make it easier to mutate

[Nathan Bedell]

I actually wanted to make a compiler plugin to do exactly this.

[Norbi]

I'm currently experimenting with something similar: https://discuss.kotlinlang.org/t/state-handling-using-immutable-objects/24423
It is a very complex topic...