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kotlinlang

if you've got a `class` with a lot of state, and you would like to clone it and only change one `val` - how should this be done? are you expected to make it into a `data class` and use the generated `copy` impl?
or move all the state into a private primary constructor?
or...?

Yes `data class` and using `copy()`  is the right way to do it

so `data class` is in no way an indication of the class being a "plain old data" - it's just opt-in for the various derived impls such as `copy` and `toString`?

If you're expecting the states to change independently fairly often then maybe keeping them separate is a better idea

You can also add your own `copy` function to any class you want.

If you only want to change one `val`, and it's always the same `val`, you can add a method to the class that creates a new instance with only the one modification, copying the rest.

One example of where this is useful is when running simulations, like the Game of Life. I add a method called `getNextState()` in order to make a new instance that has many copied values and a small subset of changed values.

I have a hunch that this might be closer to what you're looking for!

Yeah, dedicated change functions related to the domain is often better than a copy function.  It gives a name to what you are trying to do when you call it.

kotlinx.datetime defines a bunch of these, if you'd like some standard library examples.

```fun YearMonth.minusMonth(): YearMonth```
<https://kotlinlang.org/api/kotlinx-datetime/kotlinx-datetime/kotlinx.datetime/minus-month.html>