What do you think of automatically generating supe...

# language-proposals

[tschuchort]

What do you think of automatically generating super class copy functions for data classes? We already have the normal copy function to use immutability, but it gets quite annoying when you have two data classes which extend the same base class and you have to "copy" the common fields manually. When using the new MVI-architecture on android, you represent the different states of your app as data classes in a sealed class hierarchy and often times the data classes share many common fields or even only differ in their name, so this would remove a lot of bloat for many people.
I propose that the compiler generates a "copy" method for every non-interface superclass that initializes the common fields:

sealed class State {
	abstract val a: Int

	data class Default(override val a: Int, val b: Int = 0) : State() {
		companion object {
			// generated method
			fun copy(sup: State, a: Int = sup.a, b: Int = 0) = Default(a, b)
		}
	}


	data class Loading(override val a: Int, val c: Int = 0) : State() {
		companion object {
			// generated method
			fun copy(sup: State, a: Int = sup.a, c: Int = 0) = Loading(a, c)
		}
	}
}

so when transitioning from Default to Loading state you just write newState = Loading.copy(oldState, 0)

I think it would be pretty easy to implement, maybe as a compiler plugin