is there any way to say that all subtypes of a `se...

# language-proposals

[Emil Kantis]

is there any way to say that all subtypes of a sealed interface should be data types? sealed data interface could perhaps be a nice addition? I would like to use copy with the subtypes.. 🙂 apologies for the low-effort proposal.. just want to get some discussion ✌️

[Davio]

Hmm, I think the thing you are realizing here is that a data class is both a language construct but also a hidden interface, i.e. it's like data classes implement the interface Copyable , so I wonder if it would make sense or even be possible to generate the byte code for data classes in such a way that they all implement this interface.

If that were the case, you could write your sealed interface like so: sealed interface Foo : Copyable and both data classes or other classes implementing this interface would work for it

[simon.vergauwen]

That would only work for properties that are also abstract val in the sealed class though.

Since at the level of Foo you need to know which properties are common so they can be copied.

Optics solves these problem but would love to have something in the language instead of external support.

They touch upon it in the https://github.com/Kotlin/KEEP/blob/master/notes/value-classes.md KEEP.

[Davio]

Oh yeah I forgot that copy has all of the constructor parameters so you can selectively modify them in your copy function call myObj.copy(name = "something different") that makes it a lot more tricky than I thought

[Davio]

So this basically means that each data class' copy method has a different signature whereas functions like equals, toString and hashCode always have the same signature

[mcpiroman]

Though there can also be a second abstract copy() with no parameters 🤔

[Kroppeb]

You could just add a copy method to the interface and in each data class add fun copy() = copy() ... errr, wait, that doesn't work