A bit of a language question: `data class` can't b...

# announcements

[Daniele Segato]

A bit of a language question: data class can't be extended (ok).

If I chose to use an interface and implement it with a data class I can do things like this

interface MyPublicInterface {
  val x: String
  val y: Int
  val z: Whatever
}

data class MyImplementation(
  override val x: String,
  override val y: Int,
  override val z: Whatever
): MyPublicInterface

now this is ok, but if I do this then I can't use the useful copy() or componentN() functions method that comes along with a data class on my interface without casting it to my implementation.

Is there some way to make my interface declare the copy() and componentN()?

for the componentN() I guess it's possible by just defining the operators, but then I'd had to manually maintain them, still ok. But for copy() I've no idea how to do it

[Daniele Segato]

PS: can you answer in a thread?

[Daniele Segato]

According to this:
https://kotlinlang.org/docs/reference/data-classes.html

Deriving a data class from a type that already has a copy(...) function with a matching signature is deprecated in Kotlin 1.2 and is prohibited in Kotlin 1.3.

do you know where i can find the discussion about this?

[LeoColman]

What is your use case here? Why do you need interfaces for data classes?

[streetsofboston]

Alas, there is no such way; there are no such things as `data interface`s....

[streetsofboston]

For a use-case: Imagine the 'data' interface having only a subset of the `val`s that its implementing data `class`es expose.

[Daniele Segato]

yes that's the use case

data class Sub being a subset of the data in data class Over sharing the same interface matching Sub.

or, as an alternative

data class A and data class B sharing a subset of data that could be represented by an interface.

An API returning the interface.

In theory you could use copy() limited to the attributes of the common set and still create a valid A or B class.

[Daniele Segato]

I add a little bit...

I'm currently doing something like this

interface A
interface B
data class A_Data(...): A
data class B_Data(...): B

data class: X_Data(
  private val a: A,
  private val b: B
)A by a, B by b

where I manually expand this with a secondary constructor...

I'd love if I could let users of this X_Data class go and use it as if it was the sum of A_Data and B_Data.