@Flat option annotation

I offer to include @Flat (or @Flatten, for example) annotation which will be used in cases when value object of field must be included directly into parent object.

@Serializable
data class Parent(
    val stringField: String = "stringValue",
    @Flat
    val flatField: Child = Child()
)

@Serializable
class Child {
    val childString: String = "childValue"
}

fun main(args: Array<String>) {
    println(JSON.stringify(Parent.serializer(), Parent()))
}

Will give result:

{"stringField":"stringValue","childString":"childValue"}

Here I see a few problems:

• String and other primitives must be resolved like common fields (with their keys) or those fields must lead to exception like IllegalArgumentException
• Collections in this case, I think, must be interpreted as associated map: with keys numbers and values - values if list

[Coneys]

Any information about this? Is there some workaround, or will it be implemented in near future?

[pdvrieze]

There are two workarounds. One is inline classes (or tricking the serialization framework into thinking the class is inline - using a custom SerialDescriptor). The other is to use a custom serializer. If you give its descriptor as PrimitiveKind.String (there is a factory function for primitive descriptors, not even “internal”), then you can have the implementation just use encodeString, decodeString and do the “right” translation to get the objects.

What isn’t really possible is to embed multi-attribute members into a parent (neither way really works), and that would require quite some changes on the compiler plugin side.