How are value classes handled polymorphically with...

# serialization

[Zyle Moore]

How are value classes handled polymorphically with JSON serialization? I'm running into an issue. Code will be in thread, but basically, I have an open interface hierarchy, where one of the subclasses is a value class. The top level of the hierarchy is used as a property in a serializable class. Serializing is fine, deserialization is where the issue appears. When the class is value class, I get an error saying Expected JsonObject, but had JsonLiteral as the serialized body of kotlinx.serialization.Polymorphic<Name> at element: $.name. If I switch it to a data class it works better. MVCE here: https://pl.kotl.in/ojKxHZshW

[Zyle Moore]

interface Name

interface Text : Name

@JvmInline
@Serializable
value class SerializableText(val string: String) : Text

@Serializable
data class SerializablePilot(val name: Name)

val testPilot = SerializablePilot(SerializableText("asdf"))

This serializes to

{
  "type": "games.studiohummingbird.voyd.pilot.SerializablePilot",
  "name": "asdf"
}

[Zyle Moore]

The SerializersModule is registered like

SerializersModule {
    polymorphic(Name::class) {
        subclass(SerializableText::class)
    }
    polymorphic(Text::class) {
        subclass(SerializableText::class)
    }
}

[Zyle Moore]

The data class serialization is a bit bulkier

{
  "type": "games.studiohummingbird.voyd.pilot.SerializablePilot",
  "name": {
    "type": "games.studiohummingbird.skhema.datatypes.serializable.SerializableText",
    "string": "asdf"
  }
}

[Adam S]

You'll have to create a custom serializer.

Currently, when the serializer tries to decode val name: Name, it sees that Name is an interface, and so it needs the JSON to have a "type": "..." property to determine which Name implementation to decode.

But the actual value is just a string, because InlineSerializableText is a value class, so it can't have a "type" property.

[Adam S]

It's quite simple with a JsonContentPolymorphicSerializer

example: https://pl.kotl.in/pKenugJVw