How do I use sealed interfaces for union types properly? (thread)

# serialization

[kotlinforandroid]

How do I use sealed interfaces for union types properly? (thread)

[kotlinforandroid]

@Serializable(with = ValueDeserializer::class)
sealed interface Value

@JvmInline
@Serializable
value class NumberValue(val value: Int) : Value

@JvmInline
@Serializable
value class StringValue(val value: String) : Value

object ValueDeserializer :
    JsonContentPolymorphicSerializer<Value>(Value::class) {
    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out Value> {
        if (element !is JsonPrimitive) throw RuntimeException("failed to deserialize literal value")
        return if (element.jsonPrimitive.isString) StringValue.serializer() else NumberValue.serializer()
    }
}

@Test
fun `deserializes from literal value`() {
    val inputs = listOf(
        ("42", NumberValue(42)),
        ("\"42\"", StringValue("42")),
    )
    inputs.forEach { 
        // Check that the class matches.
        val decoded = Json.decodeFromString<Value>(it.first)
        assertEquals(decoded is it.second::class, true)
    }
}

[kotlinforandroid]

I get this error. The goal is to have a way to describe fields that can either be a string or a number. I need to know the type at runtime and can't use transformation to convert both to one type only.

Serializer for class 'Value' is not found.
Mark the class as @Serializable or provide the serializer explicitly.
kotlinx.serialization.SerializationException: Serializer for class 'Value' is not found.
Mark the class as @Serializable or provide the serializer explicitly.

Kotlin 1.6.10
Kotlin Serialization 1.3.3
Android Project, Source Compatibility 1.8, Java 11 (Stacktrace mentions java.base@11.0.11)

[ephemient]

https://github.com/Kotlin/kotlinx.serialization/issues/1869

[ephemient]

it should work in another structure, e.g.

@Serializable
data class Container(val value: Value)

but currently the serializer lookup for sealed interface is broken in cases such as yours

[kotlinforandroid]

Wouldn't that wrap the Value inside of an object, i.e., "42" wouldn't work, it would have to be {"value":"42"}

[ephemient]

Json.decodeFromString(ValueDeserializer, it)

[ephemient]

yes, it would wrap the value inside another object, but I find it rare that you have a top-level primitive to parse

[kotlinforandroid]

I have JSON of the form of {"label": "hello-world", "value": <"string" or number>} . And I tried to model that value type above.

[kotlinforandroid]

Maybe there is another way I can realize this. I only need deserialization though.

[ephemient]

then your real use case looks like

@Serializable
data class Container(val label: String,  val value: Value)

doesn't it? that should work

[kotlinforandroid]

haha true, that should work! I wanted to unit test the deserialization of field. Building bottom to top. Thanks for the help!

[ephemient]

as an aside, not all JsonPrimitive are string or int: in general JSON numbers can be double, and boolean and null are also primitive