PolymorphismStyle.None resulting in an error

[PhilippNowak96]

I saw that PolymorphismStyle.None has been released recently and wanted to try it out for some usecase in my project.

For the parsing process I have the following example model class:

@Serializable
data class Specification(
    val params: List<Param>,
)

@Serializable
sealed class Param {
    abstract val name: String
    @SerialName("desc") abstract val description: String
    abstract val valueType: String
}

Then I try to parse it with the following example:

Yaml(
    configuration = YamlConfiguration(
        allowAnchorsAndAliases = true,
        polymorphismStyle = PolymorphismStyle.None,
    )
).decodeFromString(
    deserializer = Specification.serializer(),
    string = """
        |params:
        |  - &param_EventStep
        |    name: event_step
        |    desc: "Something"
        |    valueType: string
    """.trimMargin(),
)

Unfortunately I receive an error which looks like:

Exception in thread "main" IncorrectTypeException at params[0] on line 2, column 5: Encountered a polymorphic map descriptor but PolymorphismStyle is 'None'
	at com.charleskorn.kaml.YamlInput$Companion.createFor$kaml(YamlInput.kt:67)
	at com.charleskorn.kaml.YamlListInput.decodeElementIndex(YamlListInput.kt:38)
	at kotlinx.serialization.internal.AbstractCollectionSerializer.merge(CollectionSerializers.kt:34)
	at kotlinx.serialization.internal.AbstractCollectionSerializer.deserialize(CollectionSerializers.kt:43)
	at kotlinx.serialization.encoding.Decoder$DefaultImpls.decodeSerializableValue(Decoding.kt:257)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableValue(AbstractDecoder.kt:16)

Can you tell me what exactly the problem is? As far as I understood the new polymorphismStyle = PolymorphismStyle.None should just skip the checks regarding polymorphism, i.e. doesn't care about sealed class and stuff like that. Am I wrong?

[charleskorn]

PolymorphismStyle.None does not support deserializing at all - it is for serialization only.

In your case, I'd suggest creating a custom serializer for the Param type that constructs the correct subtype of Param based on the value of valueType. There's more information about this in the kotlinx.serialization docs here. (Note that the examples refer to JSON, but the same ideas apply for YAML.)

[PhilippNowak96]

Oh then I misunderstood that mode 😅 Thank you very much for the explanation, I will definitely try the custom serializer way.

[mgroth0]

PolymorphismStyle.None does not support deserializing at all - it is for serialization only.

Exactly. I requested this feature because I wanted to serialize a polymorphic tree. The resulting yaml would be read by an external program that does not check types.