Contextually register in SerializersModule nullable type

[laurynas-agmis]

Registering serializer module as contextual and passing a KSerializer that has nullable type is not allowed. It is needed due to bad backend responses when field value is not null, but empty string instead.

Example:
val module = SerializersModule {
contextual(serializer = LocalDateSerializer) -> shows error Type mismatch. Required: Any Found: LocalDate?
}

LocalDateSerializer class
@Serializer(forClass = LocalDate::class)
object LocalDateSerializer : KSerializer<LocalDate?> {
private val formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd")
override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("LocalDate?", PrimitiveKind.STRING).nullable

override fun serialize(encoder: Encoder, value: LocalDate?) {
    encoder.encodeString(formatter.format(value))
}

override fun deserialize(decoder: Decoder): LocalDate? {
    val output = decoder.decodeString()
    val result = try {
        LocalDate.from(formatter.parse(output))
    } catch (e: DateTimeParseException) {
        ZonedDateTime.parse(output).toLocalDate()
    } catch (e: Exception) {
       null
    }
    return result
}

}

[sandwwraith]

The problem is in KClass<T> that requires T to be non-nullable and inferred in this overload automatically. However, I think it may be possible to change the signature of contextual(kClass: KClass<T>, serializer: KSerializer<T>) overload.

[CLOVIS-AI]

Is there a workaround? I'm currently stuck on this, I'm surprised it's not possible to serialize nullable values.

[morki]

The same problem here, registering nullable string serializer like this:

object NullableStringSerializer : KSerializer<String?> {
    private val delegate = serializer<String>().nullable
    override val descriptor: SerialDescriptor = delegate.descriptor

    override fun serialize(encoder: Encoder, value: String?) {
        delegate.serialize(encoder, value?.ifEmpty { null })
    }

    override fun deserialize(decoder: Decoder): String? {
        return delegate.deserialize(decoder)?.ifEmpty { null }
    }
}

val module = SerializersModule {
    contextual(NullableStringSerializer)
//             ~~~~~~~~~~~~~~~~~~~~~~~~
}

This results in error:

Type mismatch.
Required: Any
Found: String?

Is there any workaround available please?

[sandwwraith]

@morki Maybe contextual(NullableStringSerializer as KSerializer<String>) would work