JsonDecoder

// Class representing Either<Left|Right>
sealed class Either {
    data class Left(val errorMsg: String) : Either()
    data class Right(val data: Payload) : Either()
}

// Serializer injects custom behaviour by inspecting object content and writing
object EitherSerializer : KSerializer<Either> {
    override val descriptor: SerialDescriptor = buildSerialDescriptor("package.Either", PolymorphicKind.SEALED) {
         // ..
     }

    override fun deserialize(decoder: Decoder): Either {
        val input = decoder as? JsonDecoder ?: throw SerializationException("This class can be decoded only by Json format")
        val tree = input.decodeJsonElement() as? JsonObject ?: throw SerializationException("Expected JsonObject")
        if ("error" in tree) return Either.Left(tree["error"]!!.jsonPrimitive.content)
        return Either.Right(input.json.decodeFromJsonElement(Payload.serializer(), tree))
    }

    override fun serialize(encoder: Encoder, value: Either) {
        val output = encoder as? JsonEncoder ?: throw SerializationException("This class can be encoded only by Json format")
        val tree = when (value) {
          is Either.Left -> JsonObject(mapOf("error" to JsonPrimitive(value.errorMsg)))
          is Either.Right -> output.json.encodeToJsonElement(Payload.serializer(), value.data)
        }
        output.encodeJsonElement(tree)
    }
}