Is there any way to have this: `{ "foo": "" }` be null in this: `data class Foo(value: Int? = null)`...

# serialization

[dave08]

Is there any way to have this: { "foo": "" } be null in this: data class Foo(value: Int? = null)? (It's supposed to be a number when not an empty string...)

[Adam S]

the quickest way is probably to use JsonPrimitive and the expose the actual value as a property

data class Foo(
  private val value: JsonPrimitive? = null
) {
  val actualValue = value?.myConverterFunction()
}

[dave08]

Thanks, that's a great idea!

[ephemient]

or you can hide that behind a surrogate

[ephemient]

@Serializable(with = Foo.Serializer::class)
data class Foo(val value: Int? = null) {
    @Serializable
    data class Surrogate(val value: JsonPrimitive = JsonNull)
    object Serializer : KSerializer<Foo> {
        override val descriptor: SerialDescriptor = SerialDescriptor("Foo", Surrogate.serializer().descriptor)
        override fun serialize(encoder: Encoder, value: Foo) {
            val surrogate = Surrogate(value = JsonPrimitive(value.value))
            encoder.encodeSerializableValue(Surrogate.serializer(), value)
        }
        override fun deserialize(decoder: Decoder): Foo {
            val surrogate = decoder.decodeSerializableValue(Surrogate.serializer())
            return Foo(value = surrogate.value.intOrNull)
        }
    }
}

[dave08]

Interesting too... the only thing is that I have a bunch more fields in that class in reality. It would have been nice to be able to only apply the serializer to that one "non-standard" property and leave the rest alone.

[Adam S]

you could use a value class to encapsulate some of the logic

@Serializable
value class IntConverter(private val value: JsonPrimitive) {
  val actual get() = { value.myConverter... }
}

Then you can re-use the same class in your data class

data class Foo(
  private val value: IntConverter? = null
)

[ephemient]

I don't like that idea much; it'll make Foo(value = IntConverter(JsonPrimitive("0"))) != Foo(value = IntConverter(JsonPrimitive("-0"))) even though you would want to consider them the same value

[Adam S]

good point