One last try, I've googled until I'm blue in the f...

# serialization

[v79]

One last try, I've googled until I'm blue in the face. Given this class:

data class ResponseEntity<T>(
    val statusCode: Int,
    val body: T? = null,
    val headers: Map<String, String> = emptyMap(),
) {
    companion object {
        fun <T> ok(body: T? = null, headers: Map<String, String> = emptyMap()) =
            ResponseEntity<T>(200, body, headers)
    }
}

I absolutely, definitely need the KSerializationStrategy for T (if it exists, could be null), so that much later in the code, in another class, another method, I can call Json.encodeToString(entity.serializationStrategy, entity.body).

I think that I need to add another property to the ResponseEntity class, which stores the Class of T, or better still the KSerializer for T, or the KSerializationStrategy for T. But whatever combination of things I try - inline reified functions, typeOf(T) (which doesn't even compile) - I get stuck.

[v79]

If I explicitly supply the KClass, I can make it work:

// call site
ResponseEntity.ok(body = SimpleClass("hello from simpleClass"), clazz = SimpleClass::class)

// ResponseEntity class
data class ResponseEntity<T : Any>(
    val statusCode: Int,
    val body: T? = null,
    val headers: Map<String, String> = emptyMap(),
    val clazz: KClass<T>? = null
) {
    companion object {
        fun <T : Any> ok(body: T? = null, headers: Map<String, String> = emptyMap(), clazz: KClass<T>? = null) =
            ResponseEntity<T>(200, body, headers, clazz)

    }
}

// eventually do the serialization in another class & method:
val kSerializer = responseEntity.clazz?.serializer()
        val body: String = when (mimeType) {
            MimeType.json -> {
                kSerializer?.let {
                    Json.encodeToString(kSerializer,responseEntity.body as T)
                } ?: "no-serializer"
            }
//... etc
}

This is working. But it does require the user to to provide that clazz value, which I had hoped to avoid.

[v79]

Progress!

data class ResponseEntity<T : Any>(
    val statusCode: Int,
    val body: T? = null,
    val headers: Map<String, String> = emptyMap(),
) {
    var clazz: KClass<T>? = null
    companion object {
        inline fun <reified T : Any> ok(body: T? = null, headers: Map<String, String> = emptyMap()): ResponseEntity<T> {
            val tt = T::class
            return ResponseEntity<T>(200, body, headers).apply { clazz = tt }
        }
    }
}

[Adam S]

you could also make a fake inline constructor

inline fun <reified T : Any> ResponseEntity(
  statusCode: Int,
  body: T? = null,
  headers: Map<String, String> = emptyMap(),
) = ResponseEntity(statusCode, body, headers, T::class)

[ephemient]

@Serializable
class Foo<T>(...)

creates a

Foo.serializer(tSerializer: KSerializer<T>): KSerializer<Foo<T>>

function on the companion

[ephemient]

this works with custom serializers too, e.g.

class ResponseEntitySerializer<T>(
    private val tSerializer: KSerializer<T>
) : KSerializer<ResponseEntity<T>> {
    ...
}
@Serializable(with = ResponseEntitySerializer::class)
class ResponseEntity<T>(...)

has a ResponseEntity.serializer(tSerializer) just like the non-custom case

[ephemient]

IMO you should not be using KClass for this purpose (type erasure) and you should not be carrying a serializer per instance either (that doesn't make much sense). instead, there should be some real call site that knows the real type that it is expecting

[v79]

I'm now struggling with sealed classes with generics... but I think I might just handle those manually. Serialization is hard!

[v79]

Finally got it working. Note to self - get the KType, it's better then the KClass. My json is a bit corrupted, I think I've double-escaped it, but that's a problem for another day. I'm off to bed. I've got a flight at 6am!