KotlinxSerialization.asFormatString polymorphic serialization error

[oharaandrew314]

KotlinxSerialization.asFormatString seems incapable of properly handling polymorphic types; it fails to properly serialize the type parameter. I believe this works with raw kotlinx and with lenses because they both reifiy the base provided type, rather than rely on the runtime type.

I don't really see a way around this short of extensive modifications to asFormatString to reify the type. However, doing so would also allow the http4k-format-csv module to become an AutoMarshalling, as this was a limiting issue.

Thoughts? Live with and document the limitation? Or try to reify the type in asFormatString?

See the provided tests below, where serialize sealed class - http4k asFormatString fails.

@Serializable
sealed interface Animal {
    val name: String
}

@Serializable
@SerialName("cat")
data class Cat(
    override val name: String,
    val lives: Int,
): Animal

val json = """{"type":"cat","name":"Bandit","lives":8}"""
val obj = Cat("Bandit", 8)
val lens = KotlinxSerialization.autoBody<Animal>().toLens()

@Test
fun `serialize sealed class - kotlinx encodeToString`() {
    assertThat(
        Json.encodeToString(obj as Animal),
        equalTo(json)
    )
}
// PASS

@Test
fun `serialize sealed class - http4k asFormatString`() {
    assertThat(
        KotlinxSerialization.asFormatString(obj as Animal),
        equalTo("""{"type":"cat","name":"Bandit","lives":8}""")
    )
} // FAIL.  Was {"name":"Bandit","lives":8}"

@Test
fun `serialize sealed class - http4k lens`() {
    assertThat(
        Response(Status.OK).with(lens of obj).bodyString(),
        equalTo(json)
    )
}
// PASS

@Test
fun `deserialize sealed class - kotlinx decodeFromString`() {
    assertThat(
        Json.decodeFromString<Animal>(json),
        equalTo(obj)
    )
}
// PASS

@Test
fun `deserialize sealed class - http4k asA`() {
    assertThat(
        KotlinxSerialization.asA<Animal>(json),
        equalTo(obj)
    )
}
// PASS

@Test
fun `deserialize sealed class - http4k lens`() {
    assertThat(
        lens(Response(Status.OK).body(json)),
        equalTo(obj)
    )
}
// PASS