How can I serialize/deserialize a `Map<String, ...

# serialization

[dave08]

How can I serialize/deserialize a Map<String, Map<String, Any>> where Any can be one of String, Set<String> or Map<String, String>? I don't mind if instead of Any I'd have a sealed interface AnyValue with wrappers for those options... although I'd maybe prefer a value class that could have my own helper functions to cast and retreive one of those... I would know in advance which one it is using the String key in the inner map...

[Adam S]

it might be easier to decode to a JsonObject and then ‘manually’ decode

[Adam S]

I answered a similar question here: https://kotlinlang.slack.com/archives/C0B8MA7FA/p1672313784581899?thread_ts=1672310136.005409&cid=C0B8MA7FA

[dave08]

Thanks! I guess I could get the Ktor client to return a String and then just use decodeToJsonElement...

[Adam S]

if you wanted to set up sealed interface AnyValue then you could use value classes for the String, Set<String>, Map<String, String> - then you would have your own type hierarchy. If the value classes were @Serializable then the plugin-generated serializers would work just fine.

The slightly tricky bit would be writing the KSerializer for AnyValue. It wouldn’t be too hard if you used the JsonPolymorphicSerializer (or whatever the real name is) provided by KxS.

[dave08]

Yeah, the only thing is that I don't think you can use value classes in sealed interfaces yet, and they would probably be boxed anyways as soon as you use the interface in the map value...

[Adam S]

I’m sure you can

[Adam S]

Something like this should work, except there’s a bug…

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index -1 out of bounds for length 0
	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)

I’ve made an issue: https://github.com/Kotlin/kotlinx.serialization/issues/2159

example.kt.cpp

[dave08]

See: https://github.com/Kotlin/KEEP/blob/3adcaf7c6e5d2ff756d26d2b60ba494947a72678/proposals/sealed-inline-classes.md

[dave08]

and: https://youtrack.jetbrains.com/issue/KT-27576/Support-inline-sealed-classes which is scheduled for Kotlin 1.9

[Adam S]

ahh right, yes. Inline classes can’t extend abstract/open/sealed classes. Inline classes can extend from interfaces though.

[dave08]

I think the problem is that as soon as you use an interface, it boxes the value class and there is no real benefit... I guess the proposed KEEP is to have a special sealed value interface that doesn't box?

[dave08]

Actually, it's a sealed value class in the KEEP

[Adam S]

I’m confused by KT-27576… I thought that sealed classes were already implemented in 1.4. My guess is that it’s specifically regarding JVM. Currently Kotlin has a bespoke implementation that requires some under-the-hood workarounds, so if JVM supports value classes natively then Kotlin will migrate to that.

[Adam S]

anyway, I’m promoting the use of value classes here not because they’re more performant, but because it means KxS can automatically generate a serializer that matches the ‘internal’ type, which is very convenient.