Is it possible to deserialize an array into an obj...

# serialization

[Matthew Olsson]

Is it possible to deserialize an array into an object? For example, turning something like ["Bob", 24] into class Person(val name: String, val age: Int). Assuming of course that I already know the array corresponds to the Person class (in reality, my data looks more like { "person": ["Bob", 24] }

[jw]

There's an issue for automatically doing this, but otherwise yes that's a simple custom serializer where you read the array values and then invoke the constructor with the values

[Matthew Olsson]

Is there a way to do it more generically? I have like 50 different types of these classes

[jw]

Hmmm you may be able to write a generalized serializer which does this, but I haven't tried.

[jw]

I know how to do it with Moshi, but not kotlinx.seriazliation. But rarely can you do something with one and not the other.

[Matthew Olsson]

Ahh I haven't heard of that library before. Well I'm not particular tied to kotlinx, I'll try that if I can't get it to work

[Matthew Olsson]

Wow you were right, I was able to come up with an extremely elegant solution. All I have to do is configure the sealed class with a one-line serializer implementation. Here's the final solution, in case anyone finds this in the future: https://gist.github.com/mattco98/99a6778609e21a95d68c7dbfd949cc9f

[ephemient]

doing your own reflection bypasses a lot of kotlinx.serialization (and doesn't work on non-JVM platforms). why not work with kotlinx.serialization?

Serde_kt.cpp

[Matthew Olsson]

Because I didn't know how, that's kind of why I was here asking questions after all 😛 But that solution requires constructing and passing the serializer explicitly to the decode/encode function. Is there a way to use this solution when I have a huge tree of nested types?

[Matthew Olsson]

Ideally I would just say Serializable(with = ...), but that (obviously) overwrites the compiler-generated serializer

[Matthew Olsson]

Actually you could use the SerializersModule polymorphic I assume. If that works, then this solution is definitely much better. Thanks for improving my hack 🙂

[ephemient]

that will require manually registering them - you don't get the automatic registration that @Serializable does. but yes, it would work

[ephemient]

you could use

polymorphic(EnumVariant::class) {
    for (klass in EnumVariant::class.sealedSubclasses) subclass(klass)
}

if you're on JVM and include kotlin-reflect but otherwise that lookup is something that currently can't be done at runtime on other platforms

Serde.kt.cpp

[Matthew Olsson]

I tried doing that, but it gives me Cannot use 'CapturedType(out UncheckedType)' as reified type parameter (I do have the kotlin-reflect dependency)

[ephemient]

hmm, that's interesting. can be worked around with

@OptIn(InternalSerializationApi::class)
subclass(klass as KClass<EnumVariant>, klass.serializer())

but I don't think it's meant to be used that way

[Matthew Olsson]

Good enough for me, at least for now

[ephemient]

it's not as well documented, but you could also implement an alternative format which works like JSON except for polymorphism. then the default generated serializers will just work

Serde_kt.cpp

[ephemient]

the downside is that it misses some standard Json features (e.g. ignoreUnknownKeys, useAlternativeNames, etc.) which are definitely possible to re-implement but would take more effort

[Matthew Olsson]

Oh very interesting, I hadn't considered that approach. The lack of features is fine, I don't think I need any of them in this particular instance. This solution I think is the most appealing for me, thanks once again 🙂