Serializer for Nothing not found

[Octogonapus]

Describe the bug
Using Nothing in a generic type argument fails with an internal error Serializer for element of type Nothing has not been found.

To Reproduce
This reproduces the problem when compiling:

import kotlinx.serialization.Serializable

@Serializable
sealed class SerializableEither<out L, out R> {

    @Serializable
    data class Left<L>(val value: L) : SerializableEither<L, Nothing>()

    @Serializable
    data class Right<R>(val value: R) : SerializableEither<Nothing, R>()
}

Full compiler error: https://pastebin.com/FP6zGrDf

Expected behavior
Nothing should be serializable.

Environment

• Kotlin version: 1.3.60
• Library version: 0.14.0
• Kotlin platforms: JVM
• Gradle version: 5.6.4
• IDE version (if bug is related to the IDE): IntelliJ IDEA 2019.2.4 (Ultimate Edition)
• Runtime version: 11.0.4+10-b304.77 amd64
• Linux 5.0.0-36-generic

[sandwwraith]

I don't feel like Nothing should be serializable – since there is no legal way to have the instance of Nothing and property of type Nothing is meaningless.

However, what you want here is probably a @NotSerializable / @Serializable(NoSerializer) thing which may be useful also in cases like #607 (which slightly resembles this issue)

[Octogonapus]

I am of the perspective that Nothing is serializable because it contains no information: serizialing no information should be a trivial task, right? I originally found this problem because I was trying to serialize arrow-kt's data structures. arrow makes heavy use of Nothing throughout their data structures; most of them involve Nothing in one way or another.

@raulraja If you have a different perspective on this, please feel free to contribute it to this discussion.

[raulraja]

I think the issue here is not that Nothing is encodable or not. The issue here is that this is Nothing in type argument position. Nothing, bottom type, and subtype of all types can appear anywhere in generic arg in any data structure. For example:

sealed class Command<out A> {
  data class Message(val value: String): Command<String>()
  object Noop : Command<Nothing>()
}

Nothing in higher kind position still allows it's a wrapper to be instantiable if it Nothing appears as the type arg of a covariant argument like out A

Nothing itself does not need to be instantiated or serialized because the bottom type has 0 inhabitants.

Any? = All values
Unit = One singleton value
Nothing = No value

I think all Serialization needs for this to work with is to ignore Nothing when it appears in type argument position.

Revisiting a simple understood example which is object to this problem. One of the Arrow Data types.

A simplified version:

sealed class Option<out A> {
  data class Some<out A>(val value: A): Option<A>()
  object None : Option<Nothing>()
}

val someOne: Option<Int> = Some(1)
val noOne: Option<Int> = None

In this example where Option = Some | None that is a sealed union in order to provide a Serializer, it needs the Serializers of Some, None and A. But it does not need the serializer of Nothing to provide any of the possible values that can be created for an Option.

val kindedUpperBound: Option<Any?> = Some(1)
val kindedLowerBound: Option<Int> = Some(1)
val kindedNothing: Option<Nothing> = Some(1) //does not compile because Option<Int> is not a subtype of Option<Nothing>

In the case above since Nothing appears invariant in the receiver value it can ensure impossible values won't compile.

I believe you can support higher kinded <_ : Nothing> which is the issue at hand by simply treating Nothing in type argument position as a NOOP serializer and keep bailing for properties which specify Nothing directly. For example:

class ImpossibleToSerializeOrInstantiate(val x: Nothing = TODO())

[pdvrieze]

Perhaps a NothingSerializer would be valid if it would indeed refuse to serialize or deserialize (but does exist for type argument reasons). I think the use case is valid and implementing is probably easiest when just creating this "serializer"..

[sandwwraith]

So it would be basically like this:

object NotSerializable : KSerializer<Any> {
    override val descriptor: SerialDescriptor = SerialClassDescImpl( "kotlin.Any") 
    override fun deserialize(decoder: Decoder): Any = throw SerializationException("Not serializable")
    override fun serialize(encoder: Encoder, obj: Any) = throw SerializationException("Not serializable")
}

Such serializer can be applied for types in generic arguments (data class Left<L>(val value: L) : SerializableEither<L, @Serializable(NotSerializable::class) Nothing>()).

It even can be a default serializer for Nothing.

[pdvrieze]

@sandwwraith The way I see it yes (except perhaps the return type for deserialize could be Nothing).

[fluidsonic]

I've just started using kotlinx-serialization in a new project and this is basically the first issue I came across, with a compiler error that's not very helpful (location: "File is unknown").

Having a default serializer for Nothing would avoid that, wouldn't it?

[rubenquadros]

I am trying to use kotlinx-serialization and I ran into this error.

sealed class ApiResponse<out S, out E> {
    data class SuccessResponse<S>(val response: S): ApiResponse<S, Nothing>()

    object SuccessNoBody: ApiResponse<Nothing, Nothing>()

    data class ErrorResponse<E>(val error: E): ApiResponse<Nothing, E>()
}

Any help would be great! Thanks in advance :)

[pdvrieze]

@rubenquadros Assuming that the error you are getting refers to there not being a serializer for Nothing you would need to create a custom serializer for Nothing. Something like:

object NothingSerializer: KSerializer<Nothing> {
    override val descriptor: SerialDescriptor = buildClassSerialDescriptor("kotlin.Nothing") {}

    override fun deserialize(decoder: Decoder): Nothing =
        throw UnsupportedOperationException("Nothing does not have instances")

    override fun serialize(encoder: Encoder, value: Nothing): Nothing =
        throw UnsupportedOperationException("Nothing cannot be serialized")
}

[rubenquadros]

@pdvrieze I still get the same error. I am using kotlinx-serialization with ktor. Maybe I am doing something wrong. Here is my code

    runBlocking {
        client.responsePipeline.intercept(HttpResponsePipeline.Transform) { (info, body) ->
            val response = when {
                context.response.status == HttpStatusCode.OK -> {
                    if (context.response.contentLength() == 0L) ApiResponse.SuccessNoBody
                    else ApiResponse.SuccessResponse(body)
                }
                else -> {
                    ApiResponse.ErrorNoBody(context.response.status.value)
                }
            }
            proceedWith(HttpResponseContainer(info, response))
        }
        val response: ApiResponse<User, Nothing> = client.post("url")
    }

[pdvrieze]

@rubenquadros You will need to actually make sure that you actually specify the serializer within ApiResponse. Something like:
Every place it has Nothing as parameter you use @Serializable(NothingSerializer::class) Nothing to expose nothing. We really need the error message though.

[rubenquadros]

@pdvrieze Here is the stacktrace https://pastebin.com/pNYVKCyn
Sorry for not providing it earlier.

[pdvrieze]

@rubenquadros The problem here is that you somehow captured a Void type from the Java side. This is not a built in type. Ideally you would avoid the Void type entirely, but alternatively you could provide a serializer as a contextual serializer (registered in the module).

[ShreckYe]

Good point. I tried this first but it doesn't work for the JS target. Now the only workaround I can think of is creating a SerializableNothing class and replacing Nothing with it everywhere. I am wondering if there is a multiplatform workaround other than this and simpler than this.

[ShreckYe]

Hi everyone. I have tried both workaround solutions, using contextual serializers and mapping Nothing to SerializableNothing, and it turned out the latter works better on Multiplatform. I managed to organize some prototype code in an open-source library, the core of which is this KType.mapNothingToSerializableNothing function. I hope it can provide a good reference.

If you'd like to you can also depend the library from Maven Central. The code is still in prototype and I didn't add any tests yet so there is not much guarantee. However, we are already using it in our internal projects.

[joffrey-bion]

So it would be basically like this:

object NotSerializable : KSerializer<Any> {
    override val descriptor: SerialDescriptor = SerialClassDescImpl( "kotlin.Any") 
    override fun deserialize(decoder: Decoder): Any = throw SerializationException("Not serializable")
    override fun serialize(encoder: Encoder, obj: Any) = throw SerializationException("Not serializable")
}

Such serializer can be applied for types in generic arguments (data class Left<L>(val value: L) : SerializableEither<L, @Serializable(NotSerializable::class) Nothing>()).

It even can be a default serializer for Nothing.

@sandwwraith Any plans on moving this forward?

[phanimahesh]

Not having a Nothing implementation by default makes using kotlinx.serialisation impossible with any libraries using higher kinded types parameterised types. Since the types are not owned by application authors in such cases the workaround of annotating the definition site can not be used as well.

[ShreckYe]

I guess you mean generic/parameterized types instead of higher-kinded types. If I am not wrong, higher-kinded types are not supported in Kotlin.

[phanimahesh]

My bad, yes. I meant parameterised types that use Nothing, which isn't uncommon.