Avro4s fails to decode enums

[MateuszKubuszok]

Consider this example:

//> using scala 3.3.5
//> using dep com.sksamuel.avro4s::avro4s-core:5.0.14

import com.sksamuel.avro4s.*
import java.io.{ByteArrayInputStream, ByteArrayOutputStream}
import scala.util.{Using, Try}

sealed trait ExampleWithSealed derives Decoder, Encoder, SchemaFor
object ExampleWithSealed {
  case class Foo(value: String) extends ExampleWithSealed
  case object Bar extends ExampleWithSealed
}

enum ExampleWithEnum derives Decoder, Encoder, SchemaFor {
  case Foo(value: String)
  case Bar
}

import java.io.{ByteArrayInputStream, ByteArrayOutputStream}

def roundTrip[A: Decoder: Encoder: SchemaFor](value: A): Try[A] = {
  for {
    encoded <- Using(new ByteArrayOutputStream()) { baos =>
      val aos = AvroOutputStream.json[A].to(baos).build()
      aos.write(value)
      aos.close()
      aos.flush()
      baos.toByteArray
    }
    decoded <- Using(new ByteArrayInputStream(encoded)) { bais =>
      AvroInputStream
        .json[A]
        .from(bais)
        .build(SchemaFor[A].schema)
        .asInstanceOf[AvroJsonInputStream[A]]
        .singleEntity
    }.flatten
  } yield decoded
}

println(roundTrip[ExampleWithSealed](ExampleWithSealed.Foo("example")))

println(roundTrip[ExampleWithEnum](ExampleWithEnum.Foo("example")))

I'd expect both println to show exactly the same behavior. Meanwhile, it produces:

Success(Foo(example))
Failure(java.lang.IllegalArgumentException: requirement failed)

Changing Scala from 3.3.5 to 3.6.3 doesn't change the result.

Removing derives and using automatic derivation, doesn't change it either.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[MateuszKubuszok]

I am almost certain that this is the culprit:

avro4s/avro4s-core/src/main/scala/com/sksamuel/avro4s/typeutils/DatatypeShape.scala

Lines 15 to 16 in dc5aaca

	val allSubtypesAreObjects = ctx.subtypes.forall(_.isObject)
	if (ctx.isEnum || allSubtypesAreObjects) SealedTraitShape.Enum else SealedTraitShape.TypeUnion

Scala 3's enum parameterless cases are not objects, they are vals - Magnolia's isObject is correctly checking whether something is an object, and case Bar in the example above is not (it compiles to val Bar = new ExampleWithEnum{}). Perhaps another utility in Magnolia would be needed (@adamw ?), or some other approach to this method (e.g. checking of constructor has no parameters?).

[adamw]

There's Subtype.isObject, which I think should be true/false depending if the value is a proper class or not.

[MateuszKubuszok]

@adamw Yeah, but it seems that it returns false for parameterless cases in enum - because they are not objects. So probably Magnolia on Scala 3 would need an adaptation for it.

[adamw]

Ah right ... that would probably need another flag. Or another model altogether - retrofitting this into SealedTrait might not be the best option.