8000 Decoder not implicitly available for value classes in Scala 3 · Issue #812 · sksamuel/avro4s · GitHub
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dvgica opened this issue Dec 15, 2023 · 2 comments
Open

Decoder not implicitly available for value classes in Scala 3 #812

dvgica opened this issue Dec 15, 2023 · 2 comments
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@dvgica
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dvgica commented Dec 15, 2023

I'm upgrading some 2.13 code to 3.3.1. It makes use of value classes which Avro4s was previously able to derive a Decoder for, but in Scala 3 cannot.

In short, I think this should work with Avro4s 5.0.8, but does not (Scastie):

case class Thing(value: String) extends AnyVal

import com.sksamuel.avro4s.Decoder

println(implicitly[Decoder[Thing]])

gives

No given instance of type com.sksamuel.avro4s.Decoder[Playground.Thing] was found for parameter e of method implicitly in object Predef.
I found:

    com.sksamuel.avro4s.Decoder.autoDerived[Playground.Thing](
      /* missing */summon[deriving.Mirror.Of[Playground.Thing]])

But Failed to synthesize an instance of type deriving.Mirror.Of[Playground.Thing]:
	* class Thing is not a generic product because it is a value class
	* class Thing is not a generic sum because it is not a sealed class.

In Avro4s 4.1.1 it works fine.

Am I missing something here?
@dvgica
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dvgica commented Dec 18, 2023

It appears that this issue also applies to Encoder and SchemaFor.

I have worked around it for now by defining my own implicits:

import com.sksamuel.avro4s.Decoder
import com.sksamuel.avro4s.Encoder
import com.sksamuel.avro4s.SchemaFor
import com.sksamuel.avro4s.decoders.StringDecoder
import com.sksamuel.avro4s.encoders.StringEncoder
import com.sksamuel.avro4s.schemas.JavaStringSchemaFor

implicit val thingDecoder: Decoder[Thing] = StringDecoder.map(Thing(_))
implicit val thingEncoder: Encoder[Thing] = StringEncoder.contramap(_.value)
implicit val thingSchemaFor: SchemaFor[Thing] = JavaStringSchemaFor.forType[Thing]

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stale bot commented Apr 26, 2025

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.

@stale stale bot added the abandoned label Apr 26, 2025
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