When your avro needs a discriminator in the payload

avro_discriminator.scala

import vulcan.{AvroError, Codec}
import vulcan.generic.*
import zio.{ZIO, ZIOAppDefault}

sealed trait Foo

object Foo {
  case class A(a: Int, `type`: "A" = "A")    extends Foo
  case class B(b: String, `type`: "B" = "B") extends Foo

  implicit def singletonString[T <: String: ValueOf]: Codec[T] =
    Codec.enumeration(
      valueOf[T],
      "singleton",
      List(valueOf[T]),
      identity,
      s => Option.when(s == valueOf[T])(s.asInstanceOf[T]).toRight(AvroError("nope"))
    )
  implicit val codec: Codec[Foo] = Codec.derive[Foo]
}

object go extends ZIOAppDefault {

  override def run = {
    for {
      schema <- ZIO.fromEither(Foo.codec.schema).debug
      _      <- ZIO.fromEither(Foo.codec.encode(Foo.A(1)).flatMap(a => Foo.codec.decode(a, schema))).debug
    } yield ()
  }
}