macro implementations cannot have implicit parameters other than WeakTypeTag evidences

[vuhainguyen]

Hi, I tried to use some examples in https://github.com/softwaremill/magnolia/tree/scala2/examples/src/main/scala/magnolia1/examples with scala 2.13 but I got the compilation error below:

macro implementations cannot have implicit parameters other than WeakTypeTag evidences
  implicit def gen[T]: Print[T] = macro Magnolia.gen[T]

Did I miss out something or the issue is real?
Thanks in advance for your help.
Best regards,
Vu Hai

[adamw]

Hm did you use the exact same examples? They are part of the build so should work jsut fine. Can you post the full code that causes the problem?

[vuhainguyen]

HI thanks for the feedback, I found this discussion https://gitter.im/propensive/magnolia?at=5e82194b49e7a8130cdaf16b, turns out I just need to add "org.scala-lang" % "scala-reflect" % "2.13.8" into my build.sbt for it to work with scala 2.13.
But my current project do cross compilation for both scala 2.13 and 3 and there's no scala reflect for scala 3. I noticed that the method Magnolia.gen is removed and we use the syntax given .. with for scala 3 as well.
Is there a way to make it work for both version of scala? (it's a bit out of the initial context of this thread, let me know if you want to close this one and open a new one instead 😅 )

trait RandomObjectDerivation {
  type Typeclass[A] = RandomObject[A]

  def join[A](caseClass: CaseClass[Typeclass, A]): Typeclass[A] = new Typeclass[A] {
    def generate(): A = caseClass.construct(_.typeclass.generate())
  }

  def split[A](sealedTrait: SealedTrait[Typeclass, A]): Typeclass[A] = new Typeclass[A] {
    def generate(): A = choice(sealedTrait.subtypes).typeclass.generate()
  }

  def split[T](ctx: SealedTrait[Typeclass, T]): Typeclass[T] =
    new Typeclass[T] {
      def generate(): T = choice(ctx.subtypes).typeclass.generate()
    }

  private def choice[A](xs: Seq[A]): A =
    xs.toIndexedSeq(Random.nextInt(xs.length))

  implicit def gen[A]: Typeclass[A] = macro Magnolia.gen[A]
}

[adamw]

Yes, in Scala 3 you can use implicits or .derived. Though I don't think you'll be able to compile the same codebase using both Scala 2 and Scala 3, there are some (not very significant, but still) differences.

I suppose you'll need to have some sources scala2 and some scala3 specific (see e.g. tapir's core module): https://github.com/softwaremill/tapir

[vuhainguyen]

Awesome, I followed this tutorial https://docs.scala-lang.org/scala3/guides/migration/tutorial-macro-cross-building.html
I can compile my project with both scala 2.13 and scala 3 now. Thanks a lot for your help 🙏
I can't find in the examples directory of scala 3 one example with .derived, can you give me one? 😅
I think this line implicit def gen[A]: Typeclass[A] = macro Magnolia.gen[A] helps me to generate the case class for scala 2.13, is there an equivalent for scala 3?