dn-q: ShowLabel

build.sbt

val dottyVersion = "3.0.0-RC3"


lazy val root = project
  .in(file("."))
  .settings(
    name := "test",
    version := "0.0.1",
    scalaVersion := dottyVersion,

    scalacOptions ++= Seq( "-unchecked", "-Ycheck:macros" ),

  )

Main.scala

// in src/main/scala/x
package x


// should pass compile
case class TestMappingTemplate() derives ShowName {
  //...
}

// should fail on compile
case class SomeOtherThing() derives ShowName {
  // ...
}


object Run extends App:

   println("run")

ShowLabel.scala

// in src/main/scala/x
package x

import scala.deriving._
import scala.compiletime._
import scala.quoted.*


object ShowLabel {

  def showLabelExpr[T](label: Expr[String])(using Quotes)(using Type[T]): Expr[ShowName[T]] = {
    val docRegex = "(Test)(Mapping)([a-zA-Z]+)".r
    val simpleRegex = "([a-zA-Z]+)(Mapping)([a-zA-Z]+)".r

    given ToExpr[ShowName[T]] with {
      override def apply(x: ShowName[T])(using Quotes): Expr[ShowName[T]] =
        Expr.summon[ShowName[T]].get
    }

    label.value match {
      case Some(docRegex(doc, _, _)) =>
        Expr(
          new ShowName[T] {
            def name(source: Option[String]): String =
              s"$doc".toLowerCase
          }
        )
        
     case Some(simpleRegex(docType, _, _)) =>
        Expr(
          new ShowName[T] {
            def name(source: Option[String]): String =
              s"$docType".toLowerCase
          }
        )

      case Some(value) =>
        quotes.reflect.report.throwError(s"Can't produce such name: $value")

      case None =>
        quotes.reflect.report.throwError(s"Expected label to be a known string but was: ${label.show}")
    }
  }

}

X.scala

// in src/main/scala/x
package x

import scala.deriving._
import scala.compiletime._
import scala.quoted.*


trait ShowName[T] {
  def name(source: Option[String]): String
}

object ShowName {


  // you need helper because you can't pass '{m.MirroredLabel} directly
  inline def _helper[T](inline label: String): ShowName[T] =
    ${ ShowLabel.showLabelExpr[T]('label) }

  inline given derived[T](using m: Mirror.Of[T]): ShowName[T] =
    _helper[T](constValue[m.MirroredLabel])
}

Thank you for reply. It really works with compile check but the name method won't work if you try to use it:

object Run extends App:
   extension[T](t: T) {
    def getName(source: Option[String] = Some(""))(using ShowName[T]) =
      summon[ShowName[T]].name(source)
   }


  println(TestMappingTemplate().getName(Some("testSource")))  

It just runs the app forever without error or output.