Scala 3 Macro - Retain a generic type in runtime - scala

Question

I'm looking for an option to retain a generic type in runtime in Scala3. In Scala2 there was a TypeTag for this, however, now it is removed and the suggested option is to use macros (https://contributors.scala-lang.org/t/scala-3-and-reflection/3627).
The documentation, however, is somewhat cryptic...
This is what I'm trying to do:
Here's a macro implementation:
object TestMacroImpl {
def getClassImpl[T](using Quotes)(using t: Type[T]): Expr[Class[T]] = '{
classOf[${t}]
}
}
Here's a macro:
import macros.TestMacro.getClassMacro
class TypedBox[T] {
val staticClass: Class[T] = TypedBox.getStaticClass[T]
}
object TypedBox {
inline def getStaticClass[T] = ${ getClassMacro[T] }
}
Test:
object Test {
def main(args: Array[String]): Unit = {
val stringBox = TypedBox[String]()
println(stringBox.staticClass)
}
}
I would envision this to be resolved as val staticClass = classOf[String]
But this does not compile, I'm getting:
/workspace/macros-test/src/main/scala/macros/TestMacro.scala:7:13
t.Underlying is not a class type
classOf[${t}]
What am I missing?

Answer 1

Not really sure why but I don't think you can reliably get an Expr[Class[T]] out of macros (from what I understood, it could be that the Class does not yet exist at the time of macro execution).
Plus, a Class[T] does not retain the parameterized types: classOf[Map [String, String]] = classOf[Map[Int, Int]] for instance.
If you don't care about them, I'd use a ClassTag instead of TypeTag which is still available in Scala 3. And no need for macros.
By the way, in macros, you can write something like the following to get a Expr[ClassTag[T]]:
private def getClassTag[T](using Type[T], Quotes): Expr[ClassTag[T]] = {
import quotes.reflect._
Expr.summon[ClassTag[T]] match {
case Some(ct) =>
ct
case None =>
report.error(
s"Unable to find a ClassTag for type ${Type.show[T]}",
Position.ofMacroExpansion
)
throw new Exception("Error when applying macro")
}
}
Finally, you might find some useful things at https://github.com/gaeljw/typetrees/blob/main/src/main/scala/io/github/gaeljw/typetrees/TypeTreeTagMacros.scala#L8 (disclaimer: I wrote it for personal projects).