Since you're seeing SymbolSourceAttachment
, I assume you're using macro paradise (because it's an internal attachment used only in paradise), so I'll feel free to use quasiquotes :)
There's no easy way to get values of default parameters in Scala reflection API. Your best shot would be reverse-engineering the names of methods that are created to calculate default values and then referring to those.
SymbolSourceAttachment
would sort of work if your macro is expanding in the same compilation run that compiles the case class, but it would break under separate compilation (attachments aren't saved in class files), and it wouldn't work in vanilla Scala (because this attachment is exclusive to paradise).
=== Macros.scala ===
import scala.reflect.macros.Context
import scala.language.experimental.macros
object Macros {
def impl[T](c: Context)(T: c.WeakTypeTag[T]): c.Expr[Map[String, Any]] = {
import c.universe._
val classSym = T.tpe.typeSymbol
val moduleSym = classSym.companionSymbol
val apply = moduleSym.typeSignature.declaration(newTermName("apply")).asMethod
// can handle only default parameters from the first parameter list
// because subsequent parameter lists might depend on previous parameters
val kvps = apply.paramss.head.map(_.asTerm).zipWithIndex.flatMap{ case (p, i) =>
if (!p.isParamWithDefault) None
else {
val getterName = newTermName("apply$default$" + (i + 1))
Some(q"${p.name.toString} -> $moduleSym.$getterName")
}
}
c.Expr[Map[String, Any]](q"Map[String, Any](..$kvps)")
}
def extractor[T]: Map[String, Any] = macro impl[T]
}
=== Test.scala ===
case class C(x: Int = 2, y: String, z: Boolean = true)(t: String = "hello")
object Test extends App {
println(Macros.extractor[C])
}
17:10 ~/Projects/Paradise2103/sandbox/src/main/scala (2.10.3)$ scalac Macros.scala && scalac Test.scala && scala Test
Map(x -> 2, z -> true)