I wrote a macros, that reads class fields:
import scala.language.experimental.macros
import scala.reflect.macros.whitebox
object ArrayLikeFields {
def extract[T]: Set[String] = macro extractImpl[T]
def extractImpl[T: c.WeakTypeTag](c: whitebox.Context): c.Expr[Set[String]] = {
import c.universe._
val tree = weakTypeOf[T].decls
.collectFirst {
case m: MethodSymbol if m.isPrimaryConstructor => m
}
.map(y => y.paramLists.headOption.getOrElse(Seq.empty))
.getOrElse(Seq.empty)
.map(s => q"${s.name.decodedName.toString}")
c.Expr[Set[String]] {
q"""Set(..$tree)"""
}
}
}
I'm able to compile and run it for concrete type:
object Main extends App {
case class Person(name:String)
val res: Set[String] = ArrayLikeFields.extract[Person]
}
But i want use it with generic types like that:
object Lib {
implicit class SomeImplicit(s: String) {
def toOrgJson[T]: JSONObject = {
val arrayLikeFields: Set[String] = ArrayLikeFields.extract[T]
//some code, that uses fields, etc
null
}
}
}
Compilation error:
Error:(14, 65) type mismatch; found : scala.collection.immutable.Set[Nothing] required: Set[String] Note: Nothing <: String, but trait Set is invariant in type A. You may wish to investigate a wildcard type such as
_ <: String
. (SLS 3.2.10) val arrayLikeFields: Set[String] = ArrayLikeFields.extract[T]
I can't understund that. How can I solve my problem?
upd
I read scala 2.10.2 calling a 'macro method' with generic type not work about materialisation, but i have no instance of class