Instantiate class symbol using macro - scala

Question

I'm using the following Scala macro (heavily inspired by the code from this SO question) to get a list of all objects contained in a given package that inherit a specific trait:
object Macros {
def allObjects[T <: AnyRef](packageName: String): List[Any] = macro allObjectsImpl[T]
def allObjectsImpl[T <: AnyRef: c.WeakTypeTag](c: Context)(packageName: c.Expr[String]) = {
import c.universe._
val baseTraitSymbol = c.weakTypeOf[T].typeSymbol
val pkg = packageName.tree match {
case Literal(Constant(name: String)) => c.mirror.staticPackage(name)
}
val types = pkg.typeSignature.members.collect {
case moduleSymbol: ModuleSymbol if moduleSymbol.moduleClass.asClass.baseClasses contains baseTraitSymbol => Ident(moduleSymbol)
}.toList
val listApply = Select(reify(List).tree, TermName("apply"))
c.Expr[List[T]](Apply(listApply, types))
}
}
Which works fine.
I want to change the macro so that instead of getting all the objects in a package, it gets all the concrete classes, and provides a list containing an instance of each of them.
The AST when creating an instance of an Object looks like this:
scala> import scala.reflect.runtime.{universe => u}
import scala.reflect.runtime.{universe=>u}
scala> u showRaw ( u reify {new Object} )
res42: String = Expr(Apply(Select(New(Ident(java.lang.Object)), termNames.CONSTRUCTOR), List()))
So I thought changing my code to this would work:
object Macros {
def allInstances[T <: AnyRef](packageName: String): List[Any] = macro allInstancesImpl[T]
def allInstancesImpl[T <: AnyRef: c.WeakTypeTag](c: Context)(packageName: c.Expr[String]) = {
import c.universe._
val baseTraitSymbol = c.weakTypeOf[T].typeSymbol
val pkg = packageName.tree match {
case Literal(Constant(name: String)) => c.mirror.staticPackage(name)
}
def isConcreteChildClass(child: ClassSymbol, base: Symbol) = {
!child.isAbstract && (child.baseClasses contains base)
}
val types = pkg.typeSignature.members.collect {
case classSymbol: ClassSymbol if isConcreteChildClass(classSymbol, baseTraitSymbol) => {
Apply(Select(New(Ident(classSymbol.primaryConstructor)), termNames.CONSTRUCTOR), List())
}
}.toList
val listApply = Select(reify(List).tree, TermName("apply"))
c.Expr[List[T]](Apply(listApply, types))
}
}
However, when I try to use the updated macro code on a test package, I get the following error:
scala> Macros.allInstances[AnyRef]("test")
<console>:9: error: class type required but ()test.TestClass found
Macros.allInstances[AnyRef]("test")
From what I'm seeing, it looks like the macro is actually returning the constructor itself instead of returning the instance that should get built by the constructor, but I can't figure out what I'm missing.

Answer 1

The problem is in this line (reformatted for clarity):
Apply(
Select(New(Ident(classSymbol.primaryConstructor)), termNames.CONSTRUCTOR),
List()
)
You're essentially selecting the constructor twice. You could just drop primaryConstructor:
Apply(
Select(New(Ident(classSymbol)), termNames.CONSTRUCTOR),
List()
)
Using ApplyConstructor would also work:
ApplyConstructor(Ident(classSymbol), Nil)
Or you could just go with quasiquotes:
q"new ${Ident(classSymbol)}()"
The quasiquote solution is the most future-proof.