How do I access default parameter values via Scala reflection?

Question

Let\'s say a I have a class:

case class Foo(id: Int, name: String, note: Option[String] = None)

Both the constructor and the apply method i

Answer 1

For case when you only have Type of Foo:

% scala
Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 15-loom).

scala> :paste

import scala.reflect.runtime.currentMirror
import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe.definitions._

def defaults(tpe: Type): Seq[(Int, Any)] = {
  tpe.typeSymbol.asClass.primaryConstructor.asMethod.paramLists.flatten.
    zipWithIndex.flatMap{ case (x, i) =>
    if (x.asTerm.isParamWithDefault) {
      val m = currentMirror
      val im = m.reflect(
        m.reflectModule(tpe.typeSymbol.asClass.companion.asModule).instance
      )
      val method = tpe.companion.decl(
        TermName("apply$default$"+(i+1).toString)
      ).asMethod
      val v = im.reflectMethod(method)()
      Some(i -> v)
    } else None
  }
}

scala> case class Foo(id: Int, name: String, note: Option[String] = None)

scala> defaults(typeOf[Foo])
res0: Seq[(Int, Any)] = List((2,None))

The same way you can call any method on companion object.

Documentation about Mirrors contains example how to invoke method on class.