Get companion object instance with new Scala reflection API

Question

With Scala's new reflection API, is it possible to get a reference to a class's companion object? I am thinking something along these lines:

trait Base {
  def companion: MetaBase = someReflectionMagic(this).asInstanceOf[MetaBase]
}

trait MetaBase {
  // stuff
}

// ---

class Foo extends Base

object Foo extends MetaBase

assert(new Foo.companion == Foo)

Solution 1

Dave! Thanks for being interested in the new reflection. Early adopters have driven the development process of reflection and macros to a significant extent, and I'm very happy about being a part of our amazing community.

Before answering your question, I'd like to start with a disclaimer. In 2.10.0-M4 we have just laid the foundations of the Scala reflection API. It's still hot off the press, so the docs are very scarce and the API isn't exactly riddled with conveniences. It works, but it requires testing and feedback. Sure, messing with pre-release APIs is troublesome, but I'm always here to help.

So far we have a draft of what will in the future become the reflection SIP: https://docs.google.com/document/d/1Z1VhhNPplbUpaZPIYdc0_EUv5RiGQ2X4oqp0i-vz1qw/edit#heading=h.pqwdkl1226tc. You can read it right away, or can first skim through my answer below.

trait Base {
  def companion: MetaBase = {
    // runtime reflection is typically done
    // by importing things from scala.reflect.runtime package
    import scala.reflect.runtime._

    // the new Scala reflection API is mirror based
    // mirrors constitute a hierarchy of objects
    // that closely follows the hierarchy of the things they reflect
    // for example, for a class you'll have a ClassMirror
    // for a method you'll have a MethodMirror and so on
    // why go the extra mile?
    // because this provides more flexibility than traditional approaches
    // you can read more about mirror-based designs here:
    // https://dl.dropbox.com/u/10497693/Library/Computer%20Science/Metaprogramming/Reflection/mirrors.pdf
    // https://dl.dropbox.com/u/10497693/Library/Computer%20Science/Metaprogramming/Reflection/reflecting-scala.pdf

    // bottom line is that to do anything you will need a mirror
    // for example, in your case, you need a ClassMirror

    // remember I said that mirrors provide more flexibility?
    // for one, this means that mirror-based reflection facilities
    // might have multiple implementations
    // in a paper linked above, Gilad Bracha muses over a runtime
    // that loads things remotely over the network
    // in our case we might have different mirrors for JVM and CLR
    // well, anyways

    // the canonical (and the only one now) implementation of the mirror API
    // is Java-based reflection that uses out of the box classloaders
    // here's its root: https://github.com/scalamacros/kepler/blob/9f71e9f114c10b52350c6c4ec757159f06e55daa/src/reflect/scala/reflect/api/Mirrors.scala#L178
    // yeah, right, I've just linked a source file from trunk
    // we'll have Scaladocs for that soon, but for now take a look
    // this file is interfaces-only and is heavy on comments

    // to start with Java-based reflection implementation you need a classloader
    // let's grab one and instantiate the root mirror
    // btw, the same effect could be achieved by writing
    // `scala.reflect.runtime.currentMirror`
    val rootMirror = universe.runtimeMirror(getClass.getClassLoader)

    // now when we've finally entered the reflective world
    // we can get the stuff done
    // first we obtain a ClassSymbol that corresponds to the current instance
    // (ClassSymbols are to Scala the same as Classes are to Java)
    var classSymbol = rootMirror.classSymbol(getClass)

    // having a Scala reflection entity
    // we can obtain its reflection using the rootMirror
    val classMirror = rootMirror.reflectClass(classSymbol)

    // now we just traverse the conceptual hierarchy of mirrors
    // that closely follows the hierarchy of Scala reflection concepts
    // for example, a ClassMirror has a companion ModuleMirror and vice versa
    val moduleMirror = classMirror.companion.get

    // finally, we've arrived at our destination
    moduleMirror.instance.asInstanceOf[MetaBase]
  }
}

trait MetaBase {
  // stuff
}

// ---

class Foo extends Base

object Foo extends MetaBase

object Test extends App {
  assert(new Foo().companion == Foo)
}

Update. Please also see the excellent post by Daniel Sobral: http://dcsobral.blogspot.ch/2012/07/json-serialization-with-reflection-in.html.

Solution 2

I didn't see Eugene's last comment and came up with this. It works for scala 2.10.

trait ReflectionSugars{
  import scala.reflect.runtime.{universe => ru}
  private lazy val universeMirror = ru.runtimeMirror(getClass.getClassLoader)

  def companionOf[T](implicit tt: ru.TypeTag[T])  = {
    val companionMirror = universeMirror.reflectModule(ru.typeOf[T].typeSymbol.companionSymbol.asModule)
    companionMirror.instance
  }

}

trait X extends ReflectionSugars{
   def companion = companionOf[X]
}

https://gist.github.com/piotrga/5928581

I hope this helps!

Solution 3

Scala 2.13.3:

import scala.reflect.runtime.{universe => ru}

def determineCompanionObject[T](implicit typetag: ru.TypeTag[T]): Unit = {

  val m = ru.runtimeMirror(getClass.getClassLoader)

  val o1 = m.reflectModule(ru.typeOf[T].typeSymbol.companion.asModule).instance
  // o1 is the companion object!

}

invoke with, e.g.

determineCompanionObject[MyClass01]