Question

I add variables with Dynamic from scala 2.10.0-RC1 like this:

import language.dynamics
import scala.collection.mutable.HashMap

object Main extends Dynamic {
  private val map = new HashMap[String, Any]
  def selectDynamic(name: String): Any = {return map(name)}
  def updateDynamic(name:String)(value: Any) = {map(name) = value}
}

val fig = new Figure(...)  // has a method number

Main.figname = fig

Now, if I want to access Main.figname.number it doesn’t work, because the compiler thinks it’s of type Any.

But it’s also Main.figname.isInstanceOf[Figure] == true, so it’s Any and Figure, but doesn’t have Figures abilities. Now I can cast it like, Main.figname.asInstanceOf[Figure].number and it works! This is ugly! And I can’t present this to my domain users (I’d like to build a internal DSL.)

Note: If I use instead of Any the supertype of Figure it doesn’t work either.

Is this a bug in scala 2.10, or a feature?

Answer 1

It is quite logical. You are explicitly returning instances of Any. A workaround would be to have instances of Dynamic all along:

import language.dynamics
import scala.collection.mutable.HashMap
import scala.reflect.ClassTag

trait DynamicBase extends Dynamic {
  def as[T:ClassTag]: T
  def selectDynamic[T](name: String): DynamicBase
  def updateDynamic(name:String)(value: Any)
}

class ReflectionDynamic( val self: Any ) extends DynamicBase with Proxy {
  def as[T:ClassTag]: T = { implicitly[ClassTag[T]].runtimeClass.asInstanceOf[Class[T]].cast( self ) }
   // TODO: cache method lookup for faster access + handle NoSuchMethodError
  def selectDynamic[T](name: String): DynamicBase = {
    val ref = self.asInstanceOf[AnyRef]
    val clazz = ref.getClass
    clazz.getMethod(name).invoke( ref ) match {
      case dyn: DynamicBase => dyn
      case res => new ReflectionDynamic( res )
    }
  }
  def updateDynamic( name: String )( value: Any ) = {
    val ref = self.asInstanceOf[AnyRef]
    val clazz = ref.getClass
    // FIXME: check parameter type, and handle overloads
    clazz.getMethods.find(_.getName == name+"_=").foreach{ meth =>
      meth.invoke( ref, value.asInstanceOf[AnyRef] )
    }
  }
}

object Main extends DynamicBase {  
  def as[T:ClassTag]: T = { implicitly[ClassTag[T]].runtimeClass.asInstanceOf[Class[T]].cast( this ) }
  private val map = new HashMap[String, DynamicBase]
  def selectDynamic[T](name: String): DynamicBase = { map(name) }
  def updateDynamic(name:String)(value: Any) = {
    val dyn = value match {
      case dyn: DynamicBase => dyn
      case _ => new ReflectionDynamic( value )
    }
    map(name) = dyn
  }
}

Usage:

scala>     class Figure {
     |       val bla: String = "BLA"
     |     }
defined class Figure
scala> val fig = new Figure()  // has a method number
fig: Figure = Figure@6d1fa2
scala> Main.figname = fig
Main.figname: DynamicBase = Figure@6d1fa2
scala> Main.figname.bla
res40: DynamicBase = BLA

All instances are wrapped in a Dynamic instance.
We can recover the actual type using the as method which performs a dynamic cast.

scala> val myString: String = Main.figname.bla.as[String]
myString: String = BLA