Dynamic mixin in Scala - is it possible?

Question

What I\'d like to achieve is having a proper implementation for

def dynamix[A, B](a: A): A with B

I may know what B is, but don\'t know wha

Answer 1

I wanted to be able to construct Scala beans in my Spring application context, but I also wanted to be able to specify the mixins to be included in the constructed bean:

The difficulty is that Class.forName function does not allow me to specify the mixins. In the end, I extended the above hacky solution to Scala 2.9.1. So, here it is in its full gory; including bits of Spring.

class ScalaBeanFactory(private val beanType: Class[_ <: AnyRef],
                       private val mixinTypes: Seq[Class[_ <: AnyRef]]) {
  val loader = new DynamicClassLoader
  val clazz = loader.buildClass(beanType, mixinTypes)

   def getTypedObject[T] = getObject.asInstanceOf[T]

   def getObject = {
     clazz.newInstance()
   }

   def getObjectType = null
   def isSingleton = true

object DynamicClassLoader {
  private var id = 0
  def uniqueId = synchronized {  id += 1; "Klass" + id.toString }
}

class DynamicClassLoader extends java.lang.ClassLoader(getClass.getClassLoader) {

  def buildClass(t: Class[_ <: AnyRef], vs: Seq[Class[_ <: AnyRef]]) = {
    val id = DynamicClassLoader.uniqueId

    val classDef = new StringBuilder

    classDef.append("class ").append(id)
    classDef.append(" extends ").append(t.getCanonicalName)
    vs.foreach(c => classDef.append(" with %s".format(c.getCanonicalName)))

    val settings = new Settings(null)
    settings.usejavacp.value = true
    val interpreter = new IMain(settings)


    interpreter.compileString(classDef.toString())


    val r = interpreter.classLoader.getResourceAsStream(id)
    val o = new ByteArrayOutputStream
    val b = new Array[Byte](16384)
    Stream.continually(r.read(b)).takeWhile(_ > 0).foreach(o.write(b, 0, _))
    val bytes = o.toByteArray

    defineClass(id, bytes, 0, bytes.length)
  }

}

The code cannot yet deal with constructors with parameters and does not copy annotations from the parent class’s constructor (should it do that?). However, it gives us a good starting point that is usable in the scala Spring namespace. Of course, don’t just take my word for it, verify it in a Specs2 specification:

class ScalaBeanFactorySpec extends Specification {

  "getTypedObject mixes-in the specified traits" in {
    val f1 = new ScalaBeanFactory(classOf[Cat],
                                  Seq(classOf[Speaking], classOf[Eating]))

    val c1 = f1.getTypedObject[Cat with Eating with Speaking]

    c1.isInstanceOf[Cat with Eating with Speaking] must_==(true)

    c1.speak    // in trait Speaking
    c1.eat      // in trait Eating
    c1.meow     // in class Cat
  }

}