Question

I’m trying to create an instance of a trait using this method

val inst = new Object with MyTrait

This works well, but I’d like to move this creation in to a generator function, ie.

object Creator {
  def create[T] : T = new Object with T
}

I’m obviously going to need the manifest to somehow fix the type erasure problems, but before I get to this, I run in to 2 questions :

1. Even with an implicit manifest, Scala still demands that T be a trait. How do I add a restriction to create[T] so that T is a trait?

2. If I chose to use the Class.newInstance method to create the instance dynamically rather than using “new”, how would I specify the “with” in “new Object with T”? Is it possible to dynamically create new concrete mixin types at runtime?

Answer 1

You can’t do this (even with a Manifest). The code new Object with T involves creating a new anonymous class representing the combination of Object with T. To pass this to your create function, you would have to generate this new class (with new bytecode) at runtime, and Scala has no facilities for generating a new class at runtime.

One strategy might be to try to transfer the special functionality of the factory method into the class’s constructor instead, and then use the constructor directly.

Another possible strategy is to create conversion functions (implicit or otherwise) to the traits you’re interested in using with this class.