Instantiate a class with Scala Macro or reflection

Question

On my scala code, I want to be able to instantiate a new class. For instance, supose I have the code below:

class Foo { def foo=10 }
trait Bar { val bar=20 }

Ideally, I want to be able to do something like:

def newInstance[A <: Foo] = { new A with Bar }
newInstance[Foo]

But, of course this doesn't work. I tried to use reflection to instantiate a class, but it seems that I'm only able to instantiate a new class (and not mix-in with a trait). I think it would be possible to make this work using Macros, but I'm not sure even where to start.

What I'm trying to do is like the following Ruby code:

class SomeClass
  def create
    self.class.new
  end
end

class Other < SomeClass
end

Other.new.create # <- this returns a new Other instance

Is it possible?

Travis Brown · Accepted Answer · 2013-09-16 23:08:32Z

11

With a macro:

import scala.language.experimental.macros
import scala.reflect.macros.Context

object MacroExample {
  def newInstance[A <: Foo]: A with Bar = macro newInstance_impl[A]

  def newInstance_impl[A <: Foo](c: Context)(implicit A: c.WeakTypeTag[A]) = {
    import c.universe._

    c.Expr[A with Bar](q"new $A with Bar")
  }
}

This will work as expected, and will fail at compile time if you try to instantiate a class that doesn't have a no-argument constructor.

I've used quasiquotes here for the sake of clarity, but you could build the tree manually with a little more work. There's not really any good reason to, though, now that quasiquotes are available as a plugin for Scala 2.10.

answered Sep 16, 2013 at 23:08

Travis Brown

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5 Comments

Maurício Szabo Over a year ago

Is it possible to overcome erasure on this solution, for example: "class NewClass[A] { MacroExample.newInstance[A] }" doesn't work...

Maurício Szabo Over a year ago

Other thing, I didn't found a way to replicate the "Ruby" behavior of extending SomeClass with Other and instantiating the "Other" class in the subclass...

Travis Brown Over a year ago

@MaurícioSzabo: About your first question: the problem isn't exactly erasure—it's that A (even with the <: Foo constraint) isn't known to be a class type. If you want a nicer error message you can check that weakTypeOf[A].typeSymbol is a ClassSymbol.

Travis Brown Over a year ago

@MaurícioSzabo: If SomeClass and Other are both classes, there's no way to inherit from both of them in Scala (even with macros).

Maurício Szabo Over a year ago

I know there is no way to inherit from both of them. What I wanted is to make a "clone-like" method like the one I showed in Ruby: define a "clone" method on a superclass and subclasses automatically understands that they need to return the Subclass, not the Superclass (provided, of course, that the class implements a no-arguments constructor)

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