Scala macros: constructing an anonymous class

Question

I am fairly new to Scala macros, and am trying to write a very basic DSL.

I have the following Scala class:

abstract class SpecialFunction {
  def apply(): Unit
}

and the following Scala macro:

def mImpl(c: Context)(bodyBlock: c.Expr[Unit]): c.Expr[X] =
  c.universe.reify {
    new X(new SpecialFunction {
      override def apply() {
        bodyBlock.splice
      }
    })
  }

def m(bodyBlock: Unit): X = macro mImpl

So far so good. For example, it allows me to write:

def example = m {
  println("Hello, world")
}

And this compiles down to:

def example = new X(new SpecialFunction {
  override def apply() {
    println("Hello, world")
  }
})

But this formulation does not allow me to have local variables in such an "m block". For example, I cannot write:

def example = m {
  val x = 7
  println(x.toString)
}

At compile time I get the error:

symbol value x does not exist in example

However, what I want to achieve is this:

def example = new X(new SpecialFunction {
  override def apply() {
    val x = 7
    println(x.toString)
  }
})

(I think I understand why this is the case: sub-expressions are evaluated before being passed to the macro, and thus the reference to x is invalid).

So my question is this: How can I get the above to work? (I just want the extra code defined in the macro to be "copy-pasted" around the code in the "m block", like in a C++ macro.)

Any help would be greatly appreciated :-)

Answer 1

Your output expression is keeping a reference to the old x symbol, but instead it should be a reference to a new one. Therefore, you can reset all the junk references in the tree you aim to reuse by applying resetLocalAttrs from Context .

How about...?

def mImpl(c: Context)(bodyBlock: c.Expr[Unit]): c.Expr[X] =
  c.universe.reify {
    new X(new SpecialFunction {
      override def apply() {
        c.Expr[Unit](c.resetLocalAttrs(bodyBlock.tree)).splice
      }
    })
  }

In this question , you can read a great explanation by the Scala Macros guy.