Macro to access source code text at runtime

Question

Is there already or is it possible to have a Scala macro that gives me access to the text of the source? For instance I would like to write code like this:

v         


        

Answer 1

I was not able to reuse withSource directly to just print the source and value and return the value. The withSource macro cannot be utilized from the same object itself (so I cannot just add my slightly modified version of withSource right in that file) and I am not able to call withSource from a subclass of WithSourceHelper, limiting reuse through inheritance.

In case anyone is interested, here is a complement to Senia's answer to just log the value with the source and return the value so the rest of the computation can occur.

def logValueImpl[T](c: Context): c.Expr[T] = {
  import c.universe._
  val source = c.prefix.tree match {
    case Apply(_, List(s)) => s
    case _ => c.abort(c.enclosingPosition, "can't find source")
  }
  val freshName = newTermName(c.fresh("logValue$"))
  val valDef = ValDef(Modifiers(), freshName, TypeTree(source.tpe), source)
  val ident = Ident(freshName)
  val print = reify{
    println(c.literal(show(source)).splice + ": " + c.Expr[T](ident).splice) }
  c.Expr[T](Block(List(valDef, print.tree), ident))
}

I then define it as an implicit conversion on def p = macro Debug.logValueImpl[T]. I can then use like this:

List(1, 2, 3).reverse.p.head 
// prints: immutable.this.List.apply[Int](1, 2, 3).reverse: List(3, 2, 1)

The funny part is that I can apply it twice:

List(1, 2, 3).reverse.p.p

And it will show me what the logValueImpl macro did:

{
  val logValue$7: List[Int] = immutable.this.List.apply[Int](1, 2, 3).reverse;
  Predef.println("immutable.this.List.apply[Int](1, 2, 3).reverse: ".+(logValue$7));
  logValue$7
}

It seems to work with other macros as well:

f"float ${1.3f}%3.2f; str ${"foo".reverse}%s%n".p`
//prints:
{
  val arg$1: Float = 1.3;
  val arg$2: Any = scala.this.Predef.augmentString("foo").reverse;
  scala.this.Predef.augmentString("float %3.2f; str %s%%n").format(arg$1, arg$2)
}: float 1.30; str oof%n

Even more interestingly if I used showRaw instead of show I can even see the tree of the expanded macro, which may come handy to figure out how to write other macros.