Retrieve the name of the value a Scala macro invocation will be assigned to

Question

I\'m attempting to write a macro that would wrap a function and deducting a parameter from the value its invocation will be assigned to.

object TestMacros {
  de         


        

Answer 1

This is very possible. I know, because I've done something like it before. The trick is to search the enclosing tree for a value whose right-hand side has the same position as the macro application:

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

object TestMacros {
  def foo(name: String): String = name.toUpperCase

  def bar = macro barImpl
  def barImpl(c: Context): c.Expr[String] = {
    import c.universe._

    c.enclosingClass.collect {
      case ValDef(_, name, _, rhs)
        if rhs.pos == c.macroApplication.pos => c.literal(foo(name.decoded))
    }.headOption.getOrElse(
      c.abort(c.enclosingPosition, "Not a valid application.")
    )
  }
}

And then:

scala> object TestUsage { val baz = TestMacros.bar }
defined module TestUsage

scala> TestUsage.baz
res0: String = BAZ

scala> class TestClassUsage { val zab = TestMacros.bar }
defined class TestClassUsage

scala> (new TestClassUsage).zab
res1: String = ZAB

Note that you can apply foo at compile-time, since you know the name of the val at compile-time. If you wanted it to be applied at runtime that would also be possible, of course.