How can parameters/settings be passed to a Scala macro?

Question

How can parameters/settings be passed to a Scala macro?

These settings should not be global, but per macro invocation.

What I would like to have is something similar to this:

def a(param: Int) = macro internalMacro("setting 1")
def b(param: Int) = macro internalMacro("setting 2")

whereas setting 1 and setting 2 should then be constant values, accessible from within the macro, so I can make the internal behavior dependent on them.

Answer 1

The parameter lists of your method and the macro definition have to line up exactly, with the macro definition method having an extra initial parameter list for the context, and every other parameter from the method definition having the same name and the same type wrapped in a c.Expr. (Note however that the type parameter lists can differ.)

This means that you can't pass information to the macro implementation as arguments in your method definition. You can use a static annotation to accomplish the same thing, though (this is a trick I learned from Eugene Burmako, who used it to implement structural types without reflective access, a.k.a. vampire methods):

import scala.annotation.StaticAnnotation
import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context

class setting(value: String) extends StaticAnnotation

def internalMacro(c: Context)(param: c.Expr[Int]) = {
  import c.universe._
  val settingValue = c.macroApplication.symbol.annotations.filter(
    _.tree.tpe <:< typeOf[setting]
  ).headOption.flatMap(
    _.tree.children.tail.collectFirst {
      case Literal(Constant(s: String)) => s
    }
  ).getOrElse(
    c.abort(c.enclosingPosition, "Annotation body not provided!")
  )
  settingValue match {
    case "setting 1" => c.Expr(q"42")
    case _ => param
  }
}

And then:

scala> @setting("setting 1") def a(param: Int): Int = macro internalMacro
defined term macro a: (param: Int)Int

scala> @setting("setting 2") def b(param: Int): Int = macro internalMacro
defined term macro b: (param: Int)Int

scala> def c(param: Int): Int = macro internalMacro
defined term macro c: (param: Int)Int

scala> a(10)
res0: Int = 42

scala> b(10)
res1: Int = 10

scala> c(10)
<console>:22: error: Annotation body not provided!
              c(10)
               ^

And I didn't even examine any enclosing trees. See my blog post here for an example of this approach in action.