Scala recursive macro?

Question

I was wondering whether Scala supports recursive macro expansion e.g. I am trying to write a lens library with a lensing macro that does this:

case class C(d: Int)
case class B(c: C)
case class A(b: B)

val a = A(B(C(10))
val aa = lens(a)(_.b.c.d)(_ + 12)
assert(aa.b.c.d == 22)

Given lens(a)(_.b.c.d)(f), I want to transforms it to a.copy(b = lens(a.b)(_.c.d)(f))

EDIT: I made some decent progress here

However, I cannot figure out a generic way to create an accessor out of List[TermName] e.g. for the above example, given that I have List(TermName('b'), TermName('c'), TermName('d'))), I want to generate an anonymous function _.b.c.d i.e. (x: A) => x.b.c.d. How do I do that?

Basically, how can I write these lines in a generic fashion?

Answer 1

Actually I managed to make it work: https://github.com/pathikrit/sauron/blob/master/src/main/scala/com/github/pathikrit/sauron/package.scala

Here is the complete source:

package com.github.pathikrit

import scala.reflect.macros.blackbox

package object sauron {

  def lens[A, B](obj: A)(path: A => B)(modifier: B => B): A = macro lensImpl[A, B]

  def lensImpl[A, B](c: blackbox.Context)(obj: c.Expr[A])(path: c.Expr[A => B])(modifier: c.Expr[B => B]): c.Tree = {
    import c.universe._

    def split(accessor: c.Tree): List[c.TermName] = accessor match {    // (_.p.q.r) -> List(p, q, r)
      case q"$pq.$r" => split(pq) :+ r
      case _: Ident => Nil
      case _ => c.abort(c.enclosingPosition, s"Unsupported path element: $accessor")
    }

    def join(pathTerms: List[TermName]): c.Tree = (q"(x => x)" /: pathTerms) {    // List(p, q, r) -> (_.p.q.r)
      case (q"($arg) => $pq", r) => q"($arg) => $pq.$r"
    }

    path.tree match {
      case q"($_) => $accessor" => split(accessor) match {
        case p :: ps => q"$obj.copy($p = lens($obj.$p)(${join(ps)})($modifier))"  // lens(a)(_.b.c)(f) = a.copy(b = lens(a.b)(_.c)(f))
        case Nil => q"$modifier($obj)"                                            // lens(x)(_)(f) = f(x)
      }
      case _ => c.abort(c.enclosingPosition, s"Path must have shape: _.a.b.c.(...), got: ${path.tree}")
    }
  }
}

And, yes, Scala does apply the same macro recursively.