Scala macro how to convert a MethodSymbol to DefDef with parameter default values?

Question

The macros I’ve been writing typically need to process all the defs from other objects/modules that live in libraries etc so the trees are not available. Inside the macro, I often need to transform a MethodSymbol obtained from the other object, into a new API call in my target trait that replicates the function signature including default parameters.

However, I’ve not figured out a way to obtain default parameter values for the parameters in the MethodSymbol so when I build up the result, I cannot “replicate” some of the method signature as I would like.

Is there a way to obtain the default parameters, say for parameters obtained from the “members” property of the symbol’s “type”, for the parameters in the MethodSymbol?

Answer 1

You can use the following technique

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

def foo[A]: Unit = macro impl[A]

def impl[A: c.WeakTypeTag](c: blackbox.Context): c.Tree = {
  import c.universe._

  // just for testing, in actual use case methodSymbol is obtained from somewhere 
  val methodSymbol = weakTypeOf[A].decl(TermName("smth")).asMethod

  val traverser = new Traverser {
    override def traverse(tree: Tree): Unit = {
      tree match {
        case t @ q"$mods def $tname[..$tparams](...$paramss): $tpt = $expr" if t.symbol == methodSymbol =>
          paramss.flatten.foreach {
            case q"$mods1 val $tname1: $tpt1 = $expr1" =>
              println(s"method $tname: default value of $tname1 is $expr1")
            case _ =>
          }
        case _ =>
      }
      super.traverse(tree)
    }
  }

  c.enclosingRun.units.foreach(unit =>
    traverser.traverse(unit.body)
  )

  q"()"
}

object App {

  class MyClass {
    def smth(x: Int = 1): Unit = ()
  }

  foo[MyClass] //Warning:scalac: method smth: default value of x is 1
}