Scala Macros: Accessing members with quasiquotes

Question

I'm trying to implement an implicit materializer as described here: http://docs.scala-lang.org/overviews/macros/implicits.html

I decided to create a macro that converts a case class from and to a String using quasiquotes for prototyping purposes. For example:

case class User(id: String, name: String)
val foo = User("testid", "foo")

Converting foo to text should result in "testid foo" and vice versa.

Here is the simple trait and its companion object I have created:

trait TextConvertible[T] {
  def convertTo(obj: T): String
  def convertFrom(text: String): T
}

object TextConvertible {
  import language.experimental.macros
  import QuasiTest.materializeTextConvertible_impl
  implicit def materializeTextConvertible[T]: TextConvertible[T] = macro materializeTextConvertible_impl[T]
}

and here is the macro:

object QuasiTest {
  import reflect.macros._

  def materializeTextConvertible_impl[T: c.WeakTypeTag](c: Context): c.Expr[TextConvertible[T]] = {
    import c.universe._
    val tpe = weakTypeOf[T]

    val fields = tpe.declarations.collect {
      case field if field.isMethod && field.asMethod.isCaseAccessor => field.asMethod.accessed
    }

    val strConvertTo = fields.map {
      field => q"obj.$field"
    }.reduce[Tree] {
      case (acc, elem) => q"""$acc + " " + $elem"""
    }

    val strConvertFrom = fields.zipWithIndex map {
      case (field, index) => q"splitted($index)"
    }

    val quasi = q"""
      new TextConvertible[$tpe] {
        def convertTo(obj: $tpe) = $strConvertTo
        def convertFrom(text: String) = {
          val splitted = text.split(" ")
          new $tpe(..$strConvertFrom)
        }
      }
    """

    c.Expr[TextConvertible[T]](quasi)
  }
}

which generates

{
  final class $anon extends TextConvertible[User] {
    def <init>() = {
      super.<init>();
      ()
    };
    def convertTo(obj: User) = obj.id.$plus(" ").$plus(obj.name);
    def convertFrom(text: String) = {
      val splitted = text.split(" ");
      new User(splitted(0), splitted(1))
    }
  };
  new $anon()
}

The generated code looks fine, but yet I get the error value id in class User cannot be accessed in User in compilation while trying to use the macro.

I suspect I am using a wrong type for fields. I tried field.asMethod.accessed.name, but it results in def convertTo(obj: User) = obj.id .$plus(" ").$plus(obj.name ); (note the extra spaces after id and name), which naturally results in the error value id is not a member of User.

What am I doing wrong?

Emre Emre · Accepted Answer · 2013-10-23T14:43:51

Ah, figured it out almost immediately after sending my question.

I changed the lines

val fields = tpe.declarations.collect {
  case field if field.isMethod && field.asMethod.isCaseAccessor => field.asMethod.accessed
}

to

val fields = tpe.declarations.collect {
  case field if field.isMethod && field.asMethod.isCaseAccessor => field.name
}

which solved the problem.

Scala Macros: Accessing members with quasiquotes

2 Answers