How to create a Macro to create List of val in a case class?

Question

I am trying to create a Macro to give me a list of val for a specific case class.

object CaseClass {

  def valList[T]: List[String] = macro implValList[T]

  def implValList[T](c: whitebox.Context): c.Expr[List[String]] = {
    import c.universe._

    val listApply = Select(reify(List).tree, TermName("apply"))

    val vals = weakTypeOf[T].decls.collect {
      case m: TermSymbol if m.isVal => q"${m.name}"
    }

    c.Expr[List[String]](Apply(listApply, vals.toList))
  }

}

So given

case class AClass(
   val a: String,
   val b: Int
)

I want a list of CaseClass.valList[AClass] = List("a", "b")

Answer 1

Not an expert on macros, so take it with a grain of salt. But I tested it with Intellij.

First, to use weakTypeOf you need to take a WeakTypeTag as an implicit in your macro impl like this:

def implValList[T](c: whitebox.Context)(implicit wt: c.WeakTypeTag[T]) ...

Second, to create literals, you use this construct instead of your quasiquote, (which, I believe, actually does nothing):

Literal(Constant(m.name.toString))

Last, I recommend using this guard instead of isVal:

m.isCaseAccessor && m.isGetter

Which is properly checking for case class parameter and also being a getter (case class parameters are duplicated, one as isGetter, other one as isParam). The reason for this being that isVal names for case classes surprisingly produce a name ending in whitespace.

The final implementation that works for me is as follows:

object CaseClass {

  def valList[T]: List[String] = macro implValList[T]

  def implValList[T](c: whitebox.Context)(implicit wt: c.WeakTypeTag[T]): c.Expr[List[String]] = {
    import c.universe._

    val listApply = Select(reify(List).tree, TermName("apply"))

    val vals = weakTypeOf[T].decls.collect {
      case m: TermSymbol if m.isCaseAccessor && m.isGetter => Literal(Constant(m.name.toString))
    }

    c.Expr[List[String]](Apply(listApply, vals.toList))
  }

}

As an alternative (because macros are somewhat of a pain to set up - you cannot use macro in the same subproject that defines it), and you don't need it very often, you might be able to get away with a shapeless one-liner:

import shapeless._
import shapeless.ops.record.Keys

case class Foo(a: Int, b: String)

Keys[the.`LabelledGeneric[Foo]`.Repr].apply().toList.map(_.name) // List("a", "b")