Is it possible to have a Map[String,Any] with the name and the values of named parameters in Scala?

Question

I am writing a wrapper to a REST web service and I'd like to have strongly typed Scala APIs.

The following is what I am doing so far:

def getMentions(count: Option[Int] = None,
                sinceID: Option[TweetID] = None,
                maxID: Option[TweetID] = None,
                trimUser: Option[Boolean] = None,
                contributorDetails: Option[Boolean] = None,
                includeEntities: Option[Boolean] = None) : List[Tweet] = {
val parameters = Map("count" -> count,
                     "since_id" -> sinceID,
                     "max_id" -> maxID,
                     "trim_user" -> trimUser,
                     "contributor_details" -> contributorDetails,
                     "include_entities" -> includeEntities)
/* 
 * Convert parameters, which is a Map[String,Any] to a Map[String,String]
 * (Removing Nones) and pass it to an object in charge of generating the request.
 */
...
}

This approach is working, but it requires me to manually generate the parameters map. If I were able to access to a Map representing parameters and their values, what I am doing would be much cleaner.

Answer 1

You could do this with runtime reflection, and I'm sure you'll get answers telling you how, if you want that, but this is actually a neat use case for Scala 2.10's macros, so here goes. First assume we have a file named ParamMapMaker.scala:

object ParamMapMaker {
  def paramMap: Map[String, Any] = macro paramMapImpl

  def paramMapImpl(c: scala.reflect.macros.Context) = {
    import c.universe._

    val params = c.enclosingMethod match {
      case DefDef(_, _, _, ps :: Nil, _, _) =>
        ps.map(p =>
          reify((
            c.Expr[String](Literal(Constant(p.name.decoded))).splice,
            c.Expr[Any](Ident(p.symbol)).splice
          )).tree
        )
      case _ => c.abort(c.enclosingPosition, "Can't call paramMap here!")
    }

    c.Expr[Map[String, Any]](Apply(Select(Ident("Map"), "apply"), params))
  }
}

I'll leave snake casing the map keys as an (easy) exercise for the reader.

We also have a test file (named Test.scala):

object Test extends App {
  def foo(hello: String, answer: Int) = ParamMapMaker.paramMap

  println(foo("world", 42))
}

Now we compile both of these:

scalac -language:experimental.macros ParamMapMaker.scala
scalac Test.scala

And when we run Test we'll get the following:

Map(hello -> world, answer -> 42)

The nifty thing about this is that there's none of the overhead of runtime reflection. If we compile the test file with -Ymacro-debug-verbose, we see that the following code has been generated (in effect) for the body of foo at compile time:

Map.apply[String, Any](
  scala.Tuple2.apply[String, String]("hello", hello),
  scala.Tuple2.apply[String, Int]("answer", answer)
)

Exactly as we'd expect.