Initial attempt at using Tony Morris's ReaderWriterStateT monad transformer (see https://gist.github.com/2360532)

RWS.scala

object RWS extends App {

  trait Pointed[P[_]] {
    def point[A](a: A): P[A]
  }

  trait PointedFunctor[PF[_]] extends Pointed[PF] with Functor[PF]

  implicit def listIsMonoid[A]: Monoid[List[A]] = new Monoid[List[A]] {
    def id: List[A] = Nil
    def op(a1: List[A], a2: List[A]) = a1 ++ a2
  }

  implicit def listIsPointed: Pointed[List] = new Pointed[List] {
    def point[A](a: A) = List(a)
  }

  implicit def idIsPointedFunctor[A]: PointedFunctor[Id] = new PointedFunctor[Id] {
    def point[A](a: A) = Id(a)
    def fmap[A, B](f: A => B) =
      i => Id(f(i.a))
  }

  case class Config(port: Int)

  def log[R, S, F[_]](msg: String)(implicit FF: PointedFunctor[F]): ReaderWriterStateT[R, List[String], S, F, Unit] = ReaderWriterStateT { case (r, s) =>
    FF.point((List(msg.format(r, s)), (), s))
  }

  def invokeService: ReaderWriterStateT[Config, List[String], Int, Id, Int] = {
    ReaderWriterStateT { case (cfg, invocationCount) =>
      Id((List("Invoking service with port " + cfg.port), util.Random.nextInt(100), invocationCount + 1))
    }
  }

  val prg = for {
    _ <- log("Start - r: %s, s: %s")
    res <- invokeService
    _ <- log("Between - r: %s, s: %s")
    _ <- invokeService
    _ <- log("Done - r: %s, s: %s")
  } yield res

  val Id((logMessages, result, invocationCount)) = prg run (Config(port = 443), 0)
  println("Result: " + result)
  println("Service invocations: " + invocationCount)
  println("Log:%n%s".format(logMessages.mkString("\t", "%n\t".format(), "")))
}