Build a logging monad with scalaz, apply it to futures and reproduce functionality of the futures monad.

LoggingMonadTransformerPOC.scala

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future
import scala.language.higherKinds
import scalaz._
import Scalaz._

object LoggingMonadTransformerPOC extends App {

  type LoggedT[F[_], A] = WriterT[F, List[String], A]
  type LoggedFuture[A] = LoggedT[Future, A]

  //this is the applicative behavior that we want WriterT to preserve
  val example = for {
    z <- (Future("left") |@| Future("right")) { (x: String, y: String) => x + " " + y }
  } yield z

  example.onSuccess { case x => println(x) }

  val test: LoggedFuture[String] = for {
    z <- (Future("left").liftM[LoggedT] |@| Future("right").liftM[LoggedT]) { (x: String, y: String) => x + " " + y }
  } yield z

  test.value.onSuccess { case x => println(x) }

  //good interop with collections is also a must
  val example2 = Future.traverse(0 to 10) { i => Future(i) }

  example2.onSuccess { case x => println(x) }

  val test2:LoggedFuture[Vector[Int]] = (0 to 10).toVector.traverse[LoggedFuture, Int] {
    i: Int => Future(i).liftM[LoggedT]
  }

  test2.value.onSuccess { case x => println(x) }

}

Tested with scala version 2.11.7 and scalaz version 7.2.0