freeMonadServiceOrchestration.scala

// http://underscore.io/blog/posts/2015/04/14/free-monads-are-simple.html
// updated version for scalaz 7.2.x where Free automatically applies the
// Coyoneda transform

import scalaz.{Free, ~>, Id}
import scalaz.std.list._
import scalaz.syntax.traverse._

type UserId = Int
type UserName = String
type UserPhoto = String

final case class Tweet(userId: UserId, msg: String)
final case class User(id: UserId, name: UserName, photo: UserPhoto)

// Services represent web services we can call to fetch data
sealed trait Service[A]
final case class GetTweets(userId: UserId) extends Service[List[Tweet]]
final case class GetUserName(userId: UserId) extends Service[UserName]
final case class GetUserPhoto(userId: UserId) extends Service[UserPhoto]

// A request represents a request for data
final case class Request[A](service: Service[A])

def fetch[A](service: Service[A]): Free[Request, A] =
  Free.liftF[Request, A](Request(service) : Request[A])

object ToyInterpreter extends (Request ~> Id.Id) {
  import Id._

  def apply[A](in: Request[A]): Id[A] = in match {
    case Request(service) =>
      service match {
        case GetTweets(userId) =>
          println(s"Getting tweets for user $userId")
          List(Tweet(1, "Hi"), Tweet(2, "Hi"), Tweet(1, "Bye"))

        case GetUserName(userId) =>
          println(s"Getting user name for user $userId")
          userId match {
            case 1 => "Agnes"
            case 2 => "Brian"
            case _ => "Anonymous"
          }

        case GetUserPhoto(userId) =>
          println(s"Getting user photo for user $userId")
          userId match {
            case 1 => ":-)"
            case 2 => ":-D"
            case _ => ":-|"
          }
      }
  }
}

object Example {

  val theId: UserId = 1

  def getUser(id: UserId): Free[Request, User] =
    for {
      name  <- fetch(GetUserName(id))
      photo <- fetch(GetUserPhoto(id))
    } yield User(id, name, photo)

  val free: Free[Request, List[(String, User)]] =
    for {
      tweets <- fetch(GetTweets(theId))
      result <- (tweets map { tweet: Tweet =>
        for {
          user <- getUser(tweet.userId)
        } yield (tweet.msg -> user)
      }).sequenceU
    } yield result

  def run: List[(String, User)] =
    free.foldMap(ToyInterpreter)
}

// to try this code: Example.run

+1 !!