minimal cake + free monad

DI_free_monad_min_cake.sc

import cats.free.Free
import cats.free.Free.liftF
import cats.~>

// domain layer -----------------------------------------------------------
case class Movie(id: Int, title: String)

sealed trait Query[A]
case class GetMovie(id: Int) extends Query[Option[Movie]]

type QueryF[T] = Free[Query, T]

trait MovieRepo {
  def getMovie(id: Int): QueryF[Option[Movie]]
}
trait UsesMovieRepo {
  val movieRepo: MovieRepo
}
trait MovieService extends UsesMovieRepo {
  def getMovie(id: Int): QueryF[Option[Movie]] = movieRepo.getMovie(id)
}
object MovieRepoImpl extends MovieRepo {
  def getMovie(id: Int): QueryF[Option[Movie]] = liftF(GetMovie(id))
}
trait MixInMovieRepo {
  val movieRepo: MovieRepo = MovieRepoImpl
}
object MovieService extends MovieService with MixInMovieRepo

// application layer -------------------------------------------------
import monix.eval.Task

val db = Map[Int, Movie](42 -> Movie(42, "A Movie"))

def taskInterpreter: Query ~> Task = new (Query ~> Task) {
  def apply[A](fa: Query[A]): Task[A] = fa match {
    case GetMovie(id) => Task(db.get(id))
  }
}
val program = MovieService.getMovie(42).foldMap(taskInterpreter)

// runtime  ------------------------------------------------------
import scala.concurrent.duration._
import scala.concurrent.Await
import monix.execution.Scheduler.Implicits.global

Await.result(program.runToFuture, 1.second)