oop to fp - free monad

DI_free.sc

import cats.free.Free.liftF
import cats.free.Free
import cats.{Id, ~>}

// 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]

object MovieRepoOps {
  def getMovie(id: Int): QueryF[Option[Movie]] = liftF(GetMovie(id))
}
object MovieService {
  def getMovie(id: Int): QueryF[Option[Movie]] = for {
    movie <- MovieRepoOps.getMovie(id)
  } yield movie
}

// 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)

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

Await.result(program.runToFuture, 1.second)
// Some(Movie(42, A Movie))

// test environment -------------------------------------------------------
def testInterpreter: Query ~> Id = new (Query ~> Id) {
  def apply[A](fa: Query[A]): Id[A] = fa match {
    case GetMovie(_: Int) => Option(Movie(-1, "Test"))
  }
}
val id1 = MovieService.getMovie(42).foldMap(testInterpreter)
// Some(Movie(-1,Test))