Learning the Free monad with cats

FreeConsultations.scala

package project

import cats._
import cats.free.Free

object Main extends App {
  import Algebra._
  import ConsultationOp._

  val c = Consultation(_id = "123", title = "A consultation")

  val res = for {
    _ <- create(c)
    _ <- get("124")
    c <- get(c._id)
  } yield c

  println(res)

  res.foldMap(MockInterpreter)
}

object Algebra {
  type ConsultationId = String
  case class Consultation(_id: ConsultationId, title: String)

  sealed trait ConsultationOp[A]
  object ConsultationOp {
    case class Create(c: Consultation) extends ConsultationOp[Unit]
    case class Get(s: ConsultationId) extends ConsultationOp[Option[Consultation]]
  }

  def create(c: Consultation) =
    Free.liftF[ConsultationOp, Unit](ConsultationOp.Create(c))

  def get(id: ConsultationId) =
    Free.liftF[ConsultationOp, Option[Consultation]](ConsultationOp.Get(id))
}

object MockInterpreter extends (Algebra.ConsultationOp ~> Id) {
  import Algebra._
  import ConsultationOp._

  // the "db"
  var c: Option[Consultation] = None

  def apply[A](in: ConsultationOp[A]): Id[A] =
    in match {
      case Create(a) =>
        // storing it in the "db"
        c = Some(a)
        println(s"Creating consultation $a")
      case Get(id) =>
        println(s"Getting consultation with id: $id")
        val r = c.filter(_._id == id)
        r match {
          case Some(r) => println(s"Found consultation $c")
          case None => println("Consultation not found")
        }
        r
    }
}