Example of Coyoneda use with free monads in Scala

coyo.scala

import scalaz._, Scalaz._

// Model
case class UserId(id: String)
case class RequestId(id: String)
case class Message(body: String)
trait Authenticator { def authenticate(token: String): Option[UserId] }
trait MessageDb {
  def getMessageList(id: String): List[String]
  def addMessage(id: String, body: String): Unit
}

// DSL instructions
sealed trait AppAction[A]
case class Authenticate(token: String) extends AppAction[Option[UserId]]
case object GetRequestId extends AppAction[RequestId]
case class GetMessages(uid: UserId) extends AppAction[Seq[Message]]
case class AddMessage(uid: UserId, msg: Message) extends AppAction[Unit]

// User-facing monadic interface
object Scripts {
  type ScriptImpl[A] = Coyoneda[AppAction, A]
  type Script[A] = Free[ScriptImpl, A]


  def authenticate(token: String): Script[Option[UserId]] = toScript(Authenticate(token))
  def getRequestId: Script[RequestId] = toScript(GetRequestId)
  def getMessages(uid: UserId): Script[Seq[Message]] = toScript(GetMessages(uid))
  def addMessage(uid: UserId, msg: Message): Script[Unit] = toScript(AddMessage(uid, msg))
  
  private def toScript[A](a: AppAction[A]): Script[A] = Free.liftFC[AppAction, A](a)
  
  
  type Interpreter[F[_]] = AppAction ~> F
  def interpret[A, F[_]: Monad](script: Script[A], interp: Interpreter[F]): F[A] = Free.runFC(script)(interp)
}



// Interpreter
class AppInterpreter(auth: Authenticator, rid: RequestId, db: MessageDb) extends Scripts.Interpreter[Id] {
  
  def apply[A](action: AppAction[A]): A = action match {
    case Authenticate(token) => auth.authenticate(token)
    case GetRequestId => rid
    case GetMessages(UserId(id)) => db.getMessageList(id).map(Message.apply)
    case AddMessage(UserId(id), Message(body)) => db.addMessage(id, body)
  }

}