KVStoreExample.scala

import cats.free.Free
import cats.{ Monad, ~> }

object KVStoreExample {

  sealed trait KVStore[A]
  case class Put[T](key: String, value: T) extends KVStore[Unit]
  case class Get[T](key: String) extends KVStore[Option[T]]

  object KVStore {

    trait Interface[M[_]] {
      def put[T](key: String, value: T): M[Unit]
      def get[T](key: String): M[Option[T]]
    }

    trait Interp[M[_]] extends (KVStore ~> M) with Interface[M] {
      def run[A](f: Free[KVStore, A])(implicit monad: Monad[M]): M[A] =
        f.foldMap(this)
    }

    object Interp {
      def fromArrow[M[_]](arrow: KVStore ~> M): Interp[M] =
        new ReverseRoutedInterp[M](arrow)
    }

    //User has to implement the methods
    trait RoutedInterp[M[_]] extends Interp[M] {

      override final def apply[A](fa: KVStore[A]): M[A] = fa match {
        case Put(key, value) => put(key, value)
        case Get(key) => get(key)
      }
    }

    //User has to construct using an arrow
    case class ReverseRoutedInterp[M[_]](underlying: KVStore ~> M)
        extends Interp[M] {
      override final def put[T](key: String, value: T): M[Unit] =
        apply(Put(key, value))
      override final def get[T](key: String): M[Option[T]] = apply(Get(key))

      override final def apply[A](fa: KVStore[A]): M[A] = underlying.apply(fa)
    }
  }

}