A Key value store DSL client code example of https://gist.github.com/bqm/bee72c18fa4baa708f7cf2b146bbc872

ApFreeClientCode.scala

import cats.{Id, Functor, Monad, ~>}
import cats.std.function._
import cats.syntax.cartesian._

sealed trait KVS[A]
case class Put[A](key: String, value: String, a: A) extends KVS[A]
case class Get[A](key: String, a: String => A) extends KVS[A]

type Dsl[A] = ApFree[KVS, A]

implicit val kvsFunctor: Functor[KVS] = new Functor[KVS] {
  def map[A, B](a: KVS[A])(f: A => B) = a match {
    case Put(k, v, next) => Put(k, v, f(next))
    case Get(k, onResult) => Get(k, onResult andThen f)
  }
}

def put(key: String, value: String): Dsl[Unit] = {
  ApFree.liftF(Put(key, value, ()))
}

def get(key: String): Dsl[String] = {
  ApFree.liftF(Get(key, str => str))
}


val frenchHelloPut = put("frenchHello", "Bonjour")
val germanHelloPut = put("germanHello", "Hallo")
val spanishHelloPut = put("spanishHello", "Hola")
val hellos: Dsl[(String, String, String)] = for {
  _ <- frenchHelloPut.join(germanHelloPut).join(spanishHelloPut)
  frenchHelloGet = get("frenchHello")
  germanHelloGet = get("germanHello")
  spanishHelloGet = get("spanishHello")
  res <- frenchHelloGet.join(germanHelloGet).join(spanishHelloGet)
} yield (res._1._1, res._1._2, res._2)


type KvsDsl[A] = KVS[Dsl[A]]
type IdDsl[A] = Id[Dsl[A]]
import scala.collection.mutable.{Map => MMap}

val myStore = MMap.empty[String, String]

def interpret[A](
  dsl: Dsl[A]): Id[A] = dsl.fold2(
    new (KvsDsl ~> IdDsl) {
      def apply[B](fa: KVS[Dsl[B]]): Id[Dsl[B]] = fa match {
        case Get(key, f) => f(myStore(key))
        case Put(key, value, f) => {
          myStore.update(key, value)
          f
        }
    }
  }
)

// interpret(hellos)
// res1: (String, String, String) = ("Bonjour", "Hallo", "Hola")