An example toy algebra for Redis actions

RedisFreeAlgebra

// This should work with Scala 2.10.4 & scalaz 7.1, core, effect and concurrent packages
import scalaz.{ concurrent, Free, Functor, Monad, syntax }
import concurrent.Task
import Free.{freeMonad => _, _}
import syntax.monad._

// Describe the set of actions - which are functors

sealed trait RedisF[+A] {
  def map[B](fn: A => B): RedisF[B]
}

object RedisF {
  implicit val redisFunctor: Functor[RedisF] = new Functor[RedisF] {
    def map[A, B](fa: RedisF[A])(f: A => B): RedisF[B] = fa map f
  }
}

// and some instances of that trait describing our desired actions

case class Get[+A](k: String, next: String => A) extends RedisF[A] {
  def map[B](fn: A => B): RedisF[B] = copy(next = next andThen fn)
}
case class Set[+A](k: String, value: String, next: String => A) extends RedisF[A] {
  def map[B](fn: A => B): RedisF[B] = copy(next = next andThen fn)
}
case class Fail[A](excp: Throwable) extends RedisF[A] {
  def map[B](fn: A => B): RedisF[B] = Fail[B](excp)
}

// it's a simple dsl :-), it does getting and setting strings to Redis

object FreeRedis extends FreeRedis

trait FreeRedis {

  type RedisFree[A] = Free[RedisF, A]

  def just[A](a: => A): RedisFree[A] =
    Monad[RedisFree].pure(a)

  def noAction[A]: RedisFree[A] =
    liftF(Fail(new Throwable("Action failed"))): RedisFree[A]

  // Helper functions describing our redis commands below

  def set(k: String, v: String): RedisFree[String] =
    liftF(Set(k, v, identity))

  def get(k: String): RedisFree[String] =
    liftF(Get(k, identity))

  implicit val redisFreeMonad: Monad[RedisFree] = new Monad[RedisFree] {
    def point[A](a: => A) = Free.point(a)
    def bind[A,B](action: RedisFree[A])(f: A => RedisFree[B]) = action flatMap f
  }

}

// Ensure FreeRedis stuff is in scope...
import FreeRedis._

// Make a pure program value
val program1: RedisFree[String] = set("keyA", "valueB") >> get("keyA")

object Interpreters extends FreeRedis {

  // Take a program and turn each action into a string, aggregate those strings in a list
  def stepList[A](program: RedisFree[A], actions: List[String] = Nil): List[String] = program.resume.fold({
    case Set(k, v, next)  => stepList(next("success"), s"setting key: ${k} value = ${v}" :: actions)
    case Get(k, next)     => stepList(next("success"), s"getting key: ${k}" :: actions)
    case Fail(excp)       => ("ERROR!" :: actions).reverse
  }, { a: A => ("End of program" :: actions).reverse })

  // Interpret a step (action) of our program and produce a Task which we can run later
  def step[A](action: RedisF[RedisFree[A]]): Task[RedisFree[A]] = action match {
    case Set(k, v, next) => Task { println(s"setting key: ${k} value = ${v}"); "success" } map { next }
    case Get(k, next)    => Task { println(s"getting key: ${k}"); "I'm the key you got!" } map { next }
    case Fail(excp)      => Task.fail(excp)
  }

}