In the last post, we put to rest our use of mutable objects for good. Here we learn how to make use of our new found Functional Programming powers in the real world.

Every application of sufficient user base requires a persistent settings store. The more users one has, the more styles one is responsible for accommodating. In my experience, JSON has been the most useful format for small-scale persistence. JSON is widely understood, works on the web and is plaintext. We'll use Monocle and Argonaut to implement a persistent settings store.

The first question here is "Why not circe?". I found circe to be a bit too ethereal for most Java developers to wrap their heads around. Implicit scope (especially when its as magical as circe's auto) is an alarming feature for people who come from a language with no developer-defined implicit semantics (C++ developers are quite comfortable with this notion).

Monocle

The Monocle library provides semantics for defining simple accessors and combinators on nested data. Monocle is especially well suited for handling nested Case Classes which is what we'll focus on. Take the following data definition

case class Color(r: Byte, g: Byte, b: Byte)
case class FishTank(liters: Int, color: Color, fish: List[Fish])

We have nested Case Classes as well as a nested collection. To cover the changes that can occur here we would need to define:

eighteen operations
six of which are a composition from a Fish Tank into a Color
one of which is nested within a List structure.

Monocle makes this simple:

val (tankLiters, tankColor, tankFish) = {
  val gen = GenLens[FishTank]
  (gen(_.liters), gen(_.color), gen(_.fish))
}
val (colorR, colorG, colorB) = {
  val gen = GenLens[Color]
  (gen(_.r), gen(_.g), gen(_.b))
}
val (tankColorR, tankColorG, tankColorB) = (
  tankColor.composeLens(colorR),
  tankColor.composeLens(colorG),
  tankColor.composeLens(colorB)
)

Defining your data using Case Classes provides Monocle with the information it needs in order to generate lenses (nested views) into your data structures. Lens composition in Monocle is a single straightforward call. We can get into and out of our data with very little boilerplate.

Now that we can define settings and alter them, we need a way to persist them and communicate them to other parts of the system. We'll use JSON as our data format and Argonaut as our transcoder.

Argonaut

Argonaut provides semantics for converting between classes and JSON strings. Like Monocle, its easiest to use with Case Classes. Taking the same classes as above we would have:

//ignore the implicit keyword.
//I promise we'll get to it in the next post!
implicit def codecTank: CodecJson[FishTank] =
  casecodec3(
      FishTank.apply, FishTank.unapply
  )("liters", "color", "fish")
implicit def codecColor: CodecJson[Color] =
  casecodec3(
      Color.apply, Color.unapply
  )("r", "g", "b")
implicit def codecFish: CodecJson[Fish] =
  CodecJson(
    (f: Fish) =>
      ("name" := f.name) ->:
      ("color" := f.color) ->:
      jEmptyObject,
    (c: HCursor) => for{
      name <- (c --\ "name").as[String]
      color <- (c --\ "color").as[String]
    }yield{(name, color) match{
      case ("One Fish", "Red Fish") => OneFish
      case ("Two Fish", "Blue Fish") => TwoFish
      case _ => NotFish
    }}
  )

There are quite a few operators here. This could make things tricky for Java developers at first but, there are few of them so no big deal. For Case Classes, Argonaut has next to no boilerplate; one passes in the apply and unapply functions and names everything. Also, composition in Argonaut is implicit. It gets a little tricky with non Case Classes but, its still not much. At most Argonaut requires two functions; one from the class to JSON, the other from JSON to the class.

One more thing to note is that codecs for simple standard collections are implicit. The codec for List[Fish] is implicitly defined by the codec for Fish.

Putting it all together

A settings object would look something like:

object settings{
  private val settings: mutable.Map[String, FishTank] =
    mutable.Map()
  
  def apply(key: String): Option[FishTank] = settings.get(key)
  def update(key: String, byte: Byte): Unit = {
    settings(key) = settings.get(key) match{
      case Some(tank) =>
        tankColor.modify { _ => Color(byte, byte, byte) }(tank)
      case None =>
        FishTank(0, Color(byte, byte, byte), Nil)
    }
  }
  def update(key: String, size: Int): Unit = {
    settings(key) = settings.get(key) match{
      case Some(tank) =>
        tankLiters.modify(_ => size)(tank)
      case _ =>
        FishTank(size, Color(0,0,0), Nil)
    }
  }
  def update(key: String, fish:List[Fish]): Unit = {
    settings(key) = settings.get(key) match{
      case Some(tank) =>
        tankFish.modify(_ => fish)(tank)
      case _ =>
        FishTank(1, Color(0,0,0), fish)
    }
  }
  
  def persist(): Unit = {
    val jsonRaw = settings.toList.asJson
    val json = jsonRaw.nospaces
    putOnWire(json)
    writeToDisk(json)
  }
  def recall(): Unit = {
    val str = getFromDisk()
    val opt = str.decodeOption[List[(String, FishTank)]]
    opt.foreach{list =>
      settings ++= list.toMap
    }
  }
}

But, of course this is not threadsafe and it uses a mutable collection to perform its work. A different threadable implementation could look something like:

val actorSystem: ActorSystem = ???
implicit val timeout: akka.util.Timeout = ???
implicit val ec: ExecutionContext = ???
object asyncSettings{
  private sealed trait Message
  private case class Get(key: String)
      extends Message
  private case class SetGrey(key: String, hue: Byte)
      extends Message
      
  private class Perform extends Actor{
    override val receive: Receive = step(Map())
    def step(map: Map[String, FishTank]): Receive = {
        case Get(key) => sender ! map(key)
        case SetGrey(key, value) =>
          val newTank: FishTank = ???
          val newMap = map + (key -> newTank)
          context.become(step(newMap))
    }
    override def preStart(): Unit = ???//recall
    override def postStop(): Unit = ???//persist
  }
  
  val actor: ActorRef = actorSystem.actorOf{
    Props(new Perform())
  }
  def apply(key: String): Future[FishTank] =
    (actor ? Get(key)).collect{
      case Some(t: FishTank) => t
    }
  def update(key: String, hue: Byte) = 
    actor ! SetGrey(key, hue)
}

Here we use akka for asynchronous operations. One could also employ scalaz Task or simple Future composition or really any other asynchronous library.

Next we'll cover Type Classes to further decouple our data from functionality.