StateMonad.scala

package com.mjamesruggiero

import scalaz.State

object StateMonad {

  type StateCache[+A] = State[Cache, A]
  trait ConfigService {
    def value(k: String): StateCache[ConfigValue]
  }

  case class ConfigValue(
    key: String,
    value: String)

  case class Timestamped[A](value: A, timestamp: Long)

  case class Cache(
    vals: Map[String, Timestamped[ConfigValue]],
    hits: Int,
    misses: Int)
    {
      def get(key: String): Option[Timestamped[ConfigValue]] = {
        val desired = vals.get(key)
        println(s"pulled ${desired} from a cached map")
        desired
      }

      def update(k: String, s: Timestamped[ConfigValue]): Cache = {
        println("caching the value")
        Cache(vals + (k -> s), hits, misses)
      }
  }

  object FakeConfigService extends ConfigService {

    def value(k: String) = for {
      oConfigV <- checkCache(k)
      configV <- oConfigV match {
         case Some(configV) => State.state[Cache, ConfigValue](configV)
         case None => retrieve(k)
        }
      //_ = println(s"when I look in checkCache, I find ${checkCache(k)}")
      } yield configV
      
    private def checkCache(k: String): StateCache[Option[ConfigValue]] = State { c =>
      c.get(k) match {
        case Some(Timestamped(configV, ts))
          if (!stale(ts)) => {
            println(s"key '${k}' NOT stale!")
            (c.copy(hits = c.hits + 1), Some(configV))
          }
        case other => {
          println(s"key '${k}' stale!")
          (c.copy(misses = c.misses + 1), None)
        }
      }

    private def stale(ts: Long): Boolean = {
      val fiveMinutes = 5 * 60 * 1000L
      System.currentTimeMillis - ts > fiveMinutes
    }

    private def retrieve(k: String): StateCache[ConfigValue] = for {
      configV <- State.state(callWebService(k))
      tConfigV = Timestamped(configV, System.currentTimeMillis)
      _ <- State.modify[Cache] { println(s"updating '${k}"); _.update(k, tConfigV) }
    } yield configV

    private def callWebService(k: String): ConfigValue = {
      println("you actually called the webservice")
      ConfigValue(k, "fakeValue")
    }
  }
}

implicit val CacheMonoid = new Monoid[Cache] { 
    override def zero = Cache(Map.empty, 0, 0)
    override def append(a: Cache, b: => Cache) =
        Cache(a.vals  ++ b.vals, a.hits + b.hits, a.misses + b.misses)
}

scala> import scalaz._, Scalaz._, Unapply._
scala> import com.mjamesruggiero.StateMonad._
scala> FakeConfigService.value("bingo").replicateM(10).run(CacheMonoid.zero)
pulled None from a cached map
key 'bingo' stale!
you actually called the webservice
updating 'bingo
caching the value
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
pulled Some(Timestamped(ConfigValue(bingo,fakeValue),1428614584353)) from a cached map
key 'bingo' NOT stale!
res33: (com.mjamesruggiero.StateMonad.Cache,
 List[com.mjamesruggiero.StateMonad.ConfigValue]) = (Cache(Map(bingo -> Timestamped(ConfigValue(bingo,
fakeValue), 1428614584353)), 9, 1), List(ConfigValue(bingo, fakeValue), ConfigValue(bingo, fakeValue),
 ConfigValue(bingo, fakeValue), ConfigValue(bingo, fakeValue), ConfigValue(bingo, fakeValue),
ConfigValue(bingo, fakeValue), ConfigValue(bingo, fakeValue), ConfigValue(bingo, fakeValue),
ConfigValue(bingo, fakeValue), ConfigValue(bingo, fakeValue)))