State Monad and Validation with Scalaz

StateMonad.scala

import scalaz._
import Scalaz._

/**
 * Use the state monad to 'process a trade' and store the new trade.
 * As well as processing the trade, handle validation errors.
 */
object StateMonad extends App {

  case class Trade(info: String)
  type Store = List[Trade]

  // a function that takes a new trade and processes it yielding new state and validation
  val newTrade = (newTrade: Trade) ⇒
    for (
      _ ← log("New Trade");
      accepted ← accept(newTrade);
      _ ← log("Hedging New Trade");
      hedged ← hedge(newTrade);
      _ ← log("Validating portfolio");
      portfolio ← validatePortfolio;
      _ ← log("New trade processed")
    ) yield (accepted.liftFailNel |@| hedged.liftFailNel |@| portfolio.liftFailNel) {_ + _ + _}

  println("Haven't done anything yet!")
  
  // assume some existing state
  var globalState: Store = Nil
  
  // exercise the newTrade function with the existing state
  val (newState, validation) = newTrade(Trade("Big Trade"))(globalState)

  // assign the new state to our global state if the validation says its ok 
  globalState = validation.fold(failures ⇒ { println(failures); globalState }, msg ⇒ newState)
  
  println("Store contains " + globalState)

  // does nothing but print a messages and return the state its given
  def log(m: String) = state[Store, Unit](s ⇒ (s, println(m)))

  // accepts a trade putting it into the store
  def accept(newTrade: Trade) = state[Store, Validation[String, String]](s ⇒ (newTrade :: s, "trade accepted".success))

  // hedge against the new trade - apparently its all the rage
  def hedge(against: Trade) = state[Store, Validation[String, String]](s ⇒ (Trade("Hedge Trade against " + against) :: s, "hedge trade step".success))

  // validate the portfolio doing nothing with the state
  def validatePortfolio = state[Store, Validation[String, String]](s ⇒ {
    if (s.size > 10) (s, "Portolio is too big".fail)
    else (s, "All ok".success)
  })
}

/*
Running this produces:


Haven't done anything yet!
New Trade
Hedging New Trade
Validating portfolio
New trade processed
Store contains List(Trade(Hedge Trade against Trade(Big Trade)), Trade(Big Trade))
*/