Lens/State Example to remove repetition

lens-example-tradingday.scala

object TradingDays extends App {

  import scalaz._
  import Scalaz._

  case class Trade(sym: String, trader: String, qty: Int)

  case class TradingDay(symbols: Map[String, SymDay] = Map.empty)

  object TradingDay {
    val Syms = Lens[TradingDay, Map[String, SymDay]](_.symbols, (d, s) => d.copy(symbols = s))
  }
  case class SymDay(symbol: String, traders: Map[String, TraderSymDay] = Map.empty)
  object SymDay {
    val Traders = Lens[SymDay, Map[String, TraderSymDay]](_.traders, (d, t) => d.copy(traders = t))
  }

  case class TraderSymDay(trader: String, symbol: String, trades: List[Trade] = Nil)
  object TraderSymDay  {
    val Trades = Lens[TraderSymDay, List[Trade]](_.trades, (d, f) => d.copy(trades = f))
  }

  import TradingDay._
  import SymDay._
  import TraderSymDay._
  val trades = Trade("VOD", "John", 10) :: Trade("VOD", "Billy-Bob", 10) :: Trade("MSFT", "Billy-Bob", 20) :: Nil

  val res = buildDay(trades) //TODO; implement buildDay to reduce repetition

  def buildDay(trades: List[Trade]): TradingDays.TradingDay = {
    (TradingDay() /: trades) {
      (d, trade) =>
        def sym = trade.sym
        def trader = trade.trader
        val upd =
          for {
            _ <- (Syms member sym).mods(_ orElse SymDay(sym).some)
            _ <- (Syms at sym andThen Traders member trader).mods(_ orElse TraderSymDay(trader, sym).some)
            _ <- (Syms at sym andThen (Traders at trader) andThen Trades).mods(trade :: _)
            x <- init
          } yield x
        upd ! d
    }
  }


    val expected = TradingDay(Map(
      "VOD" -> SymDay("VOD", Map(
                  "John" -> TraderSymDay("John", "VOD", List(Trade("VOD", "John", 10))),
                  "Billy-Bob" -> TraderSymDay("Billy-Bob", "VOD", List(Trade("VOD", "Billy-Bob", 10))))),
      "MSFT" -> SymDay("MSFT", Map(
                  "Billy-Bob" -> TraderSymDay("Billy-Bob", "MSFT", List(Trade("MSFT", "Billy-Bob", 20)))
      ))
    ))

    assert(res == expected, res)
}