Reader monad example

reader.scala

object ReaderExample {
  import scalaz.Monad
  import scalaz.syntax.monad._

  type AccountId = Int
  type Account = Int
  type BetId = Int
  type Bet = Int

  final case class Config(
    accounts: Map[AccountId, Account],
    bets: Map[Account, List[Bet]]
  )

  type Reader[A,B] = A => B
  implicit def monadInstance[Config]: Monad[({type L[B] = Reader[Config,B]})#L] =
    new Monad[({type L[B] = Reader[Config,B]})#L] {
      override def bind[A, B](fa: Reader[Config,A])(f: (A) ⇒ Reader[Config,B]): Reader[Config,B] =
        (config: Config) => {
          f(fa(config))(config)
        }

      override def point[A](a: ⇒ A): Reader[Config,A] =
        (config: Config) => a
    }

  type Configured[A] = Reader[Config,A]

  def getAccount(accountId: AccountId): Configured[Account] =
    (config: Config) => config.accounts(accountId)

  def getBets(account: Account): Configured[List[Bet]] =
    (config: Config) => config.bets(account)

  def getLastBet(bets: List[Bet]): Bet =
    bets.last

  val example: AccountId => Reader[Config,Bet] =
    (accountId: AccountId) => {
      for {
        account <- getAccount(accountId)
        bets    <- getBets(account)
        last    <- getLastBet(bets).pure[Configured]
      } yield last
    }

  val result =
    example(1)(Config(accounts = Map(1 -> 10), bets = Map(10 -> List(1,2,3))))
}