Reader Monad example 1

reader-monad-example-1.scala

/**
   *
   * The classic use of Readers is to build programs that accept a configuration as a
   * parameter. Let’s ground this with a complete example of a simple login system.
   * Our configuration will consist of two databases: a list of valid users and a
   * list of their passwords:
   */

  case class Db(usernames: Map[Int, String],
                passwords: Map[String, String]
               )


  type DbReader[A] = Reader[Db, A]

  def findUsername(userId: Int): DbReader[Option[String]] = Reader(_.usernames.get(userId))

  def checkPassword(username: String,
                    password: String): DbReader[Boolean] =
    Reader(_.passwords.get(username).contains(password))

  import cats.syntax.applicative._ // for pure

  def checkLogin(userId: Int,
                 password: String): DbReader[Boolean] =
    for {
      maybeUsername <- findUsername(userId)
      passwordOk <- maybeUsername match {
        case Some(u) => checkPassword(u, password)
        case None => false.pure[DbReader]
      }
    } yield passwordOk


  val users = Map(
    1 -> "dade",
    2 -> "kate",
    3 -> "margo"
  )
  val passwords = Map(
    "dade" -> "zerocool",
    "kate" -> "acidburn",
    "margo" -> "secret"
  )

  val db = Db(users, passwords)
  checkLogin(1, "zerocool").run(db)
  // res10: cats.Id[Boolean] = true
  checkLogin(4, "davinci").run(db)
  // res11: cats.Id[Boolean] = false