import cats.data.Reader
import cats.syntax.applicative._

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(db => db.usernames.get(userId))

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

def checkLogin(userId: Int, password: String): DbReader[Boolean] =
  for {
    username <- findUsername(userId)
    isPasswordOk <- username
      .map { username =>
        checkPassword(username, password)
      }
      .getOrElse {
        false.pure[DbReader]
      }
  } yield isPasswordOk

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)
checkLogin(4, "davinci").run(db)