Reader Monad

ReaderMonad.scala

object ReaderMonad {
  class Reader[-E, +R] private (g: E => R) {
    def apply(env: E): R = g(env)
    def map[A](f: R => A): Reader[E, A] = new Reader[E, A](env => f(g(env)))
    def flatMap[A, B <: E](f: R => Reader[B, A]): Reader[B, A] = new Reader[B, A](env => f(g(env))(env))
  }

  def pure[E, R](v: R): Reader[E, R] = Reader.pure(v)
  def ask[E]: Reader[E, E] = Reader.ask

  private[this] object Reader {
    def pure[E, R](v: R): Reader[E, R] = new Reader[E, R](_ => v)
    def ask[E]: Reader[E, E] = new Reader[E, E](identity)
  }
}

object Main {
  import ReaderMonad._

  case class User(id: Int)

  trait UserRepository {
    def find(id: Int): Option[User]
  }

  object UserRepository extends UserRepository {
    private[this] val users = Map(1 -> User(1))
    def find(id: Int): Option[User] = users.get(id)
  }

  sealed trait AuthResult
  case class Success(user: User) extends AuthResult
  case class Failure(message: String) extends AuthResult

  case class Authenticator(repository: UserRepository) {
    def authenticate(username: String, password: String): Reader[Int, AuthResult] = for {
      userId <- ask
    } yield {
      if (isValid(username, password)) {
        repository find(userId) map (Success(_)) getOrElse Failure("The user doesn't exist.")
      } else {
        Failure("Incorrect username or password.")
      }
    }

    def isValid(username: String, password: String): Boolean = true
  }

  def main(args: Array[String]): Unit = {
    val authenticator = new Authenticator(UserRepository)
    val readerForUser1 = authenticator.authenticate("hoge", "")
    val readerForUser2 = authenticator.authenticate("fuba", "")

    println(readerForUser1(1))
    println(readerForUser2(2))
  }
}