Showing some functional code in Scala

EitherTest.scala

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration
import scala.concurrent.{Await, Future}

/**
  * Created by thiago on 3/10/16.
  */
object EitherTest extends App {

  //given a list of ids
  val idsToBeInserted = List(1, 2, 3, 4, 5)

  //Let's find what id is already registered or not
  val future = idsToBeInserted.map { id => //iterate over ids
    FakeDatabase.getById(id).map { //try to find on database
      case Some(oldUser) => Left(oldUser) //already registered? LEFT
      case None => Right(User(id, s"Dummy - $id", s"dummy.$id@gmail.com")) //new user? RIGHT
    }
  }

  val f = Future.sequence(future).map { listOfEither =>

    //collect what is left or right as optional, filter by non empty and get the value
    val registeredUsers = listOfEither.map(_.left.toOption).filter(_.nonEmpty).map(_.get)
    val newUsers = listOfEither.map(_.right.toOption).filter(_.nonEmpty).map(_.get)

    println("--- print already registered users ---")
    registeredUsers.foreach(println)

    println("")
    
    println("--- print new users ---")
    newUsers.foreach(println)
  }

  //block at the end to get the result
  Await.result(f, Duration.Inf)
}

case class User(id: Int, name: String, email: String)

object FakeDatabase {
  val existingUsers = List(
    User(1, "John", "[email protected]"),
    User(2, "Paul", "[email protected]"),
    User(3, "Mike", "[email protected]")
  )

  def getById(id: Int): Future[Option[User]] = {
    Future {
      existingUsers.find(_.id == id)
    }
  }
}