procedural_example.scala

import scala.collection.mutable.ListBuffer

case class User(firstName: String,
                nickName: Option[String] = None,
                lastName: String,
                happiness: Option[Boolean] = None,
                age : Int)

val allUsers: ListBuffer[User] = scala.collection.mutable.ListBuffer.empty[User]
allUsers += User("Calvin", Some("Murican"), "Ference", happiness = Some(false), age = 78)
allUsers += User("Max", Some("Mad"), "Millette-Coulombe", happiness = Some(true), age = 12)
allUsers += User("Johann", lastName = "David", age = 26)
allUsers += User("Dom", Some("Pink Scarf"), "Caron", Some(true), age = 32)

def notOldEnough(user: User): Boolean = {
  return user.age < 18
}

def oldEnough(user: User): Boolean = !notOldEnough(user)

def getHelloMessage(user: User): String = {
  var helloMessage = ""
  if(user.happiness.isDefined) {
    var isHappy = user.happiness.get
    if (isHappy) {
      helloMessage = "We are happy too! have a good day."
    } else {
      helloMessage = "Have a bad day ! You suck !"
    }
  } else {
    helloMessage = "We have no clue if you are happy but have a good day !"
  }
  return helloMessage
}

def getFullName(user: User): String = {
  var fullName = user.firstName
  if(user.nickName.isDefined) {
    fullName += s" ${user.nickName.get} "
  } else {
    fullName += " "
  }
  fullName += user.lastName
  return fullName
}

def printHelloMessage(allUsers: ListBuffer[User], checkAge: Boolean = false): Unit = {
  
  //using a new collection resolve the problem
  val filteredUsers = scala.collection.mutable.ListBuffer.empty[User]
  for( user <- allUsers ){
    if(checkAge) {
      if(oldEnough(user)) filteredUsers += user
    } else {
      filteredUsers += user
    }
  }
  for( user <- filteredUsers ){
    var fullName = getFullName(user)
    var helloMessage = getHelloMessage(user)
    println(s"Hi $fullName ! $helloMessage")
  }
  //problem highlighted by using the same collection
  /*
  for( user <- allUsers ){
    if(checkAge && notOldEnough(user)) allUsers -= user
  }
  for( user <- allUsers ){
    var fullName = getFullName(user)
    var helloMessage = getHelloMessage(user)
    println(s"Hi $fullName ! $helloMessage")
  }
   */  
}

printHelloMessage(allUsers)
printHelloMessage(allUsers, checkAge = true)
printHelloMessage(allUsers)