Question

I have to build a list whose members should be included or not based on a different condition for each.

Let’s say I have to validate a purchase order and, depending on the price, I have to notify a number of people: if the price is more than 10, the supervisor has to be notified. If the price is more than 100 then both the supervisor and the manager. If the price is more than 1000 then the supervisor, the manager, and the director.

My function should take a price as input and output a list of people to notify. I came up with the following:

def whoToNotify(price:Int) = {
  addIf(price>1000, "director",
        addIf(price>100, "manager",
              addIf(price>10, "supervisor", Nil)
        )
       )
}

def addIf[A](condition:Boolean, elem:A, list:List[A]) = {
  if(condition) elem :: list else list
}

Are there better ways to do this in plain Scala? Am I reinventing some wheel here with my addIf function?

Please note that the check on price is just a simplification. In real life, the checks would be more complex, on a number of database fields, and including someone in the organizational hierarchy will not imply including all the people below, so truncate-a-list solutions won’t work.

EDIT

Basically, I want to achieve the following, using immutable lists:

def whoToNotify(po:PurchaseOrder) = {
      val people = scala.collection.mutable.Buffer[String]()
      if(directorCondition(po)) people += "director"
      if(managerCondition(po)) people += "manager"
      if(supervisorCondition(po)) people += "supervisor"
      people.toList
}

Answer 1

You can use List#flatten() to build a list from subelements. It would even let you add two people at the same time (I’ll do that for the managers in the example below):

def whoToNotify(price:Int) =
  List(if (price > 1000) List("director") else Nil,
       if (price > 100) List("manager1", "manager2") else Nil,
       if (price > 10) List("supervisor") else Nil).flatten