Simple example of document store, one mutable the other immutable.

Catalog.scala

object stuff {

class Catalog(documentsIn:List[Document]=List.empty){
  def isEmpty=_documents.isEmpty
  def add(document:Document)= _documents::=document
  private[this] var _documents=documentsIn
  def documents= _documents
}

class ImCatalog(val documents:List[Document]=List.empty){
  def isEmpty=documents.isEmpty
  def +(document:Document)= new ImCatalog(document::documents)
}

case class Document(title:String)

val cat=new Catalog                              

assert(cat.isEmpty==true)

val helloWorld=Document("Hello world")            

cat.add(helloWorld)

assert(cat.isEmpty==false)

assert(cat.documents.head==helloWorld)

var imcat=new ImCatalog                          

assert(imcat.isEmpty==true)

imcat+=helloWorld

assert(imcat.isEmpty==false)

assert(imcat.documents.head==helloWorld)

val docs=List(helloWorld,helloWorld) 

val cat2=new Catalog(docs)   

assert(cat2.documents.size==2)

imcat=new ImCatalog(docs)

assert(imcat.documents.size==2)

}