Efficient implementation of immutable (double) LinkedList

Question

Having read this question Immutable or not immutable? and reading answers to my previous questions on immutability, I am still a bit puzzled about efficient implementation o

Answer 1

There is a way to emulate "mutation" : using immutable maps.

For a linked list of Strings (in Scala style pseudocode):

case class ListItem(s:String, id:UUID, nextID: UUID)

then the ListItems can be stored in a map where the key is UUID: type MyList = Map[UUID, ListItem]

If I want to insert a new ListItem into val list : MyList :

def insertAfter(l:MyList, e:ListItem)={ 
     val beforeE=l.getElementBefore(e)
     val afterE=l.getElementAfter(e)
     val eToInsert=e.copy(nextID=afterE.nextID)
     val beforeE_new=beforeE.copy(nextID=e.nextID)
     val l_tmp=l.update(beforeE.id,beforeE_new)
     return l_tmp.add(eToInsert)
}

Where add, update, get takes constant time using Map: http://docs.scala-lang.org/overviews/collections/performance-characteristics

Implementing double linked list goes similarly.