Functional data structures and data sharing. How does it work in Scala?

Question

Say I have a method like the following (curried for better type inference)

def dropWhile[A](l: List[A]) (f: A => Boolean): List[A] = {

  l match {
    case h::t if f(h) => dropWhile(t)(f)
    case _ => l
  }
}

and then I call it as follows

scala> val lst = (1 to 10).toList
lst: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

scala> dropWhile(lst) ( _ <= 4)
res0: List[Int] = List(5, 6, 7, 8, 9, 10)

and get the expected result back. My question is

lst is a list with its own memory address. By default, method arguments are passed by value so in

dropWhile(lst) ( _ <= 4)

lst will be copied over to method parameter l? and so the method will have its own copy in l and what is more, each recursive invocation should have its own copy? In that context, how does data sharing work?

Answer 1

lst will be copied over to method parameter l?

No, only the reference to the list will be copied as lst is passed in to dropWhile , not the elements of the list. The effect is both lst and l will each have a copy of the reference that points to the same object in memory.