Editing Persistent data structure (section)

===Linked lists===
Singly [[linked list]]s are the bread-and-butter data structure in functional languages.<ref name="auto">''This example is taken from Okasaki.  See the bibliography.''</ref> Some [[ML programming language|ML]]-derived languages, like [[Haskell (programming language)|Haskell]], are purely functional because once a node in the list has been allocated, it cannot be modified, only copied,  referenced or destroyed by the [[Garbage collection (computer science)|garbage collector]] when nothing refers to it.  (Note that ML itself is '''not''' purely functional, but supports non-destructive list operations subset, that is also true in the [[Lisp (programming language)|Lisp]] (LISt Processing) functional language dialects like [[Racket (programming language)|Scheme]] and [[Racket (programming language)|Racket]].)

Consider the two lists:
 xs = [0, 1, 2]
 ys = [3, 4, 5]
These would be represented in memory by:

[[File:Purely_functional_list_before.svg]]

where a circle indicates a node in the list (the arrow out representing the second element of the node which is a pointer to another node).

Now concatenating the two lists:
 zs = xs ++ ys
results in the following memory structure:

[[File:Purely_functional_list_after.svg]]

Notice that the nodes in list <code>xs</code> have been copied, but the nodes in <code>ys</code> are shared.  As a result, the original lists (<code>xs</code> and <code>ys</code>) persist and have not been modified.

The reason for the copy is that the last node in <code>xs</code> (the node containing the original value <code>2</code>) cannot be modified to point to the start of <code>ys</code>, because that would change the value of <code>xs</code>.