Lists as data

Scheme programs are lists, but lists can be data too. Scheme provides syntactic support for lists, and a variety of standard functions for manipulating them. The following "quoted list" is Scheme syntax for the list containing 1, 2, 3, and 4:

    '(1 2 3 4)   ; Unlike the list (1 2 3 4), we do not evaluate this list
                 ; as a special form or function call.  The single quote
                 ; in Scheme means: "This is data; do not evaluate it."

    '(define a 7) ; The list containing define, a, and 7.  This list's parts
                  ; are not evaluated, and nothing is bound to the symbol a.

The special list nil is represented by (). Below are pictures of the two lists constructed above. Each bipartite box is called a "list cell" or a "cons cell".

• How would you define the lists of data above in C or C++?
• How do you think the empty list () is implemented?
• What is the difference between () and '()?

An alternative way of defining lists of data is to use the function list. This is not exactly equivalent, because it does not prevent the elements in the list from being evaluated:

  '(1 2 (+ 1 2))     ; Evaluates to the list '(1 2 (+ 1 2))
  (list 1 2 (+ 1 2)) ; Evaluates to the list '(1 2 3)
                     ; Can you draw pictures of these two lists?

A third way to construct lists is using the cons function, which takes two arguments and constructs a single cons cell:

   (cons 1 (cons 2 (cons 3 (cons 4 ()))))  ; Equivalent to '(1 2 3 4)

   ; Cons doesn't force you to construct well-formed ("proper") lists.
   ; It is only by convention that the first element carries the "payload"
   ; while the second element points to the "tail" (rest of the list)
   (cons 1 2)
   (cons (cons 1 2) (cons 3 ()))