Question

> (cons 2 3) 
(2 . 3)

The Lisp environment needs to allocate only a single cons cell to connect the two items.

Above is from the Lisp book “Land of Lisp”. I don’t understand why this pair is only located in a single cons cell. What does the memory look like for this data?

Answer 1

A cons cell always holds two values, called car and cdr:

+-----+-----+
| car | cdr |
+-----+-----+

To represent a cons cell, Lisp has the “dot notation”:

(car . cdr)

The function cons creates such a cons cell from its two arguments:

(cons 1 2)
=> (1 . 2)

which can be thought of like this:

+-----+-----+
|  1  |  2  |
+-----+-----+

The values of a cons cell can also be “references” or “pointers” to other things. Those other things can, for example, be other cons cells:

+-----+-----+     +-----+-----+
|  1  |   ------->|  2  | nil |
+-----+-----+     +-----+-----+

This would be (1 . (2 . nil)) in dot notation. This chaining is used in Lisp to represent lists. Since lists are used for the representation of code, they are important for Lisp. Therefore, there is a shorter notation for them: (1 2).