> (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?
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).