8.4.6.1 Dotted Pair Notation

Dotted pair notation is an alternative syntax for cons cells that represents the CAR and CDR explicitly. In this syntax, (a . b) stands for a cons cell whose CAR is the object a, and whose CDR is the object b. Dotted pair notation is therefore more general than list syntax. In the dotted pair notation, the list ‘(1 2 3)’ is written as ‘(1 . (2 . (3 . nil)))’. For nil-terminated lists, the two notations produce the same result, but list notation is usually clearer and more convenient when it is applicable. When printing a list, the dotted pair notation is only used if the CDR of a cell is not a list.

Here’s how box notation can illustrate dotted pairs. This example shows the pair (rose . violet):

    ___ ___
   |___|___|--> violet
     |
     |
      --> rose

Dotted pair notation can be combined with list notation to represent a chain of cons cells with a non-nil final CDR. For example, (rose violet . buttercup) is equivalent to (rose . (violet . buttercup)). The object looks like this:

    ___ ___      ___ ___
   |___|___|--> |___|___|--> buttercup
     |            |
     |            |
      --> rose     --> violet

These diagrams make it evident why (rose . violet . buttercup) is invalid syntax; it would require a cons cell that has three parts rather than two.

The list (rose violet) is equivalent to (rose . (violet)) and looks like this:

    ___ ___      ___ ___
   |___|___|--> |___|___|--> nil
     |            |
     |            |
      --> rose     --> violet

Similarly, the three-element list (rose violet buttercup) is equivalent to (rose . (violet . (buttercup))).