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, you can use either
notation, but list notation is usually clearer and more convenient.
When printing a list, the dotted pair notation is only used if the
CDR of a cons cell is not a list.
Here's an example using boxes to illustrate dotted pair notation.
This example shows the pair (rose . violet):
--- ---
| | |--> violet
--- ---
|
|
--> rose
You can combine dotted pair notation with list notation to represent
conveniently a chain of cons cells with a non-nil final CDR.
You write a dot after the last element of the list, followed by the
CDR of the final cons cell. For example, (rose violet
. buttercup) is equivalent to (rose . (violet . buttercup)).
The object looks like this:
--- --- --- ---
| | |--> | | |--> buttercup
--- --- --- ---
| |
| |
--> rose --> violet
The syntax (rose . violet . buttercup) is invalid because
there is nothing that it could mean. If anything, it would say to put
buttercup in the CDR of a cons cell whose CDR is already
used for violet.
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))).
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