Printing Lists and Conses

Wherever possible, list notation is preferred over dot notation. Therefore the following algorithm is used to print a cons x:

1.

A left-parenthesis is printed.

2.

The car of x is printed.

3.

If the cdr of x is itself a cons, it is made to be the current cons (i.e., x becomes that cons), a space is printed, and step 2 is re-entered.

4.

If the cdr of x is not null, a space, a dot, a space, and the cdr of x are printed.

5.

A right-parenthesis is printed.

Actually, the above algorithm is only used when *print-pretty* is false. When *print-pretty* is true (or when pprint is used), additional whitespace_1 may replace the use of a single space, and a more elaborate algorithm with similar goals but more presentational flexibility is used; see section Printer Dispatching.

Although the two expressions below are equivalent, and the reader accepts either one and produces the same cons, the printer always prints such a cons in the second form.

 (a . (b . ((c . (d . nil)) . (e . nil))))
 (a b (c d) e)

The printing of conses is affected by *print-level*, *print-length*, and *print-circle*.

Following are examples of printed representations of lists:

 (a . b)     ;A dotted pair of a and b
 (a.b)       ;A list of one element, the symbol named a.b
 (a. b)      ;A list of two elements a. and b
 (a .b)      ;A list of two elements a and .b
 (a b . c)   ;A dotted list of a and b with c at the end; two conses
 .iot        ;The symbol whose name is .iot
 (. b)       ;Invalid -- an error is signaled if an attempt is made to read 
             ;this syntax.
 (a .)       ;Invalid -- an error is signaled.
 (a .. b)    ;Invalid -- an error is signaled.
 (a . . b)   ;Invalid -- an error is signaled.
 (a b c ...) ;Invalid -- an error is signaled.
 (a \. b)    ;A list of three elements a, ., and b
 (a |.| b)   ;A list of three elements a, ., and b
 (a \... b)  ;A list of three elements a, ..., and b
 (a |...| b) ;A list of three elements a, ..., and b

For information on how the Lisp reader parses lists and conses, see section Left-Parenthesis.

Go to the first, previous, next, last section, table of contents.