ANSI Common Lisp   22 Printer   22.1 The Lisp Printer   22.1.3 Default Print-Object Methods

22.1.3.5 Printing Lists and Conses

• 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 whitespace₁ may replace the use of a single space, and a more elaborate algorithm with similar goals but more presentational flexibility is used; see Section 22.1.2 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)

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 2.4.1 Left-Parenthesis.