CLHS: Section 2.4.8

2.4.8 Sharpsign

Sharpsign is a non-terminating dispatching macro character. It reads an optional sequence of digits and then one more character, and uses that character to select a function to run as a reader macro function.

The standard syntax includes constructs introduced by the # character. The syntax of these constructs is as follows: a character that identifies the type of construct is followed by arguments in some form. If the character is a letter, its case is not important; #O and #o are considered to be equivalent, for example.

Certain # constructs allow an unsigned decimal number to appear between the # and the character.

The reader macros associated with the dispatching macro character # are described later in this section and summarized in the next figure.

dispatch char  purpose                  dispatch char  purpose                
Backspace      signals error            {              undefined*             
Tab            signals error            }              undefined*             
Newline        signals error            +              read-time conditional  
Linefeed       signals error            -              read-time conditional  
Page           signals error            .              read-time evaluation   
Return         signals error            /              undefined              
Space          signals error            A, a           array                  
!              undefined*               B, b           binary rational        
"              undefined                C, c           complex number         
#              reference to = label     D, d           undefined              
$              undefined                E, e           undefined              
%              undefined                F, f           undefined              
&              undefined                G, g           undefined              
'              function abbreviation    H, h           undefined              
(              simple vector            I, i           undefined              
)              signals error            J, j           undefined              
*              bit vector               K, k           undefined              
,              undefined                L, l           undefined              
:              uninterned symbol        M, m           undefined              
;              undefined                N, n           undefined              
<              signals error            O, o           octal rational         
=              labels following object  P, p           pathname               
>              undefined                Q, q           undefined              
?              undefined*               R, r           radix-n rational       
@              undefined                S, s           structure              
[              undefined*               T, t           undefined              
\              character object         U, u           undefined              
]              undefined*               V, v           undefined              
^              undefined                W, w           undefined              
_              undefined                X, x           hexadecimal rational   
`              undefined                Y, y           undefined              
|              balanced comment         Z, z           undefined              
~              undefined                Rubout         undefined

Figure 2-19. Standard #Dispatching Macro Character Syntax

The combinations marked by an asterisk (*) are explicitly reserved to the user. No conforming implementation defines them.

Note also that digits do not appear in the preceding table. This is because the notations #0, #1, ..., #9 are reserved for another purpose which occupies the same syntactic space. When a digit follows a sharpsign, it is not treated as a dispatch character. Instead, an unsigned integer argument is accumulated and passed as an argument to the reader macro for the character that follows the digits. For example, #2A((1 2) (3 4)) is a use of #A with an argument of 2.