get-dispatch-macro-character disp-char sub-char &optional readtable → function

set-dispatch-macro-character disp-char sub-char new-function &optional readtable → t

disp-char — a character.

sub-char — a character.

readtable — a readtable designator. The default is the current readtable.

function — a function designator or nil.

new-function — a function designator.

set-dispatch-macro-character causes new-function to be called when disp-char followed by sub-char is read. If sub-char is a lowercase letter, it is converted to its uppercase equivalent. It is an error if sub-char is one of the ten decimal digits.

set-dispatch-macro-character installs a new-function to be called when a particular dispatching macro character pair is read. New-function is installed as the dispatch function to be called when readtable is in use and when disp-char is followed by sub-char.

For more information about how the new-function is invoked, see Section 2.1.4.4 (Macro Characters).

get-dispatch-macro-character retrieves the dispatch function associated with disp-char and sub-char in readtable.

get-dispatch-macro-character returns the macro-character function for sub-char under disp-char, or nil if there is no function associated with sub-char. If sub-char is a decimal digit, get-dispatch-macro-character returns nil.

(get-dispatch-macro-character #\# #\{) → NIL 
(set-dispatch-macro-character #\# #\{        ;dispatch on #{ 
   #'(lambda(s c n) 
       (let ((list (read s nil (values) t)))  ;list is object after #n{ 
         (when (consp list)                   ;return nth element of list 
           (unless (and n (< 0 n (length list))) (setq n 0)) 
           (setq list (nth n list))) 
        list))) → T 
#{(1 2 3 4) → 1 
#3{(0 1 2 3) → 3 
#{123 → 123

If it is desired that #$foo : as if it were (dollars foo).

(defun |#$-reader| (stream subchar arg) 
   (declare (ignore subchar arg)) 
   (list 'dollars (read stream t nil t))) → |#$-reader| 
 (set-dispatch-macro-character #\# #\$ #'|#$-reader|) → T

Section 2.1.4.4 (Macro Characters)

The readtable is modified.

*readtable*.

For either function, an error is signaled if disp-char is not a dispatching macro character in readtable.

*readtable*

It is necessary to use make-dispatch-macro-character to set up the dispatch character before specifying its sub-characters.