set-dispatch-macro-character, get-dispatch-macro-characterFunction


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

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

    Arguments and Values

    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 (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
    See Also

    Section (Macro Characters)

    Side Effects

    The readtable is modified.

    Affected By


    Exceptional Situations

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

    See Also



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