CLHS: Function SET-DISPATCH-MACRO-CHARACTER...

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Function SET-DISPATCH-MACRO-CHARACTER, GET-DISPATCH-MACRO-CHARACTER

Syntax:

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.

Description:

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.

Examples:

     (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 2.1.4.4 (Macro Characters)

Side Effects:

The readtable is modified.

Affected By:

*readtable*.

Exceptional Situations:

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

See Also:

*readtable*

Notes:

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

The following X3J13 cleanup issue, not part of the specification, applies to this section:

- GET-MACRO-CHARACTER-READTABLE:NIL-STANDARD

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