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| ANSI Common Lisp 23 Reader 23.2 Dictionary of Reader
23.2.10 set-macro-character, get-macro-character | Function
  Syntax: get-macro-character char `&optional` readtable function, non-terminating-p set-macro-character char new-function `&optional` non-terminating-p readtable t Arguments and Values: char - a character. non-terminating-p - a generalized boolean. The default is false. readtable - a readtable designator. The default is the current readtable. function - nil, or a designator for a function of two arguments. new-function - a function designator. Description: get-macro-character returns as its primary value, function, the reader macro function associated with char in readtable (if any), or else nil if char is not a macro character in readtable. The secondary value, non-terminating-p, is true if char is a non-terminating macro character; otherwise, it is false. set-macro-character causes char to be a macro character associated with the reader macro function new-function (or the designator for new-function) in readtable. If non-terminating-p is true, char becomes a non-terminating macro character; otherwise it becomes a terminating macro character. Examples:
       (get-macro-character #\{)   NIL, false
       (not (get-macro-character #\;))  false
  The following is a possible definition for the single-quote reader macro in standard syntax:
       (defun single-quote-reader (stream char)
         (declare (ignore char))
         (list 'quote (read stream t nil t)))   SINGLE-QUOTE-READER
       (set-macro-character #\' #'single-quote-reader)   T
  Here `single-quote-reader` reads an object following the single-quote and returns a list of quote and that object. The char argument is ignored. The following is a possible definition for the semicolon reader macro in standard syntax:
       (defun semicolon-reader (stream char)
         (declare (ignore char))
         ;; First swallow the rest of the current input line.
         ;; End-of-file is acceptable for terminating the comment.
         (do () ((char= (read-char stream nil #\Newline t) #\Newline)))
         ;; Return zero values.
         (values))   SEMICOLON-READER
       (set-macro-character #\; #'semicolon-reader)   T
Side Effects: The readtable is modified. See Also: *readtable* Allegro CL Implementation Details: None.

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