set-macro-character char new-function &optional non-terminating-p readtable    t

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.

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.

 (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