📝Common Lisp: readtable

In Common Lisp, the reader is programmable. The reader is driven by *readtable*. When reader finds a character from the readtable, it invokes the corresponding reader macro. Reader macro is a function that receives the input stream as an argument, can read more characters and returns a structure.

Here is an example of reader macro for quote ('):

(defun single-quote-reader (stream char)
  (declare (ignore char))
  (list 'quote (read stream t nil t)))

(set-macro-character #\' #'single-quote-reader)

And here is a reader macro for semicolon (comments):

(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))

(set-macro-character #\; #'semicolon-reader)

I believe that most of the Common Lisp syntax (besides symbols and numbers) is implemented as reader macros (see a list of reader macros in CLtL2 22.1.3. Macro Characters).

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