2.4.4 Semicolon

Syntax: ;«text»

A semicolon introduces characters that are to be ignored, such as comments. The semicolon and all characters up to and including the next newline or end of file are ignored.

2.4.4.1 Examples of Semicolon

 (+ 3 ; three
    4)
→ 7

2.4.4.2 Notes about Style for Semicolon

Some text editors make assumptions about desired indentation based on the number of semicolons that begin a comment. The following style conventions are common, although not by any means universal.

2.4.4.2.1 Use of Single Semicolon

Comments that begin with a single semicolon are all aligned to the same column at the right (sometimes called the “comment column”). The text of such a comment generally applies only to the line on which it appears. Occasionally two or three contain a single sentence together; this is sometimes indicated by indenting all but the first with an additional space (after the semicolon).

2.4.4.2.2 Use of Double Semicolon

Comments that begin with a double semicolon are all aligned to the same level of indentation as a form would be at that same position in the code. The text of such a comment usually describes the state of the program at the point where the comment occurs, the code which follows the comment, or both.

2.4.4.2.3 Use of Triple Semicolon

Comments that begin with a triple semicolon are all aligned to the left margin. Usually they are used prior to a definition or set of definitions, rather than within a definition.

2.4.4.2.4 Use of Quadruple Semicolon

Comments that begin with a quadruple semicolon are all aligned to the left margin, and generally contain only a short piece of text that serve as a title for the code which follows, and might be used in the header or footer of a program that prepares code for presentation as a hardcopy document.

2.4.4.2.5 Examples of Style for Semicolon

;;;; Math Utilities

;;; FIB computes the the Fibonacci function in the traditional
;;; recursive way.

(defun fib (n)
  (check-type n integer)
  ;; At this point we're sure we have an integer argument.
  ;; Now we can get down to some serious computation.
  (cond ((< n 0)
         ;; Hey, this is just supposed to be a simple example.
         ;; Did you really expect me to handle the general case?
         (error "FIB got ~D as an argument." n))
        ((< n 2) n)             ;fib[0]=0 and fib[1]=1
        ;; The cheap cases didn't work.
        ;; Nothing more to do but recurse.
        (t (+ (fib (- n 1))     ;The traditional formula
              (fib (- n 2)))))) ; is fib[n-1]+fib[n-2].