3.4.3.3 Whitespace and comments

Whitespace characters are spaces, linefeeds, carriage returns, character tabulations, form feeds, line tabulations, and any other character whose category is Zs, Zl, or Zp. Whitespace is used for improved readability and as necessary to separate lexemes from each other. Whitespace may occur between any two lexemes, but not within a lexeme. Whitespace may also occur inside a string, where it is significant.

The lexical syntax includes several comment forms. In all cases, comments are invisible to Scheme, except that they act as delimiters, so, for example, a comment cannot appear in the middle of an identifier or representation of a number object.

A semicolon (;) indicates the start of a line comment. The comment continues to the end of the line on which the semicolon appears.

Another way to indicate a comment is to prefix a ?datum (cf. section “Formal account”) with #;, possibly with ?interlexeme-space before the ?datum. The comment consists of the comment prefix #; and the ?datum together. This notation is useful for “commenting out” sections of code.

Block comments may be indicated with properly nested #| and |# pairs.

#|
   The FACT procedure computes the factorial of a
   non-negative integer.
|#
(define fact
  (lambda (n)
    ;; base case
    (if (= n 0)
        #;(= n 1)
        1       ; identity of *
        (* n (fact (- n 1))))))

The lexeme #!r6rs, which signifies that the program text that follows is written with the lexical and datum syntax described in this report, is also otherwise treated as a comment.