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3.1.3 Defining and Setting Variables

   To define a new variable, you use Scheme's define syntax like this:
(define variable-name value)

   This makes a new variable called variable-name and stores value in it as the
   variable's initial value. For example:
;; Make a variable `x' with initial numeric value 1.
(define x 1)

;; Make a variable `organization' with an initial string value.
(define organization "Free Software Foundation")

   (In Scheme, a semicolon marks the beginning of a comment that continues until the
   end of the line. So the lines beginning ;; are comments.)

   Changing the value of an already existing variable is very similar, except that
   define is replaced by the Scheme syntax set!, like this:
(set! variable-name new-value)

   Remember that variables do not have fixed types, so new-value may have a
   completely different type from whatever was previously stored in the location
   named by variable-name. Both of the following examples are therefore correct.
;; Change the value of `x' to 5.
(set! x 5)

;; Change the value of `organization' to the FSF's street number.
(set! organization 545)

   In these examples, value and new-value are literal numeric or string values. In
   general, however, value and new-value can be any Scheme expression. Even though
   we have not yet covered the forms that Scheme expressions can take (see
   [10]Expressions and Evaluation), you can probably guess what the following set!
   example does...
(set! x (+ x 1))

   (Note: this is not a complete description of define and set!, because we need to
   introduce some other aspects of Scheme before the missing pieces can be filled
   in. If, however, you are already familiar with the structure of Scheme, you may
   like to read about those missing pieces immediately by jumping ahead to the
   following references.
     * [11]Lambda Alternatives, to read about an alternative form of the define
       syntax that can be used when defining new procedures.
     * [12]Procedures with Setters, to read about an alternative form of the set!
       syntax that helps with changing a single value in the depths of a compound
       data structure.)
     * See [13]Internal definitions, to read about using define other than at top
       level in a Scheme program, including a discussion of when it works to use
       define rather than set! to change the value of an existing variable.
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References

   1. http://www.gnu.org/software/guile/manual/html_node/index.html
   2. http://www.gnu.org/software/guile/manual/html_node/Concept-Index.html
   3. http://www.gnu.org/software/guile/manual/html_node/index.html#SEC_Contents
   4. http://www.gnu.org/software/guile/manual/html_node/About-Data.html
   5. http://www.gnu.org/software/guile/manual/html_node/Values-and-Variables.html
   6. http://www.gnu.org/software/guile/manual/html_node/Values-and-Variables.html
   7. http://www.gnu.org/software/guile/manual/html_node/About-Data.html
   8. http://www.gnu.org/software/guile/manual/html_node/index.html#SEC_Contents
   9. http://www.gnu.org/software/guile/manual/html_node/Concept-Index.html
  10. http://www.gnu.org/software/guile/manual/html_node/About-Expressions.html
  11. http://www.gnu.org/software/guile/manual/html_node/Lambda-Alternatives.html
  12. http://www.gnu.org/software/guile/manual/html_node/Procedures-with-Setters.html
  13. http://www.gnu.org/software/guile/manual/html_node/Internal-Definitions.html
  14. http://www.gnu.org/software/guile/manual/html_node/Values-and-Variables.html
  15. http://www.gnu.org/software/guile/manual/html_node/About-Data.html
  16. http://www.gnu.org/software/guile/manual/html_node/index.html#SEC_Contents
  17. http://www.gnu.org/software/guile/manual/html_node/Concept-Index.html