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 14.4.2 Local Variables and Environments
 ---------------------------------------
 
 We have seen how to create top level variables using the `define'
 syntax ( Definition).  It is often useful to create variables
 that are more limited in their scope, typically as part of a procedure
 body.  In Scheme, this is done using the `let' syntax, or one of its
 modified forms `let*' and `letrec'.  These syntaxes are described in
 full later in the manual ( Local Bindings).  Here our purpose is
 to illustrate their use just enough that we can see how local variables
 work.
 
    For example, the following code uses a local variable `s' to
 simplify the computation of the area of a triangle given the lengths of
 its three sides.
 
      (define a 5.3)
      (define b 4.7)
      (define c 2.8)
 
      (define area
        (let ((s (/ (+ a b c) 2)))
          (sqrt (* s (- s a) (- s b) (- s c)))))
 
    The effect of the `let' expression is to create a new environment
 and, within this environment, an association between the name `s' and a
 new location whose initial value is obtained by evaluating `(/ (+ a b
 c) 2)'.  The expressions in the body of the `let', namely `(sqrt (* s
 (- s a) (- s b) (- s c)))', are then evaluated in the context of the
 new environment, and the value of the last expression evaluated becomes
 the value of the whole `let' expression, and therefore the value of the
 variable `area'.
 

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