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(guile.info)Local Variables

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Local Variables and Environments
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We have seen how to create top level variables using the `define'
syntax (Note: 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 (REFFIXME).  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'.