Node:Local Variables, Next:Chaining, Previous:About Environments, Up:About Closure
We have seen how to create top level variables using the define
syntax (see 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 (see 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.