setq and defvar in Lisp

Question

I see that the Practical Common Lisp uses (defvar *db* nil) for setting up a global variable. Isn\'t it OK to use setq for the same purpos

Answer 1

defvar introduces a dynamic variable while setq is used to assign a value to a dynamic or lexical variable. The value of a dynamic variable is looked up in the environment that calls the function, while the value of a lexical variable is looked up in the environment where the function was defined. The following example will make the difference clear:

;; dynamic variable sample
> (defvar *x* 100)
*X*
> (defun fx () *x*)
FX
> (fx)
100
> (let ((*x* 500)) (fx)) ;; gets the value of *x* from the dynamic scope.
500
> (fx) ;; *x* now refers to the global binding.
100

;; example of using a lexical variable
> (let ((y 200))
   (let ((fy (lambda () (format t "~a~%" y))))
     (funcall fy) ;; => 200
     (let ((y 500))
       (funcall fy) ;; => 200, the value of lexically bound y
       (setq y 500) ;; => y in the current environment is modified
       (funcall fy)) ;; => 200, the value of lexically bound y, which was 
                     ;; unaffected by setq
     (setq y 500) => ;; value of the original y is modified.
     (funcall fy))) ;; => 500, the new value of y in fy's defining environment.

Dynamic variables are useful for passing around a default value. For instance, we can bind the dynamic variable *out* to the standard output, so that it becomes the default output of all io functions. To override this behavior, we just introduce a local binding:

> (defun my-print (s)
        (format *out* "~a~%" s))
MY-PRINT
> (my-print "hello")
hello
> (let ((*out* some-stream))
    (my-print " cruel ")) ;; goes to some-stream
> (my-print " world.")
world

A common use of lexical variables is in defining closures, to emulate objects with state. In the first example, the variable y in the binding environment of fy effectively became the private state of that function.

defvar will assign a value to a variable only if it is not already assigned. So the following re-definition of *x* will not change the original binding:

> (defvar *x* 400)
*X*
> *x*
100

We can assign a new value to *x* by using setq:

> (setq *x* 400)
400
> *x*
400
> (fx)
400
> (let ((*x* 500)) (fx)) ;; setq changed the binding of *x*, but 
                         ;; its dynamic property still remains.
500
> (fx)
400