Setq and defvar in Lisp

To give the short answer, macros are used for defining language syntax extensions to Common Lisp or Domain Specific Languages (DSLs). These languages are embedded right into the existing Lisp code. Now, the DSLs can have syntax similar to Lisp (like Peter Norvig's Prolog Interpreter for Common Lisp) or completely different (e.g. Infix Notation Math for Clojure).

Here is a more concrete example:
Python has list comprehensions built into the language. This gives a simple syntax for a common case. The line

divisibleByTwo = [x for x in range(10) if x % 2 == 0]

yields a list containing all even numbers between 0 and 9. Back in the Python 1.5 days there was no such syntax; you'd use something more like this:

divisibleByTwo = []
for x in range( 10 ):
   if x % 2 == 0:
      divisibleByTwo.append( x )

These are both functionally equivalent. Let's invoke our suspension of disbelief and pretend Lisp has a very limited loop macro that just does iteration and no easy way to do the equivalent of list comprehensions.

In Lisp you could write the following. I should note this contrived example is picked to be identical to the Python code not a good example of Lisp code.

;; the following two functions just make equivalent of Python's range function
;; you can safely ignore them unless you are running this code
(defun range-helper (x)
  (if (= x 0)
      (list x)
      (cons x (range-helper (- x 1)))))

(defun range (x)
  (reverse (range-helper (- x 1))))

;; equivalent to the python example:
;; define a variable
(defvar divisibleByTwo nil)

;; loop from 0 upto and including 9
(loop for x in (range 10)
   ;; test for divisibility by two
   if (= (mod x 2) 0) 
   ;; append to the list
   do (setq divisibleByTwo (append divisibleByTwo (list x))))

Before I go further, I should better explain what a macro is. It is a transformation performed on code by code. That is, a piece of code, read by the interpreter (or compiler), which takes in code as an argument, manipulates and the returns the result, which is then run in-place.

Of course that's a lot of typing and programmers are lazy. So we could define DSL for doing list comprehensions. In fact, we're using one macro already (the loop macro).

Lisp defines a couple of special syntax forms. The quote (') indicates the next token is a literal. The quasiquote or backtick (`) indicates the next token is a literal with escapes. Escapes are indicated by the comma operator. The literal '(1 2 3) is the equivalent of Python's [1, 2, 3]. You can assign it to another variable or use it in place. You can think of `(1 2 ,x) as the equivalent of Python's [1, 2, x] where x is a variable previously defined. This list notation is part of the magic that goes into macros. The second part is the Lisp reader which intelligently substitutes macros for code but that is best illustrated below:

So we can define a macro called lcomp (short for list comprehension). Its syntax will be exactly like the python that we used in the example [x for x in range(10) if x % 2 == 0] - (lcomp x for x in (range 10) if (= (% x 2) 0))

(defmacro lcomp (expression for var in list conditional conditional-test)
  ;; create a unique variable name for the result
  (let ((result (gensym)))
    ;; the arguments are really code so we can substitute them 
    ;; store nil in the unique variable name generated above
    `(let ((,result nil))
       ;; var is a variable name
       ;; list is the list literal we are suppose to iterate over
       (loop for ,var in ,list
            ;; conditional is if or unless
            ;; conditional-test is (= (mod x 2) 0) in our examples
            ,conditional ,conditional-test
            ;; and this is the action from the earlier lisp example
            ;; result = result + [x] in python
            do (setq ,result (append ,result (list ,expression))))
           ;; return the result 
       ,result)))

Now we can execute at the command line:

CL-USER> (lcomp x for x in (range 10) if (= (mod x 2) 0))
(0 2 4 6 8)

Pretty neat, huh? Now it doesn't stop there. You have a mechanism, or a paintbrush, if you like. You can have any syntax you could possibly want. Like Python or C#'s with syntax. Or .NET's LINQ syntax. In end, this is what attracts people to Lisp - ultimate flexibility.

What's going on?

You say: feel like there is nothing special is going on here. The outer foo in bar increments the global x, and foo surrounded by let in bar increments the shadowed x. What's the big deal?

The special that's going on here is that LET can shadow the value of *x*. With lexical variables that's not possible.

The code declares *x* to be special via the DEFVAR.

In FOO now the value of *x* is looked up dynamic. FOO will take the current dynamic binding of *x* or, if there is none, the symbol value of the symbol *x*. A new dynamic binding can, for example, be introduced with LET.

A lexical variable on the other hand has to be present in the lexical environment somewhere. LET, LAMBDA, DEFUN and others can introduce such lexical variables. See here the lexical variable x introduced in three different ways:

(let ((x 3))
  (* (sin x) (cos x)))

(lambda (x)
  (* (sin x) (cos x)))

(defun baz (x)
  (* (sin x) (cos x)))

If our code were:

(defvar x 0)

(let ((x 3))
  (* (sin x) (cos x)))

(lambda (x)
  (* (sin x) (cos x)))

(defun baz (x)
  (* (sin x) (cos x)))

Then X were special in all three above cases, because of the DEFVAR declaration, which declares X to be special - globally for all levels. Because of this, there is the convention to declare special variables as *X*. Thus only variables with stars around them are special - by convention. That's a useful convention.

In your code you have then:

(defun bar ()
  (foo)
  (let ((*x* 20))
    (foo))
  (foo))

Since *x* has be declared special via the DEFVAR above in your code, the LET construct introduces a new dynamic binding for *x*. FOO is then called. Since inside FOO the *x* uses dynamic binding, it looks up the current one and finds that *x* is dynamically bound to 20.

The value of a special variable is found in the current dynamic binding.

Local SPECIAL declarations

There are also local special declarations:

(defun foo-s ()
  (declare (special *x*))
  (+ *x* 1))

If the variable had been declared special by a DEFVAR or DEFPARAMETER, then the local special declaration can be omitted.

A lexical variable directly references the variable binding:

(defun foo-l (x)
  (+ x 1))

Let's see it in practice:

(let ((f (let ((x 10))
           (lambda ()
             (setq x (+ x 1))))))
  (print (funcall f))    ; form 1
  (let ((x 20))          ; form 2
    (print (funcall f))))

Here all variables are lexical. In form 2 the LET will not shadow the X in our function f. It can't. The function uses the lexical bound variable, introduced by the LET ((X 10). Surrounding the call with another lexically bound X in form 2 has no effect on our function.

Let's try special variables:

(let ((f (let ((x 10))
           (declare (special x))
           (lambda ()
             (setq x (+ x 1))))))
  (print (funcall f))    ; form 1
  (let ((x 20))          ; form 2
    (declare (special x))
    (print (funcall f))))

What now? Does that work?

It does not!

The first form calls the function and it tries to look up the dynamic value of X and there is none. We get an error in form 1: X is unbound, because there is no dynamic binding in effect.

Form 2 would work, since the LET with the special declaration introduces a dynamic binding for X.