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From: chip@ateng.com (Chip Salzenberg)
Newsgroups: comp.lang.lisp
Subject: The answer: Undeclared free variables are special
Message-ID: <256C52AF.135@ateng.com>
Date: 23 Nov 89 20:27:27 GMT
Organization: A T Engineering, Tampa, FL
Lines: 49

Thanks to many who responded to my earlier query about free variables.  The
question was, what is the output of this Common Lisp code:

	(setq *parrot* "live")      ;; no previous declaration of *parrot*

	(defun cleese ()
	  (let ((*parrot* "dead"))
	    (declare (special *parrot*))
	    (palin)
	    *parrot*))

	(defun palin ()
	  (print *parrot*))        ;; no declaration in scope

	(cleese)

The answer:

	"dead"
	"dead"

Quoting page 55 of Common Lisp the Language:

        There are actually two kinds of variables in Common Lisp, called
        _lexical_ (or _static_) variables and _special_ (or _dynamic_)
        variables.

Thus we have defined that there is no such thing as a "global" variable --
there are only lexical and special variables.  Continuing:

        The general rule is that if the symbol occurs textually within a
        program construct that creates a binding for a variable of the same
        name, then the reference is to the variable specified by the
        binding; if no such program construct textually contains the
        reference, then it is taken to refer to the special variable of that
	name.                                       ^^^^^^^ ^^^^^^^^

Now, we can all tell from the above code that *parrot* is not a lexical
variable.  It therefore must be special.  And since (cleese) created the
special binding of *parrot* to "dead", that's the binding (palin) sees.

This construct causes warnings from some compilers, since a reference to a
non-defvar'd free variable might be a misspelling.  But it's legal.

Thanks in particular to Barry Margolin and Tim Moore for helpful answers.
-- 
You may redistribute this article only to those who may freely do likewise.
Chip Salzenberg at A T Engineering;  <chip@ateng.com> or <uunet!ateng!chip>
    "Did I ever tell you the Jim Gladding story about the binoculars?"