Path: utzoo!attcan!uunet!tektronix!tekcrl!tekchips!willc
From: willc@tekchips.TEK.COM (Will Clinger)
Newsgroups: comp.lang.lisp
Subject: Re: SETQ vs DEFVAR at the top level
Summary: DEFVAR PROCLAIMs, SETQ doesn't
Keywords: closure, HELP, DEFVAR, PROCLAIM, SPECIAL
Message-ID: <3368@tekchips.TEK.COM>
Date: 1 Jul 88 19:13:16 GMT
References: <4431@medusa.cs.purdue.edu>
Reply-To: willc@tekchips.UUCP (Will Clinger)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 19

In article <4431@medusa.cs.purdue.edu> bouma@cs.purdue.EDU (William J. Bouma) writes:
>
>     In Common Lisp, what difference does it make if one gives his 
>global variables lexical scope (via SETQ) or dynamic scope (via DEFVAR)?
>None you say, only it is better style to use DEFVAR? WRONG bucko!

    [long example deleted]

That's right, friends.  DEFVAR implicitly PROCLAIMs its variable to be
SPECIAL, while SETQ does not.  A variable PROCLAIMed SPECIAL by DEFVAR
cannot be used as a lexical variable, and thus cannot be captured by a
closure.  Common Lisp provides DEFVAR (and PROCLAIM) to make it easier
to introduce mysterious scoping bugs such as the one in Bouma's example.
Without DEFVAR and PROCLAIM, each binding instance of a special variable
would have to be explicitly declared as special, eliminating the need
to scan entire programs in order to determine that a variable that
appears lexical is in fact so.

Peace, Will