Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!ncar!ames!pasteur!ucbvax!SUMEX-AIM.STANFORD.EDU!SCHMIDT
From: SCHMIDT@SUMEX-AIM.STANFORD.EDU (Christopher Schmidt)
Newsgroups: comp.sys.xerox
Subject: Re: defvar under Lyric
Message-ID: <12384217868.75.SCHMIDT@SUMEX-AIM.Stanford.EDU>
Date: 22 Mar 88 01:00:32 GMT
References: <8803211018.aa17239@VIM.BRL.ARPA>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 27


	I don't agree with the generalization "...that the def
constructs apply to global variables."  DEFVAR proclaims a variable
special (CLtL, p 68.).  Common Lisp lacks a global variable
declaration mechanism (a serious omission, in my opinion), so Xerox
Lisp provides XCL:DEFGLOBALVAR analogous to DEFVAR, that proclaims
variables global instead of special.  (I.e. compiled code uses the
GVAR opcode which is much more efficient than the FVAR opcode.)

	Although your letter spelled out the implications of the following
inequality, I think it bears repeating that
		(DEFVAR FOO 23)
is not equivalent to 
		(PROCLAIM '(SPECIAL FOO))
		(SETQ FOO 23)
because DEFVAR sets FOO only if it is unbound.  By the same token
		(XCL:DEFGLOBALVAR FOO 23)
is not equivalent to 
		(PROCLAIM '(XCL:GLOBAL FOO))
		(SETQ FOO 23)

	People writing portable Common Lisp code must write some ugly
conditional proclamations, I suspect, if they want good performance.
I hope your letter alerts some programmers to the pitfalls of DEFVAR.
[I have little experience writing portable code, personally.]
--Christopher
-------