Path: utzoo!attcan!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!mailrus!ames!necntc!primerd!zaphod!doug
From: doug@zaphod.prime.com
Newsgroups: comp.lang.lisp
Subject: Re: SETQ vs DEFVAR at the top level
Message-ID: <26500003@zaphod>
Date: 1 Jul 88 14:31:00 GMT
References: <4431@medusa.cs.purdue.edu>
Lines: 33
Nf-ID: #R:medusa.cs.purdue.edu:-443100:zaphod:26500003:000:1380
Nf-From: zaphod.prime.com!doug    Jul  1 10:31:00 1988


I think the difference between SETQ and DEFVAR in this instance is 
easy to explain.  A SETQ at top level will refer to a global, hence
dynamically scoped variable.  A SETQ does not make any declarations 
about the referenced variable.  A DEFVAR form is not equivalent to 
a SETQ.  It differs in one unimportant and one very important way:

1) Unimportant,  it only will give a value to an already unbound variable
2) Important, it does the equivalent to (PROCLAIM '(SPECIAL variable))

That second thing is what has screwed you up.  The proclaimation is global.
It overrides the normal behavior everywhere you use the symbol.  So not
only does your binding of *Q* in the let become dynamic but any use
of *Q* as a parameter would become dynamic.

Lest you think that I'm just rationalizing the following passage from
CLtL relates to LET bindings (and therefore do DEFUN, LAMBDA, etc.):

... each binding will be a lexical binding unless there is a special
declaration to the contrary...

PROCLAIM has global extent and effects all such declarations.

--------------------
Douglas Rand
 
  Internet:  doug@zaphod.prime.com
  Usenet:    primerd!zaphod!doug 
  Phone:     (508) - 879 - 2960
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->  The above opinions are mine alone and are not influenced by
    my employer,  my cat or the clothes I wear.