Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!zaphod.mps.ohio-state.edu!think!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: Question about Macros (Common Lisp)
Message-ID: <33613@news.Think.COM>
Date: 3 Feb 90 05:15:42 GMT
References: <31480@shemp.CS.UCLA.EDU>
Sender: news@Think.COM
Distribution: usa
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 32

In article <31480@shemp.CS.UCLA.EDU> srt@maui.cs.ucla.edu (Scott Turner) writes:
>What's the solution?  I have to delay the de-reference of the symbol until 
>the macro is actually expanded.  I came up with this:
>	`(setf (structure-test foo)
>		#'(lambda (,(intern "*SPEC*")) ,@test-part))
>Which (surprise!) works, but which I find pretty ugly and disgusting.
>So, is there a better way to do this?  Or is my whole approach wrong-minded?

I have a couple of ideas.

1. Require the users of your package to (import 'rule::*spec*).

2. Don't automatically bind the variable *spec*.  Allow the macro user to
specify his variable.  I think this is a good idea regardless of the
package problem.  If you need the special variable bound in order to
communicate among internal routines of the Rule system, bind it using LET
and then pass the value to the functions created by the macro.  The only
problem with this is that if the user writes (setq <his-spec-variable>
<new-value>) it won't change *spec*.

Symbolics Genera has some facilities that work like your macro; for
instance, their ADVISE macro uses the variables ARGLIST and VALUES
similarly.  Their solution is a variant on #1 above -- these symbols are
external in the COMMON-LISP package (VALUES is a CL function name, and
ARGLIST is a CL declaration name), and most user programs are in packages
that inherit from COMMON-LISP, so they get the right symbol.  However, I
wouldn't suggest this solution for your problem.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar