Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!rutgers!aramis.rutgers.edu!atanasoff.rutgers.edu!lou
From: lou@atanasoff.rutgers.edu (Lou Steinberg)
Newsgroups: comp.lang.lisp
Subject: Re: Question about Macros (Common Lisp)
Message-ID: <Feb.5.11.39.45.1990.3151@atanasoff.rutgers.edu>
Date: 5 Feb 90 16:39:59 GMT
References: <31480@shemp.CS.UCLA.EDU>
Distribution: usa
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 46
To: srt@maui.cs.ucla.edu

In article <31480@shemp.CS.UCLA.EDU> srt@maui.cs.ucla.edu (Scott Turner) writes:

> [...] I write a simple defmacro, the important part of which looks
> like this:
> 
> 	`(setf (structure-test foo)
> 	       #'(lambda (*spec*) ,@test-part))
> 
> Let's say that I define this macro in the RULE package.  Later on, I go to 
> use this in the USER package, and looking at the macro-expansion I see 
> this:
> 
> 	(lambda (rule::*spec*) (eql *spec* 'hyper))
> 
> 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))

Even this is not guaranteed to work - it depends on the current
package being the same at macro expand time as it was when the macro
call was read.  On some systems, the interpreter expands macros at
macro call time, when the package may be different.  To really do it
right, you need to search the value of test-part for any symbol whose
print-name is "*SPEC*", and use that symbol for your lambda variable.
Of course, this means different rules may be using different lambda
variables, if they were read in under different packages.

A possibly cleaner solution is to find any symbol in test-part that
has a print-name of *SPEC* and replace it with YOUR symbol *SPEC*.

	 `(setf (structure-test foo)
 		#'(lambda (*SPEC*)
		   ,@(subst-if  '*SPEC* 
		             #'(lambda (x) (and (symbol x)
					        (string-equal (symbol-name x)
					                      "*SPEC*")))n
			     test-part)))

(Warning - the above is not debugged.)
-- 
					Lou Steinberg

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