Path: utzoo!attcan!uunet!mcsun!ukc!edcastle!aiai!jeff
From: jeff@aiai.ed.ac.uk (Jeff Dalton)
Newsgroups: comp.lang.lisp
Subject: Re: Constants in code (Ex- Virtues of Lisp syntax)
Message-ID: <3448@skye.ed.ac.uk>
Date: 25 Sep 90 17:11:25 GMT
References: <GUMBY.90Sep22150717@Cygnus.COM> <2863191137@ARTEMIS.cam.nist.gov>
Reply-To: jeff@aiai.UUCP (Jeff Dalton)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 22

I couldn't quite make out who wrote the stuff quoted below, so I
decided to avoid the risk of misattribution by not attributing it
to anyone.

>> I would be unhappy if quote performed computation each time you used 
>> it!  A very common idiom is to pass a quoted, uninterned symbol around
>> as a marker in some  structure.  (I used to  use '(())).  This is  the
>> only way  I  can  get  a  token  I  guarantee won't appear in my input
>> stream!

I always call LIST or CONS to get a unique object, rather than rely on
anything that involves QUOTE, although some people have pointed out
that, if you want an object that cannot appear in a stream, the stream
itself will do (unless, thanks to #., the stream is held in a global
variable).  That is, you do something like:

   (with-open-stream (s ...)
     (do ((input (read s nil s) (read s nil s)))
         ((eq input s))
       (process input)))

-- Jeff