User-Agent: Thunderbird 2.0.0.22 (Windows/20090605)
MIME-Version: 1.0
References: <4A7921CF.5020803@morningstar2.co.uk>                              
            <4A7A1505.4040604@residenset.net>                                  
            <DB76B425-0A88-43B4-9CF0-B92FAF159E37@gmail.com>                   
            <4A7AD930.2090106@residenset.net>                                 
            <8516B615-51AA-4D90-BB7D-A9E122AA0335@gmail.com>                   
            <4A816C31.9080401@residenset.net>           
            <4A817463.8000805@morningstar2.co.uk>           
            <4A81E3CA.6050608@residenset.net>
            <AF0E0200-8A77-491D-93E5-E08198E50B64@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Message-ID:  <4A825031.6000007@morningstar2.co.uk>
Date:         Wed, 12 Aug 2009 06:16:33 +0100
Reply-To: Mailing list for the LaTeX3 project
              <LATEX-L@LISTSERV.UNI-HEIDELBERG.DE>
Sender: Mailing list for the LaTeX3 project <LATEX-L@LISTSERV.UNI-HEIDELBERG.DE>
From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: v expansion, was xparse
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <AF0E0200-8A77-491D-93E5-E08198E50B64@gmail.com>
Precedence: list
Status: R

Will Robertson wrote:
>> Does it inspect the argument at runtime to figure out which of the two
>> cases is at hand? Looks rather fragile & syntactic sugar to me.
> 
> The heuristic is fairly safe, I believe. It uses \ifx\noexpand#1#1 to
> detect whether the argument is a macro, and if it is not inserts \the to
> extract the value of the toks/dim/whatever. The whole thing takes place
> inside a \romannumeral expansion so the number of expansion steps is
> constant.

Thus far, all seems to work okay. Whether that means it never goes wrong
 ...

> (There is no error checking at present, but may be added conditionally
> in the future.)

Will, don't forget that it does catch things like:

\csname l_does_not_exist_tl\endcsname

which using :o will not. (Variables are not meant to equal \relax under
any circumstances, according to the LaTeX3 specs.)

>>> So V is preferred for accessing variables (as the internal structure is
>>> not important),
>>
>> Wouldn't you still need to know the variable's "internal structure"
>> when setting it?
> 
> But setting does not need to be done in a known number of expansion
> steps, so the setting functions \toks_set:Nn, \tl_set:Nn, etc., all
> behave analogously.

As Will says, we have set functions for everything, and also use
functions, the idea begin that you can always know that

\<var>_use:N \l_my_var

and

\<var>_set:Nn \l_my_var {<content>}

will be correct (of coure <content> must be correct here!).
-- 
Joseph Wright