User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:9.0) Gecko/20111222
            Thunderbird/9.0.1
MIME-Version: 1.0
References: <D6C19E2C-AAB5-4237-8102-0FCE69E1F442@gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Message-ID:  <4F254832.8040906@morningstar2.co.uk>
Date:         Sun, 29 Jan 2012 13:22:58 +0000
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: Unexpandability inside \csname
To: LATEX-L@listserv.uni-heidelberg.de
In-Reply-To:  <D6C19E2C-AAB5-4237-8102-0FCE69E1F442@gmail.com>
Precedence: list
Status: R

Hello Will,

On 29/01/2012 12:56, Will Robertson wrote:
> Consider:
> 
>   \csname\ifnum 3 > 2 foo\fi\endcsname
> 
> This (probably obviously to all of you) complains with the standard "Missing \endcsname inserted." presumably because there's an unexpandable implicit \relax inserted somewhere in there.

No, it works fine :-) I assume you were thinking of something else.

> In expl3 we've discussed the concept of "restricted expandability", which refers to an expandable function that doesn't fully expand inside an "f" function (which is expandable \romannumeral-style expansion).
> 
> Does it make sense to also indicate how/where expandable functions won't behave correctly inside "c" arguments? I must admit I haven't considered the ramifications of what these mean entirely. It does seem there's not necessarily much overlap between the f-unexpandable functions and the c-unexpandable ones.

Expansion in an "f"-type argument is more 'restricted' than in a
"x"-type one as "f" expansion stops when the first non-expandable token
is left in the input stream. In that sense, "c"-type expansion should be
compared to "x"-type expansion, since TeX will keep going until it finds
an \endcsname. Now, of course there is a difference as for example a
protected macro is fine in an "x"-type expansion but no in a "c"-type.
-- 
Joseph Wright