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Message-ID:  <BANLkTi=qwNmk+ZmhbV70LCdhZkX57_QDaA@mail.gmail.com>
Date:         Wed, 11 May 2011 15:01:19 -0400
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From: Bruno Le Floch <blflatex@GMAIL.COM>
Subject: Re: xparse and space skipping
To: LATEX-L@listserv.uni-heidelberg.de
In-Reply-To:  <4DCA93CC.5020605@morningstar2.co.uk>
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Status: R

>   \foo[bar]
>   \foo [bar]
>   \foo  text
>
> TeX will always skip spaces and so things are not consistent. In the
> same way, mandatory arguments are always grabbed

Not quite true. For instance, amsmath redefined \\ not to skip spaces
when looking for an optional argument, I believe. And that works
because \\ is a control symbol, not a control word. I think this
choice of behaviour really needs to be supported, since it leads to
surprises for the end-user otherwise.

> really correct! (At the same time, the implementation would be slightly
> easier to follow if this change was made.)

On the other hand, emulating LaTeX2e arguments with space skipping
seems needed as well. Is it possible to get the best of the two worlds
with a "o_no_skip" argument specifier? Or more generally a modifier,
say K, which makes the next argument keep spaces (except perhaps for m
arguments where it should produce an error)?

\DeclareDocumentCommand {\\} { Ks Ko } {...}

Just my take on it.
Regards,
Bruno