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Sender: Mailing list for the LaTeX3 project <[log in to unmask]>
Date: Mon, 16 May 2011 14:49:20 +0200
Reply-To: Mailing list for the LaTeX3 project <[log in to unmask]>
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From: Lars Hellström <[log in to unmask]>
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My take would be as follows:

1. As several has observed: for compatibility with LaTeX2e, spaces should 
normally be skipped. This allows for formatting such as


2. Sometimes, an existing command is redefined so that an optional 
argument is added at the end. In such cases, it may be preferable to make 
that argument such that it does not skip spaces, for compatibility with 
existing documents written under the assumption that spaces would not be 
skipped at that point. In other words, if

    \bar{apa} cepa

used to have a space, and \bar for some reason in a package needs to be 
extended to support \bar{apa}[bepa], then \bar{apa} cepa should still have 
that space.

Therefore, the solution should be to provide both, but let the 
space-nonskipping variants come with a big warning in the xparse 
documentation, detailing why they are usually inappropriate. In 
particular, such documentation should suggest the argument order


as preferable to


since package authors are otherwise likely to pick one at random, never 
even considering the syntactical implications.

I also think the spacious formatting


needs to be mentioned in the documentation, because I'm sure there are 
plenty of LaTeX users out there who don't know this is normally possible, 
and therefore would be inclined to assume thight formatting if ever 
defining commands. That 2e package definitions are likely to say things like


certainly supports the mistaken impression that spaces are not allowed 
between arguments.

Lars Hellström