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Date:         Mon, 10 Aug 2009 08:15:29 +0100
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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: Should \ExplSyntax(On/Off) be \protected?
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Status: R

Will Robertson wrote:
> Considering that the things that \ExplSyntaxXYZ do are not expandable, I
> think it would be correct to define them \protected.
> 
> Speaking generally, however, I'm not sure how much expansion we should
> be restricting. Lars in particular, recently, has been advocating
> keeping things expandable (and LuaTeX should broaden the applicability
> of what can be done there) but at this stage I've got nothing much to
> say :)

I perhaps should have suggested two general cases:

1) Design functions which clearly are expandable (such as \IfBooleanTF
or the version of \IfNoValue in xparse-alt): these should left alone as
they will behave "nicely".

2) Design functions which are not expandable (at least under pdfTeX or
XeTeX: LuaTeX raises lots of issues, I suspect). This includes
\ExplSyntaxOn/Off, \DeclareDocumentCommand and so on. I'd suggest that
these should be \protected.

I'm not saying it's likely that \edef-ing things like
\DeclareDocumentCommand is likely, but that it seems more consistent.
--
Joseph Wright