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Message-ID:  <AF0E0200-8A77-491D-93E5-E08198E50B64@gmail.com>
Date:         Wed, 12 Aug 2009 09:59:57 +0930
Reply-To: Mailing list for the LaTeX3 project
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From: Will Robertson <wspr81@GMAIL.COM>
Subject: v expansion, was xparse
To: LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
In-Reply-To:  <4A81E3CA.6050608@residenset.net>
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Status: R

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On 12/08/2009, at 7:04 AM, Lars Hellstr=F6m wrote:

> Joseph Wright skrev:
>> The re-factor saw us drop "O", and sort-of downgrade "o" (we use it
>> less). The V specifier lets you write both
>> \foo_internal:V \l_my_tl  % A macro at the TeX level
>> and
>> \foo_internal:V \l_my_toks % A toks at the TeX level
>
> (o.0)
>
> Does it inspect the argument at runtime to figure out which of the =20
> 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 =20=

detect whether the argument is a macro, and if it is not inserts \the =20=

to extract the value of the toks/dim/whatever. The whole thing takes =20
place inside a \romannumeral expansion so the number of expansion =20
steps is constant.

>> whereas \foo_internal:o would need
>> \foo_internal:o {\l_my_tl}
>> \foo_internal:o { \toks_use:N \l_my_toks }
>
> I think I would rather prefer that, myself!

Only because you know what is happening underneath :)
But, furthermore, this only works if no error checking is to occur as =20=

the variable is "got". With "V", provided the error checking is =20
expandable, this can be transparently enabled without messing up code =20=

like the example above.

(There is no error checking at present, but may be added conditionally =20=

in the future.)

The idea is that the V expansions are a consistent wrapper around =20
"getting a variable", and needn't change if the underlying machinery =20
becomes different.

>> So V is preferred for accessing variables (as the internal =20
>> structure is
>> not important),
>
> Wouldn't you still need to know the variable's "internal structure" =20=

> when setting it?

But setting does not need to be done in a known number of expansion =20
steps, so the setting functions \toks_set:Nn, \tl_set:Nn, etc., all =20
behave analogously.

Will=

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