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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: Quarks: revisions of use in expl3
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On 22/09/2010 13:15, Lars Hellström wrote:
> The idea I have proposed is to change the definition of quarks from
> effectively
>
> \def\q_nil{\q_nil}
> to
> \def\q_nil{\string\q_nil}

This breaks \quark_if_<name-here>:NTF. For example, \q_no_value is used 
as a return value for functions such as

   \prop_get:NnN \l_some_prop { key } \l_return_tl

So \quark_if_no_value:NTF needs to be true for both

   \quark_if_no_value:NTF \q_no_value

and

   \quark_if_no_value:NTF \l_return_tl

That then requires two separate tests, something like:

   \tex_ifx:D \q_no_value #1
     \prg_return_true:
   \tex_else:D
     \tex_ifx:D \q_novalue_tl #1 % set to { \q_no_value }
       \prg_return_true:
     \tex_else:D
       \prg_return_false:
     \tex_fi:D
   \tex_fi:D

I'd also point out that currently an 'o' expansion of a quark gives the 
quark, whereas in the scheme suggested things do change.

(At a higher level, \NoValue in xparse is implemented as

   \cs_new_protected:Npn \NoValue { -NoValue- }

so that it _can_ be typeset without a loop occurring. However, this is 
used in 'document level' functions, so the requirements are different.)
-- 
Joseph Wright