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Subject:

Re: o and V-type functions

From:

Joseph Wright <[log in to unmask]>

Reply-To:

Mailing list for the LaTeX3 project <[log in to unmask]>

Date:

Tue, 27 Sep 2011 14:01:31 +0100

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 On 27/09/2011 13:10, Ulrike Fischer wrote: > The documentations source3 says at the start "In general, the V and > v specifiers are favoured over o for recovering stored information". > > But everytime I start to define functions which should expand their > argument I end either with x or o types. > > As an example I tried to write a function which process key-val > lists. The list can also be given through a command. With o this > works fine, with V normal and empty lists gives errors: > > \documentclass[parskip]{scrartcl} > \usepackage{expl3} > > > \begin{document} > > \section{expl3} > \ExplSyntaxOn > > \keys_define:nn {test} > {testa .code:n = {#1}, > testb .code:n = {#1}, > } > > > \cs_new:Npn \test_usekeys:n #1 {keys:~[\keys_set:nn {test}{#1}]} > \cs_generate_variant:Nn \test_usekeys:n {V} > \cs_generate_variant:Nn \test_usekeys:n {o} > > \def\mykeylist{testa=A,testb=B} > > \test_usekeys:V {\mykeylist} > %\test_usekeys:V {testa=A,testb=B} > %\test_usekeys:V {} > > \par > > \test_usekeys:o {\mykeylist}\\ > \test_usekeys:o {testa=A,testb=B}\\ > \test_usekeys:o {}\\ > \test_usekeys:o {\mykeylist}\\ > > \end{document} > > > Could someone explain me when arguments of type V are actually > useful and "better" than o and how they should be used? The idea behind the "V" specifier is that it is independent of the implementation of a variable. Over time, we've moved to using macros for most items, but historically there were several toks-based variables where the difference in expansion behaviour was important. We do have register data types for things like dimens, and there there is an advantage to a 'implementation independent' approach: see later. What perhaps we've not made clear is that "V" applies to a _single token_ variable name, while "o" applies to multiple tokens. So "\test_usekeys:V {\mykeylist}" is conceptually wrong (although in this case it will work), and should read "\test_usekeys:V \mykeylist". In cases where you always know that the variable will be macro-based, you can always use "o". However, with "V" it's possible to do something like   \my_set_function:V \l_my_clist   \my_set_function:V \l_my_int and 'coerce' the int to a clist with one item in it. (I'm using exactly this approach inside xgalley to avoid worrying about complexity in cases where the input might be a list or might be a int.) I did some performance tests, and in general we don't see too much loss in speed using "V" rather than "o". So you can gain the fact that "V" maintains the _semantic" concept of 'pass the variable value', rather than 'expand this token once', without a serious loss of performance. -- Joseph Wright

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