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% \iffalse
%% File: l3quark.dtx Copyright (C) 1990-1998 LaTeX3 project
%
%<*dtx>
\ProvidesFile{l3quark.dtx}
%</dtx>
%<package>\NeedsTeXFormat{LaTeX2e}
%<package>\ProvidesPackage{l3quark}
%<driver> \ProvidesFile{l3quark.drv}
% \fi
% \ProvidesFile{l3quark.dtx}
[1998/05/20 v1.0e L3 Experimental Quark Commands]
%
% \iffalse
%<*driver>
\documentclass{l3doc}
\begin{document}
\DocInput{l3quark.dtx}
\end{document}
%</driver>
% \fi
%
%
% \GetFileInfo{l3quark.dtx}
% \title{The \textsf{l3quark} package\thanks{This file
% has version number \fileversion, last
% revised \filedate.}\\
% ``Quarks''}
% \author{\Team}
% \date{\filedate}
% \maketitle
%
%
% \section{Quarks}
%^^A \label{sec:quarks}
%
%
% A special type of constants in \LaTeX3 are `quarks'. These are control
% sequences that expand to themselves and should therefore NEVER be
% executed directly in the code. This would result in an endless loop!
%
% They are meant to be used as delimiter is weird functions (for example
% as the stop token (i.e., "\q_stop"). They also permit the following
% ingenious trick: when you pick up a token in a temporary, and you want
% to know whether you have picked up a particular quark, all you have to
% do is compare the temporary to the quark using "\if_meaning:NN". A
% set of special quark testing functions is set up below.
%
% By convention all constants of type quark start out with "\q_".
%
% \subsection{Functions}
%
% \begin{function}{%
% \quark_new:N |
% }
% \begin{syntax}
% "\quark_new:N" <quark>
% \end{syntax}
% Defines <quark> to be a new constant of type "quark".
% \end{function}
%
% \begin{function}{%
% \quark_if_no_value:nTF |
% \quark_if_no_value:nF |
% \quark_if_no_value:nT |
% \quark_if_no_value:NTF |
% \quark_if_no_value:NF |
% }
% \begin{syntax}
% "\quark_if_no_value:nTF" "{" <token list> "}"
% " {"<true code>"}{"<false code>"}"
% "\quark_if_no_value:NTF" <tlp>
% " {"<true code>"}{"<false code>"}"
% \end{syntax}
% This tests whether or not <token list> contains only the quark
% "\q_no_value".
%
% If <token list> to be tested is stored in a token list pointer use
% "\quark_if_no_value:NTF", or "\quark_if_no_value:NF" or check the
% value directly with "\if_meaning:NN". All those cases are faster then
% "\quark_if_no_value:nTF" so should be preferred.
%
% \begin{texnote}
% But be aware of the fact that "\if_meaning:NN" can result in an
% overflow of \TeX{}'s parameter stack since it leaves the corresponding
% "\fi:" on the input until the whole replacement text is processed. It
% is therefore better in recursions to use "\quark_if_no_value:NTF" as
% it will remove the conditional prior to processing the "T" or "F" case
% and so allows tail-recursion.
% \end{texnote}
% \end{function}
%
% \subsection{Constants}
%
% \begin{variable}{\q_no_value} The canonical `missing value quark'
% that is returned by certain functions to denote that a requested value
% is not found in the data structure.
% \end{variable}
%
% \begin{variable}{\q_stop}
% This constant is used as a a marker in parameter text. This allows a
% scanning function to find the end of some input string.
% \end{variable}
%
% \begin{variable}{\q_nil}
% This constant represent the nil pointer in pointer structures.
% \end{variable}
%
% \section{Implementation}
%
% We start by ensuring that the required packages are loaded.
% We check for \texttt{l3expan} since this a basic package that is
% essential for use of any higher-level package.
% \begin{macrocode}
%<package>\RequirePackage{l3expan}\par
%<*package>
% \end{macrocode}
%
% \begin{macro}{\quark_new:N}
% Allocate a new quark.
% \begin{macrocode}
\def_new:Npn \quark_new:N #1{\tlp_new:Nn #1{#1}}
% \end{macrocode}
% \end{macro}
%
% \begin{macro}{\q_stop}
% \begin{macro}{\q_no_value}
% \begin{macro}{\q_nil}
% |\q_stop| is often used as a marker in parameter text,
% |\q_no_value| is the canonical missing value, and |\q_nil|
% represents a nil pointer in some data structures.
% \begin{macrocode}
\quark_new:N \q_stop
\quark_new:N \q_no_value
\quark_new:N \q_nil
% \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\q_error}
% \begin{macro}{\q_mark}
% We need two additional quarks. |\q_error| delimits the end of
% the computation for purposes of error recovery. |\q_mark| is
% used in parameter text when we need a scanning boundary that is
% distinct from |\q_stop|.
% \begin{macrocode}
\quark_new:N\q_error
\quark_new:N\q_mark
% \end{macrocode}
% \end{macro}
% \end{macro}
%
%
% \begin{macro}{\quark_if_no_value:NTF}
% \begin{macro}{\quark_if_no_value:NF}
% \begin{macro}{\quark_if_no_value:nTF}
% \begin{macro}{\quark_if_no_value:nT}
% \begin{macro}{\quark_if_no_value:nF}
% Here we test if we found a special quark as the first argument.
% The argument might contain an arbitrary list of tokens, therefore
% we have to wrap it up in a token list pointer.
% \begin{macrocode}
\def_new:Npn \quark_if_no_value:NTF #1{
% \end{macrocode}
% We better start with |\q_no_value| as the first argument since
% the whole thing may otherwise loop if |#1| is wrongly given
% a string like |aabc| instead of a single token.\footnote{It may
% still loop in special circumstances however!}
% \begin{macrocode}
\if_meaning:NN\q_no_value#1
\exp_after:NN\use_choice_i:nn
\else: \exp_after:NN\use_choice_ii:nn \fi:}
% \end{macrocode}
% It would be possible to speed up the following commands by
% providing individual implementations similar to the one above.
% Should perhaps be done if they are used often!
% \begin{macrocode}
\def_new:Npn \quark_if_no_value:NF #1{\quark_if_no_value:NTF {#1}\use_noop:}
\def_new:Npn \quark_if_no_value:nTF #1{\tlp_gset:Nn \g_testa_tlp {#1}
\quark_if_no_value:NTF\g_testa_tlp}
\def_new:Npn \quark_if_no_value:nF #1{\quark_if_no_value:nTF {#1}\use_noop:}
\def_new:Npn \quark_if_no_value:nT #1#2{\quark_if_no_value:nTF {#1}
{#2}\use_noop:}
% \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\quark_if_nil:NTF}
% A function to check for the presence of |\q_nil|.
% \begin{macrocode}
\def_new:Npn\quark_if_nil:NTF#1{
\if_meaning:NN#1\q_nil
\exp_after:NN\use_choice_i:nn
\else:
\exp_after:NN\use_choice_ii:nn\fi:}
% \end{macrocode}
% \end{macro}
%
% Show token usage:
% \begin{macrocode}
%</package>
%<*showmemory>
\showMemUsage
%</showmemory>
% \end{macrocode}
%
%
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