% \iffalse meta-comment
%
%% File: l3candidates.dtx
%
% Copyright (C) 2012-2023 The LaTeX Project
%
% It may be distributed and/or modified under the conditions of the
% LaTeX Project Public License (LPPL), either version 1.3c of this
% license or (at your option) any later version.  The latest version
% of this license is in the file
%
%    https://www.latex-project.org/lppl.txt
%
% This file is part of the "l3kernel bundle" (The Work in LPPL)
% and all files in that bundle must be distributed together.
%
% -----------------------------------------------------------------------
%
% The development version of the bundle can be found at
%
%    https://github.com/latex3/latex3
%
% for those people who are interested.
%
%<*driver>
\documentclass[full,kernel]{l3doc}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \title{^^A
%   The \textsf{l3candidates} package\\ Experimental additions to
%   \pkg{l3kernel}^^A
% }
%
% \author{^^A
%  The \LaTeX{} Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released 2023-08-11}
%
% \maketitle
%
% \begin{documentation}
%
% \section{Important notice}
%
% This module provides a space in which functions can be added to
% \pkg{l3kernel} (\pkg{expl3}) while still being experimental.
% \begin{quote}
%  \bfseries
% As such, the
% functions here may not remain in their current form, or indeed at all,
% in \pkg{l3kernel} in the future.
% \end{quote}
%  In contrast to the material in
% \pkg{l3experimental}, the functions here are all \emph{small} additions to
% the kernel. We encourage programmers to test them out and report back on
% the \texttt{LaTeX-L} mailing list.
%
% \medskip
%
%   Thus, if you intend to use any of these functions from the candidate module in a public package
%  offered to others for productive use (e.g., being placed on CTAN) please consider the following points carefully:
% \begin{itemize}
% \item Be prepared that your public packages might require updating when such functions
%        are being finalized.
% \item Consider informing us that you use a particular function in your public package, e.g., by
%         discussing this on the \texttt{LaTeX-L}
%    mailing list. This way it becomes easier to coordinate any updates necessary without issues
%    for the users of your package.
% \item Discussing and understanding use cases for a particular addition or concept also helps to
%         ensure that we provide the right interfaces in the final version so please give us feedback
%         if you consider a certain candidate function useful (or not).
% \end{itemize}
% We only add functions in this space if we consider them being serious candidates for a final inclusion
% into the kernel. However, real use sometimes leads to better ideas, so functions from this module are
% \textbf{not necessarily stable} and we may have to adjust them!
%
% \section{Additions to \pkg{l3seq}}
%
% \begin{function}{\seq_set_filter:NNn, \seq_gset_filter:NNn}
%   \begin{syntax}
%     \cs{seq_set_filter:NNn} \meta{sequence_1} \meta{sequence_2} \Arg{inline boolexpr}
%   \end{syntax}
%   Evaluates the \meta{inline boolexpr} for every \meta{item} stored
%   within the \meta{sequence_2}. The \meta{inline boolexpr}
%   receives the \meta{item} as |#1|. The sequence of all \meta{items}
%   for which the \meta{inline boolexpr} evaluated to \texttt{true}
%   is assigned to \meta{sequence_1}.
%   \begin{texnote}
%     Contrarily to other mapping functions, \cs{seq_map_break:} cannot
%     be used in this function, and would lead to low-level \TeX{} errors.
%   \end{texnote}
% \end{function}
%
% \section{Additions to \pkg{l3tl}}
%
% \begin{function}[added = 2018-04-01]{\tl_build_begin:N, \tl_build_gbegin:N}
%   \begin{syntax}
%     \cs{tl_build_begin:N} \meta{tl~var}
%   \end{syntax}
%   Clears the \meta{tl~var} and sets it up to support other
%   \cs[no-index]{tl_build_\ldots{}} functions, which allow accumulating
%   large numbers of tokens piece by piece much more efficiently than
%   standard \pkg{l3tl} functions.  Until \cs{tl_build_end:N}
%   \meta{tl~var} is called, applying any function from \pkg{l3tl} other
%   than \cs[no-index]{tl_build_\ldots{}} will lead to incorrect
%   results.  The |begin| and |gbegin| functions must be used for local
%   and global \meta{tl~var} respectively.
% \end{function}
%
% \begin{function}[added = 2018-04-01]{\tl_build_clear:N, \tl_build_gclear:N}
%   \begin{syntax}
%     \cs{tl_build_clear:N} \meta{tl~var}
%   \end{syntax}
%   Clears the \meta{tl~var} and sets it up to support other
%   \cs[no-index]{tl_build_\ldots{}} functions.  The |clear| and
%   |gclear| functions must be used for local and global \meta{tl~var}
%   respectively.
% \end{function}
%
% \begin{function}[added = 2018-04-01]
%   {
%     \tl_build_put_left:Nn, \tl_build_put_left:Nx,
%     \tl_build_gput_left:Nn, \tl_build_gput_left:Nx,
%     \tl_build_put_right:Nn, \tl_build_put_right:Nx,
%     \tl_build_gput_right:Nn, \tl_build_gput_right:Nx,
%   }
%   \begin{syntax}
%     \cs{tl_build_put_left:Nn} \meta{tl~var} \Arg{tokens}
%     \cs{tl_build_put_right:Nn} \meta{tl~var} \Arg{tokens}
%   \end{syntax}
%   Adds \meta{tokens} to the left or right side of the current contents
%   of \meta{tl~var}.  The \meta{tl~var} must have been set up with
%   \cs{tl_build_begin:N} or \cs{tl_build_gbegin:N}.  The |put| and
%   |gput| functions must be used for local and global \meta{tl~var}
%   respectively.  The |right| functions are about twice faster than the
%   |left| functions.
% \end{function}
%
% \begin{function}[added = 2018-04-01]{\tl_build_get:NN}
%   \begin{syntax}
%     \cs{tl_build_get:NN} \meta{tl~var_1} \meta{tl~var_2}
%   \end{syntax}
%   Stores the contents of the \meta{tl~var_1} in the \meta{tl~var_2}.
%   The \meta{tl~var_1} must have been set up with \cs{tl_build_begin:N}
%   or \cs{tl_build_gbegin:N}.  The \meta{tl~var_2} is a
%   \enquote{normal} token list variable, assigned locally using
%   \cs{tl_set:Nn}.
% \end{function}
%
% \begin{function}[added = 2018-04-01]{\tl_build_end:N, \tl_build_gend:N}
%   \begin{syntax}
%     \cs{tl_build_end:N} \meta{tl~var}
%   \end{syntax}
%   Gets the contents of \meta{tl~var} and stores that into the
%   \meta{tl~var} using \cs{tl_set:Nn} or \cs{tl_gset:Nn}.
%   The \meta{tl~var} must have
%   been set up with \cs{tl_build_begin:N} or \cs{tl_build_gbegin:N}.
%   The |end| and |gend| functions must be used for local and global
%   \meta{tl~var} respectively.  These functions completely remove the
%   setup code that enabled \meta{tl~var} to be used for other
%   \cs[no-index]{tl_build_\ldots{}} functions.
% \end{function}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3candidates} implementation}
%
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
%
% \subsection{Additions to \pkg{l3seq}}
%
%    \begin{macrocode}
%<@@=seq>
%    \end{macrocode}
%
% \begin{macro}{\seq_set_filter:NNn, \seq_gset_filter:NNn}
% \begin{macro}{\@@_set_filter:NNNn}
%   Similar to \cs{seq_map_inline:Nn}, without a
%   \cs{prg_break_point:} because the user's code
%   is performed within the evaluation of a boolean expression,
%   and skipping out of that would break horribly.
%   The \cs{@@_wrap_item:n} function inserts the relevant
%   \cs{@@_item:n} without expansion in the input stream,
%   hence in the \texttt{x}-expanding assignment.
%    \begin{macrocode}
\cs_new_protected:Npn \seq_set_filter:NNn
  { \@@_set_filter:NNNn \__kernel_tl_set:Nx }
\cs_new_protected:Npn \seq_gset_filter:NNn
  { \@@_set_filter:NNNn \__kernel_tl_gset:Nx }
\cs_new_protected:Npn \@@_set_filter:NNNn #1#2#3#4
  {
    \@@_push_item_def:n { \bool_if:nT {#4} { \@@_wrap_item:n {##1} } }
    #1 #2 { #3 }
    \@@_pop_item_def:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Additions to \pkg{l3tl}}
%
% \subsubsection{Building a token list}
%
%    \begin{macrocode}
%<@@=tl>
%    \end{macrocode}
%
% Between \cs{tl_build_begin:N} \meta{tl~var} and \cs{tl_build_end:N}
% \meta{tl~var}, the \meta{tl~var} has the structure
% \begin{quote}
%   \cs{exp_end:} \ldots{} \cs{exp_end:} \cs{@@_build_last:NNn}
%   \meta{assignment} \meta{next~tl} \Arg{left} \meta{right}
% \end{quote}
% where \meta{right} is not braced.  The \enquote{data} it represents is
% \meta{left} followed by the \enquote{data} of \meta{next~tl} followed
% by \meta{right}.  The \meta{next~tl} is a token list variable whose
% name is that of \meta{tl~var} followed by~|'|.  There are between $0$
% and $4$ \cs{exp_end:} to keep track of when \meta{left} and
% \meta{right} should be put into the \meta{next~tl}.  The
% \meta{assignment} is \cs{cs_set_nopar:Npx} if the variable is local,
% and \cs{cs_gset_nopar:Npx} if it is global.
%
% \begin{macro}{\tl_build_begin:N, \tl_build_gbegin:N}
% \begin{macro}{\@@_build_begin:NN, \@@_build_begin:NNN}
%   First construct the \meta{next~tl}: using a prime here conflicts
%   with the usual \pkg{expl3} convention but we need a name that can be
%   derived from |#1| without any external data such as a counter.
%   Empty that \meta{next~tl} and setup the structure.  The local and
%   global versions only differ by a single function
%   \cs[no-index]{cs_(g)set_nopar:Npx} used for all assignments: this is
%   important because only that function is stored in the \meta{tl~var}
%   and \meta{next~tl} for subsequent assignments.  In principle
%   \cs{@@_build_begin:NNN} could use \cs[no-index]{tl_(g)clear_new:N}
%   to empty |#1| and make sure it is defined, but logging the
%   definition does not seem useful so we just do |#3| |#1| |{}| to
%   clear it locally or globally as appropriate.
%    \begin{macrocode}
\cs_new_protected:Npn \tl_build_begin:N #1
  { \@@_build_begin:NN \cs_set_nopar:Npx #1 }
\cs_new_protected:Npn \tl_build_gbegin:N #1
  { \@@_build_begin:NN \cs_gset_nopar:Npx #1 }
\cs_new_protected:Npn \@@_build_begin:NN #1#2
  { \exp_args:Nc \@@_build_begin:NNN { \cs_to_str:N #2 ' } #2 #1 }
\cs_new_protected:Npn \@@_build_begin:NNN #1#2#3
  {
    #3 #1 { }
    #3 #2
      {
        \exp_not:n { \exp_end: \exp_end: \exp_end: \exp_end: }
        \exp_not:n { \@@_build_last:NNn #3 #1 { } }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\tl_build_clear:N, \tl_build_gclear:N}
%   The |begin| and |gbegin| functions already clear enough to make the
%   token list variable effectively empty.  Eventually the |begin| and
%   |gbegin| functions should check that |#1'| is empty or undefined,
%   while the |clear| and |gclear| functions ought to empty |#1'|,
%   |#1''| and so on, similar to \cs{tl_build_end:N}.  This only affects
%   memory usage.
%    \begin{macrocode}
\cs_new_eq:NN \tl_build_clear:N \tl_build_begin:N
\cs_new_eq:NN \tl_build_gclear:N \tl_build_gbegin:N
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \tl_build_put_right:Nn, \tl_build_put_right:Nx,
%     \tl_build_gput_right:Nn, \tl_build_gput_right:Nx,
%     \@@_build_last:NNn, \@@_build_put:nn, \@@_build_put:nw
%   }
%   Similar to \cs{tl_put_right:Nn}, but apply \cs{exp:w} to |#1|.  Most
%   of the time this just removes one \cs{exp_end:}.  When there are
%   none left, \cs{@@_build_last:NNn} is expanded instead.  It resets
%   the definition of the \meta{tl~var} by ending the \cs{exp_not:n} and
%   the definition early.  Then it makes sure the \meta{next~tl} (its
%   argument |#1|) is set-up and starts a new definition.  Then
%   \cs{@@_build_put:nn} and \cs{@@_build_put:nw} place the \meta{left}
%   part of the original \meta{tl~var} as appropriate for the definition
%   of the \meta{next~tl} (the \meta{right} part is left in the right
%   place without ever becoming a macro argument).  We use
%   \cs{exp_after:wN} rather than some \cs{exp_args:No} to avoid reading
%   arguments that are likely very long token lists.  We use
%   \cs[no-index]{cs_(g)set_nopar:Npx} rather than
%   \cs[no-index]{tl_(g)set:Nx} partly for the same reason and partly
%   because the assignments are interrupted by brace tricks, which
%   implies that the assignment does not simply set the token list to an
%   |x|-expansion of the second argument.
%    \begin{macrocode}
\cs_new_protected:Npn \tl_build_put_right:Nn #1#2
  {
    \cs_set_nopar:Npx #1
      { \exp_after:wN \exp_not:n \exp_after:wN { \exp:w #1 #2 } }
  }
\cs_new_protected:Npn \tl_build_put_right:Nx #1#2
  {
    \cs_set_nopar:Npx #1
      { \exp_after:wN \exp_not:n \exp_after:wN { \exp:w #1 } #2 }
  }
\cs_new_protected:Npn \tl_build_gput_right:Nn #1#2
  {
    \cs_gset_nopar:Npx #1
      { \exp_after:wN \exp_not:n \exp_after:wN { \exp:w #1 #2 } }
  }
\cs_new_protected:Npn \tl_build_gput_right:Nx #1#2
  {
    \cs_gset_nopar:Npx #1
      { \exp_after:wN \exp_not:n \exp_after:wN { \exp:w #1 } #2 }
  }
\cs_new_protected:Npn \@@_build_last:NNn #1#2
  {
    \if_false: { { \fi:
          \exp_end: \exp_end: \exp_end: \exp_end: \exp_end:
          \@@_build_last:NNn #1 #2 { }
        }
      }
    \if_meaning:w \c_empty_tl #2
      \@@_build_begin:NN #1 #2
    \fi:
    #1 #2
      {
        \exp_after:wN \exp_not:n \exp_after:wN
          {
            \exp:w \if_false: } } \fi:
            \exp_after:wN \@@_build_put:nn \exp_after:wN {#2}
  }
\cs_new_protected:Npn \@@_build_put:nn #1#2 { \@@_build_put:nw {#2} #1 }
\cs_new_protected:Npn \@@_build_put:nw #1#2 \@@_build_last:NNn #3#4#5
  { #2 \@@_build_last:NNn #3 #4 { #1 #5 } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \tl_build_put_left:Nn, \tl_build_put_left:Nx,
%     \tl_build_gput_left:Nn, \tl_build_gput_left:Nx, \@@_build_put_left:NNn
%   }
%   See \cs{tl_build_put_right:Nn} for all the machinery.  We could
%   easily provide \cs[no-index]{tl_build_put_left_right:Nnn}, by just
%   add the \meta{right} material after the \Arg{left} in the
%   |x|-expanding assignment.
%    \begin{macrocode}
\cs_new_protected:Npn \tl_build_put_left:Nn #1
  { \@@_build_put_left:NNn \cs_set_nopar:Npx #1 }
\cs_generate_variant:Nn \tl_build_put_left:Nn { Nx }
\cs_new_protected:Npn \tl_build_gput_left:Nn #1
  { \@@_build_put_left:NNn \cs_gset_nopar:Npx #1 }
\cs_generate_variant:Nn \tl_build_gput_left:Nn { Nx }
\cs_new_protected:Npn \@@_build_put_left:NNn #1#2#3
  {
    #1 #2
      {
        \exp_after:wN \exp_not:n \exp_after:wN
          {
            \exp:w \exp_after:wN \@@_build_put:nn
              \exp_after:wN {#2} {#3}
          }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\tl_build_get:NN}
% \begin{macro}{\@@_build_get:NNN, \@@_build_get:w, \@@_build_get_end:w}
%   The idea is to expand the \meta{tl~var} then the \meta{next~tl} and
%   so on, all within an |x|-expanding assignment, and wrap as
%   appropriate in \cs{exp_not:n}.  The various \meta{left} parts are
%   left in the assignment as we go, which enables us to expand the
%   \meta{next~tl} at the right place.  The various \meta{right} parts
%   are eventually picked up in one last \cs{exp_not:n}, with a brace
%   trick to wrap all the \meta{right} parts together.
%    \begin{macrocode}
\cs_new_protected:Npn \tl_build_get:NN
  { \@@_build_get:NNN \__kernel_tl_set:Nx }
\cs_new_protected:Npn \@@_build_get:NNN #1#2#3
  { #1 #3 { \if_false: { \fi: \exp_after:wN \@@_build_get:w #2 } } }
\cs_new:Npn \@@_build_get:w #1 \@@_build_last:NNn #2#3#4
  {
    \exp_not:n {#4}
    \if_meaning:w \c_empty_tl #3
      \exp_after:wN \@@_build_get_end:w
    \fi:
    \exp_after:wN \@@_build_get:w #3
  }
\cs_new:Npn \@@_build_get_end:w #1#2#3
  { \exp_after:wN \exp_not:n \exp_after:wN { \if_false: } \fi: }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\tl_build_end:N, \tl_build_gend:N, \@@_build_end_loop:NN}
%   Get the data then clear the \meta{next~tl} recursively until finding
%   an empty one.  It is perhaps wasteful to repeatedly use
%   \cs{cs_to_str:N}.  The local/global scope is checked by
%   \cs{tl_set:Nx} or \cs{tl_gset:Nx}.
%    \begin{macrocode}
\cs_new_protected:Npn \tl_build_end:N #1
  {
    \@@_build_get:NNN \__kernel_tl_set:Nx #1 #1
    \exp_args:Nc \@@_build_end_loop:NN { \cs_to_str:N #1 ' } \tl_clear:N
  }
\cs_new_protected:Npn \tl_build_gend:N #1
  {
    \@@_build_get:NNN \__kernel_tl_gset:Nx #1 #1
    \exp_args:Nc \@@_build_end_loop:NN { \cs_to_str:N #1 ' } \tl_gclear:N
  }
\cs_new_protected:Npn \@@_build_end_loop:NN #1#2
  {
    \if_meaning:w \c_empty_tl #1
      \exp_after:wN \use_none:nnnnnn
    \fi:
    #2 #1
    \exp_args:Nc \@@_build_end_loop:NN { \cs_to_str:N #1 ' } #2
  }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%</package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex
