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+% \iffalse meta-comment
+%
+%% File: l3flag.dtx
+%
+% Copyright (C) 2011-2024 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 \pkg{l3flag} module\\ Expandable flags^^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 2024-05-08}
+%
+% \maketitle
+%
+% \begin{documentation}
+%
+% Flags are the only data-type that can be modified in expansion-only
+% contexts. This module is meant mostly for kernel use: in almost all
+% cases, booleans or integers should be preferred to flags because they
+% are very significantly faster.
+%
+% A flag can hold any (small) non-negative value, which we call its
+% \meta{height}. In expansion-only contexts, a flag can only be
+% \enquote{raised}: this increases the \meta{height} by $1$. The \meta{height}
+% can also be queried expandably. However, decreasing it, or setting it
+% to zero requires non-expandable assignments.
+%
+% Flag variables are always local.
+%
+% A typical use case of flags would be to keep track of whether an
+% exceptional condition has occurred during expandable processing, and
+% produce a meaningful (non-expandable) message after the end of the
+% expandable processing. This is exemplified by \pkg{l3str-convert},
+% which for performance reasons performs conversions of individual
+% characters expandably and for readability reasons produces a single
+% error message describing incorrect inputs that were encountered.
+%
+% Flags should not be used without carefully considering the fact that
+% raising a flag takes a time and memory proportional to its height and
+% that the memory cannot be reclaimed even if the flag is cleared.
+% Flags should not be used unless it is unavoidable.
+%
+% In earlier versions, flags were referenced by an \texttt{n}-type
+% \meta{flag name} such as \texttt{fp_overflow}, used as part of
+% \cs{use:c} constructions. All of the commands described below have
+% \texttt{n}-type analogues that can still appear in old code, but the
+% \texttt{N}-type commands are to be preferred moving forward. The
+% \texttt{n}-type \meta{flag name} is simply mapped to
+% \cs[no-index]{l_\meta{flag name}_flag}, which makes it easier for
+% packages using public flags (such as \pkg{l3fp}) to retain backwards
+% compatibility.
+%
+% \section{Setting up flags}
+%
+% \begin{function}[added = 2024-01-12]{\flag_new:N, \flag_new:c}
+% \begin{syntax}
+% \cs{flag_new:N} \meta{flag~var}
+% \end{syntax}
+% Creates a new \meta{flag~var}, or raises an error if the name is
+% already taken. The declaration is global, but flags are always local
+% variables. The \meta{flag~var} initially has zero height.
+% \end{function}
+%
+% \begin{function}[added = 2024-01-12]{\flag_clear:N, \flag_clear:c}
+% \begin{syntax}
+% \cs{flag_clear:N} \meta{flag~var}
+% \end{syntax}
+% Sets the height of the \meta{flag~var} to zero. The assignment is local.
+% \end{function}
+%
+% \begin{function}[added = 2024-01-12]{\flag_clear_new:N, \flag_clear_new:c}
+% \begin{syntax}
+% \cs{flag_clear_new:N} \meta{flag~var}
+% \end{syntax}
+% Ensures that the \meta{flag~var} exists globally by applying
+% \cs{flag_new:N} if necessary, then applies \cs{flag_clear:N}, setting
+% the height to zero locally.
+% \end{function}
+%
+% \begin{function}[added = 2024-01-12]{\flag_show:N, \flag_show:c}
+% \begin{syntax}
+% \cs{flag_show:N} \meta{flag~var}
+% \end{syntax}
+% Displays the height of the \meta{flag~var} in the terminal.
+% \end{function}
+%
+% \begin{function}[added = 2024-01-12]{\flag_log:N, \flag_log:c}
+% \begin{syntax}
+% \cs{flag_log:N} \meta{flag~var}
+% \end{syntax}
+% Writes the height of the \meta{flag~var} in the log file.
+% \end{function}
+%
+% \section{Expandable flag commands}
+%
+% \begin{function}[EXP, pTF, added = 2024-01-12]{\flag_if_exist:N, \flag_if_exist:c}
+% \begin{syntax}
+% \cs{flag_if_exist_p:N} \meta{flag~var}
+% \cs{flag_if_exist:NTF} \meta{flag~var} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% This function returns \texttt{true} if the \meta{flag~var} is
+% currently defined, and \texttt{false} otherwise. This does not check
+% that the \meta{flag~var} really is a flag variable.
+% \end{function}
+%
+% \begin{function}[EXP, pTF, added = 2024-01-12]{\flag_if_raised:N, \flag_if_raised:c}
+% \begin{syntax}
+% \cs{flag_if_raised_p:N} \meta{flag~var}
+% \cs{flag_if_raised:NTF} \meta{flag~var} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% This function returns \texttt{true} if the \meta{flag~var} has non-zero
+% height, and \texttt{false} if the \meta{flag~var} has zero height.
+% \end{function}
+%
+% \begin{function}[EXP, added = 2024-01-12]{\flag_height:N, \flag_height:c}
+% \begin{syntax}
+% \cs{flag_height:N} \meta{flag~var}
+% \end{syntax}
+% Expands to the height of the \meta{flag~var} as an integer denotation.
+% \end{function}
+%
+% \begin{function}[EXP, added = 2024-01-12]{\flag_raise:N, \flag_raise:c}
+% \begin{syntax}
+% \cs{flag_raise:N} \meta{flag~var}
+% \end{syntax}
+% The height of \meta{flag~var} is increased by $1$ locally.
+% \end{function}
+%
+% \begin{function}[EXP, added = 2024-01-12]{\flag_ensure_raised:N, \flag_ensure_raised:c}
+% \begin{syntax}
+% \cs{flag_ensure_raised:N} \meta{flag~var}
+% \end{syntax}
+% Ensures the \meta{flag~var} is raised by making its height at least~$1$,
+% locally.
+% \end{function}
+%
+% \begin{variable}[added = 2024-01-12]{\l_tmpa_flag, \l_tmpb_flag}
+% Scratch flag for local assignment. These are never used by
+% the kernel code, and so are safe for use with any \LaTeX3-defined
+% function. However, they may be overwritten by other non-kernel
+% code and so should only be used for short-term storage.
+% \end{variable}
+%
+% \end{documentation}
+%
+% \begin{implementation}
+%
+% \section{\pkg{l3flag} implementation}
+%
+% \begin{macrocode}
+%<*package>
+% \end{macrocode}
+%
+% \begin{macrocode}
+%<@@=flag>
+% \end{macrocode}
+%
+% \TestFiles{m3flag001}
+%
+% \subsection{Protected flag commands}
+%
+% The height $h$ of a flag (which is initially zero) is stored by
+% setting control sequences of the form \cs[no-index]{\meta{flag
+% name}\meta{integer}} to \tn{relax} for $0\leq\meta{integer}<h$. These
+% control sequences are produced by \cs{cs:w} \meta{flag~var}
+% \meta{integer} \cs{cs_end:}, namely the \meta{flag~var} is actually a
+% (protected) macro expanding to its own csname.
+%
+% \begin{macro}{\flag_new:N, \flag_new:c}
+% Evaluate the csname of~|#1| for use in constructing the various
+% indexed macros.
+% \begin{macrocode}
+\cs_new_protected:Npn \flag_new:N #1
+ { \cs_new_protected:Npe #1 { \cs_to_str:N #1 } }
+\cs_generate_variant:Nn \flag_new:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{variable}{\l_tmpa_flag, \l_tmpb_flag}
+% Two flag variables for scratch use.
+% \begin{macrocode}
+\flag_new:N \l_tmpa_flag
+\flag_new:N \l_tmpb_flag
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{macro}{\flag_clear:N, \flag_clear:c}
+% \begin{macro}{\@@_clear:wN}
+% Undefine control sequences, starting from the |0| flag, upwards,
+% until reaching an undefined control sequence. We don't use
+% \cs{cs_undefine:c} because that would act globally.
+% \begin{macrocode}
+\cs_new_protected:Npn \flag_clear:N #1
+ {
+ \@@_clear:wN 0 ; #1
+ \prg_break_point:
+ }
+\cs_generate_variant:Nn \flag_clear:N { c }
+\cs_new_protected:Npn \@@_clear:wN #1 ; #2
+ {
+ \if_cs_exist:w #2 #1 \cs_end: \else:
+ \prg_break:n
+ \fi:
+ \cs_set_eq:cN { #2 #1 } \tex_undefined:D
+ \exp_after:wN \@@_clear:wN
+ \int_value:w \int_eval:w \c_one_int + #1 ; #2
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}{\flag_clear_new:N, \flag_clear_new:c}
+% As for other datatypes, clear the \meta{flag~var} or create a new one,
+% as appropriate.
+% \begin{macrocode}
+\cs_new_protected:Npn \flag_clear_new:N #1
+ { \flag_if_exist:NTF #1 { \flag_clear:N } { \flag_new:N } #1 }
+\cs_generate_variant:Nn \flag_clear_new:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}{\flag_show:N, \flag_show:c, \flag_log:N, \flag_log:c, \@@_show:NN}
+% Show the height (terminal or log file) using appropriate \pkg{l3msg}
+% auxiliaries.
+% \begin{macrocode}
+\cs_new_protected:Npn \flag_show:N { \@@_show:NN \tl_show:n }
+\cs_generate_variant:Nn \flag_show:N { c }
+\cs_new_protected:Npn \flag_log:N { \@@_show:NN \tl_log:n }
+\cs_generate_variant:Nn \flag_log:N { c }
+\cs_new_protected:Npn \@@_show:NN #1#2
+ {
+ \__kernel_chk_defined:NT #2
+ { \exp_args:Ne #1 { \tl_to_str:n { #2 height } = \flag_height:N #2 } }
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{Expandable flag commands}
+%
+% \begin{macro}[EXP, pTF]{\flag_if_exist:N, \flag_if_exist:c}
+% Copies of the \texttt{cs} functions defined in \pkg{l3basics}.
+% \begin{macrocode}
+\prg_new_eq_conditional:NNn \flag_if_exist:N \cs_if_exist:N
+ { TF , T , F , p }
+\prg_new_eq_conditional:NNn \flag_if_exist:c \cs_if_exist:c
+ { TF , T , F , p }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[EXP, pTF]{\flag_if_raised:N, \flag_if_raised:c}
+% Test if the flag has a non-zero height, by checking the |0| control sequence.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \flag_if_raised:N #1 { p , T , F , TF }
+ {
+ \if_cs_exist:w #1 0 \cs_end:
+ \prg_return_true:
+ \else:
+ \prg_return_false:
+ \fi:
+ }
+\prg_generate_conditional_variant:Nnn \flag_if_raised:N
+ { c } { p , T , F , TF }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\flag_height:N, \flag_height:c}
+% \begin{macro}[EXP]{\@@_height_loop:wN, \@@_height_end:wN}
+% Extract the value of the flag by going through all of the
+% control sequences starting from |0|.
+% \begin{macrocode}
+\cs_new:Npn \flag_height:N #1 { \@@_height_loop:wN 0; #1 }
+\cs_new:Npn \@@_height_loop:wN #1 ; #2
+ {
+ \if_cs_exist:w #2 #1 \cs_end: \else:
+ \exp_after:wN \@@_height_end:wN
+ \fi:
+ \exp_after:wN \@@_height_loop:wN
+ \int_value:w \int_eval:w \c_one_int + #1 ; #2
+ }
+\cs_new:Npn \@@_height_end:wN #1 + #2 ; #3 {#2}
+\cs_generate_variant:Nn \flag_height:N { c }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\flag_raise:N, \flag_raise:c}
+% Change the appropriate control sequence to \tn{relax} by expanding a
+% \cs{cs:w} \ldots{} \cs{cs_end:} construction, then pass it to
+% \cs{use_none:n} to avoid leaving anything in the input stream.
+% \begin{macrocode}
+\cs_new:Npn \flag_raise:N #1
+ { \exp_after:wN \use_none:n \cs:w #1 \flag_height:N #1 \cs_end: }
+\cs_generate_variant:Nn \flag_raise:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\flag_ensure_raised:N, \flag_ensure_raised:c}
+% Pass the control sequence with name \meta{flag name}\texttt{0} to
+% \cs{use_none:n}. Constructing the control sequence ensures that it
+% changes from being undefined (if it was so) to being \tn{relax}.
+% \begin{macrocode}
+\cs_new:Npn \flag_ensure_raised:N #1
+ { \exp_after:wN \use_none:n \cs:w #1 0 \cs_end: }
+\cs_generate_variant:Nn \flag_ensure_raised:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{Old \texttt{n}-type flag commands}
+%
+% Here we keep the old flag commands since our policy is to no longer
+% delete deprecated functions. The idea is to simply map \meta{flag
+% name} to \cs[no-index]{l_\meta{flag name}_flag}. When the debugging
+% code is activated, it checks existence of the \texttt{N}-type flag
+% variables that result.
+%
+% \begin{macro}[no-user-doc]{\flag_new:n, \flag_clear:n, \flag_clear_new:n}
+% \begin{macro}[EXP, pTF, no-user-doc]{\flag_if_exist:n, \flag_if_raised:n}
+% \begin{macro}[EXP, no-user-doc]{\flag_height:n, \flag_raise:n, \flag_ensure_raised:n}
+% \begin{macrocode}
+\cs_new_protected:Npn \flag_new:n #1 { \flag_new:c { l_#1_flag } }
+\cs_new_protected:Npn \flag_clear:n #1 { \flag_clear:c { l_#1_flag } }
+\cs_new_protected:Npn \flag_clear_new:n #1 { \flag_clear_new:c { l_#1_flag } }
+\cs_new:Npn \flag_if_exist_p:n #1 { \flag_if_exist_p:c { l_#1_flag } }
+\cs_new:Npn \flag_if_exist:nT #1 { \flag_if_exist:cT { l_#1_flag } }
+\cs_new:Npn \flag_if_exist:nF #1 { \flag_if_exist:cF { l_#1_flag } }
+\cs_new:Npn \flag_if_exist:nTF #1 { \flag_if_exist:cTF { l_#1_flag } }
+\cs_new:Npn \flag_if_raised_p:n #1 { \flag_if_raised_p:c { l_#1_flag } }
+\cs_new:Npn \flag_if_raised:nT #1 { \flag_if_raised:cT { l_#1_flag } }
+\cs_new:Npn \flag_if_raised:nF #1 { \flag_if_raised:cF { l_#1_flag } }
+\cs_new:Npn \flag_if_raised:nTF #1 { \flag_if_raised:cTF { l_#1_flag } }
+\cs_new:Npn \flag_height:n #1 { \flag_height:c { l_#1_flag } }
+\cs_new:Npn \flag_raise:n #1 { \flag_raise:c { l_#1_flag } }
+\cs_new:Npn \flag_ensure_raised:n #1 { \flag_ensure_raised:c { l_#1_flag } }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[no-user-doc]{\flag_show:n, \flag_log:n, \@@_show:Nn}
+% To avoid changing the output here we mostly keep the old code.
+% \begin{macrocode}
+\cs_new_protected:Npn \flag_show:n { \@@_show:Nn \tl_show:n }
+\cs_new_protected:Npn \flag_log:n { \@@_show:Nn \tl_log:n }
+\cs_new_protected:Npn \@@_show:Nn #1#2
+ {
+ \exp_args:Nc \__kernel_chk_defined:NT { l_#2_flag }
+ {
+ \exp_args:Ne #1
+ { \tl_to_str:n { flag~#2~height } = \flag_height:n {#2} }
+ }
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macrocode}
+%</package>
+% \end{macrocode}
+%
+% \end{implementation}
+%
+% \PrintIndex