在连续的宏调用之间保存值

Question

我想在宏调用之间保存一个变量值，目的是不必在连续调用中重复大量参数。如果在文本模式下，一切都会按预期工作。如果我对每次调用单独调用数学模式，即$\myMacro$，这就是我想要做的，则每次调用都会重置变量。有解决方法吗？

\documentclass{article}
\usepackage{xparse}
\usepackage{amsmath}
\ExplSyntaxOn
%   VARIABLE DECLARATIONS:
\tl_new:N \savevar_rn_tl
\tl_set:Nn \savevar_rn_tl {INIT}
\NewDocumentCommand\myMacro{O{1}O{Z}}   
{
    !*\tl_use:N \savevar_rn_tl*!        \\
    \str_case:nn {#1} 
        {
            {1}
            {
                #2\\        
                \tl_set:Nn \savevar_rn_tl {#2@#2}
            }
            {2}
            {
                \tl_use:N \savevar_rn_tl \\
                \tl_set:Nn \savevar_rn_tl {RESET}
            }
        }
}
\ExplSyntaxOff
\begin{document}
\section{The way I want to use it, but savevar is not preserved between calls:}
$\myMacro$\\
$\myMacro[2]$\\
$\myMacro[1][\alpha]$\\
$\myMacro[2]$\\
\section{savevar is preserved between calls, but only if calls are made in textmode:}
\myMacro\\
\myMacro[2]\\
\myMacro[1][W]\\
\myMacro[2]\\
\end{document} 

Answer 1

切换到数学模式会进入一个组，因此\tl_set:Nn只能在组内设置值，而不能在组外设置值。如果要保留该值，则\tl_gset:Nn必须使用（全局设置）。

表面上的‘重置’只是因为\savevar_rn_tl外部仍然具有初始设置的值并且填充#2@#2仅在本地、在组内。

\documentclass{article}
\usepackage{xparse}
\usepackage{amsmath}
\usepackage{mathtools}
\ExplSyntaxOn
%   VARIABLE DECLARATIONS:
\tl_new:N \g_savevar_rn_tl
\tl_set:Nn \g_savevar_rn_tl {INIT}
\NewDocumentCommand\myMacro{O{1}O{Z}}   
{
    !*\tl_use:N \g_savevar_rn_tl*!        \\
    \str_case:nn {#1} 
        {
            {1}
            {
                #2\\ 
                Value\space before:\tl_use:N \g_savevar_rn_tl \\
                \tl_gset:Nn \g_savevar_rn_tl {#2@#2}
                Value\space after:\tl_use:N \g_savevar_rn_tl \\
              }
            {2}
            {
              Before: \tl_use:N \g_savevar_rn_tl \\
              \tl_gset:Nn \g_savevar_rn_tl {RESET}
              Value: \tl_use:N \g_savevar_rn_tl \\
            }
        }
}
\ExplSyntaxOff
\begin{document}
\section{The way I want to use it, but savevar is not preserved between calls:}
$\myMacro$\\
$\myMacro[2]$\\
$\myMacro[1][\alpha]$\\
$\myMacro[1][\beta]$
\section{savevar is preserved between calls, but only if calls are made in textmode:}
\ExplSyntaxOn
\tl_clear:N \g_savevar_rn_tl
\ExplSyntaxOff
\myMacro[1][A]\\
\myMacro[2]\\
\myMacro[1][W]\\
\myMacro[2]\\
\end{document}