应用需求：识别传递的参数中 `{}` 的数量 about forum HOT 6 CLOSED

% !TeX program = xelatex
\documentclass{ctexart}
\ExplSyntaxOn
\NewDocumentCommand { \whudefineconstant } { m }
  {
    \keys_set:nn { whu/constant } {#1}
  }

\cs_new_protected:Nn \__whu_define_constant_family:n
  {
    \keys_define:nn { whu/constant }
      {
        #1/中文正文 .code:n =
          { \__whu_set_constant:nnn {#1} { \l_keys_key_str } {##1} }
      , #1/中文目录 .code:n =
          { \__whu_set_constant:nnn {#1} { \l_keys_key_str } {##1} }
      , #1/英文正文 .code:n =
          { \__whu_set_constant:nnn {#1} { \l_keys_key_str } {##1} }
      , #1/英文目录 .code:n =
          { \__whu_set_constant:nnn {#1} { \l_keys_key_str } {##1} }
      , #1         .code:n = 
          {
            \keys_set:nn { whu/constant/#1 } { ##1 }
            \bool_lazy_and:nnT
              { \__whu_constant_if_exist_p:nn {#1} {中文正文} }
              { ! \__whu_constant_if_exist_p:nn {#1} {中文目录} }
              {
                \__whu_set_constant:nnn {#1} {中文目录}
                  { \__whu_use_constant:nn {#1} {中文正文} }
              }
            \bool_lazy_and:nnT
              { \__whu_constant_if_exist_p:nn {#1} {英文正文} }
              { ! \__whu_constant_if_exist_p:nn {#1} {英文目录} }
              {
                \__whu_set_constant:nnn {#1} {英文目录}
                  { \__whu_use_constant:nn {#1} {英文正文} }
              }
          }
      }
  }

\__whu_define_constant_family:n {致谢}
\__whu_define_constant_family:n {摘要}
\__whu_define_constant_family:n {啥啥}

\cs_new_protected:Nn \__whu_set_constant:nnn
  {
    \tl_const:cn { c__whu_constant_ #1/#2 _tl } {#3}
  }
\cs_new_protected:Nn \__whu_use_constant:nn
  {
    \tl_use:c { c__whu_constant_ #1/#2 _tl }
  }
\cs_new_protected:Nn \__whu_get_constant_csname:nn
  {
    c__whu_constant_ #1/#2 _tl
  }
\prg_new_conditional:Nnn \__whu_constant_if_exist:nn { p , T , F , TF }
  {
    \tl_if_exist:cTF { \__whu_get_constant_csname:nn {#1} {#2} }
       \prg_return_true:
       \prg_return_false:
  }
\ExplSyntaxOff

\begin{document}
\whudefineconstant{
  致谢={
    英文正文=ACKNOWLEDGEMENT,
    英文目录=Acknowledgement,
    中文正文=致\quad 谢,
    中文目录=致谢,
  },
  摘要={
    中文正文=摘\qquad 要,
    英文正文=Abstract,
  },
  啥啥={
    中文正文=啥啥,
  },
}

\ExplSyntaxOn
\cs_new_protected:Npn \WhuPrintConstant #1
  {
    \par\noindent
    \textbf{#1}
    \__whu_print_constant_aux:nn {#1} {中文正文}
    \__whu_print_constant_aux:nn {#1} {中文目录}
    \__whu_print_constant_aux:nn {#1} {英文正文}
    \__whu_print_constant_aux:nn {#1} {英文目录}
    \par
  }

\cs_new_protected:Nn \__whu_print_constant_aux:nn
  {
    \\\null\qquad （#2）：~
    \cs_if_exist_use:cF { c__whu_constant_ #1/#2 _tl } { \fbox{未定义} }
  }
\ExplSyntaxOff

\WhuPrintConstant{致谢}
\WhuPrintConstant{摘要}
\WhuPrintConstant{啥啥}
\end{document}