CreateDocumentParameter

CreateDocumentParameter.tex

\makeatletter
\DeclareDocumentCommand{\setvalue}{m o m}{%
    \IfNoValueTF{#2}{%
        % NO INDEX
        \expandafter\DeclareDocumentCommand\csname @#1\endcsname{}{#3}%
    }{%
        % INDEX
        \expandafter\DeclareDocumentCommand\csname @#1@#2\endcsname{}{#3}%
    }%
}%
%
\DeclareDocumentCommand{\getvalue}{m o}{%
    \IfNoValueTF{#2}{%
        % NO INDEX
        \ifcsname @#1\endcsname%
            \csname @#1\endcsname%
        \else%
            \textcolor{red}{@#1}\xspace%
        \fi%
    }{%
        % INDEX
        \ifcsname @#1@#2\endcsname%
            \csname @#1@#2\endcsname%
        \else%
            \textcolor{red}{@#1@#2}%
        \fi%
    }%
}%
%
\DeclareDocumentCommand{\CreateDocumentParameter}{m o G{undefined}}{%
    \IfNoValueTF{#2}{%
        % NO INDEX
        \setvalue{#1}{#3}%
    }{%
        % INDEX
        \setvalue{#1}[#2]{#3}%
    }%
    \expandafter\DeclareDocumentCommand\csname #1\endcsname{o g}{%
        \IfNoValueTF{##2}{%
            \getvalue{#1}[##1]\xspace%
        }{%
            \setvalue{#1}[##1]{##2}%
        }%
    }%
}%
\makeatother