Function / macro / commands as tags

Question

I want to implement function as tags. For now, this code is working:

In equations_folder/equations.tex

\EQUATION{miqp-obj}
\begin{equation}
  \label{eq:miqp-obj}
  \min_{\mathbf{x}}C = \frac{1}{2} \mathbf{x}^{T}Q\mathbf{x}+\mathbf{c}^{T}\mathbf{x} 
\end{equation}
\ENDEQUATION
 
\bigskip % use this to make a space between equations

\EQUATION{miqp-cons}
\begin{equation}
  \label{eq:miqp-cons}
  A\mathbf{x} \preceq \mathbf{b},
\end{equation}
\ENDEQUATION

Using a format.cls

\newcommand{\inputeq}[2]{% #1 = file, #2 = eq name
    \long\def\EQUATION ##1#2 {}%
    \input{#1}
}
\let\ENDEQUATION\endinput

And from my main.tex

    \inputeq{equations/equations}{{miqp-obj}}
    \inputeq{equations/equations}{{miqp-cons}}

Problem is, the miqp-obj is printed. I can fix it by edit the above code into: \inputeq{equations/equations}{miqp-obj} and \EQUATION miqp-obj, but the tag will lose the bracket. Can anyone help me modify the .cls code?

I've read this but does not satisfy my need.

__

Bonus: Originally, I want to implement bibtex-like function when we can cite our equation from a single centralized file. If you can help me with this, but not answering the main question, feel free to answer it that way. This is because I want to make a standard equation naming for my research group, and everyone should only cite this file or contribute to it first.

__

Edit: above code sadly only work if I cite only one equations. Solution to multi citing is using:

\EQUATION miqp-obj
\begin{equation}
  \label{eq:miqp-obj}
  \min_{\mathbf{x}}C = \frac{1}{2} \mathbf{x}^{T}Q\mathbf{x}+\mathbf{c}^{T}\mathbf{x} 
\end{equation}
\ENDEQUATION

Problem is, \EQUATION become a command with arbitrary many input.

Answer 1

Seems you have a ⟨.tex-file⟩ with a sequence of \EQUATION{⟨tag⟩}...\ENDEQUATION-thingies and you wish a command \inputeq{⟨.tex-file⟩}{⟨tag⟩} to deliver that one \EQUATION...\ENDEQUATION-thingie where ⟨tag⟩ is matching.

In other words:

• Constellations of ⟨.tex-file⟩s form a database of equations.
• \EQUATION{⟨tag⟩}...\ENDEQUATION-thingies form entries of that database.
• ⟨tag⟩ is a primary-key/a unique identifier for an entry of that database.

As long as you are okay with obeying the conventions that

• with \EQUATION...\ENDEQUATION-thingies the same ⟨tag⟩ is used only once/is not used repeatedly across all ⟨.tex-file⟩s used by \inputeq-commands within the whole document,
• a ⟨.tex-file⟩ contains only material nested between \EQUATION{⟨tag⟩}...\ENDEQUATION

, a "poorman's approach" could be defining \EQUATION in terms of an undelimited argument trailed by an \ENDEQUATION-delimited argument.

In the following example I did some trickery for removing leading space-tokens from the \ENDEQUATION-delimited argument and for preventing the removal of curly braces that might surround the entire \ENDEQUATION-delimited argument.

The following example provides a command

\inputeq{⟨.tex-file⟩}{⟨tag⟩}

The "unstarred" variant of that command is intended to introduce an equation with equation-number and cross-referencing-label so that cross-referencing via \ref{...}/\pageref{...}/etc while using the hyperref-package yields a hyperlink to the corresponding place in the document.

The "starred" variant, i.e., \inputeq*{⟨.tex-file⟩}{⟨tag⟩} does not produce a cross-referencing label and is intended for repeating/quoting an equation introduced via the "unstarred" variant.

The "starred" variant can only be used if the "unstarred" variant is used with the same ⟨.tex-file⟩-argument and the same ⟨tag⟩-argument in the same document, too. Otherwise you get undefined-reference-errors.

% Compile with LaTeX.
%
% Just to make sure you have an external .tex-file   equations.tex   which
% can be used as   <.tex-file>  when applying 
%   \inputeq{<.tex-file>}{<tag>}%
% let's create a file   equations.tex   in the current directory via the
% filecontents*-environment:

\begin{filecontents*}{equations.tex}
\EQUATION{miqp-obj}
  \min_{\mathbf{x}}C = \frac{1}{2} \mathbf{x}^{T}Q\mathbf{x}+\mathbf{c}^{T}\mathbf{x} 
\ENDEQUATION
% 
% !!!! Nothing processable by TeX outside \EQUATION..\ENDEQUATION  !!!!!
%
\EQUATION{miqp-cons}
  A\mathbf{x} \preceq \mathbf{b},
\ENDEQUATION
\end{filecontents*}

% Between \makeatletter..\makeatother let's define "mechanisms"
%  - \EQUATION{<tag>}...\ENDEQUATION
%  - \inputeq{<.tex-file>}{<tag>}
%
\makeatletter
\RequirePackage{refcount}%
%%=============================================================================
%% PARAPHERNALIA:
%% \UD@firstoftwo, \UD@secondoftwo, \UD@PassFirstToSecond, \UD@Exchange,
%% \UD@stopromannumeral, \UD@CheckWhetherNull,
%% \UD@CheckWhetherLeadingExplicitSpace,
%% \UD@Stringify, \UD@checkstringsubsetof,
%%=============================================================================
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\newcommand\UD@Exchange[2]{#2#1}%
\@ifdefinable\UD@removespace{\UD@Exchange{ }{\def\UD@removespace}{}}%
\@ifdefinable\UD@stopromannumeral{\chardef\UD@stopromannumeral=`\^^00}%
%%-----------------------------------------------------------------------------
%% Check whether argument is empty:
%%.............................................................................
%% \UD@CheckWhetherNull{<Argument which is to be checked>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is empty>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is not empty>}%
%%
%% The gist of this macro comes from Robert R. Schneck's \ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
\newcommand\UD@CheckWhetherNull[1]{%
  \romannumeral\expandafter\UD@secondoftwo\string{\expandafter
  \UD@secondoftwo\expandafter{\expandafter{\string#1}\expandafter
  \UD@secondoftwo\string}\expandafter\UD@firstoftwo\expandafter{\expandafter
  \UD@secondoftwo\string}\expandafter\UD@stopromannumeral\UD@secondoftwo}{%
  \expandafter\UD@stopromannumeral\UD@firstoftwo}%
}%
%%-----------------------------------------------------------------------------
%% Check whether brace-balanced argument starts with a space-token
%%.............................................................................
%% \UD@CheckWhetherLeadingExplicitSpace{<Argument which is to be checked>}%
%%                                     {<Tokens to be delivered in case <argument
%%                                       which is to be checked> does have a
%%                                       leading explicit space-token>}%
%%                                     {<Tokens to be delivered in case <argument
%%                                       which is to be checked> does not have a
%%                                       a leading explicit space-token>}%
\newcommand\UD@CheckWhetherLeadingExplicitSpace[1]{%
  \romannumeral\UD@CheckWhetherNull{#1}%
  {\expandafter\UD@stopromannumeral\UD@secondoftwo}%
  {%
    % Let's nest things into \UD@firstoftwo{...}{} to make sure they are nested in braces
    % and thus do not disturb when the test is carried out within \halign/\valign:
    \expandafter\UD@firstoftwo\expandafter{%
      \expandafter\expandafter\expandafter\UD@stopromannumeral
      \romannumeral\expandafter\UD@secondoftwo
      \string{\UD@CheckWhetherLeadingExplicitSpaceB.#1 }{}%
    }{}%
  }%
}%
\@ifdefinable\UD@CheckWhetherLeadingExplicitSpaceB{%
  \long\def\UD@CheckWhetherLeadingExplicitSpaceB#1 {%
    \expandafter\UD@CheckWhetherNull\expandafter{\UD@firstoftwo{}#1}%
    {\UD@Exchange{\UD@firstoftwo}}{\UD@Exchange{\UD@secondoftwo}}%
    {\expandafter\expandafter\expandafter\UD@stopromannumeral
     \expandafter\expandafter\expandafter}%
     \expandafter\UD@secondoftwo\expandafter{\string}%
  }%
}%
%%-----------------------------------------------------------------------------
%% Remove all leading spaces:
\newcommand\UD@RemoveAllLeadingSpaces[1]{%
  \romannumeral\UD@RemoveAllLeadingSpacesLoop{#1}%
}%
\newcommand\UD@RemoveAllLeadingSpacesLoop[1]{%
  \UD@CheckWhetherLeadingExplicitSpace{#1}{%
    \expandafter\UD@RemoveAllLeadingSpacesLoop\expandafter{\UD@removespace#1}%
  }{\UD@stopromannumeral#1}%
}%
%%=============================================================================
%% \inputeq{<.tex-file>}{<tag>}
%%.............................................................................
\newcommand*\TAGPHRASE{}%
\newcommand*\CrossRefFork{}%
\newcommand\inputeq{%
  \@ifstar{\global\let\CrossRefFork\UD@secondoftwo\Innerinputeq}%
          {\global\let\CrossRefFork\UD@firstoftwo\Innerinputeq}%
}%
\newcommand\Innerinputeq[2]{%
  \xdef\TAGPHRASE{\unexpanded{#2}}%
  \input{#1}%
}%
%%=============================================================================
%% \EQUATION{<TAG>}...\ENDEQUATION
%%.............................................................................
\newcounter{equationduplicates}%
\def\theHequationduplicates{someduplicate.\number\value{equationduplicates}}%
\newcommand*\EQUATION[1]{%
  \InnerEQUATION{#1} %Prepend a space to ensure surrounding brases aren't stripped off.
}%
\@ifdefinable\InnerEQUATION{%
  \long\def\InnerEQUATION#1#2\ENDEQUATION{%
     \begingroup
     \edef\tempa{\unexpanded{#1}}%
     \expandafter\endgroup
     \ifx\tempa\TAGPHRASE\expandafter\UD@secondoftwo\else\expandafter\UD@firstoftwo\fi{}{%
       \begingroup
       \CrossRefFork{}{%
          \def\theequation{\getrefnumber{eq:#1}}%
          \refstepcounter{equationduplicates}%
          \let\theHequation\theHequationduplicates
       }%
       \begin{equation}\CrossRefFork{\label{eq:#1}}{}%
       \UD@RemoveAllLeadingSpacesLoop{#2}%
       \end{equation}%
       \CrossRefFork{}{\global\advance\c@equation by -1\relax}%
       \endgroup
     }%
  }%
}%
\makeatother

% Now let's create a nice document:

\documentclass{article}

\AtBeginDocument{%
  % Let's number equations within sections
  \def\theequation{\thesection.\arabic{equation}}%
  \csname @addtoreset\endcsname{equation}{section}%
}%

\usepackage{hyperref}

\begin{document}

\section{Referencing:}

\noindent Reference to equation \verb|miqp-obj|: \ref{eq:miqp-obj}

\noindent Reference to equation \verb|miqp-cons|: \ref{eq:miqp-cons}

\noindent\hrule\hfill

\section{Repeating:}

These are just repetitions not hyperlinked by cross-referencing-commands:

\inputeq*{equations.tex}{miqp-cons}

\inputeq*{equations.tex}{miqp-obj}

\noindent\hrule\hfill

\section{Originals:}

These are the "originals"/the targets for hyperlinks created by cross-referencing-commands:

\inputeq{equations.tex}{miqp-obj}

\inputeq{equations.tex}{miqp-cons}

\noindent\hrule\hfill

\section{Repeating again:}

These are just repetitions not hyperlinked by cross-referencing-commands:

\inputeq*{equations.tex}{miqp-obj}

\inputeq*{equations.tex}{miqp-cons}

\section{Another section}
\begin{equation}
x^2+y^2=z^2
\end{equation}

\end{document}

The example above creates a text-file equations.tex with the following content:

\EQUATION{miqp-obj}
  \min_{\mathbf{x}}C = \frac{1}{2} \mathbf{x}^{T}Q\mathbf{x}+\mathbf{c}^{T}\mathbf{x} 
\ENDEQUATION
% 
% !!!! Nothing processable by TeX outside \EQUATION..\ENDEQUATION  !!!!!
%
\EQUATION{miqp-cons}
  A\mathbf{x} \preceq \mathbf{b},
\ENDEQUATION

In the example above that file is used as ⟨.tex-file⟩ with \inputeq{⟨.tex-file⟩}{⟨tag⟩}-commands.

The resulting .pdf-file looks like this:

Some of the pitfalls:

• ⟨.tex-files⟩ containing database-entries for "named equations" must obey the convention that all material processable by TeX is nested between \EQUATION{⟨tag⟩}...\ENDEQUATION.
• As the ⟨tag⟩ also is used for creating the cross-referencing-label no two \EQUATIONs (across different equation.tex-files!!!) may use the same ⟨tag⟩.
  In other words: ⟨tag⟩ is to be a primary-key of the database that is formed by all ⟨.tex-files⟩ used via \inputeq within your document.
• If many people are involved in maintaining the .tex-files containing equation-databases, some of these people probably not being all too familiar with TeX, ensuring the above "by hand and eyes" might be a problem.
  Therefore I strongly recommend maintaining databases via professional database-management-systems, e.g., MariaDB or MySQL, instead of using "manually"-maintained .tex-files for this purpose.
• Things between \EQUATION{⟨tag⟩}...\ENDEQUATION are processed as macro-arguments. Therefore between \EQUATION{⟨tag⟩}...\ENDEQUATION you can't use commands/environments like \verb or verbatim where it is relied on temporarily changing the catcode-régime and having things read and tokenized from .tex-input-file under that changed catcode-régime.
• Nesting \EQUATION{⟨tag⟩}...\ENDEQUATION might not turn out as intended.
• I doubt that this list of pitfalls is complete. ;-)

Answer 2

You can load the file and populate a property list:

\begin{filecontents*}{\jobname-equations}
\EQUATION{miqp-obj}{
  \min_{\mathbf{x}}C = \frac{1}{2} \mathbf{x}^{T}Q\mathbf{x}+\mathbf{c}^{T}\mathbf{x} 
}
 
\EQUATION{miqp-cons}{
  A\mathbf{x} \preceq \mathbf{b},
}
\end{filecontents*}

\documentclass{article}
\usepackage{amsmath}

\ExplSyntaxOn

\prop_new:N \g_yassirroni_equations_prop

\NewDocumentCommand{\EQUATION}{mm}
 {
  \prop_gput:Nnn \g_yassirroni_equations_prop { #1 }
   {
    \begin{equation}\label{eq:#1}#2\end{equation}
   }
 }

\NewDocumentCommand{\useequation}{m}
 {
  \prop_item:Nn \g_yassirroni_equations_prop { #1 }
 }

\ExplSyntaxOff

\input{\jobname-equations} % use your own file name

\begin{document}

Here we set the first equation
\useequation{miqp-obj}
and now also the second one
\useequation{miqp-cons}
Let's see whether references work: \eqref{eq:miqp-obj} and \eqref{eq:miqp-cons}.

\end{document}

Here I use filecontents* just to make the example self-contained.

However, I'm doubtful that this would ease the workflow.