Create a macro that appends some code an existing command, with arguments

Question

As I am working on improving my answer to Different approach to literate programming for LaTeX, I have created a macro that automatically processes two arguments and formats indexes accordingly. The arguments are strings (not control sequences), with #1 being the type of macro and #2 being the name of the macro.

Here is a sample of the macro I am using. It relies on etoolbox and xstring.

\newcommand{\set@macro@type}[2]{
    \StrSubstitute{#2}{@}{"@}[\entryname]
    % Default definitions
    \def\macro@type{#1}
    \def\macro@format{#2}
    \def\macro@index{#2@\string\texttt{#2}}
    % Specific type definitions
    \ifstrequal{#1}{m}
        {\def\macro@type{}
         \def\macro@format{\textbackslash#2}
         \def\macro@index{%
            \entryname @\string\texttt{{\string\textbackslash}\entryname}}}{}
    \ifstrequal{#1}{l}
        {\def\macro@type{length}
         \def\macro@format{%
            \entryname @\string\texttt{{\string\textbackslash}\entryname}}}{}
    \ifstrequal{#1}{e}
        {\def\macro@type{environment}
         \def\macro@format{\textbackslash#2\par\textbackslash end#2}
         \def\macro@index{%
            \entryname @\string\texttt{{\string\textbackslash}\entryname}}}{}}

However, I would like to make it so that the user can add new types of macros with an interface such as:

\addmacro{<name>}{<type>}{<format>}{<index>}

which should append the following code to my existing macro:

\ifstrequal{#1}{<name>}
    {\def\macro@type{<type>}
     \def\macro@format{<format>}
     \def\macro@index{<index>}}

Ideally, the \addmacro command could be starred, too, as I have an additional switch to toggle depending on the user's wishes.

So, my question is: how can I implement a macro that appends some code at the end of the main macro, and that can be used repeatedly. I would prefer to keep using etoolbox if possible (I have seen \apptocmd in the documentation but can't get it to work). The main difficulty which I am encountering is with the arguments #1 and #2 of \set@macro@style, which the user must be able refer to in some way. How could this be implemented? An explanation would be very welcome too.

Answer 1

You could probably place an empty macro without arguments (\setmacrotypeend) at the end of \set@macro@type and append code to this with \ltx@GlobalAppendToMacro from the ltxcmds package. About necessary expansion see Comparing an argument to a string when argument is a result of a command with etoolbox. I did not test this, but for a comment it is too long and needs too much format.

\newcommand{\setmacrotypeend}{}

\newcommand{\set@macro@type}[2]{
... (unchanged as it was before)
\def\setmacrotypefirstargument{#1}
\setmacrotypeend%
}

\RequirePackage{ltxcmds}

\newcommand{\addmacro}[4]{%
\ltx@GlobalAppendToMacro{\setmacrotypeend}{%
  \expandafter\ifstrequal\expandafter{\setmacrotypefirstargument}{#1}
    {\def\macro@type{#2}
     \def\macro@format{#3}
     \def\macro@index{#4}}
}%
}