Emulate a “tuple” data structure when the syntax for creating a tuple is predefined and cannot be altered

Question

Imagine I have a macro \createcontact that accepts three parameters. Now, instead of using these during the expansion, I would like to save them separately for later use and retrieval. The above macro is used in the following context:

\newcommand{\myfirstauthor}{\createcontact{Alice}{Munich}{Germany}}
\newcommand{\mysecondauthor}{\createcontact{Bob}{London}{United Kingdom}}
% and so forth

How do I implement \createcontact so that I can extract the individual parameters that have been passed to it from \myfirstauthor, \mysecondauthor etc.? How would then the code for the extraction look like? In other words, how do I need to define \createcontact and, say, a macro \extractsecond so that \extractsecond\myfirstauthor expands to Munich?

EDIT: Note that only the definition of \createcontact can be adapted, not that of \myfirstauthor, \mysecondauthor etc.. That is, the usage pattern above must remain "as is".

The reason of this exercise is that the above pattern is used in users' documents all over the place. I would like to do more complex processing of the \createcontact parameters than simply using them once in a macro. (For example, I want to use Munich in one context, and Germany in another.)

As per Peter's request, here comes a "compilable" example that has placeholders instead of the commands I'm interested in:

\documentclass{article}

% HOW TO IMPLEMENT THIS?
\newcommand{\createcontact}[3]{...}
\newcommand{\extractfirst}[1]{...}
\newcommand{\extractsecond}[1]{...}
\newcommand{\extractthird}[1]{...}
% HOW TO IMPLEMENT THIS?

% DO NOT CHANGE OR MOVE THIS
\newcommand{\myfirstauthor}{\createcontact{Alice}{Munich}{Germany}}
\newcommand{\mysecondauthor}{\createcontact{Bob}{London}{United Kingdom}}
% DO NOT CHANGE OR MOVE THIS

\begin{document}
  \extractfirst{\myfirstauthor} lives in \extractsecond{\myfirstauthor}
  which is located in \extractthird{\myfirstauthor};
  this may or may not be true for \extractfirst{\mysecondauthor}.
\end{document}

Answer 1

Using lambda lists:

\documentclass{minimal}
\usepackage{lambda} % http://www.ctan.org/pkg/lambda-lists
%
\def\createcontact#1#2#3{\Listize[#1,#2,#3]}
\let\extractfirst\Head
\def\extractsecond#1{\extractfirst{\Tail{#1}}}
\def\extractthird#1{\extractsecond{\Tail{#1}}}
%
\newcommand{\myfirstauthor}{\createcontact{Alice}{Munich}{Germany}}
\newcommand{\mysecondauthor}{\createcontact{Bob}{London}{United Kingdom}}
%
\begin{document}
  \extractfirst{\myfirstauthor} lives in \extractsecond{\myfirstauthor}
  which is located in \extractthird{\myfirstauthor};
  (or \Head{\Tail{\Tail{\myfirstauthor}}},
  or \Head{\Reverse{\myfirstauthor}})
  this may or may not be true for \extractfirst{\mysecondauthor}.
\end{document}

Answer 2

Basic Solution:

Without more details it is difficult to know if something as simple as this will work for you: You call \myfirstauthor{Kenobi}{General}{1138}, and then to access each member you simply refer to \myfirstauthorName, \myfirstauthorRank, and \myfirstauthorSerialNum:

\documentclass{article}

\newcommand{\myfirstauthor}[3]{%
    \newcommand{\myfirstauthorName}{#1}%
    \newcommand{\myfirstauthorRank}{#2}%
    \newcommand{\myfirstauthorSerialNum}{#3}%
}

\begin{document}
\myfirstauthor{Kenobi}{General}{1138}

The details of the first author are:

\medskip
\begin{tabular}{rl}
Name: & \myfirstauthorName\\
Rank: & \myfirstauthorRank\\
Serial Number: & \myfirstauthorSerialNum
\end{tabular}
\end{document}

---

Alternate Solution:

Instead of creating a list of separate macors \myfirstauthor, \mysecondauthor, etc you could specify the differentiation between them as the first parameter to the \myauthor macro. This yields a more flexible solution as you can create the macros on the fly. So with this solution yo can use \myauthor[first]{Kenobi}{General}{1138} (or leave out the first as that is the default), but others can simply be defined by \myauthor[second]{Dooku}{Count}{66}, and accessed as in the above case to yield:

To create another one you just use \myauthor[third]{}{}{}, etc...

\documentclass{article}

\newcommand{\myauthor}[4][first]{%
    \expandafter\newcommand\csname my#1authorName\endcsname{#2}%
    \expandafter\newcommand\csname my#1authorRank\endcsname{#3}%
    \expandafter\newcommand\csname my#1authorSerialNum\endcsname{#4}%
}

\begin{document}
\myauthor{Kenobi}{General}{1138}
\myauthor[second]{Dooku}{Count}{66}

The details of the first two authors are:

\medskip
\begin{tabular}{rll}
Name: & \myfirstauthorName & \mysecondauthorName\\
Rank: & \myfirstauthorRank & \mysecondauthorRank\\
Serial Number: & \myfirstauthorSerialNum & \mysecondauthorSerialNum
\end{tabular}
\end{document}

---

Solution Based on given MWE:

With the revised restrictions, you can use \AtBeginDocument to change the definition of \createcontact based on if \myfirstauthor or \mysecondauthor is called:

\documentclass{article}

% HOW TO IMPLEMENT THIS?
\newcommand{\createcontact}[3]{}
\newcommand{\extractfirst}[1]{\csname #1Name\endcsname}
\newcommand{\extractsecond}[1]{\csname #1City\endcsname}
\newcommand{\extractthird}[1]{\csname #1Country\endcsname}

\newcommand{\myfirstauthorName}{}%
\newcommand{\myfirstauthorCity}{}%
\newcommand{\myfirstauthorCountry}{}%
\newcommand{\CreateFirstContact}[3]{%
    \renewcommand{\myfirstauthorName}{#1}%
    \renewcommand{\myfirstauthorCity}{#2}%
    \renewcommand{\myfirstauthorCountry}{#3}%
}
\newcommand{\mysecondauthorName}{}%
\newcommand{\mysecondauthorCity}{}%
\newcommand{\mysecondauthorCountry}{}%
\newcommand{\CreateSecondContact}[3]{%
    \renewcommand{\mysecondauthorName}{#1}%
    \renewcommand{\mysecondauthorCity}{#2}%
    \renewcommand{\mysecondauthorCountry}{#3}%
}


\AtBeginDocument{%
    \let\OldFirstAuthor\myfirstauthor
    \let\OldSecondAuthor\mysecondauthor
    \renewcommand{\myfirstauthor}{\let\createcontact\CreateFirstContact\OldFirstAuthor}
    \renewcommand{\mysecondauthor}{\let\createcontact\CreateSecondContact\OldSecondAuthor}
    \myfirstauthor
    \mysecondauthor
}
% HOW TO IMPLEMENT THIS?

% DO NOT CHANGE THIS
\newcommand{\myfirstauthor}{\createcontact{Alice}{Munich}{Germany}}
\newcommand{\mysecondauthor}{\createcontact{Bob}{London}{United Kingdom}}

\begin{document}
\noindent
  \extractfirst{myfirstauthor} lives in \extractsecond{myfirstauthor}
  which is located in \extractthird{myfirstauthor};
  this may or may not be true for \extractfirst{mysecondauthor}.
\end{document}

Answer 3

With credit to Bruno Le Floch for the idea, here is what I would consider "working for me":

\documentclass{standalone}
% ONE POSSIBLE SOLUTION
\newcommand{\createcontact}[3]{}
\newcommand{\extractfirst}[1]{\renewcommand{\createcontact}[3]{##1}#1}
\newcommand{\extractsecond}[1]{\renewcommand{\createcontact}[3]{##2}#1}
\newcommand{\extractthird}[1]{\renewcommand{\createcontact}[3]{##3}#1}
% ONE POSSIBLE SOLUTION

% DO NOT CHANGE THIS
\newcommand{\myfirstauthor}{\createcontact{Alice}{Munich}{Germany}}
\newcommand{\mysecondauthor}{\createcontact{Bob}{London}{United Kingdom}}
% DO NOT CHANGE THIS
\begin{document}

  \extractfirst{\myfirstauthor} lives in \extractsecond{\myfirstauthor}
  which is located in \extractthird{\myfirstauthor};
  this may or may not true for \extractfirst{\mysecondauthor}.
\end{document}