[Tex/LaTex] Define macro for sequence/list/tuple macros

expansionmacros

I would like to define a macro for defining further macros to display sequences/lists/tuples of elements by specifying the following parameters:

The name of the new command.
What to display for an empty sequence/list/tuple.
How to open a singleton sequence/list/tuple.
How to close a singleton sequence/list/tuple.
How to open a sequence/list/tuple with more than one element.
How to close a sequence/list/tuple with more than one element.
What to display as separating object between two elements in the sequence/list/tuple.

I had a look at the following questions and corresponding answers:

So I tried to code my command and came up with the following two versions (the second one is outcommented in the code below). Unfortunately, none of them works and I cannot figure out what the problem is:

\documentclass{article}

\makeatletter 
\newcommand{\definelistcommand}[7]{%
\expandafter\newcommand\csname @start#1\endcsname{\expandafter\@ifnextchar\csname @stop#1\endcsname{#2\csname @end#1\endcsname}{\csname @first#1\endcsname}}%
\expandafter\newcommand\csname @first#1\endcsname[1]{\expandafter\@ifnextchar\csname @stop#1\endcsname{#3##1#4\csname @end#1\endcsname}{#5##1\csname @next#1\endcsname}}%
\expandafter\newcommand\csname @next#1\endcsname[1]{#7##1\expandafter\@ifnextchar\csname @stop#1\endcsname{#6\csname @end#1\endcsname}{\csname @next#1\endcsname}}%
\expandafter\newcommand\csname @end#1\endcsname[1]{}% consumes the \stop command
\expandafter\newcommand\csname #1\endcsname[1]{\csname @start#1\endcsname##1\csname @stop#1\endcsname}%
}
\makeatother

% \makeatletter 
% \edef\definelistcommand#1#2#3#4#5#6#7{%
% \noexpand\newcommand\expandafter\noexpand\csname @start#1\endcsname{\noexpand\@ifnextchar\expandafter\noexpand\csname @stop#1\endcsname{#2\expandafter\noexpand\csname @end#1\endcsname}{\expandafter\noexpand\csname @first#1\endcsname}}%
% \noexpand\newcommand\expandafter\noexpand\csname @first#1\endcsname[1]{\noexpand\@ifnextchar\expandafter\noexpand\csname @stop#1\endcsname{#3##1#4\expandafter\noexpand\csname @end#1\endcsname}{#5##1\expandafter\noexpand\csname @next#1\endcsname}}%
% \noexpand\newcommand\expandafter\noexpand\csname @next#1\endcsname[1]{#7##1\noexpand\@ifnextchar\expandafter\noexpand\csname @stop#1\endcsname{#6\expandafter\noexpand\csname @end#1\endcsname}{\expandafter\noexpand\csname @next#1\endcsname}}%
% \noexpand\newcommand\expandafter\noexpand\csname @end#1\endcsname[1]{}% consumes the \stop command
% \noexpand\newcommand\expandafter\noexpand\csname #1\endcsname[1]{\expandafter\noexpand\csname @start#1\endcsname##1\expandafter\noexpand\csname @stop#1\endcsname}%
% }
% \makeatother

\definelistcommand{mylist}{empty}{[}{]}{(}{)}{,}

\begin{document}

\mylist{{a}{b}{c}}

\mylist{{a}{b}}

\mylist{a}

\mylist{{b}}

\mylist{}

\end{document}

The desired output would be:

(a,b,c)
(a,b)
[a]
[b]
empty

For the first version, I get this error message:

! Missing \endcsname inserted.
<to be read again>
                   \let
l.27 \mylist{{a}{b}{c}}

For the second version, I get this:

! Undefined control sequence.
l.27 \mylist
            {{a}{b}{c}}

What are the errors in my commands and how would a correct solution work?

EDIT (to point future readers to the fact that the accepted answer is not the only interesting one):
Although I accepted the answer by Christian Hupfer (since it contains an explanation and fix for my attempt), the other answers also contain valuable contributions. In particular, I will use egreg's solution as it allows a key-value pair specification of my desired parameters. Unfortunately, I cannot accept two answers.

Best Answer

Here's a expl3 version with seq variables.

The command \NewListCommand declares the command list macro named like the first argument and uses a global seq variable with some prefix, see \g_cryingshadow_mynewlist_seq etc.

This is filled by the \mynewlist command and the content displayed depending on the content of the other parameters from #3 to #6. The 7th. parameter is used as a delimiter

Some notes: The empty parameter could be an optional one.

\documentclass{article}


\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\NewListCommand}{mmmmmmm}{%
  \seq_new:c {g_cryingshadow_#1_seq} % Make a new sequence
  \expandafter\NewDocumentCommand\csname #1\endcsname{m}{%
    \seq_gclear:c {g_cryingshadow_#1_seq}% Clear the sequence
    \seq_gclear:N \l_tmpa_seq % Clear a temporary `\l_tmpa_seq
    \seq_set_split:Nnn \l_tmpa_seq {} {##1} % Split the sequence into tokens. 
    \seq_set_eq:cN {g_cryingshadow_#1_seq} \l_tmpa_seq % copy the temporary sequence to the real one → this would not be necessary if the sequence is not be used outside 
    \seq_if_empty:cTF {g_cryingshadow_#1_seq} {%
      #2% Checked --> is empty
    }{%
      % Display the various styles
      \int_case:nn { \seq_count:c {g_cryingshadow_#1_seq} }
      {%
        {1} {#3      \seq_use:cnnn {g_cryingshadow_#1_seq} {,} {#7} {#7} #4}
      }%
      \int_compare:nNnT { \seq_count:c {g_cryingshadow_#1_seq}}  > 1 {%
        #5      \seq_use:cnnn {g_cryingshadow_#1_seq} {#7} {#7} {#7} #6
      }
    }
  }
}
\ExplSyntaxOff


\NewListCommand{mynewlist}{empty}{[}{]}{(}{)}{,}


\begin{document}

\mynewlist{{A}{B}{C}}

\mynewlist{{A}{B}}

\mynewlist{{a}}

\mynewlist{}


\mynewlist{{A}{B}{C}{D}{E}{F}}


Astronomer's alphabet:

\mynewlist{{Oh}{be}{a}{fine}{girl}{kiss}{me}}

\end{document}

Edit: Here's the working version for usual LaTeX:

\documentclass{article}
\usepackage{etoolbox}

\makeatletter

\def\definelistcommand#1#2#3#4#5#6#7{%

  \expandafter\def\csname @firstinlist#1\endcsname##1{%
    \@ifnextchar\@stoplist{%
      #3##1#4\@endlist%
    }{%
      #5##1\csname @nextlist#1\endcsname%
    }%
  }

  \expandafter\def\csname @nextlist#1\endcsname##1{%
    \@ifnextchar\@stoplist{%
      #7##1#6\@endlist%
    }{%
      #7##1\csname @nextlist#1\endcsname%
    }%
  }


  \expandafter\def\csname @startlist#1\endcsname##1{%
    \@ifnextchar\@stoplist{%
      #3##1#4\@endlist%
    }{%
      \csname @firstinlist#1\endcsname##1% 
    }%
  }

  \expandafter\def\csname #1\endcsname##1{%
    \ifblank{##1}{%
      #2%
    }{%
      \csname @startlist#1\endcsname##1\@stoplist%
    }%
  }
}

\def\@endlist#1{}


\definelistcommand{mycmd}{empty}{[}{]}{(}{)}{,}

\definelistcommand{othercmd}{empty}{\textbraceleft}{\textbraceright}{(}{)}{+}

\begin{document}


\makeatother

\mycmd{{A}{B}{C}}

\mycmd{A}

\mycmd{{A}{B}}

\mycmd{}

\othercmd{{O}{B}}

\othercmd{{O}}


\end{document}

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So, your intention is: just one place for the file name. If needed, you only have to make a change at this place. That's very good! However, I would not choose subsection headings within the document body for such places.

I suggest to define macros for file names all at one place in the preamble.

Once you have done this, you could easily use \expandafter. Further I guess it may be harder to use \expandafter with referencing commands such as \nameref or \autoref. With normal macros it's easy. For example

\newcommand*{\expath}[1]{\expandafter\path\expandafter{#1}}

calls \path with the expanded file name variable as argument.

Complete minimal example:

\documentclass{article}
\usepackage{url}
\newcommand*{\filecities}{cities.jsp}
\newcommand*{\expath}[1]{\expandafter\path\expandafter{#1}}
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\tableofcontents
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The file name is \expath{\filecities}.
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alt text

If the file name changes, just modify \filecities.

I've tested the same way of expansion with \nameref but it didn't work - but in any case I would not depend on definitions in headings within the body, for such work I rely on the preamble.

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\ch is not provided by chemmacros but by the chemformula package.

As you've noticed it doesn't expand its argument. As a consequence when it sees \compound (unaware of its definition) it does nothing with it and just inputs it. Only afterwards \compound expands to its definition but then it is too late.

That means you either need to do that manually by adding the appropriate number of \expandafters or you can define a \chX (say) which expands its argument once before parsing it:

\documentclass{article}
\usepackage{chemformula}

\ExplSyntaxOn
\NewDocumentCommand \chX { O{}m }
  { \chemformula_ch:no {#1} {#2} }
\cs_generate_variant:Nn \chemformula_ch:nn {no}
\ExplSyntaxOff

\newcommand{\compound}{H2O}

\begin{document}

\ch{H2O} \par
\expandafter\ch\expandafter{\compound} \par
\chX{\compound}

\end{document}

If you choose the \chX way together with chemmacros you either should also load chemformula explicitly (\usepackage{chemmacros}\usepackage{chemformula}) or tell chemformula to do it for you (\chemsetup{formula = chemformula}) before defining the variant.

Best Answer

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