How to map optional arguments to functions?

Question

What is the right way to map each optional argument to a function that applies a specific style to it?

In the example below, optional arguments are typeset in bold face preceded by a colon. But I want users to be able to define what style #2 is to be typeset in, what style #3 is to be typeset in, and so on:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand {\mystyle} { m } {:\textbf{#1}}
\NewDocumentCommand {\mycs} { moooooooo }
 {
  #1
  \tl_set:Nn \l_my_tl { {#2} {#3} {#4} {#5} {#6} {#7} {#8} {#9} }
  \tl_map_inline:Nn \l_my_tl {\IfValueT{##1}{\mystyle{##1}}}
 }
\ExplSyntaxOff

\begin{document}
\mycs{1}[2][3][4][5][6][7][8][9]
\end{document}

Answer 1

The following might be an option. The \setstyle macro takes a comma separated list of commands names possibly preceded by characters. By default the list is comma separated, but the optional argument can change the separator to whatever. In the example below I used

\setstyle{:\textbf,.\emph,\textsc}

The \usestyles macro takes two mandatory arguments. The first corresponds to your mandatory argument above, the second is a list of arguments. By default the list is comma separated, but this can be changed with the optional argument. The first n styles are then mapped to the first n arguments.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn

% optional arg1 changes the separator for arg2
\NewDocumentCommand{\setstyle}{ O{,} m }{
    \ss_set_my_styles:nn {#1}{#2}
}

% optional arg1 changes the separator for arg3
\NewDocumentCommand{\usestyles}{ O{,} m m }{
    \ss_use_my_styles:nnn {#1}{#2}{#3}
}

% split style list at separator and store
\seq_new:N \g__ss_my_styles_seq
\cs_new:Npn \ss_set_my_styles:nn #1#2
    {
        \seq_set_split:Nnn \g__ss_my_styles_seq {#1}{#2}
    }

\cs_new:Npn \ss_use_my_styles:nnn #1#2#3
    {
        #2\par
        % split arg list at separator
        \seq_set_split:Nnn \l_tmpa_seq {#1}{#3}
        % map styles to list items in order
        \seq_mapthread_function:NNN \g__ss_my_styles_seq \l_tmpa_seq \__ss_use:nn
    }

\cs_new:Npn \__ss_use:nn #1#2
    {
        % applies arg1 to arg2
        #1{#2}\par
    }

\ExplSyntaxOff
\begin{document}

\setstyle{:\textbf,.\emph,\textsc}
\usestyles{first}{arg1,arg2,arg3}

\end{document}

---

---

Another option using l3regex. With this you can add characters before and after the arguments.

• \setstyle[<separator>][<string to replace>]{<style list>}

The first optional sets the separator for the <style list> which defaults to a comma, the second sets the string to be replaced by the argument and defaults to thearg.

• \usestyles[<separator>]{<1st mand>}{<list of args>}

The optional again sets the separator for the third argument.

\documentclass{article}
\usepackage{xparse}
\usepackage{l3regex}
\ExplSyntaxOn

\tl_new:N \c__ss_repl_symb_tl
\seq_new:N \g__ss_my_styles_seq

% optional #1 sets separator
% optional #2 sets string to replace
\NewDocumentCommand{\setstyle}{ O{,} O{thearg} m }{
    \ss_set_my_styles:nn {#1}{#2}{#3}
}

% split style list at separator and store
\cs_new:Npn \ss_set_my_styles:nn #1#2#3 {
    \tl_gset:Nn \c__ss_repl_symb_tl {#2}
    \seq_set_split:Nnn \g__ss_my_styles_seq {#1}{#3}
}

% optional #1 sets separator to use when
% splitting #3
\NewDocumentCommand{\usestyles}{ O{,} m m }{
    \ss_use_my_styles:nnn {#1}{#2}{#3}
}

\cs_new:Npn \ss_use_my_styles:nnn #1#2#3 {
    #2\par
    % split arg list at separator
    \seq_set_split:Nnn \l_tmpa_seq {#1}{#3}
    % map styles to list items in order
    \seq_mapthread_function:NNN \g__ss_my_styles_seq \l_tmpa_seq \__ss_use:nn
}

\cs_generate_variant:Nn \regex_replace_all:nnN {VnN}    
\cs_new:Npn \__ss_use:nn #1#2 {
    \tl_set:Nn \l_tmpa_tl {#1}
    \regex_replace_all:VnN {\c__ss_repl_symb_tl} {#2} \l_tmpa_tl
    \tl_use:N \l_tmpa_tl\par
}

\ExplSyntaxOff
\begin{document}

\setstyle{before \textbf{thearg} after,before \emph{thearg} after,before \textsc{thearg} after}
\usestyles{first}{arg1,arg2,arg3}

\end{document}