Parsing non-expandable commands like \small, using expl3

Question

Using expl3 syntax, I want in my \coolgloss command to use as an optional argument one or several formatting commands such as \small and \itshape.

With a command like \newcommand{\coolgloss}[3][\small\itshape]{text}{gloss} I have no problems, but I specifically want to use expl3 in order to have a split optional which includes the negative raise value for \rule.

It works if I only want to pass one command name (e.g. small) through \csname...\endcsname, but I want support for multiple commands (e.g. \small\itshape), and I want the arguments to be given with the backslash included.

Thus, the command must take a split optional argument, whose the second subargument (argument #2) is dedicated to the formatting:

\coolgloss[5,\small\itshape]{text}{gloss}

But when I input it, the \small command (not the \itshape one) triggers a TeX capacity exceeded error.

This has to be an issue with expansion and variable types, i.e. somewhere I should replace a n with some other type.

MWE

\documentclass{article}

\usepackage{gb4e}

\NewDocumentCommand{\coolglossaux}{mmmm}{%
    \renewcommand{\eachwordtwo}{\rule[-#1pt]{0pt}{0pt}#2}% !! #2 is the one !!
    \getwords(\lineone,\eachwordone)#3 \\%
    \getwords(\linetwo,\eachwordtwo)#4 \\%
    \loop\lastword{\eachwordone}{\lineone}{\wordone}%
    \lastword{\eachwordtwo}{\linetwo}{\wordtwo}%
    \global\setbox\gline=\hbox{\unhbox\gline
        \hskip\glossglue
        \vtop{\box\wordone
            \nointerlineskip
            \box\wordtwo
        }%
    }%
    \testdone
    \ifnotdone
    \repeat
    {\hskip -\glossglue}\unhbox\gline
}

\ExplSyntaxOn

\NewDocumentCommand{\coolgloss}{>{\SplitArgument{1}{,}}omm}
{
    \my_xcoolgloss:nnnn #1 {#2} {#3}
}
\cs_new_protected:Nn \my_xcoolgloss:nnnn
{
    \__my_xcoolgloss:eenn
    { \tl_if_novalue:nTF { #1 } { 10 } { #1 } }
    { \tl_if_novalue:nTF { #2 } {} { #2 } }
    { #3 }
    { #4 }
}

\cs_set_eq:NN \__my_xcoolgloss:nnnn \coolglossaux
\cs_generate_variant:Nn \__my_xcoolgloss:nnnn {ee}

\ExplSyntaxOff

\begin{document}
    
    \coolgloss[5,\itshape]{I like \LaTeX}{I like \LaTeX}
    
    %\coolgloss[5,\small\itshape]{I like \LaTeX}{I like \LaTeX} => TeX capacity exceeded
    
\end{document}

Answer 1

As MaxChernoff said, you can't expand \small. For this reason the following uses \exp_not:n for your two optional sub-arguments, so that the \tl_if_novalue:nTF tests are completely processed, but their results don't expand any further.

EDIT: I changed the auxiliary \coolglossaux to be defined \protected via \@ifdefinable and \protected\def. This might not be ideal as well, but unfortunately LaTeX2e has no good interface to \protected outside the expl3 and \NewDocumentCommand routes. Also I've fixed a bug in the argument splitter if the optional argument isn't used at all.

\documentclass{article}

\usepackage{gb4e}

\makeatletter
% not perfect as well, but LaTeX2e itself has no good interface to `\protected`.
% The `\@ifdefinable` makes this behave like `\newcommand` even though we use
% a primitive `\def` for the definition.
\@ifdefinable{\coolglossaux}
    {%
        \protected\def\coolglossaux#1#2#3#4{%
            \renewcommand{\eachwordtwo}{\rule[-#1pt]{0pt}{0pt}#2}% !! #2 is the one !!
            \getwords(\lineone,\eachwordone)#3 \\%
            \getwords(\linetwo,\eachwordtwo)#4 \\%
            \loop\lastword{\eachwordone}{\lineone}{\wordone}%
            \lastword{\eachwordtwo}{\linetwo}{\wordtwo}%
            \global\setbox\gline=\hbox{\unhbox\gline
                \hskip\glossglue
                \vtop{\box\wordone
                    \nointerlineskip
                    \box\wordtwo
                }%
            }%
            \testdone
            \ifnotdone
            \repeat
            {\hskip -\glossglue}\unhbox\gline
        }
    }
\makeatother

\ExplSyntaxOn

\NewDocumentCommand{\coolgloss}{>{\SplitArgument{1}{,}}omm}
{
    \tl_if_novalue:nTF {#1}
        { \my_xcoolgloss:nnnn {#1} {#1} {#2} {#3} }
        { \my_xcoolgloss:nnnn #1 {#2} {#3} }
}
\cs_new_protected:Nn \my_xcoolgloss:nnnn
{
    \__my_xcoolgloss:eenn
    { \tl_if_novalue:nTF { #1 } { 10 } { \exp_not:n {#1} } }
    { \tl_if_novalue:nTF { #2 } {} { \exp_not:n {#2} } }
    { #3 }
    { #4 }
}

\cs_set_eq:NN \__my_xcoolgloss:nnnn \coolglossaux
\cs_generate_variant:Nn \__my_xcoolgloss:nnnn {ee}

\ExplSyntaxOff

\begin{document}
    
    \coolgloss[5,\itshape]{I like \LaTeX}{I like \LaTeX}
    
    \coolgloss[5,\small\itshape]{I like \LaTeX}{I like \LaTeX} => TeX capacity exceeded
    
\end{document}

Answer 2

Obligatory self-advertisement since key=value solutions were mentioned: expkv

An implementation with a key=value solution and smartish unknown-key handling (literal numeric input will be assumed to be space, else it's assumed to be style -- if this is unwanted remove the block in \makeatletter...\makeatother) using the expkv family:

\documentclass{article}

\usepackage{expkv-unravel}

% has to be loaded before `gb4e` as the latter changes the category code of ^
\usepackage{expkv-cs}

% defining the key=value interface
\NewDocumentCommand\coolgl{O{}mm}
  {\coolglKV{#1}{#2}{#3}}
\ekvcSplitAndForward\coolglKV\coolglossaux
  {%
    space = 10,
    style = \small
  }

% defining a special rule how to handle unknown keys without a value
\makeatletter
\ekvdefunknownNoVal{\string\coolglKV}
  {%
    \coolglKV@ifnumber{#1}% will be true only for literal numerical input
      {\ekvcPass\coolglKV{space}}%
      {\ekvcPass\coolglKV{style}}%
        {#2}%
  }
% defining a smartish parser
\ExplSyntaxOn
\cs_new_eq:NN \coolglKV@ifempty \tl_if_empty:nTF
\ExplSyntaxOff
\protected\long\def\coolglKV@ifnumber#1%
  {%
    \begingroup
      \afterassignment\coolglKV@ifnumber@aux
      \count\z@=\iffalse{\fi0#1}%
  }
\protected\def\coolglKV@ifnumber@aux
  {%
    \endgroup
    \expandafter\coolglKV@ifempty\expandafter{\iffalse}\fi
  }
\makeatother

\protected\def\coolglossaux#1#2#3#4{%
    \renewcommand{\eachwordtwo}{\rule[-#1pt]{0pt}{0pt}#2}% !! #2 is the one !!
    \getwords(\lineone,\eachwordone)#3 \\%
    \getwords(\linetwo,\eachwordtwo)#4 \\%
    \loop\lastword{\eachwordone}{\lineone}{\wordone}%
    \lastword{\eachwordtwo}{\linetwo}{\wordtwo}%
    \global\setbox\gline=\hbox{%
        \unhbox\gline\hskip\glossglue\vtop{%
            \box\wordone\nointerlineskip\box\wordtwo
        }%
    }%
    \testdone
    \ifnotdone
    \repeat
    {\hskip -\glossglue}\unhbox\gline
}

% evil package changing catcodes in the preamble... :P
\usepackage{gb4e}

\begin{document}
\coolgl[style=\itshape\bfseries\footnotesize,space=30]{%
i try something very long in order to get two lines this is so wonderful and now you see the vertical space optional argument is perfectly working}{%
i try something very long in order to get two lines this is so wonderful and now you see the vertical space optional argument is perfectly working}

\coolgl[\itshape\bfseries\footnotesize,60]{%
i try something very long in order to get two lines this is so wonderful and now you see the vertical space optional argument is perfectly working}{%
i try something very long in order to get two lines this is so wonderful and now you see the vertical space optional argument is perfectly working}
\end{document}

Answer 3

I thought it could come in handy to other people, to show that, with a key=value system, no such issues arise.

Note that it is most often the better approach in many aspects.

\documentclass{article}

\usepackage{gb4e}

\protected\def\coolglossaux#1#2#3#4{%
    \renewcommand{\eachwordtwo}{\rule[-#1pt]{0pt}{0pt}#2}% !! #2 is the one !!
    \getwords(\lineone,\eachwordone)#3 \\%
    \getwords(\linetwo,\eachwordtwo)#4 \\%
    \loop\lastword{\eachwordone}{\lineone}{\wordone}%
    \lastword{\eachwordtwo}{\linetwo}{\wordtwo}%
    \global\setbox\gline=\hbox{%
        \unhbox\gline\hskip\glossglue\vtop{%
            \box\wordone\nointerlineskip\box\wordtwo
        }%
    }%
    \testdone
    \ifnotdone
    \repeat
    {\hskip -\glossglue}\unhbox\gline
}

\ExplSyntaxOn

\NewDocumentCommand{\coolgl}{O{}mm}
{
    \group_begin:
    \keys_set:nn { coolgl } { #1 }
    \coolgl:VVnn
    \coolgl_space
    \coolgl_style
    { #2 }
    { #3 }
    \group_end:
}
\keys_define:nn { coolgl }
{
    space.tl_set:N = \coolgl_space,
    style.tl_set:N = \coolgl_style,
    space.initial:n = 10,
    style.initial:n = \small,
}

\cs_set_eq:NN \coolgl:nnnn \coolglossaux

\cs_generate_variant:Nn \coolgl:nnnn { VV }

\ExplSyntaxOff


\begin{document}
        
\coolgl[space=30,style=\itshape\bfseries\footnotesize]{%
i try something very long in order to get two lines this is so wonderful and now you see the vertical space optional argument is perfectly working}{%
i try something very long in order to get two lines this is so wonderful and now you see the vertical space optional argument is perfectly working}
    
\end{document}