Using soul with indirect formatting applied

Question

I am using the soul package to underline omitting the descenders. It seems that passing the following as a parameter works fine

 \textit{special text}

but if the \textit is applied indirectly via the macro \ApplyStyle then I get the error:

Argument of \ApplyStyle has an extra }.

Question:

Is it possible to allow for this indirect application of the style by changes to the \varul macro? If not, what other options are available (except of course for applying the formatting directly within the invocation of the \varul macro -- that would just be too easy :-))?

Code:

\documentclass{article}
\usepackage{xparse}
\usepackage{xcolor}
\usepackage{soul}
\usepackage{xspace}


%% ----------------------------------------------------------------
%%                https://tex.stackexchange.com/questions/36894/
%%                    underline-omitting-the-descenders
%%
%%
\makeatletter
\ExplSyntaxOn
    \cs_new:Npn \white_text:n #1
      {
        \fp_set:Nn \l_tmpa_fp {.01}
        \fp_mul:Nn \l_tmpa_fp {#1}
        \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{\fp_to_decimal:N \l_tmpa_fp em}}
        \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{-\fp_to_decimal:N \l_tmpa_fp em}}
      }
    \NewDocumentCommand{\whiten}{ m }
        {
          \int_step_function:nnnN {1}{1}{#1} \white_text:n
        }
\ExplSyntaxOff

\NewDocumentCommand{ \varul }{ D<>{5} O{0.2ex} O{0.1ex} +m } {%
\begingroup
\setul{#2}{#3}%
\def\SOUL@uleverysyllable{%
   \setbox0=\hbox{\the\SOUL@syllable}%
   \ifdim\dp0>\z@
      \SOUL@ulunderline{\phantom{\the\SOUL@syllable}}%
      \whiten{#1}%
      \llap{%
        \the\SOUL@syllable
        \SOUL@setkern\SOUL@charkern
      }%
   \else
       \SOUL@ulunderline{%
         \the\SOUL@syllable
         \SOUL@setkern\SOUL@charkern
       }%
   \fi}%
    \ul{#4}%
\endgroup
}%
\makeatother
%% ----------------------------------------------------------------

\newcommand{\ApplyStyle}[1]{\textit{#1}}%

\begin{document}
\varul{\textit{special text}\xspace}%      Works!!

\varul{\ApplyStyle{special text}\xspace}%  Does NOT work
\end{document}

Answer 1

soul scans the text token by token, a process which can easily be messed up when the input stream also contains macros. The package already knows that it should leave font switching commands alone -- therefore \textit works in your case -- but fails with the unknown macro \ApplyStyle. Fortunately, there is an interface to make soul acquainted with new macros: \soulregister<command><identifier>, where <identifier> can be either 0 or 1 for the number of arguments (see section 5.2 in the documentation; also see this answer for an extension of \soulregister to accept more, and possibly optional, arguments).

Your example runs fine when adding

\soulregister\ApplyStyle{1}

to the preamble.