How use fancyvrb to show both result and verbatim text?

Question

The documentation for fancyvrb claims: "you can build example environments (showing both result and verbatim text)...."

How?

For example, how should commands from fancyvrb be used to construct a command such as \showboth, shown in the "toy example" below, so that the output is what is shown as "desired" rather than the "actual" output I get?

Toy example

\documentclass{article}
\usepackage{fancyvrb}

\newcommand{\showboth}[1]{{#1} --- \protect\Verb!#1!}

\newcommand{\doit}[1]{also {#1}}

\begin{document}

Actual output: \quad \showboth{\doit{this}}

\bigskip

Desired output: \quad \doit{this} --- \verb!\doit{this}!

\end{document}

Actual application

In the actual application, I need such a command \showboth that can be used with an argument that comes from the argument passed by a surrounding command.

The purpose of this is to allow me to experiment systematically with various kinds of styling in my document design. For example, I want to see readily which of various styles I prefer for cross-referencing theorems and sections, as in the following source.

In that source, the package showboth that's loaded consists simply of the code in the answer by @Ulrich Diez.

What I'd like is a 1-argument macro, whose argument is a label (thm:big, e.g.), that produces the kind of complete itemize environment shown (whose individual items in turn just call \showboth with the various referencing commands, with those referencing commands in turn taking that label as their own argument.

\documentclass{article}

\usepackage{amsthm}
\usepackage{thmtools}
\usepackage{enumitem}
\usepackage{suffix}

\usepackage{showboth} % code from Ulrich Diez's answer

\usepackage[colorlinks=true,linkcolor=red]{hyperref}
\usepackage{cleveref}

% Tefererencing macros:

\newcommand{\thmref}[1]{\nameref*{#1} (\cref{#1})}

\newcommand{\thmnameref}[1]{\hyperref[{#1}]{\nameref{#1}}}
\WithSuffix\newcommand\thmnameref*[1]{\nameref*{#1}}

\newcommand*{\fullref}[1]{\hyperref[{#1}]{\cref{#1}} \nameref*{#1}}

\newcommand*{\unitref}[1]{\cref{#1} (\nameref*{#1})}
\newcommand*{\unitnameref}[1]{\hyperref[{#1}]{\cref{#1}} (\nameref*{#1})}

% Thorerem styles

\swapnumbers

\declaretheoremstyle[
headfont=\bfseries,
notefont=\bfseries, 
headformat=\NAME\NUMBER\let\thmt@space\@empty\NOTE,
bodyfont=\mdseries\itshape,
]{thmstyle}

\makeatletter
\declaretheoremstyle[
headfont=\bfseries,
notefont=\bfseries, notebraces={}{},
headformat=\NUMBER\let\thmt@space\@empty\NOTE,
bodyfont=\mdseries\itshape,
]{namedthmstyle}
\makeatother

\theoremstyle{thmstyle}% default
\declaretheorem[name=Theorem,numberwithin=section]{theorem}

\theoremstyle{namedthmstyle}
\declaretheorem[style=namedthmstyle, name=Theorem,title = {},numberlike=theorem]{namedtheorem}

\begin{document}

\section{Stuff}\label{sec:stuff}

\begin{theorem}[refname=Theorem,name=Preliminary Result]\label{thm:prelim}
Notice this!
\end{theorem}

\begin{namedtheorem}[name=The Big Theorem]\label{thm:big}
Very important!
\end{namedtheorem}

\begin{theorem}\label{thm:another}
Another big result.
\end{theorem}

\begin{itemize}
\item  \showboth!\thmref{thm:big}! 
\item \showboth!\thmnameref{thm:big}! 
\item \showboth!\thmnameref*{thm:big}!
\item \showboth!\fullref{thm:big}! % LITTLE USE!
\item \showboth!\nameref{thm:big} \cref{thm:big}!
\item \showboth!\nameref*{thm:big} \cref{thm:big}! 
\item \showboth!\cref{thm:big}! 
\item \showboth!\cref*{thm:big}!
\end{itemize}

\smallskip

\begin{itemize}
\item  \showboth!\thmref{thm:prelim}! 
\item \showboth!\thmnameref{thm:prelim}! 
\item \showboth!\thmnameref*{thm:prelim}!
\item \showboth!\fullref{thm:prelim}! % LITTLE USE!
\item \showboth!\nameref{thm:prelim} \cref{thm:prelim}!
\item \showboth!\nameref*{thm:prelim} \cref{thm:prelim}! 
\item \showboth!\cref{thm:prelim}! 
\item \showboth!\cref*{thm:prelim}!
\end{itemize}

\smallskip

\begin{itemize}
\item  \showboth!\thmref{thm:another}! 
\item \showboth!\thmnameref{thm:another}! 
\item \showboth!\thmnameref*{thm:another}!
\item \showboth!\fullref{thm:another}! % LITTLE USE!
\item \showboth!\nameref{thm:another} \cref{thm:another}!
\item \showboth!\nameref*{thm:another} \cref{thm:another}! 
\item \showboth!\cref{thm:another}! 
\item \showboth!\cref*{thm:another}!
\end{itemize}

\smallskip

\begin{itemize}
\item \showboth!\unitnameref{sec:stuff}!
\item \showboth!\unitref{sec:stuff}!
\item  \showboth!\thmref{sec:stuff}! 
\item \showboth!\thmnameref{sec:stuff}! 
\item \showboth!\thmnameref*{sec:stuff}!
\item \showboth!\fullref{sec:stuff}! % LITTLE USE!
\item \showboth!\nameref{sec:stuff} \cref{sec:stuff}!
\item \showboth!\nameref*{sec:stuff} \cref{sec:stuff}! 
\item \showboth!\cref{sec:stuff}! 
\item \showboth!\cref*{sec:stuff}!
\end{itemize}

\end{document}

Answer 1

Let me say in advance that I am not familiar at all to the fancyvrb package as by now I never used it.

Therefore it is unknown to me whether that package already provides some infrastructure for achieving what you desire.

Off the cuff I suggest having \showboth read its argument under "verbatim catcode régime" and then having \showboth pass that argument to another macro \innershowboth which in turn will pass that argument to \scantokens—once as is and once along with tokens for calling the \Verb-command.

I just wrote a macro \UDcollectOneVerbArg which does read arguments under "verbatim catcode régime" and then pass these arguments and the verbatim-delimiter to other macros.
I will happily use it in the example below for implementing \showboth.

Like the commands \verb (from the LaTeX-kernel) and \Verb (from the fancyvrb-package) the command \showboth needs to read its argument under "verbatim catcode régime". Therefore like the commands \verb and \Verb the command \showboth does not work with arguments that are passed by other macros/that come from macro definitions where those arguments were tokenized under normal catcode régime. Best practise for the unexperienced is to use \showboth<verbatim-Arg> not within macro definitions and not within macro arguments.

\documentclass{article}
\usepackage{fancyvrb}
\makeatletter
%%----------------------------------------------------------------------
%% Check whether argument is empty:
%%......................................................................
%% \UD@CheckWhetherNull{<Argument which is to be checked>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is empty>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is not empty>}%
%%
%% The gist of this macro comes from Robert R. Schneck's \ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
%%
%% (\romannumeral expansion was introduced in order to overcome the 
%% concerns and worries about improperly balanced \if..\else..\fi constructs.)
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\newcommand\UD@CheckWhetherNull[1]{%
  \romannumeral0\expandafter\UD@secondoftwo\string{\expandafter
  \UD@secondoftwo\expandafter{\expandafter{\string#1}\expandafter
  \UD@secondoftwo\string}\expandafter\UD@firstoftwo\expandafter{\expandafter
  \UD@secondoftwo\string}\expandafter\expandafter\UD@firstoftwo{ }{}%
  \UD@secondoftwo}{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@firstoftwo}%
}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% // Snippet written by Ulrich Diez on November 30, 2016
%
% Implement generic stuff for reading arguments "verbatim":
%
% Syntax of \UDcollectOneVerbArg
%
%   \UDcollectOneVerbArg{<^^M-replacement>}{<mandatory>}<delimiter of verbatim arg><verbatim arg><delimiter of verbatim arg>
%   yields:
%   <mandatory>{<delimiter of verbatim arg>}{<verbatim arg>}
%
%   \UDcollectOneVerbArg{<^^M-replacement>}{<mandatory>}{<verbatim arg>}
%   yields:
%   <mandatory>{}{<verbatim arg>}
%
% with each character ^^M (usually=\endline-char) in <verbatim arg>
% replaced by token-sequence <^^M-replacement>.
%
% If reading <^^M-replacement> and <mandatory> from input is necessary,
% they will be read under unchanged catcode regime.
%
% The <verbatim arg> is also mandatory.
% It will be read under verbatim-catcode-conditions.
% There must be a leading character in front of it.
% If that leading character is an opening brace, it will be
% "assumed" that the <verbatim arg> is nested into braces.
% Otherwise it will be "assumed" that the <verbatim arg> is
% delimited by that leading character---which implies a
% syntax like with the argument of \verb.
%
% Empty-lines will not be ignored.
%
% <delimiter of verbatim arg> will be read under verbatim-catcode-conditions
% if present.
%
\begingroup
\catcode`\^^M=12 %
\UD@firstoftwo{%
  \endgroup%
  \newcommand\UDEndlreplace[2]{\romannumeral0\@UDEndlreplace{#2}#1^^M\relax{}}%
  \newcommand*\@UDEndlreplace{}%
  \long\def\@UDEndlreplace#1#2^^M#3\relax#4#5{%
    \UD@CheckWhetherNull{#3}%
    { #5{#4#2}}{\@UDEndlreplace{#1}#3\relax{#4#2#1}{#5}}%
  }%
}{}%
\newcommand\UDcollectOneVerbArg{\@UDOneVerbArg{\@UDcollectOneVerbArg}}%
\newcommand\@UDOneVerbArg[3]{%
  \@bsphack
  \begingroup
  \let\do\@makeother\dospecials
  \catcode`\{=1 %
  \catcode`\ =10 %
  \@ifnextchar\bgroup
  {\catcode`\}=2 \@@UDOneVerbArg{#1}{#2}{#3}{}}%
  {\do\{\@@UDOneVerbArg{#1}{#2}{#3}}%
}%
\newcommand\@@UDOneVerbArg[4]{%
  \do\ %
  \catcode`\^^M=12 %
  \long\def\@tempb##1#4{%
    \edef\@tempb{##1}%
    \@onelevel@sanitize\@tempb
    \expandafter\UDEndlreplace\expandafter{\@tempb}{#2}{\def\@tempb}%
    \expandafter#1\expandafter{\@tempb}{#3}{#4}%
  }%
  \@tempb
}%
\newcommand\@UDcollectOneVerbArg[3]{%
  \endgroup
  \@esphack
  #2{#3}{#1}%
}%
% Implementation of generic stuff for reading arguments "verbatim" done.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newcommand\showboth{\UDcollectOneVerbArg{^^J}{\innershowboth}}%
\newcommand\innershowboth[2]{%
  \UD@CheckWhetherNull{#1}%
  {\scantokens{{#2} --- \Verb{#2}\begingroup\catcode`\X=14 X}}%<-This will raise an error as \Verb does not accept opening braces as delimiter.
  {\scantokens{{#2} --- \Verb#1#2#1\begingroup\catcode`\X=14 X}}%
  \endgroup
}

\makeatother

\newcommand{\doit}[1]{also {#1}}

\begin{document}

\noindent\Verb|\doit{this  and that} --- \Verb!\doit{this  and that}!| yields:\medskip\\
\null\quad \doit{this  and that} --- \Verb!\doit{this  and that}!
\bigskip

\noindent\Verb|\showboth!\doit{this  and that}!| yields:\medskip\\    
\null\quad \showboth!\doit{this  and that}!
\bigskip    

\end{document}