Color-coding a Dialogue - Alternate Paragraphs in Different tcolorboxes

Question

• I would like to define a command \dialoguestart{black!10}{black!25} that starts setting the paragraphs into tcolorboxes, alternating between lighter (black!10) and darker (black!25) ones.
• The default setting of paragraphs should be returned to after another command \dialogueend

Edit: After discussion with @Schrödinger's cat, @siracusa and @cfr it became clear that the initial question was too big and too unspecific. It emerged that a reasonable way to tackle the issue is using an environment and I have adjusted the post accordingly.

To illustrate, the snippet

\documentclass{article}

\usepackage{lipsum}
\usepackage[breakable]{tcolorbox}
%\input{solution_to_this_problem}

\begin{document}

\lipsum[66]
\dialoguestart{black!10}{black!25}
\lipsum[75]
\lipsum[66]
\dialogueend
\lipsum[75]

\end{document}

should produce

The MWE below produces the desired result. It does not take me all the way, however, as the new environment can only handle one paragraph at a time.

\documentclass{article}
\usepackage{lipsum}
\usepackage[breakable]{tcolorbox}

\newif\iflight\lightfalse
\newcommand\toggleboxstart{%
    \iflight\begin{dark}%
    \else\begin{light}\fi}
\newcommand\toggleboxend{%
  \iflight\end{dark}\global\lightfalse%
  \else\end{light}\global\lighttrue\fi}
\newenvironment{toggling}[2]{%
    \newtcolorbox{light}{colback=#1,colframe=#1,sharp corners,breakable} 
    \newtcolorbox{dark}{colback=#2,colframe=#2,sharp corners,breakable}
    % something that ensures `\toggleboxend\toggleboxstart` being called between paragraphs
    }

\newcommand\dialoguestart[2]{\begin{toggling}{#1}{#2}\toggleboxstart}
\newcommand\dialogueend{\toggleboxend\end{toggling}}

\begin{document}
\lipsum[66]
\dialoguestart{black!10}{black!25}
\lipsum[75]
\dialogueend                       % these two commands
\dialoguestart{black!10}{black!25} % I would like to avoid
\lipsum[66]
\dialogueend
\lipsum[75]
\end{document}

To handle multiple paragraphs, \toggleboxend and \toggleboxstart have to be called between paragraphs within the toggling environment. I have tried adding

\let\origpar\par
\let\origeverypar\everypar
\renewcommand\everypar{\origeverypar\toggleboxstart}
\renewcommand\par{\toggleboxend\origpar}

to the environment's definition, which throws Paragraph ended before \renew@command was complete. And pushing my commands using the {everyhook} package, which solved this problem similar to the one at hand

\usepackage{everyhook}
\newenvironment{toggling}[2]{%
    \newtcolorbox{light}{colback=#1,colframe=#1,sharp corners,breakable} 
    \newtcolorbox{dark}{colback=#2,colframe=#2,sharp corners,breakable}
    \PushPreHook{par}{\toggleboxstart}
    \PushPostHook{par}{\toggleboxend}
    }

\newcommand\dialoguestart[2]{\begin{toggling}{#1}{#2}} % no more \toggleboxstart
\newcommand\dialogueend{\end{toggling}}                % no more \toggleboxend

results in being prompted TeX capacity exceeded, sorry [grouping levels=255]. What else can I do to execute some code between paragraphs? Within the environment, there will only be plain text.

---

Edit (by jakun who started the second bounty): My attempt to copy and paste my code into the bounty description did not work out well so I post it here again for better readability:

\documentclass{article}
\usepackage{lipsum}
\usepackage[breakable]{tcolorbox}

\newif\iflight

\newenvironment{toggling}[2]{%
    \newcommand\toggleboxstart{%
        \iflight
            %(BEGIN LIGHT)
            \begin{tcolorbox}[colback=#1,colframe=#1,sharp corners,breakable]%
        \else
            %(BEGIN DARK)
            \begin{tcolorbox}[colback=#2,colframe=#2,sharp corners,breakable]%
        \fi
    }%
    \newcommand\toggleboxend{%
        \end{tcolorbox}\relax
        %(END)
        \iflight
            \global\lightfalse
        \else
            \global\lighttrue
        \fi
    }%
    \toggleboxstart
    \everypar{\toggleboxend\toggleboxstart}%
}{%
    \toggleboxend
}

\begin{document}
    \lipsum[66]
    \begin{toggling}{black!10}{black!25}
    \lipsum[75]

    \lipsum[66]

    \lipsum[66]
    \end{toggling}
    \lipsum[75]
\end{document}

This code fails with

LaTeX Error: \begin{tcolorbox} on input line 36 ended by \end{tcb@savebox}.

or

Extra }, or forgotten \endgroup.

when commenting out the optional arguments of \begin{tcolorbox}.

Why does this not work and how could one make it work?

(When commenting out the \begin{tcolorbox} and \end{tcolorbox} and commenting in the test text in parentheses I get the expected output.)

Edit: I am using the following versions (grep -i version main.log):

This is pdfTeX, Version 3.14159265-2.6-1.40.20 (TeX Live 2019/Arch Linux) (preloaded format=pdflatex 2019.11.15)  25 NOV 2019 21:59
Package: tcolorbox 2019/09/19 version 4.21 text color boxes
(/usr/share/texmf-dist/tex/latex/pgf/compatibility/pgfcomp-version-0-65.sty
Package: pgfcomp-version-0-65 2019/08/03 v3.1.4b (3.1.4b)
(/usr/share/texmf-dist/tex/latex/pgf/compatibility/pgfcomp-version-1-18.sty
Package: pgfcomp-version-1-18 2019/08/03 v3.1.4b (3.1.4b)
Library (tcolorbox): 'tcbbreakable.code.tex' version '4.21'
[Loading MPS to PDF converter (version 2006.09.02).]

I have tried using tcolorbox 2019/11/15 version 4.22 instead and get the same result.

Answer 1

I don't know why but I couldn't get it running easily with \everypar. (Inserting a dummy text between paragraphs worked perfectly but replacing that dummy text with \end{tcolorbox}\begin{tcolorbox} caused an Extra }, or forgotten \endgroup. error.)

Instead I am offering a different approach based on replacing paragraph ends with delimited arguments. It has the disadvantage of being less efficient and that the entire "environment" body is read before hand. So don't try to use anything which relies on changing catcodes like the \verb command inside it. But the implementation is fairly easy:

\documentclass{article}

\usepackage{lipsum}
\usepackage[breakable]{tcolorbox}

% soluton_to_this_problem START
\long\def\dialoguestart#1#2#3\dialogueend{% #1: color 1, #2: color 2, #3: body
    \dialogueiter{#1}{#2}#3\par\dialogueend
}

\long\def\dialogueiter#1#2#3\par#4\dialogueend{% #1: this color, #2: alternative color, #3: next paragraph, #4: rest of body
    \begin{tcolorbox}[colback=#1,colframe=#1,sharp corners,breakable]
        #3
    \end{tcolorbox}
    \if\relax\detokenize{#4}\relax
        % #4 is empty => finish
    \else
        % note how I have swapped #1 and #2 to toggle the colors
        \dialogueiter{#2}{#1}#4\dialogueend
    \fi
}
% soluton_to_this_problem END

\begin{document}

\lipsum[66]
\dialoguestart{black!10}{black!25}
\lipsum[75]

\lipsum[66]

\lipsum[66]
\dialogueend
\lipsum[75]

\end{document}

\dialoguestart is just a wrapper around the \dialogueiter macro in order to avoid an error if the argument does not contain a paragraph end (end of paragraph = empty line = \par).

\dialogueiter is the macro which is doing the real job. It splits the argument in two with an argument delimiter. The first part is the next paragraph, the second part is the remaining rest. If the remaining rest is not empty \dialogueiter uses recursion to process it further.

\dialogueend is just an argument delimiter and is never expanded or executed.

---

Update: The xparse package provides (among others) the \NewDocumentCommand macro which is far more powerful than the LaTeX2e \newcommand macro by allowing to have delimited arguments (which is why I did not use \newcommand for this answer) and several optional arguments. Also it is more comfortable than the TeX primitive \def (which I used above) in this case because it supports optional arguments.

\NewDocumentCommand is also safer than \def because it checks whether a macro of that name exists already and prints an error if it does instead of silently overwriting the existing macro.

(If you wanted to implement optional arguments with \def you could lookahead for the next token with \kernel@ifnextchar and then use either a macro which expects this argument or another macro which does not.)

\usepackage{xparse}

\NewDocumentCommand{\dialoguestart}{O{black!10} O{black!25} +u\dialogueend}{% [#1: color 1], [#2: color 2], #3: body
    \dialogueiter[#1][#2]#3\par\dialogueend
}

\NewDocumentCommand{\dialogueiter}{r[] r[] u\par +u\dialogueend}{% #1: this color, #2: alternative color, #3: next paragraph, #4: rest of body
    \begin{tcolorbox}[colback=#1,colframe=#1,sharp corners,breakable]
        #3
    \end{tcolorbox}
    \if\relax\detokenize{#4}\relax
        % #4 is empty => finish
    \else
        % note how I have swapped #1 and #2 to toggle the colors
        \dialogueiter[#2][#1]#4\dialogueend
    \fi
}