# Color-coding a Dialogue - Alternate Paragraphs in Different tcolorboxes

• 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'


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

• Maybe something like \everypar would be a way to go, see e.g. this answer: tex.stackexchange.com/a/119760. – Schrödinger's cat Nov 16 '19 at 14:56
• I have tried modifying \everypar to begin and \par to end the respective environments. But both commands are called too often by LaTeX itself. My code now causes too many problems for me to fix. This might work, but goes beyond my knowledge and does not yet produce an MWE... – D.Roepo Nov 17 '19 at 9:45
• Could you add a MWE that demonstrates how your colorboxes should be used, and some example code that shows what contexts the paragraph hook should be able to "survive"? – siracusa Nov 18 '19 at 22:14
• I feel that this will require major efforts to produce something that most likely never will be fully robust. How much resistance do you have to having to add some very short macro or whatever at the end of each paragraph? (@siracusa You need to load tcolorbox with the breakable option to make the above boxes work.) – Schrödinger's cat Nov 19 '19 at 1:41
• If you're redefining the sectioning commands, then that will complicate things because you want to redefine them again to add a hook ending the highlighting. If you were willing to use an environment \begin{highlightingpars} ... [lots of paragraphs] \end{highlightingpars} that would be easier and potentially less fragile. (It depends what you want to highlight. Does that material itself include tcolorboxes, for example?) Right now, I think this question is either too big (i.e. can't be reasonably done here) or ill-defined (i.e. there are constraints we don't know). – cfr Nov 20 '19 at 1:12

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.


\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
}

• This works like a charm. How could \dialogestart be redefined to have the colors being optional arguments defaulting to black!10 and black!25? – D.Roepo Nov 23 '19 at 12:37
• @D.Roepo please see my updated answer – jakun Nov 23 '19 at 14:48
• @jakun Re your new bounty code: What tcolorbox version are you using? I can't reproduce the error with 4.22 by just commenting out the optional parameters (though the output is wrong). – siracusa Nov 24 '19 at 21:15