How to use markdown in todonotes?

Question

I use todonotes, especially in the coordination of bigger projects. Especially in the drafting and quick brainstorming phase I'd love to be able to just use markdown inside todonotes to not waste time with nesting itemize etc.

Sadly, i can't seem to make it work: my many attempts to put a markdown env inside a todonote like this

\todo[...]{\begin{markdown}...\end{markdown}}

have failed with many errors (mostly seems to be runaway arg) sounding something like this:

Argument of ^^M has an extra }.

Any ideas / help / pointers?

MCVE:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{markdown}
\usepackage{todonotes}

\begin{document}

% normal markdown works:
\begin{markdown}
* woah
* bullets
\end{markdown}

% todonotes work
\todo[inline,prepend,caption={WOAH}]{yay}

% todonotes with itemize work
\todo[inline,prepend,caption={WOAH}]{yay
\begin{itemize}
    \item woah
    \item complicated bullets
\end{itemize}
}

% todonotes with markdown break
\todo[inline,prepend,caption={WOAH}]{nay
\begin{markdown}
* woah
* bullets easy
* boom
\end{markdown}
}

\end{document}

Answer 1

In this case, markdown is a verbatim environment so it cannot be used inside the argument of most commands.

See https://texfaq.org/FAQ-verbwithin for more details.

A fix that does not involve much code change uses cprotect. Note the extra brace group around \todo[inline,prepend,caption={WOAH}].

\documentclass{article}
\usepackage{markdown}
\usepackage{todonotes}
\usepackage{cprotect}

\begin{document}

\cprotect{\todo[inline,prepend,caption={WOAH}]}{nay
\begin{markdown}
* woah
* bullets easy
* boom
\end{markdown}
}

\end{document}

For explanation, see the documentation of cprotect package.

Note that \cprotect[om]\todo[inline, ...]{nay ...} does not work because the optional argument must not be protected (the details what protect means is quite complex, see my answer for more details.)

---

Alternatively it's possible to redefine \todo like this

\NewCommandCopy\oldtodo\todo
\outer\def\todo[#1]{\icprotect{\oldtodo[#1]}}

• See cprotect documentation for why icprotect should be used.
• \NewCommandCopy is required because internally \todo uses some mechanism to protect it (prevent it from being expanded in expand-only context).

Answer 2

An alternative approach to avoid markdown. Not that markdown is anything to be avoided, and I would recommend the answer by user202729, but just to showcase some packages.

The following shows inline todonotes as a tcolorbox. Inside the tcolorbox is an easylist, which provides a simple syntax for (nested) lists similar to markdown. Also in the example is a raw link, a labeled link and some code.

The tcolorbox environments can be 'disabled' by redefining them as the comment environment from the verbatim package.

By copying the definition of the itemize style for easylist into the preamble as default the syntax for a list inside the document is simplified a bit (otherwise it would be \begin{easylist}[itemize]). Also the syntax for the other elements (links and code) should be relatively simple and fast to type.

MWE:

\documentclass{article}
\usepackage[at]{easylist}
% set itemize style as default
\ListProperties(Hang=true,Space=4pt,Space*=4pt,Hide=1000,%
Margin1=1.5em,Style1*=\textbullet\hskip .5em,%
Margin2=3.7em,Style2*=--\hskip .5em,%
Margin3=5.9em,Style3*=$\ast$\hskip .5em,%
Margin4=7.8em,Style4*=$\cdot$\hskip .5em)%
\usepackage{tcolorbox}
\usepackage{hyperref}
\usepackage{verbatim} % for switching off tcolorbox
\tcbset{
  colback=red!5!white,
  colframe=red!75!black,
}

% uncomment the next line to disable tcolorbox
%\renewenvironment{tcolorbox}{\comment}{\endcomment}
\begin{document}
Some text

\begin{tcolorbox}[title=WOAH]
\begin{easylist}
@ item 1
@ item2
@@ nested item
@ item3
\end{easylist}
\url{https://tex.stackexchange.com}\\
\href{https://tex.stackexchange.com}{TeX.SE}\\
\verb+return "@#$%^&"+
\end{tcolorbox}

More text
\end{document}

Result: