Why do I get a an error when using \parencite inside my custom environment (defined with Environ)?

Question

My Current Problem

I have a custom environment called revisions which underlines everything in a customisable way using soul (see the context for why), defined with the Environ. When I use \parencite inside this environment, I get the following errors (copied directly from the panel in vim). What am I doing wrong, and how can I make it work?

essay.tex|22 warning| LaTeX Warning: Citation '{McTaggart1908}' on page 0 undefined on input line 22.
essay.tex|22 error| Argument of \blx@citeargs@iii has an extra }.
essay.tex|22 error| Paragraph ended before \blx@citeargs@iii was complete.
essay.tex|22 error| You can't use `end-group character }' after \the.
essay.tex|22 error| TeX capacity exceeded, sorry [main memory size=5000000]. ...}{}{}{\endgroup \blxcitecmd {parenci
essay.tex|22 error| ==> Fatal error occurred, no output PDF file produced!
|| [37] Utils.pm:209> ERROR - .auxfiles/essay.bcf is malformed, last biblatex run probably failed. Deleted .auxfiles/essay.bbl

A minimal working example:

\documentclass{article}

\usepackage{environ}
\usepackage{soul}

\usepackage{biblatex}

\addbibresource{~/University/bibliography.bib}

\NewEnviron{revision}{\expandafter\ul\expandafter{\BODY}}

\title{Some Title}
\author{}
\date{}

\begin{document} 

\begin{revision}
\parencite[10]{McTaggart1908}
\end{revision}

\end{document}

Context

I am including this in the post because it may be that I have taken the long way round, and actually there is a much easier way to solve the whole thing which avoids the above problem altogether. It may also be useful in general. I welcome all ideas.

I am a student and I use LaTeX mainly for writing essays. My university require that in the second, revised version of each essay has all revisions and edits underlined. Initially (in the spirit of WYSIWYM) I just had a command \rev:

\newcommand{\rev}[1]{\ul{#1}}

But this threw similar errors to above when I used \parencite inside it. I don't know much about LaTeX, but I thought that perhaps the 'right' way to do it (and thus a way LaTeX would deal with) would be as an environment, hence the above. But that still doesn't work!

Answer 1

With lualatex and lua-ul (this requires a current tex system!) your example compiles without problems:

\documentclass{article}

\usepackage[soul]{lua-ul}

\usepackage{biblatex}
\addbibresource{biblatex-examples.bib}

\newcommand{\rev}[1]{\ul{#1}}
\begin{document}
\rev{\parencite[10]{doody}}
\end{document}

Answer 2

It's not a problem of environ, but of soul. You can try

\ul{\parencite{McTaggart1908}}

and you'll get the same problem.

Solution: use \mbox. I have filecontents* just to make the example self-contained.

\begin{filecontents*}{\jobname.bib}
@article{McTaggart1908,
  author={X. McTaggart},
  title={Title},
  journal={Journal},
  year={1908},
}
\end{filecontents*}

\documentclass{article}
\usepackage{environ}
\usepackage{soul}

\usepackage{biblatex}

\addbibresource{\jobname.bib}

\NewEnviron{revision}{\expandafter\ul\expandafter{\BODY}}

\title{Some Title}
\author{}
\date{}

\begin{document}

\ul{\mbox{\parencite[10]{McTaggart1908}}}

\begin{revision}
\mbox{\parencite[10]{McTaggart1908}}
\end{revision}

\end{document}