Error when using Greek babel, mparhack and \pagenumbering{roman}

Question

Using:

This is pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014) (preloaded format=pdflatex)

... I compile the following MWE:

\documentclass[10pt,twoside,openright]{book}
%\usepackage[english]{babel}
\usepackage[greek,english]{babel} %% A
\usepackage{mparhack}             %% B
\usepackage{lipsum}

\begin{document}
\pagenumbering{roman}             %% C
Testing

\lipsum[1]
\end{document}

... which triggers the following error upon pdflatex test.tex:

! Use of \\@lipsum doesn't match its definition.
\text@command #1->\def \reserved@a {
                                    #1}\ifx \reserved@a \@empty \let \check@...
l.12 \end{document}

? 
! Unbalanced write command.
<write> \string \mph@setcol {ii:\thepage }{
                                           \string \mph@nr }
l.12 \end{document}

? X

Interestingly, this error appears only when all three: A, B, and C are present; it doesn't occur if we replace, say, A with \usepackage[danish,english]{babel} (if you delete the old .aux file first before recompiling). (_{Note: in my original doc, I actually got ! Incomplete \iffalse; all text was ignored after line ... appearing at \mph@outputpage@hook which is from mparhack; but similarly, that error appeared first only after adding greek to babel})

So this seems to be a specific error occuring when Greek babel, mparhack and \pagenumbering{roman} interact. How do I get rid of it?

Answer 1

The problem is quite subtle and lipsum is not really involved.

Analysis of the problem

The mparhack package makes some annotations in the .aux files, using

\newcommand*\mph@outputpage@hook{%
    \bgroup
    \advance\c@page\m@ne
    \immediate\write\@auxout{%
        \string\mph@setcol{ii:\thepage}{\string\mph@nr}%
    }%
    \egroup
}

and the problem starts here: with \pagenumbering{roman} the command \thepage means \roman{page} and, when Greek is used, the command \@roman that's used by \roman doesn't survive \write, because it now contains \textlatin which is not fully expandable.

The error involves \\@lipsum because this macro happens to be stored in \reserved@a, but it's just a symptom, not the disease.

The cure

\documentclass[10pt,twoside,openright]{book}
\usepackage[greek,english]{babel}     
\usepackage{mparhack}                 
\usepackage{lipsum}

\makeatletter
\renewcommand*\mph@outputpage@hook{%
    \bgroup
    \let\textlatin\@firstofone % make \textlatin a no-op
    \advance\c@page\m@ne
    \immediate\write\@auxout{%
        \string\mph@setcol{ii:\thepage}{\string\mph@nr}%
    }%
    \egroup
}
\makeatother

\begin{document}
\pagenumbering{roman}                 
Testing

\lipsum[1]

\end{document}

This successfully removes any attempt to expand the original \textlatin when writing the annotation. The redefinition is performed in a group, so the meaning of \textlatin will be restored as soon as \egroup is scanned. In the .aux file you'll find

\mph@setcol{ii:i}{\mph@nr}

which should be correct, because ii:i is used only as a unique label.

Further problems

In case the twocolumn option is used, the problem reappears in a different form; a possible solution is to patch also \mph@setcol and \mph@check, because in this case it's not really possible to remove \textlatin from the annotation, so we remove it when \mph@setcol and \mph@check are executed. Adding a group around them is not a problem, because both only do global assignments anyway.

\documentclass[10pt,twoside,openright,twocolumn]{book}
\usepackage[greek,english]{babel}
\usepackage{mparhack}
\usepackage{lipsum}

\usepackage{etoolbox}
\makeatletter
% this is the hack for one column output
\patchcmd\mph@outputpage@hook
  {\bgroup}
  {\bgroup\let\textlatin\@firstofone}
  {}{}
% this is the hack for two column output
\pretocmd\mph@setcol{\bgroup\let\textlatin\@firstofone}{}{}
\apptocmd\mph@setcol{\egroup}{}{}
\pretocmd\mph@check{\bgroup\let\textlatin\@firstofone}{}{}
\apptocmd\mph@check{\egroup}{}{}
\makeatother

\begin{document}
\pagenumbering{roman}
Testing

\lipsum[1]

\end{document}

Answer 2

Ok, I got something, but I'm not sure it's correct - so more erudite responses will be appreciated. By using package {trace}, I compared the trace outputs for the correct [english] and the problematic [greek,english] options for babel. Both variants end up in this stack trace:

\mph@outputpage@hook ->\bgroup \advance \c@page \m@ne \immediate \write \@auxout {\string \mph@setcol {ii:\thepage }{\string \mph@nr }}\egroup

  \thepage ->\csname @roman\endcsname \c@page

At this point, the \@roman command is called; in the [english]-only case, it is:

\@roman #1->\romannumeral #1

... but in the [greek,english] case, it is:

\@roman #1->\expandafter \textlatin \expandafter {\romannumeral #1}

... and \textlatin calls \text@command #1->\def \reserved@a {#1}... where #1<-i (the roman numeral i for page number 1); however it turns out in this case \reserved@a ->\\@lipsum somehow, and so we'd get, I guess \@lipsum{i} trying to execute, which indeed is wrong.

I read a bit through texdoc babel-greek, and it seems \textlatin is to ensure switching to a latin language font/encoding; I am guessing that apparently \@roman is redefined by babel-greek simply to ensure that the engine is in "latin text" mode before trying to typeset roman numerals, which makes sense - but makes the unfortunate interaction in this case.

My use for [greek,...]{babel} is mostly to ensure fonts/encoding for correct display of μ in siunitx and the like (related: Possible bug in babel-greek, TexLive 2014); I will not be using sections of greek text. Thus, the likelihood for typesetting roman numerals while in greek text mode is very low, so I thought: why not redefine the \@roman command back to what it was? Here texdef helps:

$ texdef -t latex -s @roman
% latex.ltx, line 1850:
\def\@roman#1{\romannumeral #1}

... and so, the fixed MWE that works now is:

\documentclass[10pt,twoside,openright]{book}
\usepackage{trace}
\usepackage[greek,english]{babel} %% A
\usepackage{mparhack}             %% B
\usepackage{lipsum}

\begin{document}
\traceon
\pagenumbering{roman}             %% C
% latex.ltx, line 1850:         % fix
\makeatletter                   % fix
\def\@roman#1{\romannumeral #1} % fix
\makeatother                    % fix

Testing

\lipsum[1]
\end{document}