1

I want to define a general-ish \subtitle macro that simply adds to the \@title macro set by the user-level \title. Here is what I've got:

\documentclass{article}
\RequirePackage{xparse,expl3}
\makeatletter\ExplSyntaxOn
\NewDocumentCommand \subtitle { m }
  {
    \cs_set:Npx \@title
      {\@title\\[.5ex]\large#1}
  }
\ExplSyntaxOff\makeatother

\title{A New Chapter}
\subtitle{In This Short Book}

\begin{document}
\maketitle
Blah
\end{document}

This is supposed to define a document command \subtitle that takes one mandatory argument. It then fully expands the current value of \@title and \\[.5ex]\large#1, setting this expanded value to \@title, effectively appending the subtitle to it. Unfortunately, this raises the error

ERROR: Undefined control sequence.

--- TeX said ---
\\  ->\let \reserved@e 
                       \relax \let \reserved@f \relax \@ifstar {\let \reserv...
l.12 \subtitle{In This Short Book}

--- HELP ---
TeX encountered an unknown command name. You probably misspelled ...

Why am I getting this error and, more importantly, how might I fix it?

  • Are you aware that a second use of \subtitle adds another line and the use of \title will overwrite any previous subtitles? – Qrrbrbirlbel Oct 11 '13 at 2:16
  • @Qrrbrbirlbel I am, but thanks for pointing it out! – Sean Allred Oct 11 '13 at 2:19
  • Whatever your final decision for code is, please place it as argument to \AtBeginDocument; you'll know why when one of the users of your class will be asking you about the disparition of a subtitle (you'll certainly find somebody putting \subtitle before \title). Wouldn't a syntax such as \title[Subtitle]{Title} be better? – egreg Oct 11 '13 at 9:36
  • @egreg Very good point about users possibly having \subtitle before \title; the optional argument syntax is much nicer, actually. I didn't put too much thought into it, as this document class is only for a team of around five people. – Sean Allred Oct 12 '13 at 3:50
3

The x stands for “exhaustive expansion” (please see the manual, p. 2 or 13 for that). This seems like a bad idea with \\ and \large.

I’d simply expand the \@title once and append the rest:

\expandafter \cs_set:Npn \expandafter \@title \expandafter
   { \@title \\[.5ex] \large #1 }

The \tl_put_right:** (or \appto from etoolbox) macros do this without \expandafters.

I’m using :cn { @title } here but obviously you can just use :Nn \@title with correct catcodes (i.e. in a package).

Code

\documentclass{article}
\RequirePackage{xparse,expl3}
\ExplSyntaxOn
\NewDocumentCommand \subtitle { m }{
  \tl_put_right:cn { @title } % or \tl_put_right:Nn \@title with \makeatletter
    { \\[.5ex] \large #1 }
}
\ExplSyntaxOff
\title{A New Chapter}
\subtitle{In This Short Book}
\subtitle{And another line?}

\begin{document}
\maketitle
Blah
\end{document}
| improve this answer | |
  • I'd really like to avoid the use of \expandafter; this is expl3 after all. I didn't think about the issues with \\/\large though, I was focussed on \@title when I was writing my macro. \\/\large are definitely the source of the problem, but how might I resolve this without \expandafter? – Sean Allred Oct 11 '13 at 2:18
  • Also, my apologies about @; I moved this into a test file where it had been in a class file. Thanks for pointing it out, though! – Sean Allred Oct 11 '13 at 2:19
  • @SeanAllred Is \noexpand allowed? I’m no 3 expert so I wouldn’t use it in the first place. ;) (You need two ` for \\ in comments.) – Qrrbrbirlbel Oct 11 '13 at 2:19
  • 3
    Indeed, \@title is clearly equivalent to an expl3 tl rather than a function, so really \tl_put_... is the right choice. Of course, you should really only be storing the text and dealing with the design by altering \@maketitle, but that's another question! – Joseph Wright Oct 11 '13 at 6:16
  • 1
    Besides \@title being a variable -- the \expandafter's could be replaced with an o type argument: \cs_set:Npo \@title { \@title \\[.5ex] \large #1 } – cgnieder Oct 11 '13 at 7:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.