Text within ExplSyntaxOn/Off

Question

As below, I would like to test whether a user has input some custom text, and if not, then to display default text.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\tl_new:N \l_empty_tl

\NewDocumentCommand{\dostuff}{}{
    \tl_if_empty:NTF \l_empty_tl
        {Here is a large block of default text that I would like use.}
        {\tl_use:N \l_empty_tl}
}

\ExplSyntaxOff
\begin{document}

\dostuff

\end{document}

I would prefer not to type Here~is~a~large... and wonder whether there is a simple way to have the spaces recognized. I have tried

{\cctab_begin:N \c_document_cctab Here is a large..\cctab_end:}

to no avail. I feel like this might be a dupe, but can't seem to find a relevant question.

Answer 1

There are a few approaches one could use. If it's a simple one-shot where you just want that piece of text, then something like

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\tl_new:N \l_my_tl
\NewDocumentCommand { \dostuff } { }
  {
    \tl_if_empty:NTF \l_my_tl
      { \tl_use:N \c_my_tl }
      { \tl_use:N \l_my_tl }
  }
\use:n
  {
    \ExplSyntaxOff
    \tl_const:Nn \c_my_tl
  }
  {Here is a large block of default text that I would like use.}
\begin{document}

\dostuff

\end{document}

is probably easiest. The idea here is that \use:n tokenizes the input, but I leave the text part 'after' that so it is read with document catcodes.

Another approach is simply to alter the behaviour of , either using grouping:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\tl_new:N \l_my_tl
\NewDocumentCommand { \dostuff } { }
  {
    \tl_if_empty:NTF \l_my_tl
      { \tl_use:N \c_my_tl }
      { \tl_use:N \l_my_tl }
  }
\group_begin:
  \char_set_catcode_space:n {`\ }%
  \tl_const:Nn\c_my_tl{Here is a large block of default text that I would like use.}%
\group_end:
\ExplSyntaxOff
\begin{document}

\dostuff

\end{document}

or otherwise:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\tl_new:N \l_my_tl
\NewDocumentCommand { \dostuff } { }
  {
    \tl_if_empty:NTF \l_my_tl
      { \tl_use:N \c_my_tl }
      { \tl_use:N \l_my_tl }
  }
\char_set_catcode_space:n {`\ }%
\tl_const:Nn\c_my_tl{Here is a large block of default text that I would like use.}%
\char_set_catcode_ignore:n{`\ }%
\ExplSyntaxOff
\begin{document}

\dostuff

\end{document}

(It's not clear if the text asked for is a constant: if not then I'll modify the above.)

I'd strongly recommend not using a rescan method. To be honest, I've found rescanning to be a good way to break things, and would be extremely cautious about using it in any new code (to the point where I'd probably be happy if we dropped the \tl_rescan: family entirely!).

Answer 2

That won't work, because \cctab_begin:N \c_document_cctab only sets the catcodes to the "document" ones after the macro \dostuff is expanded, and that is already far too late to save the spaces in your block of text. Now, one could get around this using \scantokens or the LaTeX3 equivalent, but I might propose simply storing your text in a macro that is created outside the expl3 syntax, i.e.

\usepackage{xparse,expl3}

\NewDocumentCommand \mytext { } {%
 Here is a large...%
}

\ExplSyntaxOn
\tl_new:N \l_empty_tl

\NewDocumentCommand{\dostuff}{}{
    \tl_if_empty:NTF \l_empty_tl
        {\mytext}
        {\tl_use:N \l_empty_tl}
}

\ExplSyntaxOff

Then \mytext contains correctly formated text that is not disturbed by the LaTeX3 coding syntax.