Defining \xthinspace: Thin space only if not followed by certain characters

Question

Following this 2005 thread from the XeTeX list, I’ve defined a \spaceddash command and assigned it to the Unicode em-dash character U+2014 “—”:

\documentclass{minimal}
\usepackage[utf8]{inputenc}
\DeclareRobustCommand{\spaceddash}%
    {\unskip\nobreak\thinspace\textemdash\thinspace\ignorespaces}
\DeclareUnicodeCharacter{2014}{\spaceddash}

\begin{document}
meow — meow

meow—meow

— meow

meow —.
\end{document}

The document this produces looks something like this:

meow — meow
meow — meow
— meow
meow — .

Notice the thin-space between the em-dash on the last line and the period afterward—I’d like to get rid of it.

Following the example of xspace & xpunctuate, I’m trying to define a sort of \xthinspace command, one that will insert the \thinspace except if the dash is followed by certain punctuation marks (e.g., period, comma, close-parenthesis, close-quote).

How do I go about this?

Answer 1

Here's a possibility using expl3. The test gobbles spaces and then checks whether the following token appears in the exceptions list. If not, it applies \thinspace.

\documentclass{article}
\usepackage[utf8]{inputenc}

\usepackage{newunicodechar,xparse}

% The following is equivalent to \DeclareUnicodeCharacter{2014}{...}
\newunicodechar{—}{\spaceddash} % write only `\spaceddash` in aux files
\NewDocumentCommand{\spaceddash}{%
  \ifvmode\leavevmode\else\unskip\nobreak\thinspace\fi
  \textemdash\xthinspace}

\ExplSyntaxOn
\NewDocumentCommand{\xthinspace}{ } { \xths_main: }
\NewDocumentCommand{\addtoxthinspaceexceptions}{m}
 {
  \tl_gput_right:Nn \g_xths_exceptions_tl { #1 }
 }
\cs_new_protected:Npn \xths_main:
 {
  \bool_set_true:N \l_xths_apply_bool
  \peek_catcode_ignore_spaces:NF \c_space_token { \xths_check: }
 }
\cs_new_protected:Npn \xths_check:
 {
  \tl_map_inline:Nn \g_xths_exceptions_tl
    {
     \token_if_eq_charcode:NNT ##1 \l_peek_token
      {\bool_set_false:N \l_xths_apply_bool \prg_map_break: }
    }
  \bool_if:NT \l_xths_apply_bool { \thinspace }
 }
\tl_new:N \g_xths_exceptions_tl
\ExplSyntaxOff

\addtoxthinspaceexceptions{,.)}

\begin{document}
meow—meow

meow—.

meow— .
\end{document}

Try it with \hspace{1cm} instead of \thinspace if you want to verify it works. A limitation: if the — is followed by a macro that expands to a comma, a period or a closed parenthesis, the test will fail.

Answer 2

\@ifnextchar (thanks @egreg) is your friend here:

\DeclareRobustCommand{\spaceddash}%
    {\unskip\nobreak\thinspace\textemdash\gobblespaces}
\makeatletter\def\gobblespaces{\@ifnextchar.\relax\thinspace}\makeatother

\@ifnextchar gobbles any intervening space tokens. It compares the first non-space token found to its first argument ("." here) and executes the first macro if they are the same ("\relax'"); otherwise, it executes the second macro ("\thinspace'").

Obviously, this could be generalized to look for any number of tokens. I'm sure xspace does something similar.

A full MWE is:

\documentclass{minimal}
\usepackage[utf8]{inputenc}
\DeclareRobustCommand{\spaceddash}%
    {\unskip\nobreak\thinspace\textemdash\gobblespaces}
\makeatletter\def\gobblespaces{\@ifnextchar.\relax\thinspace}\makeatother
\DeclareUnicodeCharacter{2014}{\spaceddash}

\begin{document}
meow — meow

meow—meow

— meow

meow —.
\end{document}

And the resulting document looks like: