Spaces around Cyrillic emdash (babel)

Question

There is the command "--- (Cyrillic emdash) defined in the babel package (for the Russian language). It is shorter than the usual English dash --- and has shorter spaces around it (also they are non-breaking as I can see). What is the width of such spaces? Is it equal to the width of the \, space?

Answer 1

The command \cyrdash is responsible of printing the dash; when the current language is Russian, it prints character 22 (the same slot as the em-dash, but in T2A encoded fonts it is shorter) or does

\hbox to 0.8em{--\hss--}

when the current encoding is not T2A, which means it superimposes two en-dashes so that the overall width is 0.8em (so changing with the font). The definition of "- is a bit complicated:

\declare@shorthand{russian}{"-}{%
  \def\russian@sh@tmp{%
    \if\russian@sh@next-\expandafter\russian@sh@emdash
    \else\expandafter\russian@sh@hyphen\fi}%
  \futurelet\russian@sh@next\russian@sh@tmp}
\def\russian@sh@hyphen{\nobreak\-\bbl@allowhyphens}
\def\russian@sh@emdash#1#2{\cdash-#1#2}
\def\cdash#1#2#3{\def\tempx@{#3}%
\def\tempa@{-}\def\tempb@{~}\def\tempc@{*}%
 \ifx\tempx@\tempa@\@Acdash\else
  \ifx\tempx@\tempb@\@Bcdash\else
   \ifx\tempx@\tempc@\@Ccdash\else
    \errmessage{Wrong usage of cdash}\fi\fi\fi}
\def\@Acdash{\ifdim\lastskip>\z@\unskip\nobreak\hskip.2em\fi
  \cyrdash\hskip.2em\ignorespaces}%
\def\@Bcdash{\leavevmode\ifdim\lastskip>\z@\unskip\fi
 \nobreak\cyrdash\penalty\exhyphenpenalty\hskip\z@skip\ignorespaces}%
\def\@Ccdash{\leavevmode
 \nobreak\cyrdash\nobreak\hskip.35em\ignorespaces}%

Basically it means that "- produces \russian@sh@hyphen, whereas "-- does \russian@sh@emdash.

The former issues \nobreak so the preceding word part can be hyphenated; then inserts a discretionary hyphen, allowing the following word part to be hyphenated.

The latter looks at the next two tokens (the first is still -); there are four cases according to the third character

1. "--- issues \@Acdash;
2. "--~ issues \@Bcdash;
3. "--* issues \@Cdash;
4. None of the above, so "-- is followed by (ignored) spaces and another token than -, ~ or ~.

In the fourth case the error Wrong usage of cdash is raised.

• The case "--- removes a preceding space if present and, in this case, inserts a nonbreaking space of width 0.2em, followed by \cyrdash and a further nonbreaking space of width 0.2em.

• The case "--~ removes a preceding space if present, inserts \cyrdash and then a penalty like for an explicit hyphen, ignoring following spaces.

• The case "--* is for “dialogues”: no space before and the \cyrdash is followed by a space of 0.35em, ignoring following spaces.

There is an error in the definitions: "--- at the beginning of a paragraph can produce unexpected results. There should be \leavevmode at the beginning like for the other two macros.

\documentclass{article}
\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[russian]{babel}

%% fix the error
\makeatletter
\def\@Acdash{\leavevmode\ifdim\lastskip>\z@\unskip\nobreak\hskip.2em\fi
  \cyrdash\hskip.2em\ignorespaces}%
\makeatother

\begin{document}

\verb|"-|: a"-b

\verb|"---|: a"---b a "--- b

\verb|"--~|: a"--~b a "--~ b

\verb|"--*|: "--*a "--* a

\end{document}