When might it be helpful to use implicit braces instead of explicit braces?

Question

We've covered the question "When can one use implicit braces instead of explicit braces?"; this leads immediately in my mind to the question "Why would one want to?"

(Remember, implicit braces are e.g. \bgroup and \egroup as defined by the code \let\bgroup={ \let\egroup=} in plain TeX.)

I suppose this is basically the same as the question "When must one use implicit braces instead of explicit braces?"

Answer 1

There's a nice example in Appendix E of the TeXbook, where a letter format is described. A few lines of a possible letter:

\address
Prof.~Brian~K. Reid
Department of Electrical Engineering
Stanford University
Stanford, CA 94305

\body

Here, \address and \body are defined as follows:

\def\address{\beginlinemode\getaddress}
{\obeylines\gdef\getaddress #1
  #2
  {#1\gdef\addressee{#2}%
    \global\setbox\theaddress=\vbox\bgroup\raggedright%
    \hsize=\longindentation \everypar{\hangindent2em}#2
    \def\endmode{\egroup\endgroup \copy\theaddress \bigskip}}}
\def\body{\beginparmode}
\def\beginparmode{\endmode
  \begingroup\parskip=\medskipamount
  \def\endmode{\par\endgroup}}

Please pay attention mainly to the \vbox\bgroup in \getaddress and to the \egroup in \endmode that gets called by \beginparmode via \body. What happens is that \address starts a \vbox, which is then ended by the occurence of \body (or anything else calling \endmode). The box thus constructed can then be used "both in the letter and in the label on the envelope", as Knuth explains.

Answer 2

For example, you can do this:

\def\blah{\bgroup\bf}
\def\bloh{\egroup}

not bold \blah bold\bloh not bold

Doing this:

\def\blah{{\bf}
\def\bloh{}}

would have completely different result.

Answer 3

Among a gazillion other uses, it provides a handy way for storing paragraphs in a box for later processing.

\newbox\savedparbox
\def\saveparbox{\par\begingroup
  \def\par{\egroup\endgroup}
  \global\setbox\savedparbox\vbox\bgroup}

Ordinary paragraph.
\saveparbox
This paragraph will be saved in \string\box\string\savedparbox.
If you wish, you can unpack the box and do all kinds of processing on it.
In this demo, I won't do any processing.
Look in the log file to examine the box contents.

\showboxdepth=99
\showboxbreadth=999
\showbox\savedparbox

Another ordinary paragraph.
\bye

Answer 4

When testing for the presence of a brace using a \futurelet construction.

\makeatletter
\def\isbraced{\futurelet\next\isbraced@}
\def\isbraced@{\ifx\next\bgroup Braced\else Unbraced\fi}
\makeatother

Answer 5

Inside macros, \bgroup sometimes helps readability:

\def\IntroEntry#1#2%
 {\blank[line]
  \begingroup
    \setupalign[right]
    \setuptolerance[verytolerant]
    \setupindenting[no]
    \switchtobodyfont[9pt]
    \ss 
    \begingroup
      \bf #1\par 
    \endgroup
    #2\par
  \endgroup }

(I tend to get confused when there are too many braces)

Answer 6

\documentclass{minimal}
\def\A{%
  \bgroup
  \B}
\def\B{C\egroup}

\begin{document}
\A
\end{document}

in this case it maybe better to use \begingroup ...\endgroup instead