Message-Id: <199402012026.AA25295@mail.cs.tu-berlin.de>
Reply-To: Mailing list for the LaTeX3 project <LATEX-L@DHDURZ1.BITNET>
Date:         Tue, 1 Feb 1994 17:21:25 +0100
From: Tim Van Zandt <tvz@core.ucl.ac.be>
Sender: Mailing list for the LaTeX3 project <LATEX-L@DHDURZ1.BITNET>
To: Multiple recipients of list LATEX-L <LATEX-L@DHDURZ1.BITNET>
Subject:      Taking care when changing catcodes
Status: R

Hello.

Now that fiddling with catcodes is becoming increasingly popular,
some macros take it upon themselves to guarantee certain catcodes
before scanning arguments. For examle, from the nfsscode.ltx in
LaTeX3e:

\def\DeclareFontEncoding{%
   \begingroup
   \catcode`\ 9%
   \catcode`\^^M9%
   \DeclareFontEncoding@}
\def\DeclareFontEncoding@#1#2#3{%
  \endgroup
  \expandafter
  \ifx\csname T@#1\endcsname\relax
     \def\cdp@elt{\noexpand\cdp@elt}%
     \xdef\cdp@list{\cdp@list\cdp@elt{#1}%
                    {\default@family}{\default@series}%
                    {\default@shape}}%
  \fi
  \global\@namedef{T@#1}{#2}%
  \global\@namedef{M@#1}{\default@M#3}%
  }

The problem with this particular example is that if \DeclareFontEncoding
does not find its arguments, the \endgroup is never executed. That pretty
much destroys the rest of the job. Of course, the error in the arguments
has to be fixed before the file can be printed, but it is always better
if the user can plow ahead for a while before fixing each error.

\DeclareFontShape does the same kind of thing. This kind of problem is
more grave when the macro is something users commonly type themselves
(unlike \DeclareFontEncoding and \DeclareFontShape).

I suggest instead putting the \endgroup in the first part of the macro, so
that it gets executed no matter what:

\def\DeclareFontEncoding{%
   \begingroup
   \catcode`\ 9%
   \catcode`\^^M9%
   \expandafter\endgroup\DeclareFontEncoding@}
\def\DeclareFontEncoding@#1#2#3{%
  \expandafter
  \ifx\csname T@#1\endcsname\relax
     \def\cdp@elt{\noexpand\cdp@elt}%
     \xdef\cdp@list{\cdp@list\cdp@elt{#1}%
                    {\default@family}{\default@series}%
                    {\default@shape}}%
  \fi
  \global\@namedef{T@#1}{#2}%
  \global\@namedef{M@#1}{\default@M#3}%
  }