Occasionally, it’s nice to make one or two characters active in the argument of a command, to make it easier for authors to code the arguments.
Active characters can be used safely in such situations; but care is needed.
An example arose while this answer was being considered: an aspirant
macro writer posted to comp.text.tex asking for help to
make # and b produce musical sharp and flat signs, respectively,
in a macro for specifying chords.
The first problem is that both # and b have rather important uses
elsewhere in TeX (to say the least!), so that the characters can
only be made active while the command is executing.
Using the techniques discussed in “characters as commands”, we can define:
\begingroup
\catcode`\#=\active
\gdef#{$\sharp$}
\endgroup
and:
\begingroup
\lccode`\~=`\b
\lowercase{\endgroup
\def~{$\flat$}%
}
The second problem is one of timing: the command has to make each
character active before its arguments are read: this means that
the command can’t actually “have” arguments itself, but must be
split in two. So we write:
\def\chord{%
\begingroup
\catcode`\#=\active
\catcode`\b=\active
\Xchord
}
\def\Xchord#1{%
\chordfont#1%
\endgroup
}
and we can use the command as \chord{F\#} or
\chord{Bb minor}.
Two features of the coding are important:
\begingroup in \chord opens a group that is closed by
\endgroup in \Xchord; this group limits the change of
category codes, which is the raison d’être of the whole
exercise.
# is active while \Xchord is executed, it’s
not active when it’s being defined, so that the use of #1
doesn’t require any special attention.
Note that the technique used in such macros as \chord, here, is
analogous to that used in such commands as \verb; and, in just the
same way as \verb (see
“\verb doesn’t work in arguments”),
\chord won’t work inside the argument of another command (the
error messages, if they appear at all, will probably be rather odd).
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=actinarg