Active characters in command arguments

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

\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
}

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'\^etre of the whole exercise.
• Although # 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 analagous 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).