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 « Active characters in command arguments », 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.
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 analogous to that used in such commands as \verb; and, in just the same way as \verb (see « Pourquoi le mode verbatim ne marche pas toujours ? »), \chord won’t work inside the argument of another command (the error messages, if they appear at all, will probably be rather odd).

Sources

Active characters in command arguments