Macro arguments

It is occasionally useful to redefine a macro that takes arguments to do nothing. Eplain defines \gobble, \gobbletwo, and \gobblethree to swallow one, two, and three arguments, respectively.

For example, if you want to produce a "short" table of contents--one that includes only chapters, say--the easiest thing to do is read the entire .toc file (see section Contents), and just ignore the commands that produce section or subsection entries. To be specific:

\let\tocchapterentry = \shorttocchapter
\let\tocsectionentry = \gobbletwo
\let\tocsubsectionentry = \gobbletwo
\readtocfile

(Of course, this assumes you only have chapters, sections, and subsections in your document.)

In addition, Eplain defines \eattoken to swallow the single following token, using \let. Thus, \gobble followed by `{...}' ignores the entire brace-enclosed text. \eattoken followed by the same ignores only the opening left brace.

Eplain defines a macro \identity which takes one argument and expands to that argument. This may be useful if you want to provide a function for the user to redefine, but don't need to do anything by default. (For example, the default definition of \eqconstruct (see section Formatting equation references) is \identity.)

You may also want to read an optional argument. The established convention is that optional arguments are put in square brackets, so that is the syntax Eplain recognizes. Eplain ignores space tokens before an optional argument, via \futurenonspacelet.

You test for an optional argument by using \@getoptionalarg. It takes one argument, a control sequence to expand after reading the argument, if present. If an optional argument is present, the control sequence \@optionalarg expands to it; otherwise, \@optionalarg is \empty. You must therefore have the category code of @ set to 11 (letter). Here is an example:

\catcode`@=\letter
\def\cmd{\@getoptionalarg\finishcmd}
\def\finishcmd{%
  \ifx\@optionalarg\empty
    % No optional argument present.
  \else
    % One was present.
  \fi
}

If an optional argument contains another optional argument, the inner one will need to be enclosed in braces, so TeX does not mistake the end of the first for the end of the second.