gaynor@busboys.rutgers.edu (Silver) (01/10/90)
How do I control the expansion of macros? F'rinstance, suppose I wanted to
define the macro \foo to expand to exactly what it did before but also with a
bar on the end? In pseudolatex, this is what I'd like to say:
\newcommand{\foo}{foo} % \foo --> foo
\renewcommand{\foo}{\expand{\foo}bar} % \foo --> foobar
Several examples and references along these lines would be really appreciated!
Thanks muchly, [Ag] gaynor@topaz.rutgers.edudhosek@jarthur.Claremont.EDU (dhosek) (01/10/90)
In article <Jan.10.02.29.09.1990.1907@busboys.rutgers.edu> gaynor@topaz.rutgers.edu writes: >How do I control the expansion of macros? F'rinstance, suppose I wanted to >define the macro \foo to expand to exactly what it did before but also with a >bar on the end? In pseudolatex, this is what I'd like to say: > \newcommand{\foo}{foo} % \foo --> foo > \renewcommand{\foo}{\expand{\foo}bar} % \foo --> foobar >Several examples and references along these lines would be really appreciated! Well, part of the problem is that \newcommand is quite limited in what it can do. To do any fancy macro stuff, I fear, you have to descend to the level of (gasp) plain TeX. As for references, I highly recommend that you read the appropriate chapters of the TeXbook. It'll confuse you and you probably won't be much better off, but it'll make the rest of us feel better. Once you've accomplished that, You'll probably want to find a copy of Stephan v. Bechtolsheim's _TeX in Practice_ (aka _Another Look at TeX_) published by Springer-Verlag. I've heard rumors that actual copies of this book were present at the DECUS meeting last spring. However, to date, I have neither seen a (post-publication, not one of the Xeroxed ones svb sold himself prior to publication) copy nor an advertisement. Fortunately, again, interesting pieces of his book have been excerpted in TUGboat (in particular, TB 9#1, "A tutorial on \expandafter" pp.57-61 and TB 9#3 "A tutorial on \futurelet" pp. 276-8) and might be of interest. Actually, you may want to browse through as many TUGboat back issues as you can get your hands on. Also of interest might be the TUG courses in plain TeX and macro writing. So of course, now it's time to give you all TUG's address. phone number, and e-mail address (couldn't you see this coming? ;-): TeX Users Group P.O. Box 9506 Providence, RI 02940 (USA) 401-751-7760 tug@math.ams.com (wow! I managed to get it all (except the phone number) from memory!) But enough of this references stuff. You wanted examples. And examples ye shall have. First of all, I'll assume that you can figure out the basics of \def all by yourself. I will however point out two useful commands for debugging TeX macros. One is \message (this is also supported by \LaTeX). It takes its argument and displays it on the terminal. Thus \message{Hi mom!} Will produce Hi mom! on the terminal. The other command is \meaning. This expands to the meaning of whatever follows it. We're concerned with macros here, so we'll just look at what happens if we type: \def\foo{bar} \def\foobar{\foo bar} \message{\meaning\foo \meaning\bar} The output produced from this will be: macro:->bar macro:->\foo bar So, now, how can we get \meaning\bar to print "barbar" rather than "\foo bar"? One approach is to use \edef. This tells TeX to expand everything in the macro as much as possible when making the definition. For example, \edef\himom{\foo bar} \message{\meaning\himom} will produce the output macro:->barbar Now, however, sometimes you don't want everything in a macro expanded and instead you just want to expand one portion of the macro instead (this would be useful in making major additions to LaTeX's point size commands, for example). This gets a little trickier. To handle this, you will need to use the \expandafter command. The way \expandafter works is it causes the token after the token following it to be expanded. Thus, if you were to type \expandafter(\himom TeX would expand \himom before it processed the (. Now, let's assume that we would like to define a macro \doodah to be the expansion of \himom followed by \foo, unexpanded. The way that we would do this would be to type \expandafter\def\expandafter\doodah\expandafter{\himom \foo} 1 5 2 6 3 7 4 8 9 (the numbers indicate the order in which TeX will process each token. What we have done here is tell TeX to skip ahead to \himom, expand that, then go back and deal with the definition normally. There is one more way of dealing with partial expansion. Let's say that you had a great deal of stuff that should be expanded fully followed by a macro that shouldn't be expanded at all. You can do this by typing: \edef\expandedstuff{ lots of crud goes here} \def\mymacro{\expandedstuff \notexpanded} You should be able to see how this works now. You may want to use \expandafter to further expand \expandedstuff at this point, so you can reuse the control sequence name \expandedstuff. -dh -- "Odi et amo, quare id faciam, fortasse requiris? nescio, sed fieri sentio et excrucior" -Catullus D.A. Hosek. UUCP: uunet!jarthur!dhosek Internet: dhosek@hmcvax.claremont.edu