Re: [Groff] extending a macro (prepending commands)

[Tadziu Hoffmann]

> there must be an obvious answer to this question but I've looked and
> not seen an answer (yet). What is the best way to prepend commands to
> an existing macro. For example, say, in -ms how can one easily define
> a new `.LP' do some work and then call the original old `.LP' safely..
> 
> Here is my example code and I'm puzzled at the infinite recursion
> when LP is invoked. I'm sure this must be obvious to many people but
> (not me at present!)
> 
> .mso s.tmac
> .\" extend LP
> .rn LP LP-old
> .rm LP
> .de LP
> .  tm inside new LP
> .  LP-old
> ..
> .\" example text
> .LP
> .TL
> Hello
> .LP
> world

This is a cute example of "self-modifying code".
In s.tmac, LP starts out with this definition:

  .de LP
  .\" [stuff removed]
  .cov*ab-init
  \\*[\\$0]\\
  ..

The \$0 is what causes the recursion.  Normally, this
"LP" is only called the very first time, however,
since cov*ab-init contains the following line

  .als LP @LP

which effectively redefines LP to mean @LP, so when the
original LP evaluates \*[\$0], it is actually @LP that's
expanded, and all's well.  (After that, only @LP gets
called anyway.)

However, when you're renaming LP to LP-old, the \$0
evaluates to LP-old and not LP. Since LP-old has *not*
in the meantime been aliased to something else, LP-old
keeps on calling LP-old, and the recursion doesn't stop.

If you try the following (@LP instead of LP)

  .mso s.tmac
  .\" extend LP
  .rn @LP @LP-old
  .rm @LP
  .de @LP
  .  tm inside new LP
  .  @LP-old
  ..
  .\" example text
  .LP
  .TL
  Hello
  .LP
  world

I think everything will work as expected.