Re: Error: Symbol's value as variable is void

[Joe Corneli <jcorneli@math.utexas.edu>]

   > when is it good to use `eval-when-compile'?

   when you want to evaluate something at compilation time.

   if this is not completely clear to you, probably it is
   best to drop its use and come back to it later (if ever).

I'm OK with that, but still, I've seen it used, for example, in the
definition of `lisp-imenu-generic-expression'.  It might be helpful to
me to know why it is used there.  I guess otherwise the regexp-opt
forms would not be evaluated when the code was compiled, leading to
disaster.  And I also suppose that this situation is precipitated by
the use of `purecopy'.  So what are the benefits of `purecopy'?
One thing that is kind of quirky is that 
 
 (describe-variable 'lisp-font-lock-keywords-1)

produces a window headed off by "lisp-font-lock-keywords-1's value is
shown below."  No source file.  Wow, weird.

Anyway, I think I was mislead in thinking that it was the things
(potentially) inside of `eval-when-compile' that would precipitate
needing to use it.  Apparently that is not the case.

So, its all becoming much clearer.