Using Macros in Lisp

[mbr@breeze.bellcore.com (Mark Rosenstein)]

Like they used to say in high school shop, "use the right tool for the 
job." When permuting syntax is the right thing, macros are the right
tool.

   Sender: Gallagher@Gilgamesh.bellcore.com
   Date: Wed, 6 Mar 91  13:54:09 EST
   Reply-To: Gallagher@cs.umass.edu
   From: Kevin Gallagher <Gallagher@gilgamesh.cs.umass.edu>

I have just a small nit:

   Unfortunately, I don't know of any text that describes writing anything
   other than trivial macros.  Probably, the best thing to do is to look at
   code written by good lisp programmers. Important things to consider when
   writing macros are:

     -- Take care with local variables introduced by the macro.
	(i.e., use gensyms for any new locals)
Actually, I think using make-symbol generates easier to understand
macro expansions, eg.:
(defmacro too-cool-for-school (&body somebody)
  (let ((new-local (make-symbol "SOME-NAME-WITH-MEANING")))
     `(let ((,new-local (something-wonderful)))
          ...
     -- Take care to preserve the left to right order of argument
	evaluation.

     -- Don't evaluate argument forms more than once.

     -- Know the difference between read time, macro expansion time,
	compile time, and load time.

     -- Don't do any side effects in your macro.  A macro is just a source
	to source transformation.

     -- Make sure the macroexpansion doesn't cause spurious compiler
	warnings.
This can sometimes be tricky with a good optimizing compiler, but
a reasonable requirement.

One of the big tricks is to sketch out what you want the macro 
expansion to be, before you start getting into the backquotes,
and comma at-signs. Also try to do most of your syntax twiddling
in a function that the macro calls at expand time.

Mark.
#define is to lisp macros
as C is to lisp
(even though this sounds cool, it probably isn't right, oh well)