There is an old saying: if you are using "eval", you are
doing it wrong. So far I have yet to find any significant
exceptions to this rule.

I do not see what the eval is for. Just get rid of it
and have the definition of define-struct-getter
expand into the defmacro of the concatenated
name.

Meanwhile, there are several problems here.

(1) In define-struct-getter, are the slots intended
to be symbols, or forms that evaluate to symbols?
If they are symbols, intern fails. If they are
forms that are evaluated to symbols, symbol-name
fails because it is called on the form. I could
not get any case to work.

(2) In access-slot, you expand "object" twice,
which can cause trouble if the form to which
"object" is bound has side-effects. It is normal
good practice to use generated symbols here.

(3) Package arguments are usually a pain in the
neck. Standard practice is to just use "intern"
so that it's interned in the current package.

-- Dan

Yakov Zaytsev wrote:

My best macro so far ;-)

(defmacro access-slot (object &rest slot-names)

  `(ff:fslot-value-typed (ff:foreign-pointer-type ,object)

   :c (ff:foreign-pointer-address ,object)

   ,@slot-names))

(defmacro define-struct-getter (struct package &rest slots)

  (loop for s in slots

   do (let ((acc (intern (concatenate 'string (symbol-name struct) "-" (symbol-name s))))

   (sp (intern s package)))

   (eval `(defmacro ,acc (object)

   `(access-slot ,object ',',sp))))))

because it is a macro which writes macros which uses macro. My first real world macro.

On Sun, Sep 5, 2010 at 4:24 PM, Kazimir Majorinc <kazimir@chem.pmf.hr> wrote:

As first, good luck with this list!

I'm in search for best examples of "code is data" paradigm in Common
Lisp. For most CL-ers, it probably means "macros", but eval, backquote,
anything that processes the code as data is of interest. As "best" I
think on the most surprising, powerful, sophisticated examples, not
necessarily of a pedagogical value.

Imagine that someone invited you to write the presentation "Five best CL
macros ... I seen" or "Five best CL macros ... I wrote." What would you
chose and why?

Kazimir Majorinc

_______________________________________________
pro mailing list
pro@common-lisp.net
http://common-lisp.net/cgi-bin/mailman/listinfo/pro
_______________________________________________
pro mailing list
pro@common-lisp.net
http://common-lisp.net/cgi-bin/mailman/listinfo/pro
  