[pro] The Best Examples of "Code is Data"

[Yakov Zaytsev]

On Mon, Sep 6, 2010 at 12:59 AM, Scott L. Burson <Scott at sympoiesis.com>wrote:

> On Sun, Sep 5, 2010 at 11:56 AM, Yakov Zaytsev <zaytsev.yakov at gmail.com>wrote:
>
>>
>> (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))))))
>>
>
>>
>
> Rather than eval-ing the consed defmacro form at macroexpansion time, which
> doesn't do the right thing when a DEFINE-STRUCT-GETTER form is in a file
> being compiled, it would be better to do something like this:
>
>
> (defmacro define-struct-getter (struct package &rest slots)
>   `(progn
>      ,@(mapcar (lambda (s)
>
>          (let ((acc (intern (concatenate 'string (symbol-name struct) "-"
> (symbol-name s))))
>                (sp (intern s package))
>            `(defmacro ,acc (object)
>               `(access-slot object ',',sp))))
>            slots)))
>
> As tempting as it is to call EVAL for various purposes, the reality is that
> correct uses of EVAL are quite rare.  In fact, unless you're writing your
> own read-eval-print loop of some kind, the best rule of thumb is that if
> you're calling EVAL explicitly, you've made a mistake.  I have trouble
> coming up with any exceptions to this rule other than a REPL.
>
> Also, supplying a package to re-intern the slot names in, while not
> outright wrong, is an unusual kind of thing to do.  Is it that hard to just
> type the slot names with package prefixes?
>
> All that said -- if you're using nested backquotes successfully, you're
> clearly starting to get the hang of macros.
>
> -- Scott
>
>
>
re-interning the slot names saves typing because DEFINE-STRUCT-GETTERS form
is used for structures which are generated by swig. anyhow it is first time
I use INTERN
e.g.

(define-struct-getter image :MagickWand columns rows)

and thank you for pointing my EVAL usage mistake!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.common-lisp.net/pipermail/pro/attachments/20100906/a3b673a8/attachment.html>