Re: macro that writes macros

In article
<915225bf-8cc6-465b-8df5-f7ebc4cbd0e7@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
jordi.burguet.castell@xxxxxxxxx wrote:

Hi,

I'm learning lisp and tried to make this little program to write a web
page:


(defmacro new-tag (name)
(let ((sname (string-downcase (symbol-name name))))
`(defmacro ,name (&rest content)
`(progn
(format t "<~a>~%" ,',sname)
,@content
(format t "</~a>~%" ,',sname)))))

(new-tag html)
(new-tag head)
(new-tag title)
(new-tag body)
(new-tag p)

;; With that, now a web page can be "self-generated":
(html
(head
(title "My web page"))
(body
(p "Hello world")))


And it works. But then I thought about combining all those "(new-
tag ...)" like this:

(defmacro new-tags (&rest tags)
(dolist (tag tags)
`(new-tag ,tag)))

(new-tags html head title body p)

But it doesn't work! I tried without the "`" nor the "," in the "new-
tags" definition and it doesn't work either. I have been fighting with
this simple-looking problem for a long time, reading docs and stuff,
and I can't make it work anyway nor see what is wrong! Anyone has some
advice on what's going on or what I should check?

Remember, what matters for a macro is what it RETURNS -- this is then
executed in place of the macro call. And your NEW-TAGS macro doesn't
return anything. The body of a DOLIST is executed just for its side
effect, it's not accumulated into the result. In DOLIST, you specify
the return value as the third element of the first sub-form, and it
defaults to NIL.

What you want is:

(defmacro new-tags (&rest tags)
`(progn
,(loop for tag in tags
collect `(new-tag ,tag))))

--
Barry Margolin, barmar@xxxxxxxxxxxx
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***
.

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