[cl-who-devel] Macroexpansion of with-html-output body?

[Vsevolod]

Below is the message I've sent today to Edi Weitz on this topic.
Unfortunately, I've missed the existence of this mailing-list, that's
why I didn't consult the archives. Today I have quickly pierced the
discussion and, as far as I've seen, the solution, I propose, hadn't
being mentioned.

So, I suggest an implementation strategy for the macroexpansion inside
with-html-output and provide a patch against the current version
(0.11.1)...



I tried to use CL-WHO a couple of times in the past and always stumbled over the
fact, that it's possible to do something like that (example from the site):

(with-html-output (*http-stream*)
 (:table :border 0 :cellpadding 4
  (loop for i below 25 by 5
        do (htm
            (:tr :align "right"
             (loop for j from i below (+ i 5)
                   do (htm
                       (:td :bgcolor (if (oddp j)
                                       "pink"
                                       "green")
                            (fmt "~@R" (1+ j))))))))))

...but not something like this (i.e. make a macro, to compress some of
the pseudo-html tree):

(with-html-output (*http-stream*)
 (:table :border 0 :cellpadding 4
  (loop for i below 25 by 5
        do (htm
            (:tr :align "right"
             (loop for j from i below (+ i 5)
                   do (htm (embed-td j))))))))

(defmacro embed-td (j)
    `(:td :bgcolor (if (oddp ,j) "pink" "green")
             (fmt "~@R" (1+ ,j))))


Thus to write such a macro for with-htm-output you should do it in two
steps (at least, this is the only way I have discovered):

The original form should be like the following (with STR):
(with-html-output (*http-stream*)
 (:table :border 0 :cellpadding 4
  (loop for i below 25 by 5
        do (htm
            (:tr :align "right"
             (loop for j from i below (+ i 5)
                   do (STR (embed-td j))))))))

...and the macro should itself use w-h-o-to-string:
(defmacro embed-td (j)
 (let ((str (gensym)))
   `(with-html-output-to-string (,str)
      (:td :bgcolor (if (oddp ,j)
                        "pink"
                        "green")
           (fmt "~@R" (1+ ,j))))))


I think it's quite a common case in the usage of the w-h-o macro to
want such macros (at least for me it always comes up). So I suggest to
add another keyword to a list of STR, ESC and HTM -- EMB -- specially
for such cases, which will mean "macroexpand-1 the enclosed form".

With it the above code may be expressed like:

(with-html-output (*http-stream*)
 (:table :border 0 :cellpadding 4
  (loop for i below 25 by 5
        do (htm
            (:tr :align "right"
             (loop for j from i below (+ i 5)
                   do (htm (emb (embed-td j)))))))))

(defmacro embed-td (j)
   `(:td :bgcolor (if (oddp ,j)
                        "pink"
                        "green")
           (fmt "~@R" (1+ ,j))))

or like this:
(with-html-output (*http-stream*)
 (:table :border 0 :cellpadding 4
  (loop for i below 25 by 5
        do (htm
            (:tr :align "right" (emb (embed-tr i)))))))

(defmacro embed-tr (i)
 (let ((j (gensym)))
 `(loop for ,j from ,i below (+ ,i 5)
     do (htm (:td :bgcolor (if (oddp ,j)
                               "pink"
                               "green")
                  (fmt "~@R" (1+ ,j))))))


I think, sometimes such flexibility is necessary. Although, I'm not
absolutely sure, that the solution, I've hacked, is flawless...

The patch is attached. I tried to follow the guidelines for patches
with this one. The only thing, that I didn't do, is provide examples
of the usage of EMB keyword in the doc (only a brief reference). I
think, that the examples in the letter might suit, but I'm not sure.

Best regards,
Vsevolod Dyomkin
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