[Bese-devel] "Miss behavior" :action on loop

[szergling]

On Sat, May 23, 2009 at 6:41 PM, Fausto Santin <fawxtin at gmail.com> wrote:
> Hi all,
>
> Im updating things from ucw_dev to ucw-core, and simply got 1 hours
> with a little bug bothering me. Right when I was suposed to send a help
> note (this email), I found a solution to the probable bug, but Im
> still sending
> this email to see/know why, and where things happened wrong.
>
> The context: I am updating a simple blog like app, and the main render
> method (that renders the list of posts) is no good at "passing actions".
>
> ;; DO NOT work ok
> (defmethod render-html-body ((self lisdit))
>   (<:p (<ucw:a :action (add-post self) "new post"))
>   (loop :for post :in (lisdit-posts self)
>      :do
>      (<:div :id "lisdit-post"
>            (render post)
>            "post: " (<:as-html (lisdit-post-id post))
>            (<ucw:a :action (remove-post (lisdit-post-id post) self) "remove
> it")))) ; XXX here
>


Ah hah, a very common misconception for newbies. In case it's still
not clear enough, loop is doing something like this (pseudo-code only
for illustration)

(let ((post ...))
  (loop
    (setf post (some-element-of (lisdit-posts self)))
    ...body...))

All closures (or UCW actions) in body will share the same binding to
post. mapcar uses a different binding time, that's why it worked. To
solve your issue, you just need to rebind post.

(defmethod render-html-body ((self lisdit))
  (<:p (<ucw:a :action (add-post self) "new post"))
  (loop :for post :in (lisdit-posts self)
        :do
        ;; New binding here, optionally, use a different name
        (let ((post post))
          (<:div :id "lisdit-post"
             (render post)
             "post: " (<:as-html (lisdit-post-id post))
             (<ucw:a :action (remove-post (lisdit-post-id post) self)
"remove it")))))

This is untested (I don't have UCW handy at the moment), but I hope it
is still useful.




Theam Yong Chew