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Re: Need some education on defmacro define-toggle

From: Lennart Borgman
Subject: Re: Need some education on defmacro define-toggle
Date: Tue, 29 Dec 2009 01:49:30 +0100

On Tue, Dec 29, 2009 at 1:37 AM, Pascal J. Bourguignon
<address@hidden> wrote:
> Lennart Borgman <address@hidden> writes:
>> On Mon, Dec 28, 2009 at 10:33 PM, Pascal J. Bourguignon
>> <address@hidden> wrote:
>>> Lennart Borgman <address@hidden> writes:
>>>> On Mon, Dec 28, 2009 at 6:16 AM, Lennart Borgman
>>>> <address@hidden> wrote:
>>>>> I just got into some trouble because of a badly written macro I
>>>>> believe. After the long discussions about defmacro here I wonder if
>>>>> anyone wants to help me out. How should I write the defmacro below?
>>>> I resorted to self education (which is not too bad). However I still
>>>> got problems - just on a more complicated level. The macro now looks
>>>> like this:
>>>> (defmacro define-toggle (symbol value doc &rest args)
>>>>   "Declare SYMBOL as a customizable variable with a toggle function."
>>>>   (declare   (doc-string 3)   (debug t))
>>>>   (let* ((SYMBOL-toggle (intern (concat (symbol-name symbol) "-toggle")))
>>>>          (SYMBOL-name (symbol-name symbol))
>>>>          (var-doc doc)
>>>>          (fun-doc (concat "Toggles the \(boolean) value of `" SYMBOL-name
>>>>                           "'.\nFor how to set it permanently see this
>>>> variable.\n")))
>>>>     (let ((var (append `(defcustom ,symbol ,value ,var-doc)
>>>>                 args
>>>>                 nil))
>>>>           (fun `(defun ,SYMBOL-toggle ()
>>>>                    ,fun-doc
>>>>                    (interactive)
>>>>                    (customize-set-variable (quote ,symbol) (not 
>>>> ,symbol)))))
>>>>       `(list 'progn ,var ,fun))))
>>> That said, why doesn't your macro just return
>>>   `(progn ,var ,fun)
>>> instead of returning a list form that evaluates to something such as
>>>   (progn rngalt-display-validation-header 
>>> rngalt-display-validation-header-toggle)
>>> that needs to be evaluated (and of course fails since you didn't
>>> define these symbols as variables)?
>> It fails sometimes, but not always. It looks like it has something to
>> do with those eval above, but I do not understand how. If I just do
>> eval-buffer it works as I expect it to.
> Of course, when you evaluate `(list 'progn ,defcustom-form ,defun-form)
> the defcustom and the defun are evaluated, and they return their name 
> argument.

Hm, in that case the defcustom and defun must have been evaluated
earlier because they did exist. Now I have changed this so many times
so I am not sure when.

> That's why I advise you to write instead:
>     `(progn ,defcustom-form ,defun-form)

Thanks, yes, I understand that. I tried a lot of alternatives though
to get evaluation at the right time. Surprisingly enough it sometimes
worked with `(list 'prog ,var ,fun) and sometimes not.

I did not dig deep in this, because I know there are other people here
who understands this much better than me ... ;-)

In the current implementation (which seems to work) I have this

    (let ((var (append `(defcustom ,symbol ,value ,var-doc)
          (fun `(defun ,SYMBOL-toggle ()
                  (customize-set-variable (quote ,symbol) (not ,symbol)))))
      `(progn ,fun ,var)

Does that look correct?

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