Re: custom: how do I augment an option?

[Sam Steingold <sds@gnu.org>]

This was originally discussed on gnu-emacs-help, but it appears that the
further discussion should take place on emacs-devel.

I said:

If I want to _modify_ a custom variable, I have to resort to something like
--8<---------------cut here---------------start------------->8---
(add-hook 'message-load-hook
  (lambda ()
    (add-to-list 'message-syntax-checks
                 '(long-lines . disabled))))
--8<---------------cut here---------------end--------------->8---
which will break if `message-syntax-checks' ever acquires a setter
or
--8<---------------cut here---------------start------------->8---
(custom-set-variables
 '(message-syntax-checks
   (cl-adjoin '(long-lines . disabled)
    (eval (car (get 'message-syntax-checks 'standard-value)))
    :test 'equal)))
--8<---------------cut here---------------end--------------->8---
which uses a CL function _and_ also eval.

> * Stefan Monnier <zbaavre@veb.hzbagerny.pn> [2012-09-06 09:14:55 -0400]:
>
>> So, what is the official method?
>
> There are 3 official methods:
> 1- forget about flexibility and just set the var to a constant value
>    that does not depend on the default value.  That can be done from the GUI.

this sucks as it ignores the changes in the default value, as discussed
on gnu-emacs-help.

> 2- if you want to use actual code (instead of a mere constant), then
>    don't use Customize, and use something like the add-hook code you showed.

this sucks in case the custom variable in question ever acquires a
setter, as mentioned above.

> 3- submit a patch for Customize which lets the user specify not just
>    a new value but a change (like a "diff") to the default value.
>    For lists representing sets, a way for the user to specify elements
>    to add and elements to remove would be great.  For lists where order
>    matters, the user should additionally have some control over where to
>    add elements.
> Point 3 would be *really* welcome.

How about changing `custom-set-variables' to accept

(SYMBOL EXP [NOW [REQUEST [COMMENT [SETTER]]]])

instead of

(SYMBOL EXP [NOW [REQUEST [COMMENT]]])

where SETTER would be a function acceptable as the :set argument to
defcustom.

(Actually, I would prefer to replace the positional optional arguments
with keywords, like in `defcustom', but that would be an incompatible
change)

Also, semi-independently, I propose a function

(defun custom-standard-value (symbol)
  "Return the standard value of the customizable option."
  (eval (car (get symbol 'standard-value))))

A cheap ad-hoc fix for `message-syntax-checks' (and other set variables)
would be to give it a setter which would accept lists like (add ...) or
(delete ...).

-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
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