bug#24923: 25.1; Lisp watchpoints

[npostavs@users.sourceforge.net]

Daniel Colascione <dancol@dancol.org> writes:

> On Fri, Dec 02 2016, Clément Pit--Claudel wrote:
>> On 2016-12-02 20:47, npostavs@users.sourceforge.net wrote:
>>> Pushed as 88fefc3 (56c8178, e7cd98b, d3faef9, cfd2b9e, 459a234, 2272131)
>>
>> I wonder: would it make sense to extend defcustom to use this, too?
>> Wouldn't it be great if I could tag my defcustom variables in some
>> way, and then setq would automatically invoke the corresponding
>> defcustom setter?
>
> setf maybe. setq should not be magical.

There were indeed some concerns about that kind usage when this feature
was first proposed, e.g.:

- http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg01064.html

    I like the idea of such hooks (which I've always thought of as
    "watchers" rather than hooks), actually, but only for purposes such
    as debugging.

    If we want to use such hooks for purposes such as "automatically
    recompute values of dependent vars", then I think the right way is to
    introduce a new layer which checks&runs these hooks, using the "raw"
    `setq' underneath.