On Wed, Feb 1, 2017 at 8:14 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > Cc: monnier@iro.umontreal.ca, emacs-devel@gnu.org
> > From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> > Date: Wed, 1 Feb 2017 02:33:14 -0500
> >
> > Well, it's not uncommon to want to extend existing behavior, and
> add-function lets you do it easily — more easily than if the variable may
> be nil.
>
> Easier than using setq?  I doubt that.
>

Using add-function instead of setq has these advantages:

- You do not need to copy-paste the original variable definition, and
*then* add your tweaks around it. (add-function :around ..) does that for
you. Given that the *-function definitions can be multiple lines long, it
saves some clutter from your emacs config.
- If the variable customization does not work out, or you discovered a bug
in the customization (and you do not have time to debug it), you can remove
that safely using (remove-function ..) and get back the original
definition.. No need to find the original definition and re-eval that.
- If the original definition changes upstream, there are lesser chances of
your config breaking if you did add-function than setq. And if it breaks
due to an upstream change, just do remove-function till you get time to fix
your customization.
- Added convenience that you can edebug your customization function that
you "add" to the original *-function variable. So you debug focuses only on
the customization and not also on the original definition.

-- 

Kaushal Modi