RE: defvar interactively, user-variable-p

[Drew Adams]

> BTW pitty that this functionality is gone. Anyhow

Note, though, that you have commands `customize-set-variable'
and `customize-set-value', each of which does a part of what
`set-variable' does (a prefix arg to `set-variable' gives the
buffer-local behavior) - and more.

Those commands have the advantage that they not only type-check
your input (respecting the defcustom `:type') but they also
respect other defcustom keywords, such as `:set'.

`set-variable' respects `:type', but unfortunately not `:set':

http://debbugs.gnu.org/cgi/bugreport.cgi?bug=6578
http://lists.gnu.org/archive/html/emacs-devel/2010-10/msg00867.html
http://lists.gnu.org/archive/html/emacs-devel/2010-10/msg00839.html

My impression is that too few users are aware of
`customize-set-variable' and `customize-set-value'.  If the
(complete) behaviors of those commands were rolled into the
single command `set-variable' then users might take more
advantage of them.

And users who today still use `setq' in their init files
to set user options might then take to using that instead,
reducing problems due to ignoring `:type', `:set', and
`:initialize'.

I would also like to see defvar allow keywords `:type' and
`:set'.  That is, let you optionally specify the types of
allowable values and specify a setter function.

http://lists.gnu.org/archive/html/emacs-devel/2009-10/msg00668.html

Not only are defvar variables sometimes set by users, but
they can be set by other libraries.  Letting the defining
library control the allowable types would be a definite
benefit, IMO.  Use of such keywords would be optional, of
course.