Re: don't understand setq-default

[Jean Louis <bugs@gnu.support>]

* Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> [2021-04-19 01:29]:
> Jean Louis wrote:
> 
> > Here is how I have defined a buffer local variable:
> >
> > (defvar-local rcd-tabulated-marked-items nil
> >   "Collects IDs for tabulated list modes")
> > (put 'rcd-tabulated-marked-items 'permanent-local t)
> 
> Oh, great! No, didn't find/have any `defvar-local', good, for
> me that is, I'm sure it's good for something but to me it
> just isn't my style. And my style confirms it, even!
> 
> But... from that code above it looks you set it to local
> twice, first by defvar-local, then by setting the
> permanent-local property to t?
> 
> permanent-local, sounds like the default.

There is in manual what it is:

(info "(elisp) Creating Buffer-Local")

   A buffer-local variable is “permanent” if the variable name (a
symbol) has a ‘permanent-local’ property that is non-‘nil’.  Such
variables are unaffected by ‘kill-all-local-variables’, and their local
bindings are therefore not cleared by changing major modes.  Permanent
locals are appropriate for data pertaining to where the file came from
or how to save it, rather than with how to edit the contents.

As I am using the variable to extend the tabulated-list-mode, I have
been following the example and explanation in file `tabulated-list.el'
as below:

;; The reason `tabulated-list-format' and other variables are
;; permanent-local is to make it convenient to switch to a different
;; major mode, switch back, and have the original Tabulated List data
;; still valid.  See, for example, ebuff-menu.el.

That may happen sometimes.

-- 
Jean

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