bug#75170: add-to-alist: new function

[Eli Zaretskii <eliz@gnu.org>]

> From: Roland Winkler <winkler@gnu.org>
> Date: Sat, 28 Dec 2024 23:33:56 -0600
> 
> On Mon, 12 Feb 2001, Stephen Gildea wrote:
> > Here's a handy function I'd like to see added to Emacs 21: add-to-alist.
> > It is like add-to-list, but it looks only at the cars of the element to
> > be added and the existing list elements when considering a match.  An
> > additional optional argument to add-to-alist says what to do if the car
> > matches but the cdr does not.
> > 
> > I use this function in my .emacs to update values in alists such as
> > default-frame-alist and auto-mode-alist; I'm sure it has other uses.
> > My goal in proposing this function is to allow .emacs files to be
> > shorter and easier to write.  The functions add-to-list and add-hook
> > were important steps in that direction; here is another such step.
> > 
> > 
> > (defun add-to-alist (alist-var elt-cons &optional no-replace)
> >   "Add to the value of ALIST-VAR an element ELT-CONS if it isn't there yet.
> > If an element with the same car as the car of ELT-CONS is already present,
> > replace it with ELT-CONS unless NO-REPLACE is non-nil; if a matching
> > element is not already present, add ELT-CONS to the front of the alist.
> > The test for presence of the car of ELT-CONS is done with `equal'."
> >   (let ((existing-element (assoc (car elt-cons) (symbol-value alist-var))))
> >     (if existing-element
> >         (or no-replace
> >             (rplacd existing-element (cdr elt-cons)))
> >       (set alist-var (cons elt-cons (symbol-value alist-var))))))
> > 
> > 
> > The no-replace argument is useful for setting auto-mode-alist when you
> > don't know whether Emacs supports a particular programming language.
> > For example, the following suppresses using text-mode for m4 files in
> > Emacs 19 but doesn't override using m4-mode in Emacs 20.
> > 
> > (setq default-major-mode 'text-mode)
> > (add-to-alist 'auto-mode-alist '("\\.m4\\'" . fundamental-mode) t)
> 
> While I thought about a function add-to-alist I found the above thread
> from 24 years ago.  Stephen's message describes nicely when such a
> function can be useful.  I suggest to add such a function to subr.el.
> 
> The code below follows the conventions of add-to-list.
> 
> (defun add-to-alist (alist-var elt-cons &optional no-replace append compare-fn)
>   "Add ELT-CONS to the value of ALIST-VAR if it isn't there yet.
> If an element with the same car as the car of ELT-CONS is already present
> in ALIST-VAR, replace it with ELT-CONS unless NO-REPLACE is non-nil.
> If a matching element is not yet present, add ELT-CONS at the beginning
> of ALIST-VAR.  If APPEND is non-nil, add ELT-CONS at the end of ALIST-VAR.
> The test for presence of ELT-CONS is done with `equal', or with COMPARE-FN
> if that's non-nil.
> ALIST-VAR should not refer to a lexical variable.

Thanks.

What is the advantage of adding this function, given that add-to-list
can be used with alists, and given that alist-get can nowadays be used
as a generalize variable?