bug#64656: 29.0.91; Doc of minibuffer histories and completing-read - automatic addition of completions to DEFAULT list

[Juri Linkov <juri@linkov.net>]

>> > Try this:
>> > `C-h v org TAB'
>> > `M-n'
>> > `M-n'
>> > ...
>>
>> Why candidates are inserted in a random order?
>> It would make sense to insert them in the same
>> order as they are sorted in the *Completions* buffer.
>
> That's one reasonable possibility.
> It's not the only one.

I tried to use 'completions-sort' to sort 'C-h v M-n' default values.
By default, it sorts alphabetically that makes sense for 'C-h v M-n'.
But this broke many other completions.

For example, currently 'C-x b M-n M-n ...'
provides the default values sorted by the order
of recently used buffers that is very useful.
It keeps the order of 'buffer-alist'
in 'internal-complete-buffer'.

Another example is 'C-x p p M-n M-n ...'
that currently uses the order of recently
accessed projects from 'project--list'.

This means that we can't change this default behavior.

So currently there are three different sorting orders
used by default:
1. 'TAB' uses the alphabetical order;
1. 'M-p' uses the historical order;
2. 'M-n' is unsorted and follows the order of the caller.

> The fact is that the candidates are in
> a useless order, particularly when the
> completion table is just obarray or an
> unsorted, filtered subset of obarray.

This means that the caller should take care about
sorting completions is a meaningful order.
But then a new metadata type similar to
'display-sort-function' should be added
such as 'minibuffer-default-sort-function'
that might be a hassle.

So maybe this could be improved with a simper fix?
This is why I added such condition below:
(eq minibuffer-completion-table 'help--symbol-completion-table)

Please try this modified function, it should work with 'C-h v M-n':

#+begin_src emacs-lisp
(defun minibuffer-default-add-completions ()
  "Return a list of all completions without the default value.
This function is used to add all elements of the completion table to
the end of the list of defaults just after the default value."
  (let ((def minibuffer-default)
	(all (all-completions ""
			      minibuffer-completion-table
			      minibuffer-completion-predicate)))
    (when (eq minibuffer-completion-table 'help--symbol-completion-table)
      (setq all (pcase completions-sort
        	  ('nil all)
        	  ('alphabetical (sort all #'string-lessp))
        	  (_ (funcall completions-sort all)))))
    (if (listp def)
	(append def all)
      (cons def (delete def all)))))
#+end_src

> Why are all candidates inserted into the
> `M-n' queue at all?  And why no ability
> to filter them or sort them - during
> completion (i.e., taking the current
> completion state into account).

Because 'M-n' is not completion.