Re: Updating *Completions* as you type

[Spencer Baugh]

Juri Linkov <juri@linkov.net> writes:
>>>> - Again, the user is still able to configure the display-sort-function
>>>>   by configuring the individual completion table.
>>>
>>> Does this mean that every individual completion table should have
>>> a separate user option?
>>
>> No: only the completion tables which specify a display-sort-function in
>> their metadata.  All such completion tables should have a user option to
>> configure that display-sort-function.
>
> How then users could change the sorting order for individual tables
> that don't specify a display-sort-function to use an order different
> from completions-sort?

They can use the category if the table specifies one.

If the table neither specifies a category nor provides a table-specific
option, the display sort function for that table isn't currently
configurable.  Which I think we're both fine with?

So: we already accept that for some completion tables, it won't be
possible to customize their display sort function out of the box.

>> Well, yes.  So then we agree that a user option for an individual
>> completion table, if it exists, should take precedence over
>> completion-category-overrides?
>
> The problem is that we can't distinguish two cases:
>
> 1. when display-sort-function is hard-coded in metadata
>    by the author of the completion table;
> 2. when display-sort-function in metadata
>    gets the value from the user option.

I think we should just eliminate any instances of case 1.

Case 1 just means the completion table's display sort function isn't
currently configurable.  Which I think we've already accepted will be
the case for some tables, until we go and make them configurable either
by adding a table-specific option, adding a category, or both.

> Since we can't distinguish these cases, then it makes more sense
> when completion-category-overrides overrides everything:
>
>   (alist-get 'display-sort-function (alist-get category 
> completion-category-overrides))
>   (alist-get 'display-sort-function metadata) ;; metadata with/out individual 
> options
>   (alist-get 'display-sort-function (alist-get category 
> completion-category-defaults))
>
> There is no problem with this because completion-category-overrides
> is a user option as well, so everything still is under user control.

Only if the completion table specifies a category.  Which most do not.

So we'd need to change it to specify a category.  And if we're doing
that, we could also change it to not hard-code a display-sort-function
at the same time.

That is, for any tables where the display-sort-function is currently
hardcoded, we can add a category, and remove the hardcoded
display-sort-function from the table metadata, and add the
display-sort-function to completion-category-defaults.

>> So then we're only disagreeing over whether such options should exist?
>
> Yes, I think we should add individual options only in exceptional cases.
>
>> These individual options would also provide a natural place to document
>> behavior like "if you configure the display-sort-function for buffer
>> completion to 'identity, then the buffer sort order will match
>> (buffer-list)".  But the user could still make use of that information
>> by configuring the category.
>
> I agree that an option with documentation could help in such cases
> when a non-trivial sorting function is provided for a completion table.
>
>>> I see no need to add individual options as all.  Every completion table
>>> that significantly differs from other tables so that it needs a separate
>>> display-sort-function, could provide a separate category.  For example,
>>> there is a category 'buffer'.  If 'switch-to-buffer' needs another
>>> display-sort-function it could provide a category 'buffer-for-switching'.
>>
>> That won't work with the scenario I described before with sorting
>> file-name completion by mtime, where changing the sorting requires also
>> changing the completion table.
>
> I agree that individual options are required in such rare cases when
> their values affect the completion table and its formatting.

OK, I think I can agree with that, if we agree that in those rare cases,
the individual options should take precedence over the category-based
configuration.

>> Also, this would require adding a category for essentially every
>> completion table.  For example, I see that read-from-kill-ring specifies
>> a display-sort-function, currently set to 'identity.
>
> It's much simpler to add an extra line with a category.
>
>> If we wanted to make that configurable, it seems much easier to just do
>>
>>           (if (eq action 'metadata)
>>               ;; Keep sorted by recency
>> -             '(metadata (display-sort-function . identity))
>> +             `(metadata (display-sort-function . ,read-from-kill-ring-sort))
>>             (complete-with-action action completions string pred)))
>
> This is an incomplete patch, there should be also a dozen of lines
> with defcustom, its docstring, the version number and a list
> of choices, etc.  And all this for a very small percent of users
> who would like to change this order.  This is too wasteful.
> It would be much more efficient to allow doing the same
> by customizing completion-category-overrides.

The docstring and list of choices for read-from-kill-ring-sort are
something we want anyway - we would like to document that 'identity for
read-for-kill-ring keeps the kill ring sorted by recency, for example.
I see no better place to document that.

The version number is also something we want anyway: if we just add a
category to read-from-kill-ring in Emacs 30, this will work only in
Emacs 30 and not in Emacs 29, and there's no way for a user to know that
other than by reading NEWS.

For such tables, I see three good possibilities (in order of my own
preference):

A.
- Add read-from-kill-ring-sort defaulting to identity (with docstring)
(diff is 1 line + defcustom)

B.
- Add read-from-kill-ring-sort defaulting to nil (with docstring)
- add the 'read-kill category to the metadata
- add 'read-kill to completion-category-defaults
(diff is 3 lines + defcustom)

C.
- Remove display-sort-function from the metadata
- add the 'read-kill category to the metadata
- add 'read-kill to completion-category-defaults
(diff is 3 lines)

If you really don't want the defcustom and associated documentation, I'm
OK with C.

The option which I think is not good is:

D.
- add the 'read-kill category to the metadata
- make completion-category-overrides take precedence over what is
  specified in the table metadata

(diff is 1 line)

This is a slightly smaller diff than option C, but I think it's a
fundamentally worse approach than C, because in the rare cases where we
do want an individual option for the table, we won't have a way for that
option to take precedence over the broader category-based configuration.