Re: Updating *Completions* as you type

[Juri Linkov]

> For buffers, for example, sorting by the buffer-list can be done by just
> setting display-sort-function to identity, but we probably shouldn't
> expose that fact to the user.  (We could expose a generic "sort by
> defaults" but that would be less efficient.)

It should be fine to use 'identity' as a value when the Customization UI
will show a more meaningful tag like "Unsorted".

> The sorting style might even require the sorting option to actually
> change the completion table's behavior.  File name completion could be
> sorted by mtime, for example, by having the completion table include
> mtime as a text property when read-file-name-sort=mtime, and then
> setting display-sort-function to something which reads that property.  I
> think that can only work if we have separate sorting options.

Also a separate option might be needed when a completion table
supports a non-standard sorting order such as
'mule--ucs-names-sort-by-code' when 'read-char-by-name-sort'
is customized to 'code'.  But the users still should have freedom
to override all metadata: display-sort-function, affixation-function,
group-function, etc.

>>   (setopt completion-category-overrides
>>     '((buffer (display-sort-function . minibuffer-sort-by-history))
>>       (project-file (display-sort-function . minibuffer-sort-by-history))
>>
>> And default values could be specified in completion-category-defaults.
>
> I think the default values would be part of the completion table itself;
> a completion table can just set display-sort-function itself, it doesn't
> need completion-category-defaults to do that.

OTOH, the completion table needs only to return its category in metadata.
Otherwise every completion table of the same category would need to
duplicate display-sort-function in its completing-read.

> Maybe we should decide what things are supposed to be configured by
> category and what things are supposed to be configured by the table
> itself.  Because in theory we could allow setting completion-styles and
> completion-cycle-threshold with completion table metadata, and I'm not
> sure why we didn't do it that way - it could even be nice to have a
> completion style which is specific to an individual completion table.

I guess when an individual completion table needs to use metadata
slightly different from other similar cases then it should provide
a different category/subcategory.