Re: Allowing completion sources to customize completion display

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 23 Nov 2023 13:20:27 -0800
> Cc: sbaugh@janestreet.com, emacs-devel@gnu.org
> From: Jim Porter <jporterbugs@gmail.com>
> 
> On 11/23/2023 12:47 AM, Eli Zaretskii wrote:
> > IMNSHO, it will be a sad day when different completion sources will
> > present different completion UI.  There will be no end to user
> > confusion.
> 
> I think Spencer's suggestion below would be ok (assuming it actually 
> works). Then it's up to the users themselves to opt-in to different UI 
> for different sources:
> 
> On 11/22/2023 3:05 PM, Spencer Baugh wrote:
> > - Maybe completion-category-overrides could change
> >   completion-in-region-function based on the category?

I don't see how this resolves the issue I had in mind, which is that
users should NOT be presented with different completion-related
displays and use different completion-related commands depending on
the source of the completion data.  Quite the opposite: each source of
the completion data should be able to adapt its presentation to the
user preferences, as expressed via user options (if we have such
UI-related options).  Users should not re-learn how to use completion
in Emacs each time we add some new source of completion data to some
command that offers completion.

E.g., consider the following half-theoretical example: suppose the
file-name completion learns to offer completion candidates from some
source other than the list of files in default-directory -- my point
is that those additional files that come from those other sources
should be displayed in exactly the same way as the "traditional"
source, and let users use the same commands to complete.  Anything
else would be terribly confusing, and we should not do it.