bug#48841: [PATCH] Add new `completion-filter-completions` API and deferred highlighting

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 48841@debbugs.gnu.org, dgutov@yandex.ru, joaotavora@gmail.com,
>  monnier@iro.umontreal.ca, 47711@debbugs.gnu.org
> From: Daniel Mendler <mail@daniel-mendler.de>
> Date: Thu, 12 Aug 2021 10:47:17 +0200
> 
> >> The `completions` value is the list of completion strings *without*
> >> applied highlighting.  The completion strings are returned unmodified,
> >> which avoids allocations and results in performance gains for
> > 
> > This is unclear: how can you return a list of strings which you
> > produce without allocating the strings?
> 
> The function 'completion-filter-completions' receives a completion table
> as argument.  The strings produced by this table are returned
> unmodified, but of course the completion table has to produce them.  For
> a static completion table (e.g., in the simplest case a list of strings)
> the completion table itself will not allocate strings.  In this scenario
> 'completion-filter-completions' will not perform any string allocations,
> only the list will be allocated.  This is what leads to major
> performance gains.

My point was that at least some of this should be in the description,
otherwise it will leave the reader wondering.

> >> +(defvar completion--filter-completions nil
> >> +  "Enable the new completions return value format.
> >
> > Btw, why is this an internal variable?  Shouldn't all completion
> > styles ideally support it?  If so, it should be a public variable,
> > documented in the ELisp manual.  And the name should also end with -p,
> > since it's a boolean.  How about completion-filter-completions-format-p?
> 
> (As I understood the style guide '-p' is not a good idea for boolean
> variables, since a value is not a predicate in a strict sense.)
> 
> To address your technical comment - this variable is precisely what one
> of the technical difficulties mentioned in my other mail is about.  The
> question is how we can retain backward compatibility of the completion
> style 'all' functions, e.g., 'completion-basic-all-completions', while
> still allowing the function to return the newly introduced alist format
> with more data, which enables 'completion-filter-completions' to perform
> the efficient deferred highlighting.

I understand, but given that we provide this for other packages, it
shouldn't be an internal variable.

> > Also, the "This function has been superseded..." part should be a new
> > paragraph, so that it stands out.  (And I'm not yet sure we indeed
> > want to say "superseded" here, but that's part of the on-going
> > discussion.  maybe use a more neutral language here, like "See also".)
> 
> The new API 'completion-filter-completions' will substitute the existing
> API 'completion-all-completions'.

That's your hope, and I understand.  But we as a project didn't yet
decide to deprecate the original APIs, so talking about superseding is
premature.

> > Is "filter" really the right word here (in the doc string)?  "Filer"
> > means you take a sequence and produce another sequence with some
> > members removed.  That's not what this API does, is it?  Suggest to
> > use a different name, like completion-completions-alist or
> > completion-all-completions-as-alist.
> 
> "Filter" seems like exactly the right word to me.  The function takes a
> list of strings (or a completion table) and returns a subset of matching
> completion strings without further modifications to the strings. See
> above what I wrote about allocations.

But the name says "filter completions".  Which would mean you take a
list of completions and filter out some of them.  A completion table
is much more general object than a list of strings.  Thus, I think
using "filter" here is sub-optimal.

> >> +Only the elements of table that satisfy predicate PRED are considered.
> >> +POINT is the position of point within STRING.  The METADATA may be
> >> +modified by the completion style.  The return value is a alist with
> >> +the keys:
> >> +
> >> +- base: Base position of the completion (from the start of STRING)
> > 
> > "Base" here means the beginning?  If so, why not call it "beg" or
> > somesuch?
> 
> Base position is a fixed term which is already used in minibuffer.el for
> completions.  See also 'completion-base-position' for example.

Well, we don't have to keep bad habits indefinitely.  It's okay to
lose them and use better terminology.  Or at least to explain that
terminology in parentheses the first time it is used in some context.

> > Are we really losing the completion-score property here?  If so, why?
> 
> Yes, the property is removed in the current patch.  It is not actually
> used for anything in the new implementation.  But it is possible to
> restore the property such that 'completion-all-completions' always
> returns scored candidates as it does now.  See my other mail regarding
> the caveats of the current patch.

I'd prefer not to lose existing features, because that'd potentially
make the changes backward-incompatible.

Thanks.