Subject: bug#47711: bug#48841: [PATCH] Add new `completion-filter-completions` API and deferred highlighting

From: Daniel Mendler <[email-masked]>
Date: Thu, 12 Aug 2021 10:47:17 +0200

Eli, thank you for your feedback and for pointing me to the mode which
helps with the formatting.  I will address the documentation and
formatting issues as soon as the discussion has concluded.

In the following I answer to a few of your questions about technical
details.

>> 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.

>> +(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.

> Also, the "This function has been superseded..."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 want to say "superseded" here, but that's part of the on-going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'.  I only didn't go as far as
deprecating the 'completion-all-completions' API right away, but we
could also do this.

> 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.

>> +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.

> 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.

Daniel