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

[Daniel Mendler]

Hello Eli,

here I respond to the comments you sent out after I've already sent the
overhauled patch.

On 8/14/21 8:27 AM, Eli Zaretskii wrote:
>> 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.

I agree with this. The documentation should be improved.

>>>> +(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.

No, we explicitly don't provide this variable for other packages. It is
explicitly only meant to be used for the existing completion styles
emacs21, emacs22, basic, substring, partial-completion, initials and
flex to opt-in in a backward-compatible/calling convention preserving
way to the alist return format. The idea is to keep the existing APIs
fully backward compatible.

Other packages should select the format returned from the completion
styles differently. They should return the alist format on Emacs 28 or
if the API 'completion-filter-completions' API is present. In the not so
near future external packages which support only Emacs 28 and upwards
will then only return the alist format and don't have to perform any
detection anymore.

>>> 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.

It is not the hope - it is the explicit goal. The API has been designed
to replace the existing API 'completion-all-completions'. We can of
course tone this down. However I, as a package author, would appreciate
if Emacs tells me when a newer API aims to replace another API and when
the documentation is explicit about it. Of course if you decide to have
the doc strings written in a different tone, I will adapt my patch
accordingly. Here I am just explaining why I chose the word "superseded"
instead of a more neutral word.

>>> 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.

Okay, you are right about this. But I think even if the name
'completion-filter-completions' is not 100% precise it still conveys
what the API is about. 'completion-completions-alist' or
'completion-all-completions-as-alist' are valid names of course, but I
dislike them for their verbosity. Already 'completion-all-completions'
is quite verbose. A strong argument to use this long name is that the
completion style functions are still called
'completion-basic-all-completions' etc. But if we accept that the new
API 'completion-filter-completions' will actually supersede the existing
API 'completion-all-completions' it makes sense to use a name which will
not hurt our eyes in the long run. However this is of course just a
personal preference of mine. I don't want to spent much time with name
bikeshedding discussions. If you decide on a name, I will adapt my patch
accordingly.

>>>> +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.

Okay, I agree. However I tried to avoid including superfluous changes
with my patch set. We should add more context and documentation and then
rename the variables in another patch if we decide that we want to go
through with it.

>>> 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.

The overhauled patch (version 2) ensures that no features are lost. The
patch is fully backward compatible. There is a potential performance
regression in 'completion-all-completions' as identified by João. I have
yet to confirm this regression. In any case, it should be fixable since
the refactored 'completion-all-completions' API does precisely the same
amount of work as it does now.

Furthermore more documentation should be added to my patch. As of now
'completion-all-completions' is not mentioned in the info manual and
'completion-filter-completions' is also not added there. We may want to
improve the documentation of that. But for now I would like to limit my
documentation improvements to expanding the doc strings of the functions
involved in my patch.

Daniel