Re: [PATCH] `completing-read`: Add `group-function` support to completion metadata

[Dmitry Gutov <dgutov@yandex.ru>]

On 25.04.2021 22:47, Daniel Mendler wrote:
> On 4/25/21 9:35 PM, Dmitry Gutov wrote:
>> The result looks nice (when enabled), though this function still 
>> doesn't work very well with the default completing read. So whether 
>> this feature works okay will depend on the alternative UI used.
> 
> Dmitry, thank you for looking at the patch!
> 
> What do you mean exactly by "it does not work well"?

Sorry, I was basically referring to an earlier discussions where the 
consensus was that xref-show-definitions-completiong-read doesn't play 
very well with the default completing-read. Its completion table is odd, 
one could say. The proposed feature simply doesn't change that.

Perhaps if all currently planned uses of group-function are similarly 
"odd" (and no additional uses in the core are going to be added in the 
foreseeable future), you don't need to worry/care about having 
:group-function added to the core, or at least not yet. Or about 
updating the *Completions* UI.

And keep it like "unofficial extension", which I'll be happy to support 
in Xref anyway (and Xref is in ELPA Core, so users will always be able 
to install the latest version). There are benefits to being such 
extension: once you're a proper part of the protocol, you become much 
more set in stone.

>> Speaking of group-function's implementation there, the text-properties 
>> approach seems like an overkill since we can reliably string-match 
>> anyway. But it's a minor thing.
> 
> I've chosen the text property approach such that the group title 
> retrieval does not lead to allocations (transform=nil). The 
> transform=nil call is performance critical for continuously updating UIs 
> like Icomplete, Vertico etc., since the candidates are grouped after 
> sorting.

But when the list is updated, the elements are basically recreated from 
the external process's output every time, right? So this only helps if 
you want to cache the result for repeated invocations of group-function 
on the same result set.

I'd be curious to see some benchmark results for both versions.

Also, xref-find-definitions usually deals with a limited number of 
search results. But I guess some of your users set 
xref-show-xrefs-function to xref-show-definitions-completiong-read too.