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Re: Improvement proposals for `completing-read'

From: Daniel Mendler
Subject: Re: Improvement proposals for `completing-read'
Date: Thu, 8 Apr 2021 23:30:26 +0200

On 4/8/21 10:44 PM, Dmitry Gutov wrote:
I tried to integrate `fzf` once with Consult async, like generating a list outside Emacs, pushing it through `fzf` for fuzzy-filtering and presenting it to the user via completion. But it turned out that most of the external implementations are not good enough for this use case. They don't have an option to open a pipe to update the filtering input for example. I could write my own fuzzy matcher external backend which would work perfectly with async completion. However then I can also just wait for gccemacs :)

I was thinking more about interactions over network, with HTTP requests sent and received asynchronously. Mainly the cases where one uses the LSP protocol or similar.

Yes, this is all possible with async completion tables in Consult. There is a consult-spotify package which queries some web api and there is also consult-lsp in the works which accesses the lsp api (https://github.com/minad/consult/issues/263).

You may want to take a look at my Consult package, specifically the async functionality. I believe that this functionality can easily be provided on top of the current infrastructure, and actually in a nice way.

You can check out Company's asynchronous convention for backends:

https://github.com/company-mode/company-mode/blob/f3aacd77d0135c09227400fef45c54b717d33f2e/company.el#L456-L467 It's a very simple lambda-based future-like value. It can be updated to use a named type, and with other features too. I think it's a clean and simple base to build on, though.

Yes, this looks very simple. I actually prefer the functional style in contrast to some named types as you have it in Company. So how is this used? When completing the fetcher is called and as soon as it returns via the callback the results are displayed? But chunking is not possible and probably also not desired? See below for my response regarding chunking in Consult.

In Consult I am using closures which hold the asynchronously acquired data. The closure function must accept a single argument, it can either be a string (the new input) or it can be a list of newly obtained candidates.

I'm not sure where to look, sorry.

Take a look at `consult--async-sink` at

These `consult--async-*` functions can be chained together to produce an async pipeline. The goal here was to have reusable functions which I can glue together to create different async backends. See for example the pipeline for asynchronous commands: https://github.com/minad/consult/blob/3121b34e207222b2db6ac96a655d68c0edf1a449/consult.el#L1505-L1513.

I'm not 100% clear, but it sounds like chunked iteration. Which is a good feature to have. Though perhaps not always applicable to every UI (blinking with new results in a completion popup might be not user-friendly).

Indeed, the UI receives the candidates as soon as they come in. One has to ensure that this does not freeze the UI with some timer. Then there is also throttling when the user changes the input. It works very well for the `consult-grep` style commands. You may want to try those. They are similar to `counsel-grep` but additionally allow to filter using the Emacs completion style. Take a look here, in case you are interested https://github.com/minad/consult#asynchronous-search. Note that you don't have to use chunking necessarily. You can use a single chunk if the whole async result arrives in bulk.

Now a single problem remains - if new data is incoming the async data source must somehow inform completion that new candidates are available. In order to do this the async source must trigger the UI for example via icomplete-exhibit/selectrum-exhibit and so on. It would be good to have a common "completion-refresh" entry point for that. In Consult I have to write a tiny bit of integration code for each supported completion system.

See my link, perhaps.

Or in general, a Future/Promise API has a way to subscribe to the value's update(s) (and the completion frontend can do that).

Having to use a global variable seems pretty inelegant in comparison.

It is not a global variable but a function. But for sure, one could also design the async future such that it receives a callback argument which should be called when new candidates arrive. The way I wrote it in Consult is that the `consult-async-sink` handles a 'refresh action, which then informs the completion UI.

No hurry at all. Sometimes, though, a big feature like that can inform the whole design from the outset.

Yes, sure. When planning to do a big overhaul you are certainly right. But currently I am more focused on fixing a few smaller pain points with the API, like retaining text properties and so on.

Daniel Mendler

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