async functions in completion at point functions?

Question

in my library, i have completion backends that i add to completion-at-point-functions as per the docs, but my completion candidates are fetched from a server with an http request. the call is very slow, and i would like to make it asynchronous. ideally completion wouldn't interrupt user input at all.

from what i can see, completion-at-point-functions can't seem to handle an async response, say with aio.el and writing callbacks, for example with url-retrieve won't work with it either.

is there any way the two features can be made to play with each other nicely?

Answer 1

looks like using while-no-input is the simplest way to go. i received advice from capf guru extraordinaire minad to this effect:

(list start
      end
      ;; only search when necessary:
      (completion-table-dynamic
       (lambda (_)
         ;; Interruptible candidate computation
         (let ((result
                (while-no-input
                  (your-fetch-candidates-function start end))))
           (and (consp result) result))))
      :exclusive 'no
      :annotation-function
      (lambda (candidate)
        (concat " "
                (your-annotation-function candidate))))