Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> writes:

>> (add-hook 'completion-at-point-functions 'org-contacts-org-complete-function nil 'local)
>
> I assume you're careful to run this `add-hook` in the appropriate buffer
> (otherwise, of course it won't do anything useful ;-)
>

I test this `add-hook` in org-mode buffer and elisp-mode buffer.

>> When I evaluated upper code, and test by typing in "@Chri", I have not seen org-contacts related
>> complete popup candidates.
>
> Try `M-x trace-function RET org-contacts-org-complete-function RET`.
> Also you may prefer to first just try
>
>     M-: (org-contacts-org-complete-function) RET
>
> and make sure that what it returns makes sense.
> Then
>
>     M-: (all-completions "Chr" (nth 2 (org-contacts-org-complete-function))) RET
>
> Maybe try also
>
>     M-: (all-completions "@Chr" (nth 2 (org-contacts-org-complete-function))) RET
>
> then check whether the BEG..END area returned by
> `org-contacts-org-complete-function` contains "@Chr" or "Chr".

I followed your steps. Got following trace output:

#+begin_example
======================================================================
1 -> (org-contacts-org-complete-function)
1 <- org-contacts-org-complete-function: nil
======================================================================
1 -> (org-contacts-org-complete-function)
1 <- org-contacts-org-complete-function: nil
======================================================================
1 -> (org-contacts-org-complete-function)
1 <- org-contacts-org-complete-function: nil
#+end_example

I found ~org-contacts-org-complete-function~ returned a special value:

#+begin_example
#f(compiled-function (string pred action) #<bytecode -0x9e1a398d61d3acb>)
#+end_example


And this code,

#+begin_src emacs-lisp
(completion-table-dynamic
             (lambda (_)
               (mapcar
                (lambda (contact) (plist-get contact :name))
                (org-contacts--all-contacts))))
#+end_src

returned following:

#+begin_example
#[771 "....." [(lambda (_) (mapcar ... ...)) nil boundaries metadata minibuffer-selected-window window-live-p window-buffer complete-with-action] 8 "

(fn STRING PRED ACTION)"]
#+end_example

Don't know whether this is correct.

I can confirm the bellowing basic completion with ~try-completion~ code works fine:

#+begin_src emacs-lisp
(try-completion
 "tri"
 (mapcar
  (lambda (contact) (plist-get contact :name))
  (org-contacts--all-contacts)))
#+end_src

# ==================================================================================================

At last, I paste my latest simple code version:

#+begin_src emacs-lisp
(defun org-contacts-org-complete-function ()
  "Function used in `completion-at-point-functions' in `org-mode' to complete @name."
  (when-let* ((bounds (bounds-of-thing-at-point 'symbol))
              (begin (1- (car bounds)))
              (end (cdr bounds))
              (symbol (buffer-substring-no-properties begin end))
              (org-contacts-prefix-p (string-prefix-p "@" symbol))
              ;; (prefix (substring-no-properties symbol 1 nil))
              )
    (when org-contacts-prefix-p
      (list begin
            end
            (completion-table-dynamic
             (lambda (_)
               (mapcar
                (lambda (contact) (plist-get contact :name))
                (org-contacts--all-contacts))))))))
#+end_src

And test with execute following ~add-hook~ in org-mode buffer or emacs-lisp-mode buffer:

#+begin_src emacs-lisp
(add-hook 'completion-at-point-functions 'org-contacts-org-complete-function nil 'local)
#+end_src



-- 
[ stardiviner ]
       I try to make every word tell the meaning that I want to express.

       Blog: https://stardiviner.github.io/
       IRC(freenode): stardiviner, Matrix: stardiviner
       GPG: F09F650D7D674819892591401B5DF1C95AE89AC3