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Company adapter ​

Wrap your Company backend in a Cape and turn it into a Capf!

Cape provides the adapter cape-company-to-capf for Company backends. The adapter transforms Company backends to Capfs which are understood by the built-in Emacs completion mechanism. The function is approximately the inverse of the company-capf backend from Company. The adapter can be used as follows:

emacs-lisp
;; Use Company backends as Capfs.
(setq-local completion-at-point-functions
  (mapcar #'cape-company-to-capf
    (list #'company-files #'company-keywords #'company-dabbrev)))

Note that the adapter does not require Company to be installed or enabled. Backends implementing the Company specification do not necessarily have to depend on Company, however in practice most backends do. The following shows a small example completion backend, which can be used with both completion-at-point (Corfu, default completion) and Company.

emacs-lisp
(defvar demo-alist
  '((":-D" . "😀")
    (";-)" . "😉")
    (":-/" . "😕")
    (":-(" . "🙁")
    (":-*" . "😙")))

(defun demo-backend (action &optional arg &rest _)
  (pcase action
    ('prefix
     (when-let* ((beg (save-excursion
                        (and (re-search-backward "[;:]" (pos-bol) t) (point)))))
       (list (buffer-substring-no-properties beg (point)) "" t)))
    ('candidates (all-completions arg demo-alist))
    ('annotation (concat " " (cdr (assoc arg demo-alist))))
    ('post-completion
     (let ((str (buffer-substring (- (point) 3) (point))))
       (delete-region (- (point) 3) (point))
       (insert (cdr (assoc str demo-alist)))))))

;; Register demo backend with `completion-at-point'
(setq completion-at-point-functions
      (list (cape-company-to-capf #'demo-backend)))

;; Register demo backend with Company.
(setq company-backends '(demo-backend))

It is possible to merge multiple Company backends and use them as a single Capf using the company--multi-backend-adapter function from Company. The adapter transforms multiple Company backends into a single Company backend, which can then be used as a Capf via cape-company-to-capf. Capfs can be merged directly with cape-capf-super.

emacs-lisp
(require 'company)
;; Use the company-dabbrev and company-elisp backends together.
(setq completion-at-point-functions
      (list
       (cape-company-to-capf
        (apply-partially #'company--multi-backend-adapter
                         '(company-dabbrev company-elisp)))))