3.1 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:

;; 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.

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

(defun demo-backend (action &optional arg &rest _)
  (pcase action
    ('prefix (and (memq (char-before) '(?: ?\;))
                  (cons (string (char-before)) 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.

(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)))))

3.2 Super-Capf - Merging multiple Capfs

Throw multiple Capfs under the Cape and get a Super-Capf!

Cape supports merging multiple Capfs using the function cape-capf-super. Due to some technical details, not all Capfs can be merged successfully. Merge Capfs one by one and make sure that you get the desired outcome.

Note that cape-capf-super is not needed if multiple Capfs should betried one after the other, for example you can use cape-file together with programming mode Capfs by adding cape-file to the completion-at-point-functions list. File completion will then be available in comments and string literals, but not in normal code. cape-capf-super is only necessary if you want to combine multiple Capfs, such that the candidates from multiple sources appear together in the completion list at the same time.

Capf merging requires completion functions which are sufficiently well-behaved and completion functions which do not define completion boundaries. cape-capf-super has the same restrictions as completion-table-merge and completion-table-in-turn. As a simple rule of thumb, cape-capf-super works for static completion functions like cape-dabbrev, cape-keyword, cape-dict, etc., but not for multi-step completions like cape-file.

;; Merge the dabbrev, dict and keyword capfs, display candidates together.
(setq-local completion-at-point-functions
            (list (cape-capf-super #'cape-dabbrev #'cape-dict #'cape-keyword)))

;; Alternative: Define named Capf instead of using the anonymous Capf directly
(defun cape-dabbrev-dict-keyword ()
  (cape-wrap-super #'cape-dabbrev #'cape-dict #'cape-keyword))
(setq-local completion-at-point-functions (list #'cape-dabbrev-dict-keyword))

See also the aforementioned company--multi-backend-adapter from Company, which allows you to merge multiple Company backends.

3.3 Capf-Buster - Cache busting

The Capf-Buster ensures that you always get a fresh set of candidates!

If a Capf caches the candidates for too long we can use a cache busting Capf-transformer. For example the Capf merging function cape-capf-super creates a Capf, which caches the candidates for the whole lifetime of the Capf. Therefore you may want to combine a merged Capf with a cache buster under some circumstances. It is noteworthy that the company-capf backend from Company refreshes the completion table frequently. With the cape-capf-buster we can achieve a similarly refreshing strategy.

(setq-local completion-at-point-functions
            (list (cape-capf-buster #'some-caching-capf)))

3.4 Capf transformers

Cape provides a set of additional Capf transformation functions, which are mostly meant to used by experts to fine tune the Capf behavior and Capf interaction. These can either be used as advices (‘cape-wrap-*)’ or to create a new Capf from an existing Capf (‘cape-capf-*’). You can bind the Capfs created by the Capf transformers with ‘defalias’ to a function symbol.

• cape-capf-accept-all, cape-wrap-accept-all: Create a Capf which accepts every input as valid.
• cape-capf-case-fold, cape-wrap-case-fold: Create a Capf which is case insensitive.
• cape-capf-debug, cape-wrap-debug: Create a Capf which prints debugging messages.
• cape-capf-inside-code, cape-wrap-inside-code: Ensure that Capf triggers only inside code.
• cape-capf-inside-comment, cape-wrap-inside-comment: Ensure that Capf triggers only inside comments.
• cape-capf-inside-faces, cape-wrap-inside-faces: Ensure that Capf triggers only inside text with certain faces.
• cape-capf-inside-string, cape-wrap-inside-string: Ensure that Capf triggers only inside a string literal.
• cape-capf-interactive, cape-interactive: Create a Capf which can be called interactively.
• cape-capf-nonexclusive, cape-wrap-nonexclusive: Mark Capf as non-exclusive.
• cape-capf-noninterruptible, cape-wrap-noninterruptible: Protect a Capf which does not like to be interrupted.
• cape-capf-passthrough, cape-wrap-passthrough: Defeat entire completion style filtering.
• cape-capf-predicate, cape-wrap-predicate: Add candidate predicate to a Capf.
• cape-capf-prefix-length, cape-wrap-prefix-length: Enforce a minimal prefix length.
• cape-capf-properties, cape-wrap-properties: Add completion properties to a Capf.
• cape-capf-purify, cape-wrap-purify: Purify a broken Capf and ensure that it does not modify the buffer.
• cape-capf-silent, cape-wrap-silent: Silence Capf messages and errors.
• cape-capf-super, cape-wrap-super: Merge multiple Capfs into a Super-Capf.

In the following we show a few example configurations, which have come up on the Cape or Corfu issue tracker or the Corfu wiki. I use some of these tweaks in my personal configuration.

;; Example 1: Sanitize the `pcomplete-completions-at-point' Capf.  The Capf has
;; undesired side effects on Emacs 28 and earlier.  These advices are not needed
;; on Emacs 29 and newer.
(when (< emacs-major-version 29)
  (advice-add 'pcomplete-completions-at-point :around #'cape-wrap-silent)
  (advice-add 'pcomplete-completions-at-point :around #'cape-wrap-purify))

;; Example 2: Configure a Capf with a specific auto completion prefix length
(setq-local completion-at-point-functions
            (list (cape-capf-prefix-length #'cape-dabbrev 2)))

;; Example 3: Create a Capf with debugging messages
(setq-local completion-at-point-functions (list (cape-capf-debug #'cape-dict)))

;; Example 4: Named Capf
(defalias 'cape-dabbrev-min-2 (cape-capf-prefix-length #'cape-dabbrev 2))
(setq-local completion-at-point-functions (list #'cape-dabbrev-min-2))

;; Example 5: Define a defensive Dabbrev Capf, which accepts all inputs.  If you
;; use Corfu and `corfu-auto=t', the first candidate won't be auto selected if
;; `corfu-preselect=valid', such that it cannot be accidentally committed when
;; pressing RET.
(defun my-cape-dabbrev-accept-all ()
  (cape-wrap-accept-all #'cape-dabbrev))
(add-hook 'completion-at-point-functions #'my-cape-dabbrev-accept-all)

;; Example 6: Define interactive Capf which can be bound to a key.  Here we wrap
;; the `elisp-completion-at-point' such that we can complete Elisp code
;; explicitly in arbitrary buffers.
(keymap-global-set "C-c p e" (cape-capf-interactive #'elisp-completion-at-point))

;; Example 7: Ignore :keywords in Elisp completion.
(defun ignore-elisp-keywords (sym)
  (not (keywordp sym)))
(setq-local completion-at-point-functions
            (list (cape-capf-predicate #'elisp-completion-at-point
                                       #'ignore-elisp-keywords)))