Cape provides Completion At Point Extensions which can be used in combination with the Corfu completion UI or the default completion UI. The completion backends used by completion-at-point are so called completion-at-point-functions (Capfs). In principle, the Capfs provided by Cape can also be used by Company.

You can register the cape-* functions in the completion-at-point-functions list. This makes the backends available for completion, which is usually invoked by pressing TAB or M-TAB. The functions can also be invoked interactively to trigger the respective completion at point. You can bind them directly to a key in your user configuration. Notable commands/Capfs are cape-line for completion of a line from the current buffer and cape-file for completion of a file name. The command cape-symbol is particularly useful for documentation of Elisp packages or configurations, since it completes Elisp symbols anywhere.

Cape has the super power to transform Company backends into Capfs and merge multiple Capfs into a Super-Capf! These transformers allow you to still take advantage of Company backends even if you are not using Company as frontend.

• cape-dabbrev: Complete word from current buffers (see also dabbrev-capf on Emacs 29)
• cape-file: Complete file name
• cape-history: Complete from Eshell, Comint or minibuffer history
• cape-keyword: Complete programming language keyword
• cape-symbol: Complete Elisp symbol
• cape-abbrev: Complete abbreviation (add-global-abbrev, add-mode-abbrev)
• cape-ispell: Complete word from Ispell dictionary
• cape-dict: Complete word from dictionary file
• cape-line: Complete entire line from current buffer
• cape-tex: Complete unicode char from TeX command, e.g. \hbar.
• cape-sgml: Complete unicode char from Sgml entity, e.g., &alpha.
• cape-rfc1345: Complete unicode char using RFC 1345 mnemonics.

Cape is available on GNU ELPA and MELPA. You can install the package with package-install. In the long term some of the Capfs provided by this package could be upstreamed into Emacs itself.

;; Enable Corfu completion UI
;; See the Corfu README for more configuration tips.
(use-package corfu
  :init
  (global-corfu-mode))

;; Add extensions
(use-package cape
  ;; Bind dedicated completion commands
  ;; Alternative prefix keys: C-c p, M-p, M-+, ...
  :bind (("C-c p p" . completion-at-point) ;; capf
         ("C-c p t" . complete-tag)        ;; etags
         ("C-c p d" . cape-dabbrev)        ;; or dabbrev-completion
         ("C-c p h" . cape-history)
         ("C-c p f" . cape-file)
         ("C-c p k" . cape-keyword)
         ("C-c p s" . cape-symbol)
         ("C-c p a" . cape-abbrev)
         ("C-c p i" . cape-ispell)
         ("C-c p l" . cape-line)
         ("C-c p w" . cape-dict)
         ("C-c p \\" . cape-tex)
         ("C-c p _" . cape-tex)
         ("C-c p ^" . cape-tex)
         ("C-c p &" . cape-sgml)
         ("C-c p r" . cape-rfc1345))
  :init
  ;; Add `completion-at-point-functions', used by `completion-at-point'.
  (add-to-list 'completion-at-point-functions #'cape-dabbrev)
  (add-to-list 'completion-at-point-functions #'cape-file)
  ;;(add-to-list 'completion-at-point-functions #'cape-history)
  ;;(add-to-list 'completion-at-point-functions #'cape-keyword)
  ;;(add-to-list 'completion-at-point-functions #'cape-tex)
  ;;(add-to-list 'completion-at-point-functions #'cape-sgml)
  ;;(add-to-list 'completion-at-point-functions #'cape-rfc1345)
  ;;(add-to-list 'completion-at-point-functions #'cape-abbrev)
  ;;(add-to-list 'completion-at-point-functions #'cape-ispell)
  ;;(add-to-list 'completion-at-point-functions #'cape-dict)
  ;;(add-to-list 'completion-at-point-functions #'cape-symbol)
  ;;(add-to-list 'completion-at-point-functions #'cape-line)
)

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-ispell #'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 emojis
  '((":-D" . "😀")
    (";-)" . "😉")
    (":-/" . "😕")
    (":-(" . "🙁")
    (":-*" . "😙")))

(defun emoji-backend (action &optional arg &rest _)
  (pcase action
    ('prefix (and (memq (char-before) '(?: ?\;))
                  (cons (string (char-before)) t)))
    ('candidates (all-completions arg emojis))
    ('annotation (concat " " (cdr (assoc arg emojis))))
    ('post-completion
     (let ((str (buffer-substring (- (point) 3) (point))))
       (delete-region (- (point) 3) (point))
     (insert (cdr (assoc str emojis)))))))

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

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

It is possible to merge/group 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.

(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)))))

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

Cape supports merging multiple Capfs using the function cape-super-capf. This feature is EXPERIMENTAL and should only be used in special scenarios. Don’t use cape-super-capf if you are not 100% sure that you need it! If you decide to use the function, you are on UNSUPPORTED TERRITORY.

Note that cape-super-capf is not needed if you want to use multiple Capfs which are tried one by one, e.g., it is perfectly possible to use cape-file together with the Lsp-mode Capf or other programming mode Capfs by adding cape-file to the completion-at-point-functions list. The file completion will be available in comments and string literals. cape-super-capf is only needed if you want to combine multiple Capfs, such that the candidates from multiple sources appear together in the completion list at the same time.

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

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

;; Alternative: Define named Capf instead of using the anonymous Capf directly
(defalias 'cape-dabbrev+dict+keyword
  (cape-super-capf #'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.

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-super-capf 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)))

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-interactive-capf, cape-interactive: Create a Capf which can be called interactively.
• cape-wrap-accept-all, cape-capf-accept-all: Create a Capf which accepts every input as valid.
• cape-wrap-silent, cape-capf-silent: Wrap a chatty Capf and silence it.
• cape-wrap-purify, cape-capf-purify: Purify a broken Capf and ensure that it does not modify the buffer.
• cape-wrap-nonexclusive, cape-capf-nonexclusive: Mark Capf as non-exclusive.
• cape-wrap-noninterruptible, cape-capf-noninterruptible: Protect a Capf which does not like to be interrupted.
• cape-wrap-case-fold, cape-capf-case-fold: Create a Capf which is case insensitive.
• cape-wrap-properties, cape-capf-properties: Add completion properties to a Capf.
• cape-wrap-predicate, cape-capf-predicate: Add candidate predicate to a Capf.
• cape-wrap-prefix-length, cape-capf-prefix-length: Enforce a minimal prefix length.
• cape-wrap-inside-comment, cape-capf-inside-comment: Ensure that Capf triggers only inside comment.
• cape-wrap-inside-string, cape-capf-inside-string: Ensure that Capf triggers only inside a string literal.

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: 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 4: 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 even
;; if `corfu-preselect=first'. You can use this instead of `cape-dabbrev'.
(defun my-cape-dabbrev-accept-all ()
  (cape-wrap-accept-all #'cape-dabbrev))
(add-to-list 'completion-at-point-functions #'my-cape-dabbrev-accept-all)

;; Example 5: 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-interactive-capf #'elisp-completion-at-point))

;; Example 6: 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)))

Since this package is part of GNU ELPA contributions require a copyright assignment to the FSF.