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minad / cape

🦸cape.el - Completion At Point Extensions
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+title: cape.el - Let your completions fly!

+author: Daniel Mendler

+language: en

+export_file_name: cape.texi

+texinfo_dir_category: Emacs misc features

+texinfo_dir_title: Cape: (cape).

+texinfo_dir_desc: Completion At Point Extensions

+html: GNU Emacs

+html: GNU ELPA

+html: GNU-devel ELPA

+html: MELPA

+html: MELPA Stable

Cape provides Completion At Point Extensions which can be used in combination with [[https://github.com/minad/corfu][Corfu]], [[https://github.com/company-mode/company-mode][Company]] or the default completion UI. The completion backends used by ~completion-at-point~ are so called ~completion-at-point-functions~ (Capfs).

+html:

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, ~cape-history~ for history completion in shell or Comint modes and ~cape-file~ for completion of file names. The commands ~cape-elisp-symbol~ and ~cape-elisp-block~ are useful for documentation of Elisp packages or configurations, since they complete Elisp 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.

+toc: headlines 8

Cape is available on GNU ELPA and MELPA. You can install the package with ~package-install~. In the following we present a sample configuration based on the popular ~use-package~ macro.

I recommend to bind the =cape-*= completion commands to keys such that you can invoke them explicitly. This makes particular sense for special Capfs which you only want to trigger in rare circumstances. See the =:bind= specification below.

Furthermore the =cape-*= functions are Capfs which you can add to the =completion-at-point-functions= list. Take care when adding Capfs to the list since each of the Capfs adds a small runtime cost. Note that the Capfs which occur earlier in the list take precedence, such that the first Capf returning a result will win and the later Capfs may not get a chance to run. In order to merge Capfs you can try the function =cape-capf-super=.

One must distinguish the buffer-local and the global value of the =completion-at-point-functions= variable. The buffer-local value of the list takes precedence, but if the buffer-local list contains the symbol =t= at the end, it means that the functions specified in the global list should be executed afterwards. The special meaning of the value =t= is a feature of the =run-hooks= function, see the section [[info:elisp#Running Hooks]["Running Hooks" in the Elisp manual]] for further information.

+begin_src emacs-lisp

;; 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 prefix keymap providing all Cape commands under a mnemonic key. ;; Press C-c p ? to for help. :bind ("C-c p" . cape-prefix-map) ;; Alternative keys: M-p, M-+, ... ;; Alternatively bind Cape commands individually. ;; :bind (("C-c p d" . cape-dabbrev) ;; ("C-c p h" . cape-history) ;; ("C-c p f" . cape-file) ;; ...) :init ;; Add to the global default value of completion-at-point-functions' which is ;; used bycompletion-at-point'. The order of the functions matters, the ;; first function returning a result wins. Note that the list of buffer-local ;; completion functions takes precedence over the global list. (add-hook 'completion-at-point-functions #'cape-dabbrev) (add-hook 'completion-at-point-functions #'cape-file) (add-hook 'completion-at-point-functions #'cape-elisp-block) ;; (add-hook 'completion-at-point-functions #'cape-history) ;; ... )

+end_src

/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:

+begin_src 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)))

+end_src

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.

+begin_src emacs-lisp

(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))

+end_src

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~.

+begin_src 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)))))

+end_src

** 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~.

+begin_src emacs-lisp

;; 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))

+end_src

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

** 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.

+begin_src emacs-lisp

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

+end_src

** 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.

In the following we show a few example configurations, which have come up on the [[https://github.com/minad/cape/issues][Cape]] or [[https://github.com/minad/corfu/issues][Corfu issue tracker]] or the [[https://github.com/minad/corfu/wiki][Corfu wiki.]] I use some of these tweaks in my personal configuration.

+begin_src emacs-lisp

;; Example 1: Sanitize the `pcomplete-completions-at-point' Capf. The Capf has ;; undesired side effects on Emacs 28. 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)))

+end_src

Since this package is part of [[https://elpa.gnu.org/packages/cape.html][GNU ELPA]] contributions require a copyright assignment to the FSF.