Cape

By Kristoffer Balintona | Published | 1461 words, 7 minutes
Guides Emacs
Table of Contents

What is cape?

I will be going over the basic usages of Cape, namely how to add completion functions to completion-at-point-functions and how to use cape's built-in completion utilities (e.g. cape-company-to-capf and cape-capf-buster) to create backends with desired behavior. (Also see u/JDRiverRun’s informative comment about the advantages of using completion-at-point-functions over company.)

Cape is to corfu as company-backends are to company:

Cape provides a bunch of Completion At Point Extensions which can be used in combination with my 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.

Consequently, cape is only used if you utilize the built-in completion-at-point, which is best complemented by corfu text-completion 1.

You can also see the list of built-in completion-at-point-functions in the README. Several of these completion-at-point-functions are quite niche but others, such as cape-file and cape-symbol have common use cases.

Note: I use straight.el for package management and general.el to set my keybindings. Both of these packages have integration with use-package which come in the form of the :straight and :general keywords, respectively.

Basic usage: keybinds

The most basic way to use cape is to bind its built-in completion-at-point-functions to their own keys. For instance:

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(use-package cape
  :general (:prefix "M-c"              ; Choose a particular completion function
                    "p" 'completion-at-point
                    "t" 'complete-tag   ; etags
                    "d" 'cape-dabbrev   ; basically `dabbrev-completion'
                    "f" 'cape-file
                    "k" 'cape-keyword
                    "s" 'cape-symbol
                    "a" 'cape-abbrev
                    "i" 'cape-ispell
                    "l" 'cape-line
                    "w" 'cape-dict
                    "\\" 'cape-tex
                    "_" 'cape-tex
                    "^" 'cape-tex
                    "&" 'cape-sgml
                    "r" 'cape-rfc1345))

Additionally, if having completion-at-point-functions readily available through keybinds is desirable, then one can use cape-interactive-capf to turn an already existing completion-at-point-function into a command (i.e. interactive function) that can be bound.

Adding backends to completion-at-point-functions

However, cape is powerful because these functions can be added to completion-at-point-functions, meaning you can configure when each functions is used and where. The simplest way to accomplish this is by adding backends to completion-at-point-functions in a hook. Here is a simple example:

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(defun kb/cape-capf-setup-git-commit ()
  (let ((result))
    (dolist (element '(cape-symbol cape-dabbrev) result)
      (add-to-list 'completion-at-point-functions element))))

I then add this to the appropriate hook:

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:hook (git-commit-mode . kb/cape-capf-setup-git-commit)

Consequently, when making commits to git, via magit, for instance, completion-at-point-functions looks like this2:

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'(cape-symbol
  cape-dabbrev
  tags-completion-at-point-function)

There are a few additional things to keep in mind when adding backends to completion-function-at-point:

  1. add-to-list prepends elements to a list, that is, place an element at the front of a list3.
  2. add-to-list is almost always preferable to push because push adds an element to a list even if it already in the list, whereas add-to-list will not.
  3. Elements earlier in dolist will be added to the list before later elements. This means that elements which should be deeper within completion-at-point-functions should be placed first. (Notice how cape-dabbrev is added after cape-symbol.)

Cape-company-to-capf and cape-super-capf

I think the killer feature of cape is cape-company-to-capf. This function is able to convert any company backend and convert it into a completion-at-point-function which corfu can use4. For this reason, I regard cape as quite an underrated package since it achieves almost full feature parity with company. Here is an example with company-yasnippet:

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(defun kb/cape-capf-setup-lsp ()
  "Replace the default `lsp-completion-at-point' with its
`cape-capf-buster' version. Also add `cape-file' and
`company-yasnippet' backends."
  (setf (elt (cl-member 'lsp-completion-at-point completion-at-point-functions) 0)
        (cape-capf-buster #'lsp-completion-at-point))
  (add-to-list 'completion-at-point-functions (cape-company-to-capf #'company-yasnippet))
  (add-to-list 'completion-at-point-functions #'cape-dabbrev t))

Another useful function is cape-super-capf. This function combines multiple completion-at-point-functions into a single function. Effectively, this means candidates from multiple backends can appear jointly. For instance, one can combine cape-ispell and cape-dabbrev:

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(defun kb/cape-capf-setup-org ()
  (require 'org-roam)
  (if (org-roam-file-p)
      (org-roam--register-completion-functions-h)
    (let (result)
      (dolist (element (list
                        (cape-super-capf #'cape-ispell #'cape-dabbrev)
                        (cape-company-to-capf #'company-yasnippet))
                       result)
        (add-to-list 'completion-at-point-functions element)))
    ))

For other cape transformers, see the appropriate section of the README.

pcomplete nicety

Finally, I have the following advice to make usage with pcomplete, what eshell uses for completion:

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:config
;; For pcomplete. For now these two advices are strongly recommended to
;; achieve a sane Eshell experience. See
;; https://github.com/minad/corfu#completing-with-corfu-in-the-shell-or-eshell

;; Silence the pcomplete capf, no errors or messages!
(advice-add 'pcomplete-completions-at-point :around #'cape-wrap-silent)
;; Ensure that pcomplete does not write to the buffer and behaves as a pure
;; `completion-at-point-function'.
(advice-add 'pcomplete-completions-at-point :around #'cape-wrap-purify)

My completion-at-point-functions

I have very hesitantly included my WIP code which leverages cape and completion-at-point utilities. I have not described in detail each of the following functions. I have, however, attempted to provide useful docstrings and comments.

I hesitate to publish this code because it was very haphazardly written and highly dependent on my configuration. The reason for this is the order in which the completion-at-point-functions are added: any peculiarities in another’s configuration may lead to undesirable results. As a result, do not directly copy-and-paste this code and expect proper functionality. Rather, I put it here as a reference for what can be done.

Here they are
Warning! This code may produce undesirable effects! Copy at your own risk.
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(use-package cape
  :hook ((emacs-lisp-mode .  kb/cape-capf-setup-elisp)
         (lsp-completion-mode . kb/cape-capf-setup-lsp)
         (org-mode . kb/cape-capf-setup-org)
         (eshell-mode . kb/cape-capf-setup-eshell)
         (git-commit-mode . kb/cape-capf-setup-git-commit)
         (LaTeX-mode . kb/cape-capf-setup-latex)
         (sh-mode . kb/cape-capf-setup-sh)
         )
  :general (:prefix "M-c"               ; Particular completion function
                    "p" 'completion-at-point
                    "t" 'complete-tag           ; etags
                    "d" 'cape-dabbrev           ; or dabbrev-completion
                    "f" 'cape-file
                    "k" 'cape-keyword
                    "s" 'cape-symbol
                    "a" 'cape-abbrev
                    "i" 'cape-ispell
                    "l" 'cape-line
                    "w" 'cape-dict
                    "\\"' cape-tex
                    "_" 'cape-tex
                    "^" 'cape-tex
                    "&" 'cape-sgml
                    "r" 'cape-rfc1345
                    )
  :custom
  (cape-dabbrev-min-length 3)
  :init
  ;; Elisp
  (defun kb/cape-capf-ignore-keywords-elisp (cand)
    "Ignore keywords with forms that begin with \":\" (e.g.
:history)."
    (or (not (keywordp cand))
        (eq (char-after (car completion-in-region--data)) ?:)))
  (defun kb/cape-capf-setup-elisp ()
    "Replace the default `elisp-completion-at-point'
completion-at-point-function. Doing it this way will prevent
disrupting the addition of other capfs (e.g. merely setting the
variable entirely, or adding to list).

Additionally, add `cape-file' as early as possible to the list."
    (setf (elt (cl-member 'elisp-completion-at-point completion-at-point-functions) 0)
          #'elisp-completion-at-point)
    (add-to-list 'completion-at-point-functions #'cape-symbol)
    ;; I prefer this being early/first in the list
    (add-to-list 'completion-at-point-functions #'cape-file)
    (require 'company-yasnippet)
    (add-to-list 'completion-at-point-functions (cape-company-to-capf #'company-yasnippet)))

  ;; LSP
  (defun kb/cape-capf-setup-lsp ()
    "Replace the default `lsp-completion-at-point' with its
`cape-capf-buster' version. Also add `cape-file' and
`company-yasnippet' backends."
    (setf (elt (cl-member 'lsp-completion-at-point completion-at-point-functions) 0)
          (cape-capf-buster #'lsp-completion-at-point))
    ;; TODO 2022-02-28: Maybe use `cape-wrap-predicate' to have candidates
    ;; listed when I want?
    (add-to-list 'completion-at-point-functions (cape-company-to-capf #'company-yasnippet))
    (add-to-list 'completion-at-point-functions #'cape-dabbrev t))

  ;; Org
  (defun kb/cape-capf-setup-org ()
    (require 'org-roam)
    (if (org-roam-file-p)
        (org-roam--register-completion-functions-h)
      (let (result)
        (dolist (element (list
                          (cape-super-capf #'cape-ispell #'cape-dabbrev)
                          (cape-company-to-capf #'company-yasnippet))
                         result)
          (add-to-list 'completion-at-point-functions element)))
      ))

  ;; Eshell
  (defun kb/cape-capf-setup-eshell ()
    (let ((result))
      (dolist (element '(pcomplete-completions-at-point cape-file) result)
        (add-to-list 'completion-at-point-functions element))
      ))

  ;; Git-commit
  (defun kb/cape-capf-setup-git-commit ()
    (general-define-key
     :keymaps 'local
     :states 'insert
     "<tab>" 'completion-at-point)      ; Keybinding for `completion-at-point'
    (let ((result))
      (dolist (element '(cape-dabbrev cape-symbol) result)
        (add-to-list 'completion-at-point-functions element))))

  ;; LaTeX
  (defun kb/cape-capf-setup-latex ()
    (require 'company-auctex)
    (let ((result))
      (dolist (element (list
                        ;; First add `company-yasnippet'
                        (cape-company-to-capf #'company-yasnippet)
                        ;; Then add `cape-tex'
                        #'cape-tex
                        ;; Then add `company-auctex' in the order it adds its
                        ;; backends.
                        (cape-company-to-capf #'company-auctex-bibs)
                        (cape-company-to-capf #'company-auctex-labels)
                        (cape-company-to-capf
                         (apply-partially #'company--multi-backend-adapter
                                          '(company-auctex-macros company-auctex-symbols company-auctex-environments))))
                       result)
        (add-to-list 'completion-at-point-functions element))))


  ;; Sh
  (defun kb/cape-capf-setup-sh ()
    (require 'company-shell)
    (add-to-list 'completion-at-point-functions (cape-company-to-capf #'company-shell)))
  :config
  ;; For pcomplete. For now these two advices are strongly recommended to
  ;; achieve a sane Eshell experience. See
  ;; https://github.com/minad/corfu#completing-with-corfu-in-the-shell-or-eshell

  ;; Silence the pcomplete capf, no errors or messages!
  (advice-add 'pcomplete-completions-at-point :around #'cape-wrap-silent)
  ;; Ensure that pcomplete does not write to the buffer and behaves as a pure
  ;; `completion-at-point-function'.
  (advice-add 'pcomplete-completions-at-point :around #'cape-wrap-purify))

Changelog

  1. See Corfu, Kind-icon, and Corfu-doc to see the basic usage of corfu and several accessory packages. ↩︎

  2. tags-completion-at-point-function is a default completion-at-point-function ↩︎

  3. See its docstring to learn how to append to a list ↩︎

  4. This feature is currently listed as experimental but, for the most part, the results are as expected. If anything, rare edge cases are the only points of missing functionality. ↩︎

Text Completion and Minibuffer UI series

  1. Cape this post!
  2. Corfu, Kind-icon, and Corfu-doc
  3. Vertico, Marginalia, All-the-icons-completion, and Orderless