corfu.el - Completion Overlay Region FUnction

Introduction

Corfu enhances the default completion in region function with a completion overlay. The current candidates are shown in a popup overlay below or above the point. Corfu can be considered the minimalistic completion-in-region counterpart of the Vertico minibuffer UI.

Icomplete implements both completion-in-region and minibuffer completion in a single package. While Corfu and Vertico are technically similar to Icomplete, Corfu and Vertico are two separate packages in order to optimize the UI for the two distinct use cases, also leading to code bases which are easier to understand.

Corfu is a minimal package of less than 500 lines of code (my arbitrary limit for small components providing a single specific feature). In contrast to the featureful and complex Company package, Corfu only provides the completion UI and does not include custom completion backends. Completions are generated by calling the completion-at-point-functions (Capfs). Many code completion backends provide such a Capf, such that relying only on the default completion is often sufficient. For example the language server packages, Eglot and Lsp-mode, both implement a Capf.

Features

• Popup display with scrollbar indicator and arrow key navigation
• The popup must be summoned explicitly by TAB
• The current candidate is inserted with TAB and selected with RET
• Candidates sorting by prefix, string length and alphabetically
• Completion is automatically terminated after candidate selection
• Filter string can contain arbitrary characters and spaces (needed when filtering with the Orderless completion style)
• Deferred completion style highlighting for performance
• Jumping to location/documentation of current candidate (Company extension)
• Support for annotation-function and affixation-function

Notable non-feature: Timer-based automatic completions are not supported.

Configuration

Corfu is available from GNU ELPA, such that it can be installed directly via package-install. After installation, the local minor mode can be enabled with M-x corfu-mode. In order to configure Corfu and other packages in your init.el, you may want to use use-package. I recommend to give orderless completion a try, which is different from the familiar prefix TAB completion. Here is an example configuration:

;; Configure corfu
(use-package corfu
  ;; Optionally use TAB for cycling, default is `corfu-complete'.
  ;; :bind (:map corfu-map
  ;;        ("TAB" . corfu-next)
  ;;        ("S-TAB" . corfu-previous))

  ;; Enable the overlay only for certain modes.
  ;; For example it is not a useful UI for completions at point in the
  ;; minibuffer.
  :hook ((prog-mode . corfu-mode)
         (eshell-mode . corfu-mode))

  :config

  ;; Optionally enable cycling for `corfu-next' and `corfu-previous'.
  ;; (setq corfu-cycle t)
)

;; Use the `orderless' completion style.
;; Enable `partial-completion' for files to allow path expansion.
;; You may prefer to use `initials' instead of `partial-completion'.
(use-package orderless
  :init
  (setq completion-styles '(orderless)
        completion-category-defaults nil
        completion-category-overrides '((file (styles . (partial-completion))))))

;; A few more useful configurations...
(use-package emacs
  :init
  ;; TAB cycle if there are only few candidates
  (setq completion-cycle-threshold 3)

  ;; Enable indentation+completion using the TAB key.
  ;; Completion is often bound to M-TAB.
  (setq tab-always-indent 'complete))

Key bindings

Corfu uses a transient keymap corfu-map which is active while the popup is shown. The keymap defines the following remappings and bindings:

• beginning-of-buffer -> corfu-first
• end-of-buffer -> corfu-last
• scroll-down-command -> corfu-scroll-down
• scroll-up-command -> corfu-scroll-up
• next-line, down, M-n -> corfu-next
• previous-line, up, M-p -> corfu-previous
• completion-at-point, TAB -> corfu-complete
• RET -> corfu-insert
• M-g -> corfu-show-location
• M-h -> corfu-show-documentation
• C-g, ESC ESC ESC -> corfu-abort

Complementary packages

Corfu works well together with all packages providing code completion via the completion-at-point-functions. Furthermore it supports various completion styles, including the advanced Orderless completion style, where the filtering expressions are separated by spaces.

You may also want to look into my Vertico package. Vertico is the minibuffer counterpart of Corfu.

Caveats

This package is experimental and new. I am not yet claiming that this package works correctly. There are a few known technical caveats.

• No additional caching, the completion table is called repeatedly.
• The overlay popup is brittle (Alternatives to consider: Posframe, Postip)
• The thin popup borders are only drawn if line-spacing=nil.
• The abort handling could be improved, for example the input could be undone.
• The completion-in-region-mode-predicate is ignored in order to give the completion style full control. The predicate asks the backend if if the starting point of the completion changed.
• Completion is terminated if there are no matches. Add optional confirmation?
• Company kind icons and metadata are not supported (company-kind, company-docsig)

Contributions

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