GNU ELPA - corfu

Auto completion is disabled by default, but can be enabled by setting corfu-auto to t. Furthermore you may want to configure Corfu to quit completion eagerly, such that the completion popup stays out of your way when it appeared unexpectedly.

;; Enable auto completion and configure quitting
(setq corfu-auto t
      corfu-quit-no-match 'separator) ;; or t

I suggest to experiment with the various settings and key bindings to find a configuration which works for you. There is no one perfect configuration which fits all. Some people like auto completion, some like manual completion, some want to cycle with TAB and some with the arrow keys.

In case you like auto completion settings, where the completion popup appears immediately, better use a cheap completion style like basic, which performs prefix filtering. In this case Corfu completion should still be fast in buffers with efficient completion backends. You can try the following settings in an Elisp buffer or the Emacs scratch buffer. Note that such settings can slow down Emacs due to the high load on the Lisp runtime and garbage collector.

(setq-local corfu-auto        t
	    corfu-auto-delay  0 ;; TOO SMALL - NOT RECOMMENDED
	    corfu-auto-prefix 1 ;; TOO SMALL - NOT RECOMMENDED
	    completion-styles '(basic))

If you want to combine fast prefix filtering and Orderless filtering you can still do that by defining a custom Orderless completion style via orderless-define-completion-style. We use a custom style dispatcher, which enables efficient prefix filtering for input shorter than 4 characters. Note that such a setup is advanced. Please refer to the Orderless documentation and source code for further details.

(defun orderless-fast-dispatch (word index total)
  (and (= index 0) (= total 1) (length< word 4)
       (cons 'orderless-literal-prefix word)))

(orderless-define-completion-style orderless-fast
  (orderless-style-dispatchers '(orderless-fast-dispatch))
  (orderless-matching-styles '(orderless-literal orderless-regexp)))

(setq-local corfu-auto        t
	    corfu-auto-delay  0 ;; TOO SMALL - NOT RECOMMENDED
	    corfu-auto-prefix 1 ;; TOO SMALL - NOT RECOMMENDED
	    completion-styles '(orderless-fast basic))

Corfu can be used for completion in the minibuffer, since it relies on child frames to display the candidates. The Corfu popup can be shown even if it doesn't fully fit inside the minibuffer.

global-corfu-mode activates corfu-mode in the minibuffer if the variable global-corfu-minibuffer is non-nil. In order to avoid interference with specialised minibuffer completion UIs like Vertico or Mct, Corfu is only enabled if the minibuffer sets the variable completion-at-point-functions locally. This way minibuffers with completion can be detected, such that minibuffer commands like M-: (eval-expression) or M-! (shell-command) are enhanced with Corfu completion.

If needed, one can also enable Corfu more generally in all minibuffers, as long as no completion UI is active. In the following example we set global-corfu-minibuffer to a predicate function, which checks for Mct and Vertico. Furthermore we ensure that Corfu is not enabled if a password is read from the minibuffer.

(setq global-corfu-minibuffer
      (lambda ()
	(not (or (bound-and-true-p mct--active)
		 (bound-and-true-p vertico--input)
		 (eq (current-local-map) read-passwd-map)))))

When completing in the Eshell I recommend conservative local settings without auto completion, such that the completion behavior is similar to widely used shells like Bash, Zsh or Fish.

(add-hook 'eshell-mode-hook
	  (lambda ()
	    (setq-local corfu-auto nil)
	    (corfu-mode)))

When pressing RET while the Corfu popup is visible, the corfu-insert command will be invoked. This command does inserts the currently selected candidate, but it does not send the prompt input to Eshell or the Comint process. Therefore you often have to press RET twice which feels like an unnecessary double confirmation. Fortunately it is easy to improve this by using the command corfu-send instead.

(keymap-set corfu-map "RET" #'corfu-send)

Shell completion uses the flexible Pcomplete mechanism internally, which allows you to program the completions per shell command. If you want to know more, look into this blog post, which shows how to configure Pcomplete for git commands. Since Emacs 29, Pcomplete offers the pcomplete-from-help function which parses the --help output of a command and produces completions for command line options.

Pcomplete has a few bugs on Emacs 28 and older. We can work around the issues with the Cape library (Completion at point extensions). Cape provides wrappers which sanitize the Pcomplete function. On Emacs 29 the advices should not be necessary anymore, since most relevant bugs have been fixed. In case you discover any remaining Pcomplete issues, please report them upstream.

;; 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))

Orderless is an advanced completion style that supports multi-component search filters separated by a configurable character (space, by default). Normally, entering characters like space which lie outside the completion region boundaries (words, typically) causes Corfu to quit. This behavior is helpful with auto-completion, which may pop-up when not desired, e.g. on entering a new variable name. Just keep typing and Corfu will get out of the way.

But orderless search terms can contain arbitrary characters; they are also interpreted as regular expressions. To use orderless, set corfu-separator (a space, by default) to the primary character of your orderless component separator.

Then, when a new orderless component is desired, use M-SPC (corfu-insert-separator) to enter the first component separator in the input, and arbitrary orderless search terms and new separators can be entered thereafter.

To treat the entire input as Orderless input, you can set the customization option corfu-quit-at-boundary to nil. This disables the predicate which checks if the current completion boundary has been left. In contrast, if you always want to quit at the boundary, set corfu-quit-at-boundary to t. By default corfu-quit-at-boundary is set to separator which quits at completion boundaries as long as no separator has been inserted with corfu-insert-separator.

Finally, there exists the user option corfu-quit-no-match which is set to separator by default. With this setting Corfu stays alive as soon as you start advanced filtering with a corfu-separator even if there are no matches, for example due to a typo. As long as no separator character has been inserted with corfu-insert-separator, Corfu will still quit if there are no matches. This ensures that the Corfu popup goes away quickly if completion is not possible.

In the following we show two configurations, one which works best with auto completion and one which may work better with manual completion if you prefer to always use SPC to separate the Orderless components.

;; Auto completion example
(use-package corfu
  :custom
  (corfu-auto t)          ;; Enable auto completion
  ;; (corfu-separator ?_) ;; Set to orderless separator, if not using space
  :bind
  ;; Another key binding can be used, such as S-SPC.
  ;; (:map corfu-map ("M-SPC" . corfu-insert-separator))
  :init
  (global-corfu-mode))

;; Manual completion example
(use-package corfu
  :custom
  ;; (corfu-separator ?_) ;; Set to orderless separator, if not using space
  :bind
  ;; Configure SPC for separator insertion
  (:map corfu-map ("SPC" . corfu-insert-separator))
  :init
  (global-corfu-mode))

By default, Corfu steals both the RET and TAB keys, when the Corfu popup is open. This can feel intrusive, in particular in combination with auto completion. RET may accidentally commit an automatically selected candidate, while you actually wanted to start a new line. As an alternative we can unbind the RET key completely from corfu-map or reserve the RET key only in shell modes using a menu-item filter.

;; TAB-only configuration
(use-package corfu
  :custom
  (corfu-auto t)               ;; Enable auto completion
  (corfu-preselect 'directory) ;; Select the first candidate, except for directories

  ;; Free the RET key for less intrusive behavior.
  :bind
  (:map corfu-map
	;; Option 1: Unbind RET completely
	;;; ("RET" . nil)
	;; Option 2: Use RET only in shell modes
	("RET" . (menu-item
		  "" nil :filter
		  (lambda (&optional _)
		    (and (or (derived-mode-p 'eshell-mode) (derived-mode-p 'comint-mode))
			 #'corfu-send)))))

  :init
  (global-corfu-mode))

You may be interested in configuring Corfu in TAB-and-Go style. Pressing TAB moves to the next candidate and further input will then commit the selection. Note that further input will not expand snippets or templates, which may not be desired but which leads overall to a more predictable behavior. In order to force snippet expansion, confirm a candidate explicitly with RET.

(use-package corfu
  ;; TAB-and-Go customizations
  :custom
  (corfu-cycle t)           ;; Enable cycling for `corfu-next/previous'
  (corfu-preselect 'prompt) ;; Always preselect the prompt

  ;; Use TAB for cycling, default is `corfu-complete'.
  :bind
  (:map corfu-map
	("TAB" . corfu-next)
	([tab] . corfu-next)
	("S-TAB" . corfu-previous)
	([backtab] . corfu-previous))

  :init
  (global-corfu-mode))

If you leave the default configuration of the completion styles, such that the basic completion style is still present, then pressing M-TAB (corfu-expand) will expand the current input to the common prefix of all completion candidates. In contrast, TAB (corfu-complete) behaves differently and expands input to the currently selected candidate.

If you use the orderless completion style, then expansion works differently by default. Orderless only expands to single matching candidates, since due to its multi-component input, there does not necessarily exist an expansion to a common candidate prefix. However it is possible to define a separate tab completion style. The tab completion style will only take over TAB completion (if prefix expansion is possible), but besides that won't affect Orderless candidate filtering.

(add-to-list 'completion-styles-alist
	     '(tab completion-basic-try-completion ignore
	       "Completion style which provides TAB completion only."))
(setq completion-styles '(tab orderless basic)))

Sometimes it is useful to transfer the Corfu completion session to the minibuffer, since the minibuffer offers richer interaction features. In particular, Embark is available in the minibuffer, such that you can act on the candidates or export/collect the candidates to a separate buffer. We could add Corfu support to Embark in the future, such that export or collect is possible directly from Corfu. Nevertheless, the ability to transfer the Corfu completion to the minibuffer is even more powerful, since further completion is possible.

The command corfu-move-to-minibuffer is defined here in terms of consult-completion-in-region, which uses the minibuffer completion UI via completing-read.

(defun corfu-move-to-minibuffer ()
  (interactive)
  (pcase completion-in-region--data
    (`(,beg ,end ,table ,pred ,extras)
     (let ((completion-extra-properties extras)
	   completion-cycle-threshold completion-cycling)
       (consult-completion-in-region beg end table pred)))))
(keymap-set corfu-map "M-m" #'corfu-move-to-minibuffer)
(add-to-list 'corfu-continue-commands #'corfu-move-to-minibuffer)