#+title: corfu.el - Completion Overlay Region FUnction
#+author: Daniel Mendler
#+language: en
#+export_file_name: corfu.texi
#+texinfo_dir_category: Emacs
#+texinfo_dir_title: Corfu: (corfu).
#+texinfo_dir_desc: Completion Overlay Region FUnction

* Introduction

Corfu provides a completion overlay for the default completion in region
function. 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 Vertico.

Icomplete provides 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. No custom backends are provided. The completions are provided by the
~completion-at-point-functions~. However many code completion backends provide
~completion-at-point-functions~, such that relying only on the default
completion may often be sufficient.

[[https://github.com/minad/corfu/blob/main/screenshot.png?raw=true]]

* 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 [[https://github.com/oantolin/orderless][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 [[http://elpa.gnu.org/packages/corfu.html][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:

#+begin_src emacs-lisp
  ;; 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))
#+end_src

* 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 [[https://github.com/oantolin/orderless][Orderless]] completion style, where the filtering
expressions are separated by spaces.

You may also want to look into my [[https://github.com/minad/vertico][Vertico]] package, which provides a vertical
minibuffer completion system. 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 caveats.

- No caching, the completion table is called directly.
- The =:exit-function= handling is probably insufficient.
- The overlay popup is brittle (Alternatives: Posframe, Postip?)
- The thin popup borders are only drawn if =line-spacing=nil=.
- The abort handling could be improved. Undo the completion input?
- The ~completion-in-region-mode-predicate~ is deliberatly ignored in order to
  give the completion style full control. This predicate checks if the start
  position of the completion changed by calling the backend.
- Completion is terminated if there are no matches. Add optional confirmation?
- Company kind icons are not supported (~company-kind~)
- Company metadata is not shown (~company-docsig~)

* Contributions

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