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Minibuffer Confines Transcended (mct.el)

Enhancement of the default Emacs minibuffer completion UI

This manual, written by Protesilaos Stavrou, describes the customization options for mct (or mct.el and variants), and provides every other piece of information pertinent to it.

The documentation furnished herein corresponds to stable version 1.0.0, released on 2023-09-24. Any reference to a newer feature which does not yet form part of the latest tagged commit, is explicitly marked as such.

Current development target is 1.1.0-dev.

If you are viewing the README.org version of this file, please note that the GNU ELPA machinery automatically generates an Info manual out of it.

1. COPYING

Copyright (C) 2021 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License.”

(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU manual.”

2. Overview of MCT

Minibuffer and Completions in Tandem, also known as “MCT”, “Mct”, mct, or mct.el, is a package that enhances the default minibuffer and *Completions* buffer of Emacs 28 (or higher) so that they work together as part of a unified framework. The idea is to make the presentation and overall functionality be consistent with other popular, vertically aligned completion UIs while leveraging built-in functionality.

The main feature set that unifies the minibuffer and the *Completions* buffer consists of commands that cycle between the two, making it seem like they are part of a contiguous space (Basic usage).

MCT tries to find a middle ground between the frugal defaults and the more opinionated completion UIs. This is most evident in its approach on how to present completion candidates. Instead of showing them outright or only displaying them on demand, MCT implements a minimum input threshold as well as a slight delay before it pops up the *Completions* buffer and starts updating it to respond to user input.

3. Customizations

MCT is highly configurable to adapt to the varying needs of users. This section documents each user option.

3.1. Live completion

Brief: Control auto-display and live-update of the *Completions* buffer.

Symbol: mct-live-completion (choice type)

Possible values:

  1. nil
  2. t (default)
  3. visible

This user option governs the overall behaviour of MCT with regard to how it uses the Completions’ buffer:

  • When nil, the user has to manually request completions, using the regular activating commands (Usage). The Completions’ buffer is never updated live to match user input. Updating has to be handled manually. This is like the out-of-the-box minibuffer completion experience.
  • When set to visible, the Completions’ buffer is live updated only if it is visible (present in a window). The actual display of the completions is still handled manually. For this reason, the visible style does not read the mct-minimum-input, meaning that it will always try to live update the visible completions, regardless of input length (Minimum input threshold).
  • When non-nil (the default), the Completions’ buffer is automatically displayed once the mct-minimum-input is met and is hidden if the input drops below that threshold. While visible, the buffer is updated live to match the user’s input.

Note that every command or completion category that is declared in the mct-completion-passlist ignores this option altogether. This means that every such symbol will always show the Completions’ buffer automatically and will always update its contents live to match any further user input (Passlist for commands or completion categories). Same principle for the mct-completion-blocklist, which will always disable both the automatic display and live updating of the Completions’ buffer (Blocklist for commands or completion categories).

Size boundaries of the Completions.

3.2. Minimum input threshold

Brief: Try to live update completions when input is >= N.

Symbol: mct-minimum-input (natnum type)

By default, MCT expects the user to type 3 characters before it tries to compute completion candidates, display the *Completions* buffer and keep it updated live to match any subsequent input.

Setting this user option to a value greater than 1 can help reduce the total number of candidates that are being computed. That is because the Completions can consist of thousands of items that all need to be rendered at once in a buffer.

In terms of the user experience, the minimum input threshold can make sessions feel less visually demanding when the user (i) knows what they are looking for and (ii) types fast enough so that the *Completions* never have the time to pop up.

This variable is ignored for commands or completion categories that are specified in the mct-completion-passlist and mct-completion-blocklist.

Live updates per command or completion category.

3.3. Delay between live updates

Brief: Delay in seconds before updating the Completions’ buffer.

Symbol: mct-live-update-delay (number type)

The delay in seconds between live updates of the Completions’ buffer. The default value is 0.3.

This variable is ignored for commands or completion categories that are specified in the mct-completion-passlist and mct-completion-blocklist.

Live updates per command or completion category.

3.4. Live updates per command or completion category

By default, MCT has the same behaviour across all types of completion. Specifically, it respects the mct-live-completion option on whether and when to perform live completion, the mct-minimum-input threshold before doing so, and the mct-live-update-delay between changes to the *Completions* buffer.

Live completion.

Minimum input threshold.

Delay between live updates.

A passlist and a blocklist can override those options for the commands or categories specified.

3.4.1. Passlist for commands or completion categories

Brief: List of symbols where live completions are always enabled.

Symbol: mct-completion-passlist (repeat symbol type)

The value of this user option is a list of symbols. Those can refer to commands like find-file or completion categories such as file, buffer, or what other packages define like Consult’s consult-location category.

Any entry in the passlist ignores the value of mct-live-completion and the mct-minimum-input. It also bypasses any possible delay introduced by mct-live-update-delay. In other words, it immediately displays the *Completions* buffer and instantly updates it to match user input.

When the mct-completion-blocklist and the mct-completion-passlist are in conflict, the former takes precedence.

Known completion categories.

3.4.2. Blocklist for commands or completion categories

Brief: List of symbols where live completions are always disabled.

Symbol: mct-completion-blocklist (repeat symbol type)

The value of this user option is a list of symbols. Those can refer to commands like find-file or completion categories such as file, buffer, or what other packages define like Consult’s consult-location category.

This means that they ignore mct-live-completion. They do not automatically display the Completions’ buffer, nor do they update it to match user input.

The Completions’ buffer can still be accessed with commands that place it in a window (such as mct-list-completions-toggle, mct-switch-to-completions-top).

When the mct-completion-blocklist and the mct-completion-passlist are in conflict, the former takes precedence.

Perhaps a less drastic measure is to set mct-minimum-input to an appropriate value. Or better use mct-completion-passlist.

Known completion categories.

3.4.3. Known completion categories

Below are the known completion categories that can be added to the mct-completion-passlist and mct-completion-blocklist (and relevant custom code). This resource is non-exhaustive and will be updated to match available information.

  • bookmark
  • buffer
  • charset
  • coding-system
  • color
  • command (e.g. M-x)
  • customize-group
  • environment-variable
  • expression
  • face
  • file
  • function (the describe-function command bound to C-h f)
  • info-menu
  • imenu
  • input-method
  • kill-ring
  • library
  • minor-mode
  • multi-category
  • package
  • project-file
  • symbol (the describe-symbol command bound to C-h o)
  • theme
  • unicode-name (the insert-char command bound to C-x 8 RET)
  • variable (the describe-variable command bound to C-h v)

From the consult package:

  • consult-grep
  • consult-isearch
  • consult-isearch
  • consult-kmacro
  • consult-location

From the embark package:

  • embark-keybinding

In general, it is best not to add symbols which include several thousands of candidates to the passlist. So no command, function, symbol, unicode-name, variable.

When in doubt, do not add a symbol to either the pass- or block- list.

Find completion category.

3.4.4. Find completion category

While using a command that provides a minibuffer prompt, type M-: (the eval-expression command) and evaluate (mct--completion-category). It will return the completion category, if any. Note that this only works when the variable enable-recursive-minibuffers is non-nil.

To review echo area messages, use C-h e (view-echo-area-messages).

Known completion categories.

3.5. Size boundaries of the Completions

Brief: Set the maximum and minimum height of the Completions’ buffer.

Symbol: mct-completion-window-size (choice type between nil and cons cell)

The value is a cons cell in the form of (max-height . min-height) where each value is either a natural number or a function which returns such a number.

The default maximum height of the window is calculated by the function mct--frame-height-fraction, which finds the closest round number to 1/3 of the frame’s height. While the default minimum height is 1. This means that during live completions the Completions’ window will shrink or grow to show candidates within the specified boundaries. To disable this bouncing effect, set both max-height and min-height to the same number.

If nil, do not try to fit the Completions’ buffer to its window.

Live completion.

3.6. Dynamic completion tables in mct-mode

Brief: Whether to keep dynamic completion live.

Symbol: mct-persist-dynamic-completion (boolean type)

Possible values:

  1. nil
  2. t (default)

Without any intervention from MCT, the default Emacs behavior for commands such as find-file or for a file completion category is to hide the *Completions* buffer after updating the list of candidates in a non-exiting fashion (e.g. select a directory and expect to continue typing the path). This, however, runs contrary to the interaction model of MCT when it performs live completions, because the user expects the Completions’ buffer to remain visible while typing out the path to the file (Live completion).

When this user option is non-nil (the default) it makes all non-exiting commands keep the *Completions* visible when updating the list of candidates.

This applies to prompts in the file completion category whenever the user selects a candidate with mct-choose-completion-no-exit, mct-edit-completion, minibuffer-complete, minibuffer-force-complete (i.e. any command that does not exit the minibuffer).

Selecting candidates with mct-mode.

The two exceptions are (i) when the current completion session runs a command or category that is blocked by the mct-completion-blocklist or (ii) the user option mct-live-completion is nil.

Blocklist for commands or completion categories.

The underlying rationale:

Most completion commands present a flat list of candidates to choose from. Picking a candidate concludes the session. Some prompts, however, can recalculate the list of completions based on the selected candidate. A case in point is find-file (or any command with the file completion category) which dynamically adjusts the completions to show only the elements which extend the given file system path. We call such cases “dynamic completion”. Due to their particular nature, these need to be handled explicitly. The present user option is provided primarily to raise awareness about this state of affairs.

3.7. Hide the Completions mode line

Brief: Do not show a mode line in the Completions’ buffer.

Symbol: mct-hide-completion-mode-line (boolean type)

By default, the *Completions* buffer has its own mode line, just like every other window. Set this user option to non-nil to remove the mode line.

3.8. Remove shadowed file paths

Brief: Delete shadowed parts of file names from the minibuffer.

Symbol: mct-remove-shadowed-file-names (boolean type)

When the built-in file-name-shadow-mode is enabled and this user option is non-nil, MCT will delete the part of the file path that is shadowed (meaning that it is overriden) by the given input.

For example, if the user types ~/ after a long path name, everything preceding the ~/ is removed so the interactive selection process starts again from the user’s $HOME.

3.9. MCT in the minibuffer and completion in regular buffers

Emacs draws a distinction between two types of completion sessions:

  • Completion where the minibuffer is involved (such as to switch buffers or find a file).
  • Completion in a regular buffer to expand the text before point. The minibuffer is not active. We call this “in-buffer completion” or allude to the underlying function: completion-in-region.

The former scenario is what MCT has supported since its inception. Enable mct-mode to get started. There was a time where MCT also supported in-buffer completion but this was discontinued in version 1.0.0 of the package as Emacs 29 gained the requisite capabilities. To get the familiar MCT key bindings for in-buffer completion, use these in your init file:

;; Define the small wrapper functions
(defun my-mct-next-line-or-completion (n)
  "Select next completion or move to next line N times.
Select the next completion if `completion-in-region-mode' is
active and the Completions window is on display."
  (interactive "p")
  (if (and completion-in-region-mode (mct--get-completion-window))
      (minibuffer-next-completion n)
    (next-line n)))

(defun my-mct-previous-line-or-completion (n)
  "Select previous completion or move to previous line N times.
Select the previous completion if `completion-in-region-mode' is
active and the Completions window is on display."
  (interactive "p")
  (if (and completion-in-region-mode (mct--get-completion-window))
      (minibuffer-previous-completion n)
    (previous-line n)))

(defun my-mct-return-or-choose-completion (n)
  "Choose current completion or create N newlines.
Choose the current completion if `completion-in-region-mode' is
active and the Completions window is on display."
  (interactive "p")
  (if (and completion-in-region-mode (mct--get-completion-window))
      (minibuffer-choose-completion)
    (newline n :interactive)))

;; Get the key bindings
(let ((map completion-in-region-mode-map))
  (define-key map (kbd "C-n") #'my-mct-next-line-or-completion)
  (define-key map (kbd "C-p") #'my-mct-previous-line-or-completion)
  (define-key map (kbd "RET") #'my-mct-return-or-choose-completion))

;; Tweak the appearance
(setq completions-format 'one-column)
(setq completion-show-help nil)
(setq completion-auto-help t)

;; Optionally, tweak the appearance further
(setq completions-detailed t)
(setq completion-show-inline-help nil)
(setq completions-max-height 6)
(setq completions-highlight-face 'completions-highlight)

Note that the in-buffer completions will produce a new buffer window below the current one. Some users find this intrusive. In such a case, the use of a popup box is better. Consider the corfu package by Daniel Mendler, which uses such a popup (Alternatives).

4. Usage

This section outlines the various patterns of interaction that MCT establishes.

4.1. Cyclic behaviour for mct-mode

When mct-mode is enabled, some new keymaps are activated which add commands for cycling between the minibuffer and the completions. Suppose the following standard layout:

-----------------
|        |      |
| Buffers| Buf  |
|        |      |
-----------------
|        |      |
| Buf    | Buf  |
|        |      |
-----------------
-----------------
|               |
|  Completions  |
|               |
-----------------
-----------------
|  Minibuffer   |
-----------------

When inside the minibuffer, pressing C-n (or down arrow) takes you to the top of the completions, while C-p (or up arrow) moves to the bottom. The commands are mct-switch-to-completions-top for the former and mct-switch-to-completions-bottom for the latter. If the *Completions* are not shown, then the buffer pops up automatically and point moves to the given position.

Similarly, while inside the *Completions* buffer, C-p (or up arrow) at the top of the buffer switches to the minibuffer, while C-n (or down arrow) at the bottom of the buffer also goes to the minibuffer. If point is anywhere else inside the buffer, those key bindings perform a regular line motion (if the *Completions* are set to a grid view, then the left and right arrow keys perform the corresponding lateral motions). The commands are mct-previous-completion-or-mini and mct-next-completion-or-mini. Both accept an optional numeric argument. If the Nth line lies outside the boundaries of the completions’ buffer, they move the point to the minibuffer.

The display of the *Completions* can be toggled at any time from inside the minibuffer with C-l (mnemonic is “[l]ist completions” and the command is mct-list-completions-toggle).

4.2. Selecting candidates with mct-mode

There are several ways to select a completion candidate with mct-mode.

  1. Suppose that you are typing mod with the intent to select the modus-themes.el buffer. To complete the candidate follow up mod with the TAB key (minibuffer-complete). If the match is unique, the text will be expanded. Otherwise the *Completions* buffer will appear. This does not exit the minibuffer, meaning that it does not confirm your choice. To confirm your choice, use RET. If you ever make a mistake and expand the wrong candidate, just use undo. Lastly note that if the candidates meet the completion-cycle-threshold hitting TAB again will switch between them.

  2. While cycling through the completions’ buffer, type RET to select and confirm the current candidate (mct-choose-completion-exit). This works for all types of completion prompts.

  3. Similar to the above, but without exiting the minibuffer (i.e. to confirm your choice) is mct-choose-completion-no-exit which is bound to TAB in the completions’ buffer. This is particularly useful for certain contexts where selecting a candidate does not necessarily mean that the process has to be finalised (e.g. when using find-file). In those cases, the event triggered by TAB is followed by the renewal of the list of completions, where relevant (e.g. TAB over a directory in find-file, which then shows the contents of that directory).

    The command can correctly expand completion candidates even when the active style in completion-styles is partial-completion. In other words, if the minibuffer contains input like ~/G/P/m and the point is in the completions’ buffer over Git/Projects/mct/ the minibuffer’ contents will become ~/Git/Projects/mct/ and then show the contents of that directory.

  4. Type M-e (mct-edit-completion) in the completions’ buffer to place the current candidate in the minibuffer, without exiting the session. This allows you to edit the text before confirming it. If point is in the minibuffer before performing this action, the current candidate is either the one at the top of the completions’ buffer or that which is under the last known point in said buffer (the last known position is reset when the window is deleted). Internally, mct-edit-completion uses mct-choose-completion-no-exit to expand the completion candidate, so it retains its behaviour (as explained right above).

    Sometimes there is a need to switch to the minibuffer without selecting the candidate at point, such as to retype some part of the input. In those cases, type e in the completions’ buffer to move to the minibuffer. The command is called mct-focus-minibuffer, which can also be assigned to the global keymap, though MCT leaves such a decision up to the user (same for mct-focus-mini-or-completions).

  5. In prompts that allow the selection of multiple candidates (internally via the completing-read-multiple function) using M-RET (mct-choose-completion-dwim) in the *Completions* will append the candidate at point to the list of selections and keep the completions available so that another item may be selected. Any of the aforementioned applicable methods can confirm the final selection. If, say, you want to pick a total of three candidates, do M-RET for the first two and RET (mct-choose-completion-exit) for the last one. In contexts that are not CRM-powered, the M-RET has the same effect as TAB (mct-choose-completion-no-exit).

    Indicator for completing-read-multiple.

  6. When point is at the minibuffer, select the current candidate in the completions buffer with C-RET (mct-complete-and-exit), which has the same effect as first completing with TAB and then immediately exit the minibuffer with the completed candidate as the selected one.

  7. Emacs 28 has the ability to group candidates inside the completions’ buffer under headings. For example, the Consult package makes use of those (Third-party extensions). MCT provides motions that jump between such headings, placing the point at the first candidate right below the heading’s text. Use M-n (mct-next-completion-group) and M-p (mct-previous-completion-group) to move to the next or previous one, respectively (also work with they keys for forward-paragraph and backward-paragraph). Both commands accept an optional numeric argument. For the sake of avoiding surprises, these commands do not cycle between the completions and the minibuffer: they stop at the first or last heading.

5. Installation

5.1. Install the package

mct is available on the official GNU ELPA archive for users of Emacs version 27 or higher. One can install the package without any further configuration. The following commands shall suffice:

M-x package-refresh-contents
M-x package-install RET mct

A package is also available via Guix:

guix package -i emacs-mct

5.2. Manual installation method

Assuming your Emacs files are found in ~/.emacs.d/, execute the following commands in a shell prompt:

cd ~/.emacs.d

# Create a directory for manually-installed packages
mkdir manual-packages

# Go to the new directory
cd manual-packages

# Clone this repo and name it "mct"
git clone https://git.sr.ht/~protesilaos/mct mct

Finally, in your init.el (or equivalent) evaluate this:

;; Make Elisp files in that directory available to the user.
(add-to-list 'load-path "~/.emacs.d/manual-packages/mct")

Everything is in place to set up the package.

6. Sample setup

Minimal setup for the minibuffer and in-buffer completion:

(require 'mct)
(mct-mode 1)

And with more options:

(require 'mct)

(setq mct-completion-window-size (cons #'mct-frame-height-third 1))
(setq mct-remove-shadowed-file-names t) ; works when `file-name-shadow-mode' is enabled
(setq mct-hide-completion-mode-line t)
(setq mct-minimum-input 3)
(setq mct-live-completion t)
(setq mct-live-update-delay 0.6)
(setq mct-persist-dynamic-completion t)

;; This is for commands or completion categories that should always pop
;; up the completions' buffer.  It circumvents the default method of
;; waiting for some user input (see `mct-minimum-input') before
;; displaying and updating the completions' buffer.
(setq mct-completion-passlist
      '(;; Some commands
        Info-goto-node
        Info-index
        Info-menu
        vc-retrieve-tag
        ;; Some completion categories
        imenu
        file
        buffer
        kill-ring
        consult-location))

;; The blocklist follows the same principle as the passlist, except it
;; disables live completions altogether.
(setq mct-completion-blocklist nil)

(mct-mode 1)

Other useful extras from the Emacs source code (read their doc strings):

(setq completion-styles
      '(basic substring initials flex partial-completion))
(setq completion-category-overrides
      '((file (styles . (basic partial-completion initials substring)))))

(setq completion-cycle-threshold 2)
(setq completion-ignore-case t)
(setq completion-show-inline-help nil)

(setq completions-detailed t)

(setq enable-recursive-minibuffers t)
(setq minibuffer-eldef-shorten-default t)

(setq read-buffer-completion-ignore-case t)
(setq read-file-name-completion-ignore-case t)

(setq resize-mini-windows t)
(setq minibuffer-eldef-shorten-default t)

(file-name-shadow-mode 1)
(minibuffer-depth-indicate-mode 1)
(minibuffer-electric-default-mode 1)

;; Do not allow the cursor in the minibuffer prompt
(setq minibuffer-prompt-properties
      '(read-only t cursor-intangible t face minibuffer-prompt))

(add-hook 'minibuffer-setup-hook #'cursor-intangible-mode)

;;; Minibuffer history
(require 'savehist)
(setq savehist-file (locate-user-emacs-file "savehist"))
(setq history-length 500)
(setq history-delete-duplicates t)
(setq savehist-save-minibuffer-history t)
(add-hook 'after-init-hook #'savehist-mode)

;;; Third-party extensions

;;;; Enable Consult previews in the Completions buffer.
;; Requires the `consult' package.
(add-hook 'completion-list-mode-hook #'consult-preview-at-point-mode)

;;;; Setup for Orderless
;; Requires the `orderless' package

;; We make the SPC key insert a literal space and the same for the
;; question mark.  Spaces are used to delimit orderless groups, while
;; the quedtion mark is a valid regexp character.
(let ((map minibuffer-local-completion-map))
  (define-key map (kbd "SPC") nil)
  (define-key map (kbd "?") nil))

;; Because SPC works for Orderless and is trivial to activate, I like to
;; put `orderless' at the end of my `completion-styles'.  Like this:
(setq completion-styles
      '(basic substring initials flex partial-completion orderless))
(setq completion-category-overrides
      '((file (styles . (basic partial-completion orderless)))))

7. Keymaps

MCT defines its own keymaps, which extend those that are active in the minibuffer and the *Completions* buffer, respectively:

  • mct-completion-list-mode-map
  • mct-minibuffer-local-completion-map

You can invoke describe-keymap to learn more about them.

If you want to edit any key bindings, do it in these keymaps, not in those they extend and override (the names of the original ones are the same as above, minus the mct- prefix).

8. User-level tweaks or custom code

In this section we cover custom code that builds on what MCT offers.

8.1. Sort completion candidates on Emacs 29

Starting with Emacs 29 (current development target), the user option completions-sort controls the sorting method of candidates in the *Completions* buffer. Beside the default of using string-lessp, it accepts a custom function. Consider any of the following examples:

;; Some sorting functions...
(defun my-sort-by-alpha-length (elems)
  "Sort ELEMS first alphabetically, then by length."
  (sort elems (lambda (c1 c2)
                (or (string-version-lessp c1 c2)
                    (< (length c1) (length c2))))))

(defun my-sort-by-history (elems)
  "Sort ELEMS by minibuffer history.
Use `mct-sort-sort-by-alpha-length' if no history is available."
  (if-let ((hist (and (not (eq minibuffer-history-variable t))
                      (symbol-value minibuffer-history-variable))))
      (minibuffer--sort-by-position hist elems)
    (my-sort-by-alpha-length elems)))

(defun my-sort-multi-category (elems)
  "Sort ELEMS per completion category."
  (pcase (mct--completion-category)
    ('nil elems) ; no sorting
    ('kill-ring elems)
    ('project-file (my-sort-by-alpha-length elems))
    (_ (my-sort-by-history elems))))

;; Specify the sorting function.
(setq completions-sort #'my-sort-multi-category)

Known completion categories.

8.2. Indicator for completing-read-multiple

Previous versions of MCT would prepend a [CRM] tag to the minibuffer prompt of commands powered by completing-read-multiple. While this is a nice usability enhancement, it is not specific to MCT and thus should not be part of mct.el. Use this in your init file instead:

;; Add prompt indicator to `completing-read-multiple'.  We display
;; [`completing-read-multiple': <separator>], e.g.,
;; [`completing-read-multiple': ,] if the separator is a comma.  This
;; is adapted from the README of the `vertico' package by Daniel
;; Mendler.  I made some small tweaks to propertize the segments of
;; the prompt.
(defun crm-indicator (args)
  (cons (format "[`crm-separator': %s]  %s"
                (propertize
                 (replace-regexp-in-string
                  "\\`\\[.*?]\\*\\|\\[.*?]\\*\\'" ""
                  crm-separator)
                 'face 'error)
                (car args))
        (cdr args)))

(advice-add #'completing-read-multiple :filter-args #'crm-indicator)

8.3. Ido-style navigation through directories

Older versions of MCT had a command for file navigation that would delete the whole directory component before point, effectively going back up one directory. While the functionality can be useful, it is not integral to the MCT experience and thus should not belong in mct.el. Add this to your own configuration file instead:

;; Adaptation of `icomplete-fido-backward-updir'.
(defun my-backward-updir ()
  "Delete char before point or go up a directory."
  (interactive nil mct-mode)
  (cond
   ((and (eq (char-before) ?/)
         (eq (mct--completion-category) 'file))
    (when (string-equal (minibuffer-contents) "~/")
      (delete-minibuffer-contents)
      (insert (expand-file-name "~/"))
      (goto-char (line-end-position)))
    (save-excursion
      (goto-char (1- (point)))
      (when (search-backward "/" (minibuffer-prompt-end) t)
        (delete-region (1+ (point)) (point-max)))))
   (t (call-interactively 'backward-delete-char))))

(define-key minibuffer-local-filename-completion-map (kbd "DEL") #'my-backward-updir)

9. Third-party extensions

MCT only tweaks the default minibuffer. To get more out of it, consider these exceptionally well-crafted extras:

Consult by Daniel Mendler
Adds several commands that make interacting with the minibuffer more powerful. There also are multiple packages that build on it, such as consult-dir by Karthik Chikmagalur and consult-notmuch by José Antonio Ortega Ruiz.
Embark by Omar Antolín Camarena
Provides configurable contextual actions for completions and many other constructs inside buffers. A genius package!
Marginalia by Daniel and Omar
Displays informative annotations for all known types of completion candidates.
Orderless by Omar
A completion style that matches a variety of patterns (regexp, flex, initialism, etc.) regardless of the order they appear in.
all-the-icons-completion by Itai Y. Efrat
Glue code that adds icons from the all-the-icons package to the *Completions* buffer. It can make things prettier and/or more informative, while it can also be combined with Marginalia.

MCT does not support the use-case of completion-in-region. This is the kind of completion session that is done inside the buffer and does not involve the minibuffer. However, you may prefer:

Corfu by Daniel Mendler
An interface for the completion-in-region which uses a child frame (basically a pop-up) at the position of the cursor to display candidates. As with all of Daniel’s packages, Corfu aims for a clean implementation that does the right thing by being consistent with core Emacs mechanisms.
Cape also by Daniel
Additional completion-at-point-functions (CAPFs) that extend those of core Emacs. These backends can be used by packages that visualise completion-in-region.

9.1. Enable Consult previews

One of the nice features of the Consult package is the ability to preview the candidate at point. All we need to enable it in the *Completions* buffer is the following snippet:

(add-hook 'completion-list-mode-hook #'consult-preview-at-point-mode)

10. Alternatives

In the grand scheme of things, it may be helpful to think of MCT as proof-of-concept on how the default Emacs completion can become more expressive. MCT’s value rests in its potential to inspire developers to (i) patch Emacs so that its out-of-the-box completion is more interactive, and (ii) expose the shortcomings in the current implementation of the *Completions* buffer, which should again provide an impetus for further changes to Emacs. Otherwise, MCT is meant for users who can tolerate the status quo and simply want a thin layer of interactivity for minibuffer completion, in-buffer completion, and their intersection with the Completions’ buffer.

Like MCT, these alternatives provide a thin layer of functionality over the built-in infrastructure. Unlike MCT, they are not constrained by the design of the *Completions* buffer and concomitant functionality. They all make for a natural complement to the standard Emacs experience (also Extensions).

Vertico by Daniel Mendler

this is a more mature and feature-rich package with a large user base and a highly competent maintainer.

Vertico has some performance optimizations on how candidates are sorted and presented, which means that it displays results right away without any noticeable performance penalty. Whereas MCT does not change the underlying behaviour of how candidates are displayed. As such, MCT will be slower in scenaria where there are lots of candidates because core Emacs lacks those optimizations. One such case is with the describe-symbol (C-h o) prompt. If the user asks for the completions’ buffer without inputting any character (so without narrowing the list), there will be a noticeable delay before the buffer is rendered. This is mitigated in MCT by the requirement for mct-minimum-input, though the underlying mechanics remain intact.

In terms of the interaction model, the main difference between Vertico and MCT is that the former uses the minibuffer by default and shows the completions there. The minibuffer is expanded to show the candidates in a vertical list. Whereas MCT keeps the *Completions* buffer and the minibuffer as separate entities, the way standard Emacs does it.

The presence of a fully fledged buffer means that the user can invoke all relevant commands at their disposal, such as to write the buffer to a file for future review, use Isearch to move around, copy a string or rectangle to a register, and so on.

Vertico has official extensions which can make it work exactly like MCT without any of MCT’s drawbacks. These extensions can also expand Vertico’s powers such as by providing granular control over the exact style of presentation for any given completion category (e.g. display Imenu in a separate buffer, show the switch-to-buffer list horizontally in the minibuffer, and present find-file in a vertical list—whatever the user wants).

All things considered, there is no compelling reason why one may prefer MCT over Vertico in terms of the available functionality: Vertico is better.

Icomplete and fido-mode (built-in, multiple authors)

Icomplete is closer in spirit to Vertico, as it too uses the minibuffer to display completion candidates. By default, it presents the list horizontally, though there exists icomplete-vertical-mode (and fido-vertical-mode).

For our purposes, Icomplete and Fido are the same in terms of the paradigm they follow. The latter is a re-spin of the former, as it adjusts certain variables and binds some commands for the convenience of the end-user. fido-mode and its accoutrements are defined in icomplete.el.

What MCT borrows from Icomplete is for the input delay (explained elsewhere in this document). Internally, I also learnt how to extend local keymaps by studying icomplete.el.

I had used Icomplete for several months before moving to what now has become mct.el. I think it is excellent at providing a thin layer over the built-in infrastructure.

11. Acknowledgements

MCT is meant to be a collective effort. Every bit of help matters.

Author/maintainer
Protesilaos Stavrou.
Contributions to code or documentation
Daniel Mendler, James Norman Vladimir Cash, José Antonio Ortega Ruiz, Juri Linkov, Philip Kaludercic, Tomasz Hołubowicz.
Ideas and user feedback
Andrew Tropin, Benjamin (@zealotrush), Case Duckworth, Chris Burroughs, Jonathan Irving, José Antonio Ortega Ruiz, Kostadin Ninev, Manuel Uberti, Morgan Willcock, Philip Kaludercic, Theodor Thornhill, Tomasz Hołubowicz, Z.Du. As well as users: danrobi11.
Packaging
Andrew Tropin and Nicolas Goaziou (Guix).
Inspiration for certain features
icomplete.el (built-in—multiple authors), Daniel Mendler (vertico), Omar Antolín Camarena (embark, live-completions).

13. GNU Free Documentation License


                GNU Free Documentation License
                 Version 1.3, 3 November 2008


 Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     
 Everyone is permitted to copy and distribute verbatim copies
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0. PREAMBLE

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You may not copy, modify, sublicense, or distribute the Document
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10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the
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ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
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license notices just after the title page:

    Copyright (c)  YEAR  YOUR NAME.
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If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
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If you have Invariant Sections without Cover Texts, or some other
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