Re: Updating *Completions* as you type

[Juri Linkov]

> It would be nice if there was a built-in customization which caused
> *Completions* to update as you type, as long as that buffer is visible.
> I imagine such a request has been made before, so what is the obstacle
> to adding it?

I don't remember what was the obstacle, but here is the previous patch
that implements the behavior of zsh and is based on icomplete-mode.

> I would like to figure out a solution which everyone is happy with, and
> then I would be happy to implement it.  It seems to me that it's just a
> matter of, after each keystroke, triggering (for minibuffer completion)
> minibuffer-completion-help or (for buffer completion) some new function
> to populate *Completions* with all-completions output.

There is one difference between icomplete-mode and zcomplete-mode:
the former uses completion-all-sorted-completions where the
recently used items are at the top of the completion list.
Whereas the latter uses the same alphabetical sorting as TAB
shows in the *Completions* buffer.  Maybe a new function
should allow any sorting order?

You are welcome to implement such a new function
to populate *Completions* with all-completions output.
Currently I have no idea how to do this.

> This could (I guess) be done with after-change-functions, although
> maybe others have a better idea.

icomplete-mode uses post-command-hook, so zcomplete-mode does the same.

diff --git a/lisp/zcomplete.el b/lisp/zcomplete.el
new file mode 100644
index 00000000000..75a40c0afd3
--- /dev/null
+++ b/lisp/zcomplete.el
@@ -0,0 +1,317 @@
+;;; zcomplete.el --- zsh-like minibuffer completion based on icomplete -*- 
lexical-binding: t -*-
+
+;; Copyright (C) 2022 Free Software Foundation, Inc.
+
+;; Author: Juri Linkov <juri@linkov.net>
+;; Keywords: completion
+;; Maintainer: emacs-devel@gnu.org
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Like `icomplete' but provides feedback in the *Completions* window
+;; instead of the minibuffer.
+
+;;; Code:
+
+(defgroup zcomplete nil
+  "Show completions dynamically in *Completions* window from minibuffer."
+  :prefix "zcomplete-"
+  :link '(info-link "(emacs)Zcomplete")
+  :group 'minibuffer)
+
+(defcustom zcomplete-show-matches-on-no-input nil
+  "When non-nil, show completions when first prompting for input.
+This means to show completions even when the current minibuffer contents
+is the same as was the initial input after minibuffer activation.
+This also means that if you traverse the list of completions with
+commands and just hit RET without typing any characters,
+the match under point will be chosen instead of the default."
+  :type 'boolean)
+
+(defcustom zcomplete-with-completion-tables t
+  "Specialized completion tables with which zcomplete should operate.
+If this is t, zcomplete operates on all tables.
+Otherwise this should be a list of the completion tables (e.g.,
+`internal-complete-buffer') on which zcomplete should operate."
+  :type '(choice (const :tag "All" t)
+                 (repeat function)))
+
+(defcustom zcomplete-compute-delay .15
+  "Completions-computation stall, used only with large-number completions.
+See `zcomplete-delay-completions-threshold'."
+  :type 'number)
+
+(defcustom zcomplete-delay-completions-threshold 400
+  "Pending-completions number over which to apply `zcomplete-compute-delay'."
+  :type 'integer)
+
+(defcustom zcomplete-max-delay-chars 2
+  "Maximum number of initial chars to apply `zcomplete-compute-delay'."
+  :type 'integer)
+
+(defcustom zcomplete-minibuffer-setup-hook nil
+  "Zcomplete-specific customization of minibuffer setup.
+This hook is run during minibuffer setup if Zcomplete is active."
+  :type 'hook)
+
+
+(defvar zcomplete--initial-input nil
+  "Initial input in the minibuffer when `zcomplete-mode' was activated.
+Used to implement the option `zcomplete-show-matches-on-no-input'.")
+
+(defvar zcomplete--previous-input nil
+  "Previous input in the minibuffer before editing it.
+Used to optimize `zcomplete-exhibit' to not be fired while
+moving point in the minibuffer.")
+
+(defun zcomplete-post-command-hook ()
+  (let ((non-essential t)) ;E.g. don't prompt for password!
+    (unless (memq this-command '( previous-history-element next-history-element
+                                  previous-line-or-history-element
+                                  next-line-or-history-element))
+      (zcomplete-exhibit))))
+
+(defcustom zcomplete-auto-exhibit 'visible
+  "Non-nil means to use pop up completions on minibuffer edit."
+  :type '(choice (const :tag "Don't auto pop up completions" nil)
+                 (const :tag "Pop up completions window" t)
+                 (const :tag "Update completions window only when visible"
+                        visible)))
+
+(defcustom zcomplete-arrows 'visible
+  "Non-nil means to use arrows to browse completions from the minibuffer."
+  :type '(choice (const :tag "Don't use arrows" nil)
+                 (const :tag "Use arrows" t)
+                 (const :tag "Use arrows when completions window is visible"
+                        visible)))
+
+(defun zcomplete-visible ()
+  (get-buffer-window "*Completions*" 0))
+
+(defun zcomplete-bind-arrows (binding &optional horizontal)
+  `(menu-item
+    "" ,binding
+    :filter ,(lambda (cmd)
+               (when (or (eq zcomplete-arrows t)
+                         (and (eq zcomplete-arrows 'visible)
+                              (zcomplete-visible)
+                              (or (not horizontal)
+                                  (eq completions-format 'one-column))))
+                 cmd))))
+
+(defun zcomplete-bind-visible (binding)
+  `(menu-item
+    "" ,binding
+    :filter ,(lambda (cmd)
+               (when (zcomplete-visible)
+                 cmd))))
+
+(defvar-keymap zcomplete-minibuffer-mode-map ;; zcomplete-minibuffer-map
+  :doc "Keymap used by `zcomplete-mode' in the minibuffer."
+
+  "<remap> <minibuffer-complete-and-exit>" (zcomplete-bind-visible 
#'zcomplete-ret)
+  "<remap> <minibuffer-keyboard-quit>"     (zcomplete-bind-visible 
#'zcomplete-quit)
+  "<remap> <abort-minibuffers>"            (zcomplete-bind-visible 
#'zcomplete-quit)
+
+  "<left>"    (zcomplete-bind-arrows #'zcomplete-previous-completion t)
+  "<right>"   (zcomplete-bind-arrows #'zcomplete-next-completion t)
+  "<up>"      (zcomplete-bind-arrows #'zcomplete-previous-line-completion)
+  "<down>"    (zcomplete-bind-arrows #'zcomplete-next-line-completion)
+  "<home>"    (zcomplete-bind-arrows #'zcomplete-first-completion)
+  "<end>"     (zcomplete-bind-arrows #'zcomplete-last-completion)
+  "<next>"    (zcomplete-bind-arrows #'scroll-other-window)
+  "<prior>"   (zcomplete-bind-arrows #'scroll-other-window-down))
+
+(defun zcomplete-ret (&optional no-exit no-quit)
+  "Choose the completion from the minibuffer in its completions window."
+  (interactive "P")
+  (condition-case nil
+      (minibuffer-choose-completion no-exit no-quit)
+    (error (minibuffer-complete-and-exit))))
+
+(defun zcomplete-quit ()
+  "Exit minibuffer for zcomplete."
+  (interactive)
+  (minibuffer-hide-completions))
+
+(defcustom zcomplete-auto-choose nil
+  "Non-nil means to automatically insert completions to the minibuffer.
+It affects the variable `minibuffer-completion-auto-choose'.
+This variable is usable only when `zcomplete-auto-exhibit' is nil."
+  :type 'boolean)
+
+(defun zcomplete-next-completion (&optional n)
+  "Run `minibuffer-next-completion' without auto choosing."
+  (interactive "p")
+  (let ((minibuffer-completion-auto-choose zcomplete-auto-choose))
+    (minibuffer-next-completion n)))
+
+(defun zcomplete-previous-completion (&optional n)
+  "Run `minibuffer-previous-completion' without auto choosing."
+  (interactive "p")
+  (let ((minibuffer-completion-auto-choose zcomplete-auto-choose))
+    (minibuffer-previous-completion n)))
+
+(defun zcomplete-next-line-completion (&optional n)
+  "Run `minibuffer-next-line-completion' without auto choosing."
+  (interactive "p")
+  (let ((minibuffer-completion-auto-choose zcomplete-auto-choose))
+    (minibuffer-next-line-completion n)))
+
+(defun zcomplete-previous-line-completion (&optional n)
+  "Run `minibuffer-previous-line-completion' without auto choosing."
+  (interactive "p")
+  (let ((minibuffer-completion-auto-choose zcomplete-auto-choose))
+    (minibuffer-previous-line-completion n)))
+
+(defun zcomplete-first-completion ()
+  "Run `first-completion' from the minibuffer in its completions window."
+  (interactive)
+  (with-minibuffer-completions-window
+    (when completions-highlight-face
+      (setq-local cursor-face-highlight-nonselected-window t))
+    (let ((minibuffer-completion-auto-choose zcomplete-auto-choose))
+      (first-completion))))
+
+(defun zcomplete-last-completion ()
+  "Run `last-completion' from the minibuffer in its completions window."
+  (interactive)
+  (with-minibuffer-completions-window
+    (when completions-highlight-face
+      (setq-local cursor-face-highlight-nonselected-window t))
+    (let ((minibuffer-completion-auto-choose zcomplete-auto-choose))
+      (last-completion))))
+
+
+;;;###autoload
+(define-minor-mode zcomplete-mode
+  "Toggle incremental minibuffer completion (Zcomplete mode).
+
+When this global minor mode is enabled, typing in the minibuffer
+continuously displays a list of possible completions that match
+the string you have typed.  The list of completions is displayed
+in the *Completions* window.
+
+For more information, see Info node `(emacs)Zcomplete'.
+For options you can set, `\\[customize-group] zcomplete'.
+
+You can use the following key bindings to navigate and select
+completions:
+
+\\{zcomplete-minibuffer-map}"
+  :global t :group 'zcomplete
+  (remove-hook 'minibuffer-setup-hook #'zcomplete-minibuffer-setup)
+  (when zcomplete-mode
+    (add-hook 'minibuffer-setup-hook #'zcomplete-minibuffer-setup)
+    ;; (setq-default completion-show-help nil
+    ;;               completions-header-format nil)
+    ))
+
+(define-minor-mode zcomplete-minibuffer-mode
+  "Enable arrows in the minibuffer.
+The only purpose of this mode is to activate
+`zcomplete-minibuffer-mode-map' in the minibuffer."
+  :global nil)
+
+(defun zcomplete--completion-table ()
+  (if (window-minibuffer-p) minibuffer-completion-table
+    (or (nth 2 completion-in-region--data)
+        (message "In %S (w=%S): %S"
+                 (current-buffer) (selected-window) (window-minibuffer-p)))))
+(defun zcomplete--field-string ()
+  (if (window-minibuffer-p)
+      (minibuffer-contents)
+    (buffer-substring-no-properties
+     (nth 0 completion-in-region--data)
+     (nth 1 completion-in-region--data))))
+(defun zcomplete--field-beg ()
+  (if (window-minibuffer-p) (minibuffer-prompt-end)
+    (nth 0 completion-in-region--data)))
+(defun zcomplete--field-end ()
+  (if (window-minibuffer-p) (point-max)
+    (nth 1 completion-in-region--data)))
+
+(defun zcomplete-simple-completing-p ()
+  "Non-nil if current window is a minibuffer that's doing simple completion."
+  (unless executing-kbd-macro
+    (let ((table (zcomplete--completion-table)))
+      (and table
+           (or (not (functionp table))
+               (eq zcomplete-with-completion-tables t)
+               (member table zcomplete-with-completion-tables))))))
+
+(defun zcomplete-minibuffer-setup ()
+  "Run in minibuffer on activation to establish incremental completion.
+Usually run by inclusion in `minibuffer-setup-hook'."
+  (when zcomplete-mode
+    (setq-local zcomplete--initial-input (zcomplete--field-string))
+    (setq-local zcomplete--previous-input nil)
+    (zcomplete-minibuffer-mode 1)
+    (add-hook 'post-command-hook #'zcomplete-post-command-hook nil t)
+    (zcomplete-exhibit)
+    (run-hooks 'zcomplete-minibuffer-setup-hook)))
+
+(defun zcomplete-exhibit ()
+  "Update zcomplete completions display.
+Should be run via minibuffer `post-command-hook'.
+See `zcomplete-mode' and `minibuffer-setup-hook'."
+  (when (and zcomplete-mode
+             (zcomplete-simple-completing-p)) ;Shouldn't be necessary.
+    (when (and (or zcomplete-show-matches-on-no-input
+                   (not (equal (zcomplete--field-string)
+                               zcomplete--initial-input)))
+               (not (equal (zcomplete--field-string)
+                           zcomplete--previous-input))
+               (or (eq zcomplete-auto-exhibit t)
+                   (and (eq zcomplete-auto-exhibit 'visible)
+                        (zcomplete-visible)))
+               (or
+                ;; Don't bother with delay after certain number of chars:
+                (> (- (point) (zcomplete--field-beg))
+                   zcomplete-max-delay-chars)
+                ;; Don't delay if the completions are known.
+                completion-all-sorted-completions
+                ;; Don't delay if alternatives number is small enough:
+                (and (sequencep (zcomplete--completion-table))
+                     (< (length (zcomplete--completion-table))
+                        zcomplete-delay-completions-threshold))
+                ;; Delay - give some grace time for next keystroke, before
+                ;; embarking on computing completions:
+                (sit-for zcomplete-compute-delay)))
+      (save-excursion
+        (goto-char (point-max))
+        (setq-local zcomplete--previous-input (zcomplete--field-string))
+        (minibuffer-completion-help)
+        (unless zcomplete-auto-exhibit
+          (zcomplete-first-completion))))))
+
+(let ((keymap completion-in-region-mode-map))
+  (keymap-set keymap "<left>"  #'zcomplete-previous-completion)
+  (keymap-set keymap "<right>" #'zcomplete-next-completion)
+  (keymap-set keymap "<up>"    #'zcomplete-previous-line-completion)
+  (keymap-set keymap "<down>"  #'zcomplete-next-line-completion)
+  (keymap-set keymap "<home>"  #'zcomplete-first-completion)
+  (keymap-set keymap "<end>"   #'zcomplete-last-completion)
+  (keymap-set keymap "<next>"  #'scroll-other-window)
+  (keymap-set keymap "<prior>" #'scroll-other-window-down)
+  (keymap-set keymap "RET"     #'zcomplete-ret))
+
+
+(provide 'zcomplete)
+
+;;; zcomplete.el ends here