About zcomplete

[Ergus]

Hi recently I have been trying to move back to default completion. As
I had many issues with fido and icomplete.

The default completion system received a nice improvement recently with
the completion-autoselect... and I am wondering a simple mode like
this may be added to vanilla (or at least it gives an idea to a better
lisper to implement something better, this is just a proof of concept.)

```
(require 'simple)
(require 'minibuffer)

(defvar-keymap zcomplete-map
  :doc "Keymap used in *Completions* on zcomplete"
  :parent completion-list-mode-map
  "z" nil
  "n" nil
  "p" nil
  "q" nil
  "g" nil
  "h" nil
  "DEL" nil
  )

(defun zcomplete--try-on-minibuffer ()
  "Try to execute the binding on minibuffer."
  (switch-to-minibuffer)
  (if-let ((command (lookup-key (current-active-maps)
                                (this-single-command-keys))))
      (progn
        (minibuffer-hide-completions)
        (call-interactively command)
        t)
    ;; back to completions
    (switch-to-completions)
    nil))

(defun zcomplete--completions-pre-hook ()
  "Try on minibuffer when the command is not in *Completions* map."
  (when (eq this-command 'undefined)
    (zcomplete--try-on-minibuffer)))

(defun zcomplete--hack (data context signal)
  "Alternative to command-error-default-function.
This will try to execute on minibuffer, else emits the error"
  (unless (and (string= (buffer-name) "*Completions*")
               (zcomplete--try-on-minibuffer))
    (command-error-default-function data context signal)))

(defun zcomplete--completions-setup-hook ()
  "To call on completions setup."
  (add-hook 'pre-command-hook #'zcomplete--completions-pre-hook nil t)

  (setq-local command-error-function #'zcomplete--hack)
  (setq-local mode-line-format nil)
  (use-local-map zcomplete-map))

(define-minor-mode zcomplete-mode
  "Completion highlight mode to enable candidates highlight in the minibuffer."
  :global t
  :group 'minibuffer

  (if zcomplete-mode
      (progn
        (setq completion-auto-select t)
        ;; (overlay-put zcomplete-overlay 'face 'zcomplete)
        (add-hook 'completion-setup-hook #'zcomplete--completions-setup-hook t))

    (remove-hook 'completion-setup-hook #'zcomplete--completions-setup-hook)))

(provide 'zcomplete)
```

It lacks some features (like highlight the current candidate or
automatically update completion buffers when visible) to be really a
zsh-like feature, but at least we don't need to press C-g every time we
want to edit the minibuffer.

With this a tab shows the completions and goes there, and any attempt to
edit or not defined command tries to execute on the minibuffer... 

Parts of this could be implemented with advises, but I know we try to
avoid those on vanilla code.

WDYT?
Best,
Ergus