* which-key 
[[http://melpa.org/#/which-key][http://melpa.org/packages/which-key-badge.svg]]
** Introduction
=which-key= is a minor mode for Emacs that displays the keybindings following your currently
entered incomplete command (a prefix) in a popup. For example, after enabling the minor mode
if you enter =C-x= and wait for the default of 1 second the minibuffer will expand with all of
the available keybindings that follow =C-x= (or as many as space allows given your settings).
This includes prefixes like =C-x 8= which are shown in a different face. Screenshots of what
the popup will look like are included below. =which-key= started as a rewrite of
[[https://github.com/kai2nenobu/guide-key][guide-key-mode]], but the feature sets have diverged
to a certain extent. 

With respect to =guide-key=, the intention is to provide the
following features:
1. A different polling mechanism to make it lighter on resources than guide-key
2. An improved display of keys with more keys being shown by default and a nicer
   presentation
3. Customization options that allow for the rewriting of command names on the
   fly through easily modifiable alists
4. Good default configurations that work well with most themes
5. A well configured back-end for displaying keys (removing the popwin
   dependency) that can be easily customized by writing new display functions

Many of these have been implemented and are described below.

** Table of Contents                                                 :TOC@4:
 - [[#which-key-][which-key ]]
     - [[#introduction][Introduction]]
     - [[#install][Install]]
         - [[#melpa][MELPA]]
         - [[#manually][Manually]]
     - [[#initial-setup][Initial Setup]]
         - [[#minibuffer-option][Minibuffer Option]]
         - [[#side-window-right-option][Side Window Right Option]]
         - [[#side-window-bottom-option][Side Window Bottom Option]]
         - [[#side-window-right-then-bottom][Side Window Right then Bottom]]
     - [[#special-features-and-configuration-options][Special Features and Configuration Options]]
         - [[#several-popup-types][Several Popup Types]]
             - [[#minibuffer][minibuffer]]
             - [[#side-window][side window]]
             - [[#frame][frame]]
             - [[#custom][custom]]
         - [[#custom-string-replacement][Custom String Replacement]]
             - [[#key-based-replacement]["Key-Based" replacement]]
             - [[#key-and-description-replacement][Key and Description replacement]]
         - [[#sorting][Sorting]]
         - [[#paging][Paging]]
         - [[#other-options][Other Options]]
     - [[#more-examples][More Examples]]
         - [[#nice-display-with-split-frame][Nice Display with Split Frame]]
     - [[#status][Status]]
     - [[#thanks][Thanks]]

** Install
*** MELPA
After setting up [[http://melpa.org][MELPA]] as a repository, use =M-x package-install which-key= or
your preferred method. You will need to call =which-key-mode= to enable the
minor mode of course.

*** Manually
Add which-key.el to your =load-path= and require. Something like 

#+BEGIN_SRC emacs-lisp
(add-to-list 'load-path "path/to/which-key.el")
(require 'which-key)
(which-key-mode)
#+END_SRC

** Initial Setup
No further setup is required if you are happy with the default setup. To try
other options, there are 3 choices of default configs that are preconfigured
(then customize to your liking). The main choice is where you want the which-key
buffer to display. Screenshots of the default options are shown in the next
sections.

In each case, we show as many key bindings as we can fit in the buffer within
the constraints. The constraints are determined by several factors, including
your emacs settings, the size of the current emacs frame, and the which-key
settings, most of which are described below. 

By default which-key makes substitutions for text all with the aim of saving
space. The most noticeable are the "special keys" like SPC, TAB, RET, etc. This
can be turned off (see [[#other-options][Other Options]]), but the default is to
truncate these keys to one character and display them using =:inverse-video=
(flips foreground and background colors). You can see the effect in the
screenshots.

There are other substitution abilities included, which are quite flexible
(ability to use regexp for example). This makes which-key very customizable.
This functionality is targeting [[https://github.com/syl20bnr/spacemacs][spacemacs]].

*** Minibuffer Option
Take over the minibuffer. For the recommended configuration use 

#+BEGIN_SRC emacs-lisp
(which-key-setup-minibuffer)
#+END_SRC

[[./img/which-key-minibuffer.png]]

Note the maximum height of the minibuffer is controlled through the built-in
variable =max-mini-window-height=.

*** Side Window Right Option
Popup side window on right. For defaults use

#+BEGIN_SRC emacs-lisp
(which-key-setup-side-window-right)
#+END_SRC

Note the defaults are fairly conservative and will tend to not display on
narrower frames. If you get a message saying which-key can't display the keys,
try making your frame wider or adjusting the defaults related to the maximum
width (see =M-x customize-group which-key=).

[[./img/which-key-right.png]]

*** Side Window Bottom Option
Popup side window on bottom. This is the current default. To restore this setup use

#+BEGIN_SRC emacs-lisp
(which-key-setup-side-window-bottom)
#+END_SRC

[[./img/which-key-bottom.png]]

*** Side Window Right then Bottom
This is a combination of the previous two choices. It will try to use the right
side, but if there is no room it will switch to using the bottom, which is
usually easier to fit keys into. This setting can be helpful if the size of 
the Emacs frame changes frequently, which might be the caes if you are using
a dynamic/tiling window manager.

#+BEGIN_SRC emacs-lisp
(which-key-setup-side-window-right-bottom)
#+END_SRC

** Special Features and Configuration Options
There are more options than the ones described here. All of the configurable
variables are available through =M-x customize-group which-key=.
*** Several Popup Types
There are three different popup types that which-key can use by default to
display the available keys. The variable =which-key-popup-type= decides which
one is used.
**** minibuffer
#+BEGIN_SRC emacs-lisp
(setq which-key-popup-type 'minibuffer)
#+END_SRC
Show keys in the minibuffer.
**** side window
#+BEGIN_SRC emacs-lisp
(setq which-key-popup-type 'side-window)
#+END_SRC
Show keys in a side window. This popup type has further options:
#+BEGIN_SRC emacs-lisp
;; location of which-key window. valid values: top, bottom, left, right, 
;; or a list of any of the two. If it's a list, which-key will always try
;; the first location first. It will go to the second location if there is
;; not enough room to display any keys in the first location
(setq which-key-side-window-location 'bottom)

;; max width of which-key window, when displayed at left or right.
;; valid values: number of columns (integer), or percentage out of current
;; frame's width (float larger than 0 and smaller than 1)
(setq which-key-side-window-max-width 0.33)

;; max height of which-key window, when displayed at top or bottom.
;; valid values: number of lines (integer), or percentage out of current
;; frame's height (float larger than 0 and smaller than 1)
(setq which-key-side-window-max-height 0.25)
#+END_SRC
**** frame

#+BEGIN_SRC emacs-lisp
(setq which-key-popup-type 'frame)
#+END_SRC
Show keys in a popup frame. This popup won't work very well in a terminal,
where only one frame can be shown at any given moment. This popup type has
further options:
#+BEGIN_SRC emacs-lisp
;; max width of which-key frame: number of columns (an integer)
(setq which-key-frame-max-width 60)

;; max height of which-key frame: number of lines (an integer)
(setq which-key-frame-max-height 20)
#+END_SRC

**** custom
Write your own display functions! This requires you to write three functions,
=which-key-custom-popup-max-dimensions-function=,
=which-key-custom-show-popup-function=, and
=which-key-custom-hide-popup-function=. Refer to the documentation for those
variables for more information, but here is a working example (this is the
current implementation of side-window bottom).


#+BEGIN_SRC emacs-lisp
(setq which-key-popup-type 'custom)
(defun which-key-custom-popup-max-dimensions-function (ignore)
  (cons
   (which-key-height-or-percentage-to-height which-key-side-window-max-height)
   (frame-width)))
(defun fit-horizonatally ()
  (let ((fit-window-to-buffer-horizontally t))
    (fit-window-to-buffer)))
(defun which-key-custom-show-popup-function (act-popup-dim)
  (let* ((alist '((window-width . fit-horizontally)
                  (window-height . fit-window-to-buffer))))
    (if (get-buffer-window which-key--buffer)
        (display-buffer-reuse-window which-key--buffer alist)
      (display-buffer-in-major-side-window which-key--buffer 'bottom 0 alist))))
(defun which-key-custom-hide-popup-function ()
  (when (buffer-live-p which-key--buffer)
    (quit-windows-on which-key--buffer)))
#+END_SRC

*** Custom String Replacement
You can customize the way the keys show in the buffer using three different
replacement methods, each of which corresponds replacement alist. The basic idea
of behind each alist is that you specify a selection string in the =car= of each
cons cell and the replacement string in the =cdr=.

**** "Key-Based" replacement
The relevant variable is the awkwardly named
=which-key-key-based-description-replacement-alist=. In this alist you can have
cons cells of two types. An example of the first type is

#+BEGIN_SRC emacs-lisp
("C-x C-f" . "find files")
#+END_SRC

where the string on the left is the key combination whose description you want
to replace. For that key combination, which-key overwrites the description with
the second string, "find files". In the second type of entry you can restrict
the replacements to a major-mode. For example, 

#+BEGIN_SRC emacs-lisp
(org-mode . (("C-c C-c" . "Org C-c C-c") ("C-c C-a" . "Org Attach"))
#+END_SRC

Here the first entry is the major-mode and the second is a list of the first
type of entries. In case the same key combination is listed under a major-mode
and by itself, the major-mode version will take precedence.

There are two helper functions to add entries to this list,
=which-key-add-key-based-replacements= and
=which-key-add-major-mode-key-based-replacements=. You can modify the alist
directly or use these.

**** Key and Description replacement

The second and third methods target the text used for the keys and the
descriptions directly. The relevant variables are
=which-key-key-replacement-alist= and =which-key-description-replacement-alist=.
Here's an example of one of the default key replacements

#+BEGIN_SRC emacs-lisp
("<\\([[:alnum:]-]+\\)>" . "\\1")
#+END_SRC

The =car= takes a string which may use emacs regexp and the =cdr= takes a string
with the replacement text. As shown, you can specify a sub-expression of the
match. The replacements do not need to use regexp and can be as simple as

#+BEGIN_SRC emacs-lisp
("left" . "lft")
#+END_SRC

You can add this element to the key list with (there are no helper functions for
these alists)

#+BEGIN_SRC emacs-lisp
(add-to-list 'which-key-key-replacement-alist '("left" . "lft"))
#+END_SRC

*** Sorting
By default the output is sorted by the key in a custom order. The default order
is to sort lexicographically within each "class" of key, where the classes and
their order are

=Special (SPC, TAB, ...) < Single Character (a, ...) < Modifier (C-, M-, ...) < Other=

You can control the order by setting this variable. 

#+BEGIN_SRC emacs-lisp
(setq which-key-sort-order 'which-key-key-order)
;; or (setq which-key-sort-order 'which-key-description-order)
#+END_SRC

The only other built-in option at the moment (besides using nil to turn off
sorting completely) is =which-key-description-order=, which orders by the key's
description based on the usual ordering of strings after applying =downcase=.

*** Paging
This is a new feature and may have bugs, so it is disabled by default. There are
at least several prefixes that have many keys bound to them, like =C-x=.
which-key displays as many keys as it can given your settings, but for these
prefixes this may not be enough. The paging feature gives you the ability to
bind a key to the function =which-key-show-next-page= which will cycle through
the pages without changing the key sequence you were in the middle of typing.
Essentially, all you need to do to enable this for a prefix like =C-x= is the
following which will bind =<f5>= to the command.

#+BEGIN_SRC emacs-lisp
(define-key which-key-mode-map (kbd "C-x <f5>") 'which-key-show-next-page)
#+END_SRC

This is completely equivalent to 

#+BEGIN_SRC emacs-lisp
(setq which-key-paging-prefixes '("C-x"))
(setq which-key-paging-key "<f5>")
#+END_SRC

where the latter are provided for convenience if you have a lot of prefixes.

*** Other Options
The options below are also available through customize. Their defaults are
shown.

#+BEGIN_SRC emacs-lisp
  ;; Set the time delay (in seconds) for the which-key popup to appear.
  (setq which-key-idle-delay 1.0) 

  ;; Set the maximum length (in characters) for key descriptions (commands or
  ;; prefixes). Descriptions that are longer are truncated and have ".." added
  (setq which-key-max-description-length 27)

  ;; Set the separator used between keys and descriptions. Change this setting to
  ;; an ASCII character if your font does not show the default arrow. The second
  ;; setting here allows for extra padding for unicode characters. which-key uses
  ;; characters as a means of width measurement, so wide unicode characters can
  ;; throw off the calculation.
  (setq which-key-separator " → " )
  (setq which-key-unicode-correction 3)

  ;; Set the special keys. These are automatically truncated to one character and
  ;; have which-key-special-key-face applied. Set this variable to nil to disable
  ;; the feature
  (setq which-key-special-keys '("SPC" "TAB" "RET" "ESC" "DEL"))

  ;; Show the key prefix on the left or top (nil means hide the prefix). The
  ;; prefix consists of the keys you have typed so far. which-key also shows the
  ;; page information along with the prefix.
  (setq which-key-show-prefix 'left)

  ;; Set to t to show the count of keys shown vs. total keys in the mode line.
  (setq which-key-show-remaining-keys nil)
#+END_SRC
** More Examples
*** Nice Display with Split Frame
Unlike guide-key, which-key looks good even if the frame is split into several
windows.
#+CAPTION: which-key in a frame with 3 horizontal splits
[[./img/which-key-right-split.png]]

#+CAPTION: which-key in a frame with 2 vertical splits
[[./img/which-key-bottom-split.png]]

** Status
It requires testing on different platforms with different configurations, which
is beyond my capabilities. The default configuration has been reasonably stable
for me. 
** Thanks
Thanks to @bmag for helping with the initial development and finding many bugs.