#+title:                Devil Mode
#+author:               Susam Pal
#+email:                susam@susam.net
#+language:             en
#+options:              ':t toc:nil author:t email:t num:t
#+texinfo_dir_category: Emacs misc features
#+texinfo_dir_title:    Devil: (devil)
#+texinfo_dir_desc:     Minor mode for Devil-like command entering
#+export_file_name:     devil

#+texinfo: @insertcopying

Devil mode trades your comma key in exchange for a modifier-free
editing experience in Emacs.  Yes, the comma key!  The key you would
normally wield for punctuation in nearly every corner of text.  Yes,
this is twisted!  It would not be called the Devil otherwise, would
it?  If it were any more rational, we might call it something divine,
like, uh, the God mode?  But alas, there is nothing divine to be found
here.  Welcome, instead, to the realm of the Devil!  You will be
granted the occasional use of the comma key for punctuation, but only
if you can charm the Devil.  But beware, for in this sinister domain,
you must relinquish your comma key and embrace an editing experience
that whispers wicked secrets into your fingertips!

* Introduction
:PROPERTIES:
:CUSTOM_ID: introduction
:END:
Devil mode intercepts our keystrokes and translates them to Emacs key
sequences according to a configurable set of translation rules.  For
example, with the default translation rules, when we type =, x , f=,
Devil translates it to =C-x C-f=.

The choice of the comma key (=,=) to mean the control modifier key
(=C-=) may seem outrageous.  After all, the comma is a very important
punctuation both in prose as well as in code.  Can we really get away
with using =,= to mean the =C-= modifier?  It turns out, this terrible
idea can be made to work without too much of a hassle.  At least it
works for me.  It might work for you too.  If it does not, Devil can
be configured to use another key instead of =,= to mean the =C-=
modifier.  See the section [[*Custom Devil Key]] and the subsequent
sections for a few examples.

A sceptical reader may rightfully ask: If =,= is translated to =C-=,
how on earth are we going to insert a literal =,= into the text when
we need to?  The section [[*Typing Commas]] answers this.  But before we
get there, we have some fundamentals to cover.  Take the plunge and
see what unfolds.  Maybe you will like this.  Maybe you will not.  If
you do not like this, you can always retreat to God mode, Evil mode,
the vanilla key bindings, or whatever piques your fancy.

* Notation
:PROPERTIES:
:CUSTOM_ID: notation
:END:
A quick note about the notation used in the document: The previous
example shows that =, x , f= is translated to =C-x C-f=.  What this
really means is that the keystrokes =,x,f= is translated to =ctrl+x
ctrl+f=.  We do not really type any space after the commas.  The key
=,= is directly followed by the key =x=.  However, the key sequence
notation used in this document contains spaces between each keystroke.
This is consistent with how key sequences are represented in Emacs in
general and how Emacs functions like =key-description=,
=describe-key=, etc.  represent key sequences.  When we really need to
type a space, it is represented as =SPC=.

* Install
:PROPERTIES:
:CUSTOM_ID: install
:END:
Devil is available via [[https://elpa.nongnu.org/nongnu/devil.html][NonGNU ELPA]] and [[https://melpa.org/#/devil][MELPA]].  You may already have a
preferred way of installing packages from ELPA/MELPA.  If so, install
the package named =devil= to get Devil.  If you have Emacs 28.1 or a
more recent version, it has NonGNU ELPA enabled by default, so you can
install Devil quite easily with =M-x package-install RET devil RET=
without having to perform any further steps.  However, for the sake of
completeness, a few very different and basic ways of installing Devil
are presented in the next few subsections.

** Install Interactively from MELPA
:PROPERTIES:
:CUSTOM_ID: install-interactively-from-melpa
:END:
To install the latest version of Devil from MELPA, perform the
following steps:

1. Add the following to the Emacs initialization file (i.e.,
   =~/.emacs= or =~/.emacs.d/init.el= or =~/.config/emacs/init.el=):

   #+begin_src elisp
     (require 'package)
     (add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)
     (package-initialize)
   #+end_src

2. Start Emacs with the updated initialization file.  Then type these
   commands:

   #+begin_example
     M-x package-refresh-contents RET
     M-x package-install RET devil RET
   #+end_example

3. Confirm that Devil is installed successfully with this command:

   #+begin_example
     C-h f devil RET
   #+end_example

4. Enable Devil mode with this command:

   #+begin_example
     M-x global-devil-mode RET
   #+end_example

5. Type =, x , f= and watch Devil translate it to =C-x C-f= and invoke
   the corresponding command.

** Install Automatically from MELPA
:PROPERTIES:
:CUSTOM_ID: install-automatically-from-melpa
:END:
Here is yet another basic way to install and enable Devil using Elisp:

#+begin_src elisp
  (require 'package)
  (add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)
  (package-initialize)
  (unless package-archive-contents
    (package-refresh-contents))
  (unless (package-installed-p 'devil)
    (package-install 'devil))
  (global-devil-mode)
  (global-set-key (kbd "C-,") 'global-devil-mode)
#+end_src

Now type =, x , f= and watch Devil translate it to =C-x C-f= and
invoke the corresponding command.  Type =C-,= to disable Devil mode.
Type =C-,= again to enable it.

** Install from Git Source
:PROPERTIES:
:CUSTOM_ID: install-from-git-source
:END:
If you prefer obtaining Devil from its Git repository, follow these
steps:

1. Clone Devil to your system:

   #+begin_src shell
     git clone https://github.com/susam/devil.git
   #+end_src

2. Add the following to your Emacs initialization:

   #+begin_src elisp
     (add-to-list 'load-path "/path/to/devil/")
     (require 'devil)
     (global-devil-mode)
     (global-set-key (kbd "C-,") 'global-devil-mode)
   #+end_src

3. Start the Emacs editor.  Devil mode should now be enabled in all
   buffers.  The modeline of each buffer should show the =Devil=
   lighter.

4. Type =, x , f= and watch Devil translate it to =C-x C-f= and invoke
   the corresponding command.  Type =C-,= to disable Devil mode.  Type
   =C-,= again to enable it.

* Use Devil
:PROPERTIES:
:CUSTOM_ID: use-devil
:END:
Assuming vanilla Emacs key bindings have not been changed and Devil
has not been customised, here are some examples that demonstrate how
Devil may be used:

1. Type =, x , f= and watch Devil translate it to =C-x C-f= and invoke
   the =find-file= command.

2. Type =, p= to move up one line.

3. To move up multiple lines, type =, p p p= and so on.  Some Devil
   key sequences are repeatable keys by default.  The repeatable Devil
   key sequences can be repeated by typing the last key of the Devil
   key sequence over and over again.

4. Each repeatable key sequence belongs to a repeatable key sequence
   groups.  Like before, type =, p p p= to move the cursor up by a few
   lines.  But then immediately type =n n= to move the cursor down by
   a couple of lines.  Then immediately type =p n b f= to move the
   cursor up, down, left, and right.  The key sequences =, p= and =,
   n= and =, f= and =, b= form a single repeatable key sequence group.
   Therefore after we type any one of them, we can repeat that key
   sequence or any other key sequence in the same group over and over
   again merely by typing the last character of that key sequence.
   Typing any other key stops the repetition and the default behaviour
   of that other key is then observed.  Type =C-h v
   devil-repeatable-keys RET= to see the complete list of all
   repeatable key sequence groups.

5. Sometimes a repeatable key sequence may be the only key sequence in
   a repeatable key sequence group.  An example of such a key sequence
   is =, m ^= which translates to =M-^= and joins the current line to
   the previous line.  In a text buffer with multiple lines type =, m
   ^= to join the current line to the previous line.  Then type =^=
   repeatedly to continue joining lines.  Typing any other key stops
   the repetition.

6. Type =, s= and watch Devil translate it to =C-s= and invoke
   incremental search.

7. Type =, m x= and watch Devil translate it to =M-x= and invoke the
   corresponding command.  Yes, =, m= is translated to =M-=.

8. Type =, m m s= and watch Devil translate it to =C-M-s= and invoke
   regular-expression-based incremental search.  The key sequence =, m
   m= is translated to =C-M-=.

9. Type =, u , f= and watch Devil translate it to =C-u C-f= and move
   the cursor forward by 4 characters.

10. Type =, u u , f= and the cursor moves forward by 16 characters.
    Devil uses its translation rules and an additional keymap to make
    this input key sequence behave like =C-u C-u C-f= which moves the
    cursor forward by 16 characters.

11. Type =, SPC= to type a comma followed by space.  This is a special
    key sequence to make it convenient to type a comma in the text.
    Note that this sacrifices the use of =, SPC= to mean =C-SPC= which
    could have been a convenient way to set a mark.  See the section
    [[*Reclaim , SPC to Set Mark]] if you do not want to make this
    sacrifice.

12. Type =, z SPC= and watch Devil translate it to =C-SPC= and set a
    mark.  Yes, =, z= is translated to =C-= too.

13. Similarly, type =, RET= to type a comma followed by the =enter=
    key.  This is another special key.

14. Type =, ,= to type a single comma.  This special key is useful for
    cases when you really need to type a single literal comma.

15. Type =, h , k= to invoke =devil-describe-key=.  This is a special
    key that invokes the Devil variant of =describe-key= included in
    vanilla Emacs.  When the key input prompt appears, type the Devil
    key sequence =, x , f= and Devil will display the documentation of
    the function invoked by this Devil key sequence.  Note: The key
    sequence =, h k= translates to =C-h k= and invokes the vanilla
    =describe-key=.  It is the Devil key sequence =, h , k= that
    invokes =devil-describe-key=.

* Typing Commas
:PROPERTIES:
:CUSTOM_ID: typing-commas
:END:
Devil makes the questionable choice of using the comma as its
activation key.  As illustrated in the previous section, typing =, x ,
f= produces the same effect as typing =C-x C-f=.  One might naturally
wonder how then we are supposed to type literal commas.

Most often when we edit text, we do not really type a comma in
isolation.  Often we immediately follow the comma with a space or a
newline.  This assumption usually holds good while editing regular
text.  However, this assumption may not hold in some situations, like
while working with code when we need to add a single comma at the end
of an existing line.

In scenarios where the above assumption holds good, typing =, SPC=
inserts a comma and a space.  Similarly, typing =, RET= inserts a
comma and a newline.

In scenarios where we do need to type a single comma, type =, ,=
instead.

Note that you could also type a single comma with =, q ,= which
translates to =C-q ,= and inserts a literal comma.  The Emacs key
sequence =C-q= invokes the command =quoted-insert= which inserts the
next input character.  The =, ,= special key sequence is probably
easier to type than this.

Also, it is worth mentioning here that if all this fiddling with the
comma key feels clumsy, we could always customise the Devil key to
something else that feels better.  We could also disable Devil mode
temporarily and enable it again later with =C-,= as explained in
section [[*Install]].

* Devil Reader
:PROPERTIES:
:CUSTOM_ID: devil-reader
:END:
The following points briefly describe how Devil reads Devil key
sequences, translates them to Emacs key sequences, and runs commands
bound to the key sequences:

1. As soon as the Devil key is typed (which is =,= by default), Devil
   wakes up and starts reading Devil key sequences.  Type =C-h v
   devil-key RET= to see the current Devil key.

2. After each keystroke is read, Devil checks if the key sequence
   accumulated is a special key.  If it is, then the special command
   bound to the special key is executed immediately.  Note that this
   step is performed before any translation rules are applied to the
   input key sequence.  This is how the Devil special key sequence =,
   SPC= inserts a comma and a space.  Type =C-h v devil-special-keys
   RET= to see the list of special keys and the commands bound to
   them.

3. If the key sequence accumulated so far is not a special key, then
   Devil translates the Devil key sequence to a regular Emacs key
   sequence.  If the regular Emacs key sequence turns out to be a
   complete key sequence and some command is found to be bound to it,
   then that command is executed immediately.  This is how the Devil
   key sequence =, x , f= is translated to =C-x C-f= and the
   corresponding binding is executed.  If the translated key sequence
   is a complete key sequence but no command is bound to it, then
   Devil displays a message that the key sequence is undefined.  Type
   =C-h v devil-translations RET= to see the list of translation
   rules.

4. After successfully translating a Devil key sequence to an Emacs key
   sequence and executing the command bound to it, Devil checks if the
   key sequence is a repeatable key sequence.  If it is found to be a
   repeatable key sequence, then Devil sets a transient map so that
   the repeatable key sequences that belong to the same group as the
   typed Devil key sequence can be invoked merely by typing the last
   character of the input key sequence.  This is how =, p p p f f=
   moves the cursor up by three lines and then by two characters
   forward.  Type =C-h v devil-repeatable-keys RET= to see the list of
   repeatable Devil key sequences.

The variables =devil-special-keys=, =devil-translations=, and
=devil-repeatable-keys= may contain keys or values with the string
=%k= in them.  This is a placeholder for =devil-key=.  While applying
the special keys, translation rules, or repeat rules, each =%k= is
replaced with the actual value of =devil-key= before applying the
rules.

* Translation Mechanism
:PROPERTIES:
:CUSTOM_ID: translation-mechanism
:END:
The following points provide an account of the translation mechanism
that Devil uses in order to convert a Devil key sequence entered by
the user to an Emacs key sequence:

1. The input key vector read from the user is converted to a key
   description (like the string produced by functions like
   =describe-key= and =key-description=).  For example, if the user
   types =,x,f= it is converted to =, x , f=.

2. Now the resulting key description is translated with simple string
   replacements.  If any part of the string matches a key in
   =devil-translations=, then it is replaced with the corresponding
   value.  For example, =, x , f= is translated to =C- x C- f=.  Then
   Devil normalises the result to =C-x C-f= by removing the stray
   spaces after the modifier keys.

3. If the simple string based replacement discussed in the previous
   point leads to an invalid Emacs key sequence, it skips the
   replacement that causes the resulting Emacs key sequence to become
   invalid.  For example =, m m ,= results in =C-M-C-= after the
   simple string replacement because the default translation rules
   replace the leading =, m m= with =C-M-= and the trailing =,= with
   =C-=.  However, =C-M-C-= is an invalid key sequence, so the
   replacement of the trailing =,= to =C-= is skipped.  Therefore, the
   input =, m m ,= is translated to =C-M-,= instead.

4. Finally, Devil looks for key chords in the key sequence that
   contain both the =C-= modifier and an uppercase letter.  If such a
   key chord occurs, then it replaces the uppercase letter with its
   shifted form, e.g., =, m m V= first translates to =C-M-V= according
   to the previous points and then the result is translated to
   =C-M-S-v= according to this point.

* Default Translation Rules
:PROPERTIES:
:CUSTOM_ID: default-translation-rules
:END:
By default, Devil supports a small but peculiar set of translation
rules that can be used to avoid modifier keys while typing various
types of key sequences.  See =C-h v devil-translations RET= for the
translation rules.  Here are some examples that demonstrate the
default translation rules.  The obvious ones are shown first.  The
more peculiar translations come later in the table.  The concluding
paragraph of this subsection offers a guide on how to gradually and
gently adopt these key sequences into your daily routine.

| Input     | Translated | Remarks                                   |
|-----------+------------+-------------------------------------------|
| =, s=     | =C-s=      | Rule 1: =,= is replaced with =C-=         |
| =, m x=   | =M-x=      | Rule 2: =, m= is replaced with =M-=       |
| =, [ x=   | =C-[ x=    | equivalent to =M-x=                       |
| =, m m s= | =C-M-s=    | Rule 3: =, m m= is replaced with =C-M-=   |
| =, m ,=   | =M-,=      | Rule 4: =, m ,= is replaced with =M-,=    |
| =, m z m= | =M-m=      | Rule 5: =, m z= is replaced with =M-= too |
| =, c , ,= | =C-c ,=    | Rule 6: =, ,= is replaced with =,=        |
| =, z SPC= | =C-SPC=    | Rule 7: =, z= is replaced with =C-= too   |
| =, z z=   | =C-z=      | ditto                                     |
| =, z ,=   | =C-,=      | ditto                                     |

Note how we cannot use =, SPC= to set a mark because that key sequence
is already reserved as a special key sequence in =devil-special-keys=.
In order to conveniently set a mark, Devil translates =, z= to =C-=
too, so that we can type =, z SPC= and have Devil translate it to
=C-SPC=.

Also, note that while =, m= may be used to type =M-= we have =, [= as
yet another way to type a key sequence that contains =M-= because =,
[= translates to =C-[= and =C-[ <key>= is equivalent to =ESC <key>=
which in turn is equivalent to =M-<key>=.

The default translation examples presented in the table above look
weirder and weirder as we go down the table.  But they are not as
arbitrary as they might initially appear to be.  They are arranged in
such a way that overall, we get the following effect:

- Devil translates the input =,= to =C-=.  Similarly it translates =,
  m= to =M-= and =, m m= to =C-M-=.

- When we really want to type the Devil key =,= we need to double type
  it in the Devil key sequence.  Doubling the special character serves
  as an escape mechanism to avoid the special meaning of the Devil key
  and get its literal form instead.

- Now since =, ,= translates to =,= we need another escape mechanism
  to type =C-,=.  Typing =z= in between serves as this escape
  mechanism, i.e., within a Devil key sequence =, z ,= translates to
  =C-,=.

- Similarly since =, m m= translates to =C-M-= we need an escape
  mechanism to type =M-m=.  Again, typing =z= in between serves as
  this escape mechanism, i.e., =, m z m= translates to =M-m=.

Here is a gentle guide to adopting these key sequences: For beginners
using Devil, it is not necessary to memorise all of them right away.
Understanding that =,= translates to =C-= and =, m= translates to =M-=
is sufficient to begin.  Subsequently, learning that =, m m=
translates to =C-M-= unlocks several more key sequences like =, m m s=
(=C-M-s=), =, m m f= (=C-M-f=), etc.  As you encounter more key
sequences that are not covered by these initial rules, revisit the
above table to pick up new translation rules and adopt them in your
day-to-day usage of Devil.

* Describe Devil Key
:PROPERTIES:
:CUSTOM_ID: devil-describe-key
:END:
Devil offers a command named =devil-describe-key= that can be used to
describe a Devil key sequence.  It works similarly to the
=describe-key= command of vanilla Emacs that can be invoked with =C-h
k=.  The =devil-describe-key= command can be invoked with the special
key sequence =, h , k=.  Type =, h , k= and a prompt appears to read a
key sequence.  Type any Devil key sequence, say, =, x , f= and Devil
immediately shows the documentation for the function invoked by this
key sequence.

Note that =, x , f= (=devil-describe-key=) can also be used to look up
documentation for vanilla Emacs key sequences like =C-x C-f=.

Also note that the Devil key sequence is =, h k= is still free to
invoke =C-h k= (=describe-key= of vanilla Emacs).

* Bonus Key Bindings
:PROPERTIES:
:CUSTOM_ID: bonus-key-bindings
:END:
Devil adds the following additional key bindings only when Devil is
enabled globally with =global-devil-mode=:

- Adds the Devil key to =isearch-mode-map=, so that Devil key
  sequences work in incremental search too.

- Adds =u= to =universal-argument-more= to allow repeating the
  universal argument command =C-u= simply by repeating =u=.

As mentioned before these features are available only when Devil is
enabled globally with =global-devil-mode=.  If Devil is enabled
locally with =devil-mode=, then these features are not available.

* Custom Configuration Examples
:PROPERTIES:
:CUSTOM_ID: custom-configuration-examples
:END:
In the examples presented below, the =(require 'devil)= calls may be
omitted if Devil has been installed from a package archive like ELPA
or MELPA.  There are appropriate autoloads in place in the Devil
package that would ensure that it is loaded automatically on enabling
Devil mode.  However, the =require= calls have been included in the
examples below for the sake of completeness.

** Local Mode
:PROPERTIES:
:CUSTOM_ID: local-mode
:END:
While the section [[*Install]] shows how we enable Devil mode globally,
this section shows how we can enable it locally.  Here is an example
initialization code that enables Devil locally only in text buffers.

#+begin_src elisp
  (require 'devil)
  (add-hook 'text-mode-hook 'devil-mode)
  (global-set-key (kbd "C-,") 'devil-mode)
#+end_src

This is not recommended though because this does not provide a
seamless Devil experience.  For example, with Devil enabled locally in
a text buffer like this, although we can type =, x , f= to launch the
=find-file= minibuffer, we cannot use Devil key sequences in the
minibuffer.  Further the special keymaps described in the previous
section work only when Devil is enabled globally.

** Mode as a buffer default
:PROPERTIES:
:CUSTOM_ID: buffer-default
:END:
While enabling =global-devil-mode= is typically enough for using Devil
in every buffer the user encounters, buffer creation which bypasses
the traditional setup hooks will not have the mode enabled by default.
The most likely place to encounter this problem is with the default
Emacs startup screen, which is created after the user's init file has
been processed but does not run any hooks which will enable Devil.

To solve this particular problem it is recommended to either customise
the startup screen to one which does run the required hooks, or to
advise the =display-startup-screen= function to enable Devil in the
buffer once created.  Here is an example of enabling
=global-devil-mode= and ensuring it is activated in the default Emacs
startup screen.

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)
  (advice-add 'display-startup-screen
              :after (lambda (&optional _) (devil-mode 1)))
#+end_src

This solves the problem most likely to be encountered.  But what if
you do genuinely need Devil active in *every* buffer?  When
=devil-global-sets-buffer-default= is set to =t= and
=global-minor-mode= is called, Devil will be enabled in all new
buffers by default.  Enabling this functionality may seem appealing,
but consider that not all buffers are intended to be interacted with,
and the decision to opt-out of hooks which could apply a minor-mode is
likely to have been intentional.  Even though it is not recommended,
the following example demonstrates how to have Devil active in all
buffers by default.

#+begin_src elisp
  (require 'devil)
  (setq devil-global-sets-buffer-default t)
  (global-devil-mode)
#+end_src

** Custom Appearance
:PROPERTIES:
:CUSTOM_ID: custom-appearance
:END:
The following initialization code shows how we can customise Devil to
show a Devil smiley (😈) in the modeline and in the Devil prompt.

#+begin_src elisp
  (require 'devil)
  (setq devil-lighter " \U0001F608")
  (setq devil-prompt "\U0001F608 %t")
  (global-devil-mode)
  (global-set-key (kbd "C-,") 'global-devil-mode)
#+end_src

** Reclaim , SPC to Set Mark
:PROPERTIES:
:CUSTOM_ID: reclaim-comma-space-to-set-mark
:END:
The default configuration for special keys reserves =, SPC= to insert
a literal comma followed by space.  This default makes it easy to type
comma in various contexts.  However, this means that =, SPC= does not
translate to =C-SPC=.  Therefore =, SPC= cannot be used to set mark.
Instead, the default translation rules offer =, z SPC= as a way to set
mark.

If you would rather set mark using =, SPC= and you are happy with
typing the special key =, ,= to insert a literal comma, then use the
following configuration:

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)
  (global-set-key (kbd "C-,") 'global-devil-mode)
  (assoc-delete-all "%k SPC" devil-special-keys)
#+end_src

This removes the special key =, SPC= from =devil-special-keys= so that
it is now free to be translated to =C-SPC= and invoke =set-mark-command=.

** Custom Devil Key
:PROPERTIES:
:CUSTOM_ID: custom-devil-key
:END:
The following initialization code shows how we can customise Devil to
use a different Devil key.

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)
  (global-set-key (kbd "C-;") 'global-devil-mode)
  (devil-set-key (kbd ";"))
#+end_src

The above example sets the Devil key to the semicolon, perhaps another
dubious choice for the Devil key.  With this configuration, we can use
=; x ; f= and have Devil translate it to =C-x C-f=.

** Yet Another Custom Devil Key
:PROPERTIES:
:CUSTOM_ID: yet-another-custom-devil-key
:END:
The following initialization code shows how we can customise Devil to
use yet another different Devil key.

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)
  (global-set-key (kbd "C-<left>") 'global-devil-mode)
  (devil-set-key (kbd "<left>"))
  (dolist (key '("%k SPC" "%k RET" "%k <return>"))
    (assoc-delete-all key devil-special-keys))
#+end_src

The above example sets the Devil key to the left arrow key.  With this
configuration, we can use =<left> x <left> f= and have Devil translate
it to =C-x C-f=.  We can type the special key =<left> <left>= to
produce the same effect as the original =<left>=.

The above example removes some special keys that are no longer useful.
In particular, =<left> SPC= is no longer reserved as a special key, so
we can use it now to set a mark.

** Multiple Devil Keys
:PROPERTIES:
:CUSTOM_ID: multiple-devil-keys
:END:
While this package provides the comma (=,=) as the default and the
only Devil key, nothing stops you from extending the mode map to
support multiple Devil keys.  Say, you decide that in addition to
activating Devil with =,= which also plays the role of =C-=, you also
want to activate Devil with =.= which must now play the role of =M-=.
To achieve such a result, you could use this initialization code as a
starting point and then customise it further based on your
requirements:

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)
  (define-key devil-mode-map (kbd ".") #'devil)
  (add-to-list 'devil-special-keys `(". ." . ,(devil-key-executor ".")))
  (setq devil-translations '((", z" . "C-")
                             (". z" . "M-")
                             (", ," . ",")
                             (". ." . ".")
                             ("," . "C-")
                             ("." . "M-")))
#+end_src

With this configuration, we can type =, x , f= for =C-x C-f= like
before.  But now we can also type =. x= for =M-x=.  Similarly, we can
type =, . s= for =C-M-s= and so on.  Also =, ,= inserts a literal
comma and =. .= inserts a literal dot.  Further we can type =, z ,= to
get =C-,= and =. z .= to get =M-.=.

Note that by default Devil configures only one activation key (=,=)
because the more activation keys we add, the more intrusive Devil
becomes during regular editing tasks.  Every key that we reserve for
activating Devil loses its default function and then we need
workarounds to somehow invoke the default function associated with
that key (like repeating =.= twice to insert a single =.= in the above
example).  Therefore, it is a good idea to keep the number of Devil
keys as small as possible.

** Make All Keys Repeatable
:PROPERTIES:
:CUSTOM_ID: make-all-keys-repeatable
:END:
By default Devil has a small list of key sequences that are considered
repeatable.  This list is defined in the variable
=devil-repeatable-keys=.  Type =C-h v devil-repeatable-keys RET= to
view this list.  For example, consider the repeatable key sequence
group =("%k p" "%k n" "%k f" "%k b")= in this list.  Assuming that the
default Devil and Emacs key bindings have not been changed, this means
that after we type =, p= and move the cursor to the previous line, we
can repeat this operation by typing =p= over and again.  We can also
immediately type =f= to move the cursor right by one character.  The
repetition occurs as long as the last character of any repeatable key
sequence in the group is typed again.  Typing any other key stops the
repetition and the default behaviour of the other key is then
observed.

It is possible to make all key sequences repeatable by setting the
variable =devil-all-keys-repeatable= to =t=.  Here is an example
configuration:

#+begin_src elisp
  (require 'devil)
  (setq devil-all-keys-repeatable t)
  (global-devil-mode)
#+end_src

With this configuration, the repeatable key sequence groups still
function as described above.  However, in addition to that now all
other Devil key sequences that end up executing Emacs commands also
become repeatable, i.e., any Devil key sequence that does not belong
to =devil-all-keys-repeatable= but invokes an Emacs command is now
repeatable and it can be repeated by merely repeating the last
character of the key sequence.

Note that only Devil key sequences that get translated to a regular
Emacs key sequence and result in the execution of an Emacs command can
be repeatable.  The special keys defined in =devil-special-keys= are
never repeatable.

** Interaction with Repeat Mode
:PROPERTIES:
:CUSTOM_ID: interaction-with-repeat-mode
:END:
Repeatable keys in Devil function somewhat like =repeat-mode=
introduced in Emacs 28.1.  Here is an example configuration that
disables repeatable keys in Devil and shows how to use =repeat-mode=
instead to define repeatable commands.

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)
  (setq devil-repeatable-keys nil)

  (defvar movement-repeat-map
    (let ((map (make-sparse-keymap)))
      (define-key map (kbd "p") #'previous-line)
      (define-key map (kbd "n") #'next-line)
      (define-key map (kbd "b") #'backward-char)
      (define-key map (kbd "f") #'forward-char)
      map))

  (dolist (cmd '(previous-line next-line backward-char forward-char))
    (put cmd 'repeat-map 'movement-repeat-map))

  (repeat-mode)
#+end_src

Now if we type =C-p= to move the cursor up by one line, we can repeat
it by merely typing =p= again and we can also type or repeat =n=, =b=,
or =f=, to move the cursor down, left, or right respectively.

Repeat mode works fine with Devil too, so with the above
configuration, when we type =, p= to move the cursor to the previous
line, we can type or repeat =p=, =n=, =b=, or =f= to move the cursor
up, down, left, or right again.

We do not really need to disable Devil's repeatable keys while using
repeat mode.  Both can be enabled together.  However, the results can
be surprising due to certain differences between the two.  For
example, consider the following configuration:

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)

  (defvar movement-repeat-map
    (let ((map (make-sparse-keymap)))
      (define-key map (kbd "p") #'previous-line)
      (define-key map (kbd "n") #'next-line)
      map))

  (dolist (cmd '(previous-line next-line))
    (put cmd 'repeat-map 'movement-repeat-map))

  (repeat-mode)
#+end_src

Now both Devil repeatable keys and repeat mode are active.  If we now
type =, p= we can repeat =p= and =n= to move the cursor up and down.
Repeat mode makes this repetition possible.  Additionally, after
typing =, p= we can also type or repeat =b= and =f= to move the cursor
left and right.  Devil makes this repetition possible.  We can tell
the difference between repeat mode handling repeatable commands and
Devil mode handling repeatable keys by looking at the echo area.  When
we repeat =p= which is handled by repeat mode, we see a message
"Repeat with p, n" in the echo area.  But when we repeat =b= which is
handled by Devil, we see no such message; Devil sets up repeatable
keys silently.

** Comparison with Repeat Mode
:PROPERTIES:
:CUSTOM_ID: comparison-with-repeat-mode
:END:
The previous section demonstrates how much of what Devil accomplishes
with its support for repeatable key sequences can also be accomplished
with =repeat-mode= that comes out of the box in Emacs 28.1 and later
versions.

However, there is a crucial difference between Devil's repeatable keys
and =repeat-mode=.  Repeat mode provides repeatable /commands/ but
Devil supports repeatable /keys/.  This different is crucial and
arguably makes repeatable key sequences easier to configure in Devil.
To demonstrate the difference, let us consider the key sequence =M-e=.
The command =forward-sentence= is bound to it by default in the global
map.  However, in Org mode, the command =org-forward-sentence= is
bound to it.  The corresponding Devil key sequence is =, m e= and this
is a repeatable key sequence in Devil.  Therefore, we can type =, m e=
followed by =e e e= and so on to move the cursor forward by multiple
sentences in text mode as well as in Org mode.

To emulate the same behaviour using repeat mode, we need a
configuration like this:

#+begin_src elisp
  (require 'devil)
  (global-devil-mode)
  (setq devil-repeatable-keys nil)

  (defvar forward-sentence-repeat-map
    (let ((map (make-sparse-keymap)))
      (define-key map (kbd "e") #'forward-sentence)
      map))

  (defvar org-forward-sentence-repeat-map
    (let ((map (make-sparse-keymap)))
      (define-key map (kbd "e") #'org-forward-sentence)
      map))

  (put #'forward-sentence 'repeat-map 'forward-sentence-repeat-map)
  (put #'org-forward-sentence 'repeat-map 'org-forward-sentence-repeat-map)

  (repeat-mode)
#+end_src

Note how we need to configure repeat mode for both commands that are
bound to =M-e=.  With the above configuration, we can now type =, m e=
followed by =e e e= to move forward by multiple sentences in both text
mode as well as Org mode.  However, we can never be sure if we missed
configuring repeat mode for some other command that might be bound to
=M-e= in some mode.  For example, in C mode, the command
=c-end-of-statement= is bound to =M-e=.  The above configuration is no
good for repeating this command by typing =e e e=.

Devil, however, can repeat the command bound to =M-e= in any mode.
Devil does not merely make the command bound to it in a particular
mode repeatable.  Instead Devil makes the key sequence =, m e= itself
repeatable.  Therefore, with Devil's own support for repeatable key
sequences, we can type =, m e= and then =e e e= to repeat the command
bound to =M-e= regardless of which mode is active or which command is
bound to this key sequence.

* Why?
:PROPERTIES:
:CUSTOM_ID: why
:END:
Why go to the trouble of creating and using something like this?  Why
not just remap =caps lock= to =ctrl= like every other sane person
does?  Or if it is so important to avoid modifier keys, why not use
something like God mode or Evil mode?

Well, for one, both God mode and Evil mode are modal editing modes.
Devil, on the other hand, retains the non-modal editing experience of
Emacs.

Devil mode began as a fun little experiment.  From the outset, it was
clear that using something as crucial as the comma for specifying the
modifier key is asking for trouble.  However, I still wanted to see
how far I could go with it.  It turned out that in a matter of days, I
was using it full-time for all of my Emacs usage.

This experiment was partly motivated by Macbook keyboards which do not
have a =ctrl= key on the right side of the keyboard.  Being a
touch-typist myself, I found it inconvenient to type key combinations
like =C-x=, =C-s=, =C-r=, =C-d=, =C-f=, =C-w=, =C-a=, =C-e=,
etc. where both the modifier key and the modified key need to be
pressed with the left hand fingers.  I am not particularly fond of
remapping =caps lock= to behave like =ctrl= because that still suffers
from the problem that key combinations like =C-x=, =C-a= require
pressing both the modifier key and the modified key with the left hand
fingers.  I know many people remap both their =caps lock= and =enter=
to behave like =ctrl=.  While I think that is a fine solution, I was
not willing to put up with the work required to make that work
seamlessly across all the various operating systems I work on.

What began as a tiny whimsical experiment a few years ago turned out
to be quite effective, at least to me.  I like that this solution is
implemented purely as Elisp and therefore does not have any external
dependency.  I am sharing this solution in the form of a minor mode,
just in case, there is someone out there who might find this useful
too.

* Comparison with God Mode
:PROPERTIES:
:CUSTOM_ID: comparison-with-god-mode
:END:
God mode provides a modal editing experience but Devil does not.
Devil has the same underlying philosophy as that of God mode, i.e.,
the user should not have to learn new key bindings.  However, Devil
does not have a hard separation between insert mode and command mode
like God mode has.  Instead, Devil waits for an activation key (=,= by
default) and as soon as it is activated, it intercepts and translates
keys, runs the corresponding command, and then gets out of the way.
So Devil tries to retain the non-modal editing experience of vanilla
Emacs.

Now it is worth mentioning that some of this non-modal editing
experience can be reproduced in god-mode too using its
=god-execute-with-current-bindings= function.  Here is an example:

#+begin_src elisp
  (global-set-key (kbd ",") #'god-execute-with-current-bindings)
#+end_src

With this configuration, God mode translates =, x f= to =C-x C-f=.
Similarly =, g x= invokes =M-x= and =, G s= invokes =C-M-x=.  This
provides a non-modal editing experience in God mode too.  However,
this experience does not extend seamlessly to minibuffers.  Devil does
extend its Devil key translation to minibuffers.

Further note that in God mode the =ctrl= modifier has sticky
behaviour, i.e., the modifier remains active automatically for the
entire key sequence.  Therefore in the above example, we type =,= only
once while typing =, x f= to invoke =C-x C-f=.  However, this sticky
behaviour implies that we need some way to disambiguate between key
sequences like =C-x C-f= (=find-file=) and =C-x f=
(=set-fill-column=).  God mode solves this by introducing =SPC= to
deactivate the modifier, e.g., =, x f= translates to =C-x C-f= but =,
x SPC f= translates to =C-x f=.  Devil does not treat the modifier key
as sticky which leads to simpler key sequences at the cost of a little
additional typing, i.e., =, x , f= translates to =C-x C-f= and =, x f=
translates to =C-x f=.

To summarize, there are primarily four things that Devil does
differently:

- Provide a non-modal editing experience from the outset.
- Seamlessly extend the same editing experience to minibuffer,
  incremental search, etc.
- Translate key sequences using string replacements.  This allows for
  arbitrary and sophisticated key translations for the adventurous.
- Choose non-sticky behaviour for the modifier keys.

These differences could make Devil easier to use than God mode for
some people but clumsy for other people.  It depends on one's tastes
and preferences.

* Frequently Asked Questions
:PROPERTIES:
:CUSTOM_ID: frequently-asked-questions
:END:
01. Why was the comma (=,=) chosen as the default Devil key?  Isn't
    the semicolon (=;=) a better choice since it belongs to the home
    row?

    Opinions vary.  As the author and maintainer of this minor mode, I
    made a choice to use the comma as the default Devil key.
    Although, the semicolon belongs to the home row on most keyboards
    and the comma does not, I find the vertical movement to reach the
    comma key with the long finger more convenient than the horizontal
    movement necessary to reach the semicolon with the little finger.

    As a touch typist, my fingers rest on the eight home row keys when
    idle.  The horizontal movement necessary to type the semicolon
    leads to a significant angular movement of the wrist.  Curling my
    long finger to reach the comma key helps me avoid this wrist
    strain.  If you do not like this default, it is quite easy to
    customise the Devil key to be the semicolon or any other key of
    your choice.  See the section [[*Custom Devil Key]] and the subsequent
    sections to learn how to do this.

02. I am happy with typing =, ,= every time, I need to type a comma.
    Can I free up =, SPC= to invoke =set-mark-command=?

    Yes, this can be done by removing the special key =, SPC= from
    =devil-special-keys=.  See the section [[*Reclaim , SPC to Set Mark]]
    to find out how to do this.

03. Can I make the Devil key sticky, i.e., can I type =, x f= instead
    of =, x , f= to invoke =C-x C-f=?

    Devil does not support sticky keys.  Say, Devil were to translate
    =, x f= to =C-x C-f=, how would we invoke =C-x f= then?  We need
    some way to disambiguate between =C-x C-f= and =C-x f=.  Different
    tools take different approaches to disambiguate the two key
    sequences.  God-mode translates =x f= to =C-x C-f= and =x SPC f=
    to =C-x f=, i.e., God-mode treats the =C-= modifier as sticky by
    default but when we want to make it non-sticky, we need to type
    =SPC= in god-mode.  This makes some key sequences like =C-x C-f=
    shorter to type but some other key sequences like =C-x f= longer
    to type.

    Devil treats the Devil key as non-sticky, so that there is no need
    for additional peculiar rules to switch between sticky and
    non-sticky behaviour to disambiguate key sequences like =C-x C-f=
    and =C-x f=.  With Devil =, x , f= translates to =C-x C-f= and
    similarly =, x f= translates to =C-x f=.  The translation rules
    are simpler at the cost of a little additional typing in some
    cases.  In most such cases, Devil requires typing an additional
    comma that one could have avoided if the comma were sticky.
    However, in other cases, Devil eliminates the need to type an
    extra key to make the modifier key non-sticky.

04. Are there some things that are easier to do with Devil than
    god-mode?

    Devil is not necessarily easier than god-mode.  It is different.
    Preferences vary, so some may find Devil easier to use while some
    others may find god-mode easier to use.  See the section
    [[*Comparison with God Mode]] for more details on the differences
    between the two modes.

05. I've called the function =global-devil-mode= in my config file,
    but when Emacs opens Devil does not seem to active.  Why isn't it
    working?

    The most likely issue is that the first buffer shown is the
    default Emacs startup screen, which is created without running any
    hooks which would give Devil the opportunity to be applied.  See
    the section [[*Mode as a buffer default]] for further details and
    possible solutions.

* Conclusion
:PROPERTIES:
:CUSTOM_ID: conclusion
:END:
Devil is a minor mode to translate key sequences.  Devil utilises this
translation capability to provide a modifier-free editing experience
and it does so without resorting to modal-editing.  Devil retains the
non-modal editing of vanilla Emacs.  This mode was written as a quirky
experiment to make it easier to use Emacs without modifier keys.
However, the resulting mode turned out to be quite convenient to use,
in general.  You might find Devil comfortable.  Or you might find
Devil to be a terrible idea.  It is also possible that you might find
Devil useful but intrusive.  In such cases, there are plenty of
customisable options that you can modify to configure Devil according
to your preferences.  If you need any help or if you find any issues,
please create an issue at [[https://github.com/susam/devil/issues]].
