Devil Mode

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.

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.

Devil is available via NonGNU ELPA and 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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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:

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

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.

1. 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.

2. 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.

3. 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.

4. 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.

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.