Let's Write an Emacs Mode for Inform 7!

So, my love affair with Inform continues. Whatever love I housed for the in-built editor however, has withered and died. The keybindings all felt wrong, I kept accidentally narrowing the scope of the code, and had to use the mouse way too much for it to be a pleasant writing experience. So I thought, Hey, why not write an emacs mode for Inform? I’ve been meaning to learn about major modes and syntax highlighting anyway, and this is as good an excuse as any. After a bit of research, it turned out that creating a new major mode was pretty straightforward.

First, since we want headings in the source code to stand out, we’ll define a new font face, based on font-lock-preprocessor-face. It could be inherited from any face, really, but let’s pick one that’s likely to have been included in most colour themes.

(defface inform7-heading-face
'((t (:inherit font-lock-preprocessor-face :weight bold :height 1.2)))
"Face for Inform 7 headings"
:group 'font-lock-highlighting-faces)

Then some keybindings for our mode. There’s not much for us to do here, but let’s make RET automatically jump to the correct indentation level; that’s always nice:

(defvar inform7-mode-map
(let ((map (make-sparse-keymap)))
(define-key map (kbd "RET") 'newline-and-indent)
(define-key map "\C-j" 'newline-and-indent)
"Keymap for inform7 major mode")

The easiest way to add syntax highlighting for our mode is to use regexp-based matchers. Inform 7 hasn’t got that much of a formal syntax, but it would be nice to highlight keywords like if, then and otherwise and for section headings in the text. We should also find some way of supporting indexed text (i.e. anthing inside a string surrounded by []).

The last one is a bit tricky, as the same delimiters are also used for comments (see below), and it’s not actually possible to distinguishing between them using only regexps. The solution here, prepending a ‘.’ to the matcher, is a workaround that’s good enough for most cases though.

One thing to watch out for is that that the names of the faces must be symbols. For the built-in faces there are variables defined for each of these, but we must be careful to quote the name of the face we defined above.

(defconst inform7-font-lock-keywords
`(( ,(regexp-opt '("Include" "Use" "let" "say" "if" "otherwise") 'words) . font-lock-keyword-face)
("^\\(\\(?:Book\\|Chapter\\|Part\\|Section\\|Volume\\) - .*\\)" . 'inform7-heading-face)
(".\\(\\[.*?\\]\\)." 0 font-lock-variable-name-face t)
"Highlighting expressions for inform7-mode")

We now have everything we need to define our new mode. Current wisdom holds that it’s a good idea to extend an existing mode using define-derived-mode, and have someone else do all the heavy lifting. In our case, we want to extend sws-mode, which handles significant whitespace and provides functions for intelligent indentation of lines and blocks of code. It is part of jade-mode.

Defining our major mode is deceptively simple; we just pass in a mode name, the name of the mode we’re deriving from, a printable name that will be displayed in the status line, and a body of code that will be executed when the mode is initialized. define-derived-mode will take care of the rest and set up a keymap, syntax table, mode hooks and ensure that the parent mode and the hooks are called on init. In our case, we’ve already created the keymap manually, so define-derived-mode will use that one instead.

(define-derived-mode inform7-mode sws-mode "Inform7"
"Major mode for editing inform 7 story files."
(set (make-local-variable 'font-lock-defaults) '(inform7-font-lock-keywords nil t)))

Two things remain, though. First, the line indent function does always increases the indentation of new/empty lines by one step. For inform code, we only want that if the preceding line ends with a ‘:’, otherwise we’d like the new line to have the same indentation level as the previous one.

It turns out, that can be accomplished by adding some advice around sws-do-indent-line, which is the function in sws-mode that actually carries out the indentation:

(defadvice sws-do-indent-line (around inform7-indent-blank () activate)
"Ensure proper indentation of blank lines according to Inform7 conventions."
(if (and (eq major-mode 'inform7-mode) (sws-empty-line-p))
(if (save-excursion
(looking-at "^.*:[ \t]*$"))
(indent-to (sws-max-indent))
(indent-to (sws-previous-indentation)))

The other thing we need to fix is highlighting of comments in the text. We don’t want to do this with regexps, as it’s tricky to get this working reliably for multi-line comments. Instead, we’ll take advantage of the built-in facilities for syntactic highlighting. Incidentally, you may have noticed that we didn’t have to do anything to get highlighting of strings in the code; the syntactic parser gives us that functionality for free.

These two lines modifies the syntax table, telling the parser to treat [ and ] as comment delimiters.

(modify-syntax-entry ?\[ "<]" inform7-mode-syntax-table)
(modify-syntax-entry ?\] ">[" inform7-mode-syntax-table)

So there we are, 41 lines of code all in all, not bad for a start.