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Emacs Lisp Tutorial

How to Write a Emacs Major Mode for Syntax Coloring

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Xah Lee, 2008-11, 2009, 2010, 2010-05-25

This page gives a practical example of writing a emacs major mode to do syntax coloring of your own language. You should have few months experience of coding emacs lisp. If you don't know elisp, first take a look at Emacs Lisp Basics.

The Problem

Your company uses its own in-house language. You want to write a major mode for that language, so that the keywords of the language will be highlighted.

Solution

Suppose your language source code looks like this:

Sin[x]^2 + Cos[y]^2 == 1
Pi^2/6 == Sum[1/x^2,{x,1,Infinity}]

You want the words “Sin”, “Cos”, “Sum”, colored as functions, and “Pi” and “Infinity” colored as constants.

Here's how you define the mode:

(setq myKeywords
 '(("Sin\\|Cos\\|Sum" . font-lock-function-name-face)
   ("Pi\\|Infinity" . font-lock-constant-face)
  )
)

(define-derived-mode math-lang-mode fundamental-mode
  (setq font-lock-defaults '(myKeywords))
  (setq mode-name "math lang")
)

The string "Sin\\|Cos\\|Sum" is a regex, the “font-lock-function-name-face” is a pre-defined variable that holds the value for the default font face used for function keywords.

The line “define-derived-mode” defines your mode, named “math-lang-mode”, based on the “fundamental-mode” (which is the most basic mode). The line (setq font-lock-defaults '(myKeywords)) sets up the syntax highlighting for your mode.

The line (setq mode-name "math lang") gives a easy name to be displayed on the status line, so users know what mode they are in. Otherwise it'll show as *invalid*.

That's all there is to it. (don't forget to select your code and call “eval-region” to let emacs know about it.) Now, when you call “math-lang-mode”, emacs will now syntax color the buffer's text. (you must have font-lock-mode on, if not, call font-lock-mode.) Here's what it looks like:

Sin[x]^2 + Cos[y]^2 == 1
Pi^2/6 == Sum[1/x^2,{x,1,Infinity}]

OMG, Emacs is beautiful!

Here's another simple example: Emacs Lisp: html6-mode.

Hundreds Of Keywords

Typically, a language may have hundreds of keywords. Elisp has a way to generate regex for your keywords.

Suppose you are writing a mode for the Linden Scripting Language (LSL). LSL has about 553 keywords. First, here's a sample of LSL source code so you get some idea of how we want it colored.

// comment starts with two slashes

// Examples of variable declaration and assignment:
integer score = 0;
string mySay = "i ♥ you";
vector v = <3,4,5>;
list myList= [2,4,7,3];

// Example of defining a function.
// most built-in function's names start with “ll” (Linden Library).
integer sum(integer a, integer b)
{
    integer result = a + b;
    return result;
}

 default
 {
     state_entry()
     {
         llSay(0, mySay);
     }

     touch_start(integer total_number)
     {
         if (score == 1) {
             llSay(0, mySay);
         } else {
             llWhisper(0, "Ouch!");
         }
     }
 }

In the above, the red are comments, green are the language's “type” keywords, purple is the lang's “keyword” keywords, pink is the lang's “event” keywords, blue is the lang's “function” keywords.

In the following, first, we define the group of words to be colored differently.

;; define several class of keywords
(defvar mylsl-keywords
  '("break" "default" "do" "else" "for" "if" "return" "state" "while")
  "LSL keywords.")

(defvar mylsl-types
  '("float" "integer" "key" "list" "rotation" "string" "vector")
  "LSL types.")

(defvar mylsl-constants
  '("ACTIVE" "AGENT" "ALL_SIDES" "ATTACH_BACK")
  "LSL constants.")

(defvar mylsl-events
  '("at_rot_target" "at_target" "attach")
  "LSL events.")

(defvar mylsl-functions
  '("llAbs" "llAcos" "llAddToLandBanList" "llAddToLandPassList")
  "LSL functions.")

In the above, we defined several variables that hold lists. Each list is a class of keywords in LSL language. Each list may have hundreds of elements.

;; create the regex string for each class of keywords
(defvar mylsl-keywords-regexp (regexp-opt mylsl-keywords 'words))
(defvar mylsl-type-regexp (regexp-opt mylsl-types 'words))
(defvar mylsl-constant-regexp (regexp-opt mylsl-constants 'words))
(defvar mylsl-event-regexp (regexp-opt mylsl-events 'words))
(defvar mylsl-functions-regexp (regexp-opt mylsl-functions 'words))

In the above, we generate the regex for each keyword class, using the built-in function “regexp-opt”. We gave regexp-opt a second optional argument 'words. This will create a regex that match only if it is a complete word. So that, when a word is contained inside a longer word, it will not be highlighted. (For example, “for” is usually a keyword for looping, but if you have a user-defined function named “inform”, you don't want part of the word colored as “for”.)

(info "(elisp) Regexp Functions")

;; clear memory
(setq mylsl-keywords nil)
(setq mylsl-types nil)
(setq mylsl-constants nil)
(setq mylsl-events nil)
(setq mylsl-functions nil)

In the above, we clear the lists to save memory, because we don't need it anymore.

;; create the list for font-lock.
;; each class of keyword is given a particular face
(setq mylsl-font-lock-keywords
  `(
    (,mylsl-type-regexp . font-lock-type-face)
    (,mylsl-constant-regexp . font-lock-constant-face)
    (,mylsl-event-regexp . font-lock-builtin-face)
    (,mylsl-functions-regexp . font-lock-function-name-face)
    (,mylsl-keywords-regexp . font-lock-keyword-face)
    ;; note: order above matters. “mylsl-keywords-regexp” goes last because
    ;; otherwise the keyword “state” in the function “state_entry”
    ;; would be highlighted.
))

In the above, we create a list in preparation to feed it to “font-lock-defaults”.

Note that the highlighting mechanism of font-lock-defaults is based on first-come-first-serve basis, and once a piece of text got its coloring, it won't be changed. So, the order of your list is important. Make sure the smallest lengthed text goes last. (this won't fix all cases where a keyword matches part of other keywords. If your language has a lot such keywords, you need to use other forms to solve this problem. (info "(elisp) Search-based Fontification"))

The `( ,a ,b …) is a lisp special syntax to evaluate parts of elements inside the list. Inside the paren, elements preceded by a , will be evaluated.

Finally, we define our mode like this:

;; define the mode
(define-derived-mode mylsl-mode fundamental-mode
  "lsl mode"
  "Major mode for editing LSL (Linden Scripting Language)…"

  ;; code for syntax highlighting
  (setq font-lock-defaults '((mylsl-font-lock-keywords)))

  ;; clear memory
  (setq mylsl-keywords-regexp nil)
  (setq mylsl-types-regexp nil)
  (setq mylsl-constants-regexp nil)
  (setq mylsl-events-regexp nil)
  (setq mylsl-functions-regexp nil)
)

In the above, we based our mode on fundamental-mode, which is the most basic mode. If you are actually writing a mode for LSL, it makes sense to base it on c-mode, because the syntax is similar. Basing on a similar language's mode will save you time in coding many features, such as handling comment and indentation.

Syntax Coloring For Comments

For comment syntax coloring, you need to use syntax table. To have a command that does commenting and uncommenting, you'll need to write your own function. See: How To Add Comment Handling In Your Major Mode.

How to Name Your Mode

A major mode has a associated command name that invokes the mode, has a “major-mode” and “mode-name” buffer local variables associated with it. Of course, you also need to create the file name of the source code file. Also, all the symbols in your source code should start with some prefix such as “mylsl-” , because emacs does not have namespaces or a module system. You need to understand the basics of these issues. See: How To Name Your Emacs Major Mode.

Full Featured Language Mode

In this tutorial, we only covered syntax coloring of fixed strings. For many language, the syntax coloring are not fixed set of strings. For example, in XML, you have <xyz>…</xyz> pattern where the “xyz” can be anything.

In many languages, you have both fixed strings as keywords as well as complex patterns. For example, in HTML, when you have <h1>Heading</h1> or <b>important</b>, <i>slanted</i>, you usually also want to color the enclosed text. Here's a example of how emacs html-mode colors html:

<h1>Some String Inside Pattern Needs Color Depending On Tag</h1>
<a href="http://example.org/">complexity in coloring links</a>
<p>Note nesting issues <b>here</b>.</p>

Besides syntax coloring, a full featured language mode should also handle comments, indentation, keyword completion, function documentation lookup, function template insertion, graphical menus, supporting emacs's customize-group scheme, or any other features that may be useful for coding the language your mode is designed for.

This tutorial only gives a basic template for writing a language mode with syntax coloring. There are a lot details and conventions you need to know if you want to make your mode full featured.

(info "(elisp) Major Mode Conventions")

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