How to Write a Emacs Major Mode for Syntax Coloring

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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.

Problem Description

You want to write a major mode for a new 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 the code:

(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. Now, just select the above 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}]

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

Writing a Mode for a Language that Has Hundreds of Keywords

Typically, a language has 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.
// 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!");
         }
     }
 }

Each type of keyword uses a different color:

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

;; define several class of keywords
(setq mylsl-keywords '("break" "default" "do" "else" "for" "if" "return" "state" "while") )
(setq mylsl-types '("float" "integer" "key" "list" "rotation" "string" "vector"))
(setq mylsl-constants '("ACTIVE" "AGENT" "ALL_SIDES" "ATTACH_BACK"))
(setq mylsl-events '("at_rot_target" "at_target" "attach"))
(setq mylsl-functions '("llAbs" "llAcos" "llAddToLandBanList" "llAddToLandPassList"))

In the above, each variable holds a list of keywords. (For real LSL mode, each list may have hundreds of elements.)

Now we generate the regex for each keyword group:

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

We use the built-in function regexp-opt. We give 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 them 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. 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 length 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.

Provide Your Mode to the Features List

At the end of your mode, you should add a provide, like this:

(provide 'mylsl-mode)

When a file with this line is loaded, emacs will add the symbol 'mylsl-mode to the variable named features (its value is a list). When some file calls (require 'mylsl-mode), emacs will first check if that symbol is in the variable features list, if not, then proceed to load it.

For detail, see: Emacs Lisp's Library System: What's require, load, load-file, autoload, feature?.

Now, that's it! Gather the above code and put them in a file, and you'll have a basic LSL mode!

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: Emacs Lisp: Implementing Comment Handling in a Major Mode.

How to Name Your Mode

There are several names associated with a major mode:

Also, all the symbols in your source code should start with some prefix such as “mylsl-”, because elisp does not have namespace 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.

Complex Syntax Coloring

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.

emacs html-mode syntax coloring screenshot 2013-07-31
emacs html-mode syntax coloring screenshot

Note color and underline for text inside the “h1” tag. Even though the text isn't any of the keyword in HTML language, but it needs to be syntax colored in a particular way.

Features of a Major Mode

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.

The following will help you implement other features for a major mode:

(info "(elisp) Major Mode Conventions")

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