Xah Lee, 2008-11
This page gives a practical example of writing a emacs major mode to do syntax coloring of your own language. You should have at least few months experience of coding emacs lisp.
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.
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)))
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)) tells emacs that when your mode is active, the syntax coloring should be set according to your keywords.
That's all there is to it. Now, when you invoke “math-lang-mode”, emacs will now syntax color the buffer's text. (you must have font-lock-mode on, if not, do “Alt+x 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}]
O My GOD, Emacs is beautiful!
Typically, a language may have hundreds of keywords. Elisp provide a way to generate regex for your keywords.
Suppose the you are writing a mode for the Linden Scripting Language, which has close to ~553 keywords. Here's a example of how to code it.
;; 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, first we define several lists, each one is a class of keywords in the language. Note that the keyword list in the above is truncated. Each list can have hundreds of keywords.
;; 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 also match on word boundary. 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 created 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, since 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, since 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.
For comment syntax coloring, you need to use syntax table. To have a commant that does commenting and uncommenting, you'll need to write your own function. For detail, see: How To Add Comment Handling In Your Major Mode.
Emacs lisp does not support Namespace, nor does it support lexical scope. This means, the symbols in your package should have unique names to avoid multiple packages having the same name that can override each other. This is done by adding a unique prefix to all names in your symbols.
If your mode is for public distribution, try to come up with a name that uniquely identify your mode. (examples: there's perl-mode and cperl-mode, xml-mode and nxml-mode, and there's html-mode, html-helper-mode, nxhtml-mode, several javascript modes named javascript-mode, js-mode, js2-mode, several irc modes variously named rcirc-mode, erc-mode, etc.)
Also, there is no technical relation of your mode's name and the file name. For example, your can have your mode named “mylsl-mode”, while the file name can be “lsl_mode_by_John.el”. This is because elisp does not enforce a relation between the package name and file name, unlike Java.
Normally, you just name your file the same as the value of the variable “major-mode” defined in your package. (“major-mode” is a predefined buffer local variable. It is set to the first argument of define-derived-mode.) So, for example, if you have “mylsl-mode”, then the file can be “mylsl-mode.el”. This is how majority of major mode package's files are named.
Besides syntax coloring, a full featured language mode should also handle comments, indentation, keyword completion, integrated documentation lookup, function template insertion, graphical menus, supporting emacs's customize-group scheme, or any other features that may be useful for coding your language.
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")
For a example source code of a full featured mode that are written from scratch and does not depend on other modes, see: xlsl-mode.
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