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24.6.10 Parser-based Font Lock

Besides simple syntactic font lock and regexp-based font lock, Emacs also provides complete syntactic font lock with the help of a parser. Currently, Emacs uses the tree-sitter library (see Parsing Program Source) for this purpose.

Parser-based font lock and other font lock mechanisms are not mutually exclusive. By default, if enabled, parser-based font lock runs first, replacing syntactic font lock, then the regexp-based font lock.

Although parser-based font lock doesn’t share the same customization variables with regexp-based font lock, it uses similar customization schemes. The tree-sitter counterpart of font-lock-keywords is treesit-font-lock-settings.

In general, tree-sitter fontification works as follows:

For more information about queries, patterns, and capture names, see Pattern Matching Tree-sitter Nodes.

To setup tree-sitter fontification, a major mode should first set treesit-font-lock-settings with the output of treesit-font-lock-rules, then call treesit-major-mode-setup.

Function: treesit-font-lock-rules &rest query-specs

This function is used to set treesit-font-lock-settings. It takes care of compiling queries and other post-processing, and outputs a value that treesit-font-lock-settings accepts. Here’s an example:

 :language 'javascript
 :feature 'constant
 :override t
 '((true) @font-lock-constant-face
   (false) @font-lock-constant-face)
 :language 'html
 :feature 'script
 "(script_element) @font-lock-builtin-face")

This function takes a series of query-specs, where each query-spec is a query preceded by one or more :keyword/value pairs. Each query is a tree-sitter query in either the string, s-expression or compiled form.

For each query, the :keyword/value pairs that precede it add meta information to it. The :language keyword declares query’s language. The :feature keyword sets the feature name of query. Users can control which features are enabled with treesit-font-lock-level and treesit-font-lock-feature-list (described below). These two keywords are mandatory.

Other keywords are optional:

:overridenilIf the region already has a face, discard the new face
tAlways apply the new face
appendAppend the new face to existing ones
prependPrepend the new face to existing ones
keepFill-in regions without an existing face

Lisp programs mark patterns in query with capture names (names that starts with @), and tree-sitter will return matched nodes tagged with those same capture names. For the purpose of fontification, capture names in query should be face names like font-lock-keyword-face. The captured node will be fontified with that face.

Capture names can also be function names, in which case the function is called with 4 arguments: node and override, start and end, where node is the node itself, override is the override property of the rule which captured this node, and start and end limits the region in which this function should fontify. (If this function wants to respect the override argument, it can use treesit-fontify-with-override.)

Beyond the 4 arguments presented, this function should accept more arguments as optional arguments for future extensibility.

If a capture name is both a face and a function, the face takes priority. If a capture name is neither a face nor a function, it is ignored.

Variable: treesit-font-lock-feature-list

This is a list of lists of feature symbols. Each element of the list is a list that represents a decoration level. treesit-font-lock-level controls which levels are activated.

Each element of the list is a list of the form (feature …), where each feature corresponds to the :feature value of a query defined in treesit-font-lock-rules. Removing a feature symbol from this list disables the corresponding query during font-lock.

Common feature names, for many programming languages, include definition, type, assignment, builtin, constant, keyword, string-interpolation, comment, doc, string, operator, preprocessor, escape-sequence, and key. Major modes are free to subdivide or extend these common features.

Some of these features warrant some explanation: definition highlights whatever is being defined, e.g., the function name in a function definition, the struct name in a struct definition, the variable name in a variable definition; assignment highlights the whatever is being assigned to, e.g., the variable or field in an assignment statement; key highlights keys in key-value pairs, e.g., keys in a JSON object, or a Python dictionary; doc highlights docstrings or doc-comments.

For example, the value of this variable could be:

((comment string doc) ; level 1
 (function-name keyword type builtin constant) ; level 2
 (variable-name string-interpolation key)) ; level 3

Major modes should set this variable before calling treesit-major-mode-setup.

For this variable to take effect, a Lisp program should call treesit-font-lock-recompute-features (which resets treesit-font-lock-settings accordingly), or treesit-major-mode-setup (which calls treesit-font-lock-recompute-features).

Variable: treesit-font-lock-settings

A list of settings for tree-sitter based font lock. The exact format of each setting is considered internal. One should always use treesit-font-lock-rules to set this variable.

Multi-language major modes should provide range functions in treesit-range-functions, and Emacs will set the ranges accordingly before fontifing a region (see Parsing Text in Multiple Languages).

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