# Syntax Highlighting Syntax highlighting is a very common feature in applications that deal with code. Tree-sitter has built-in support for syntax highlighting via the [`tree-sitter-highlight`][highlight crate] library, which is now used on GitHub.com for highlighting code written in several languages. You can also perform syntax highlighting at the command line using the `tree-sitter highlight` command. This document explains how the Tree-sitter syntax highlighting system works, using the command line interface. If you are using `tree-sitter-highlight` library (either from C or from Rust), all of these concepts are still applicable, but the configuration data is provided using in-memory objects, rather than files. ## Overview All the files needed to highlight a given language are normally included in the same git repository as the Tree-sitter grammar for that language (for example, [`tree-sitter-javascript`][js grammar], [`tree-sitter-ruby`][ruby grammar]). To run syntax highlighting from the command-line, three types of files are needed: 1. Per-user configuration in `~/.config/tree-sitter/config.json` (see the [init-config][init-config] page for more info). 2. Language configuration in grammar repositories' `tree-sitter.json` files (see the [init][init] page for more info). 3. Tree queries in the grammars repositories' `queries` folders. For an example of the language-specific files, see the [`tree-sitter.json` file][ts json] and [`queries` directory][queries] in the `tree-sitter-ruby` repository. The following sections describe the behavior of each file. ## Language Configuration The `tree-sitter.json` file is used by the Tree-sitter CLI. Within this file, the CLI looks for data nested under the top-level `"grammars"` key. This key is expected to contain an array of objects with the following keys: ### Basics These keys specify basic information about the parser: - `scope` (required) — A string like `"source.js"` that identifies the language. We strive to match the scope names used by popular [TextMate grammars][textmate] and by the [Linguist][linguist] library. - `path` (optional) — A relative path from the directory containing `tree-sitter.json` to another directory containing the `src/` folder, which contains the actual generated parser. The default value is `"."` (so that `src/` is in the same folder as `tree-sitter.json`), and this very rarely needs to be overridden. - `external-files` (optional) — A list of relative paths from the root dir of a parser to files that should be checked for modifications during recompilation. This is useful during development to have changes to other files besides scanner.c be picked up by the cli. ### Language Detection These keys help to decide whether the language applies to a given file: - `file-types` — An array of filename suffix strings. The grammar will be used for files whose names end with one of these suffixes. Note that the suffix may match an *entire* filename. - `first-line-regex` — A regex pattern that will be tested against the first line of a file to determine whether this language applies to the file. If present, this regex will be used for any file whose language does not match any grammar's `file-types`. - `content-regex` — A regex pattern that will be tested against the contents of the file to break ties in cases where multiple grammars matched the file using the above two criteria. If the regex matches, this grammar will be preferred over another grammar with no `content-regex`. If the regex does not match, a grammar with no `content-regex` will be preferred over this one. - `injection-regex` — A regex pattern that will be tested against a *language name* ito determine whether this language should be used for a potential *language injection* site. Language injection is described in more detail in [a later section](#language-injection). ### Query Paths These keys specify relative paths from the directory containing `tree-sitter.json` to the files that control syntax highlighting: - `highlights` — Path to a *highlight query*. Default: `queries/highlights.scm` - `locals` — Path to a *local variable query*. Default: `queries/locals.scm`. - `injections` — Path to an *injection query*. Default: `queries/injections.scm`. The behaviors of these three files are described in the next section. ## Queries Tree-sitter's syntax highlighting system is based on *tree queries*, which are a general system for pattern-matching on Tree-sitter's syntax trees. See [this section][pattern matching] of the documentation for more information about tree queries. Syntax highlighting is controlled by *three* different types of query files that are usually included in the `queries` folder. The default names for the query files use the `.scm` file. We chose this extension because it commonly used for files written in [Scheme][scheme], a popular dialect of Lisp, and these query files use a Lisp-like syntax. ### Highlights The most important query is called the highlights query. The highlights query uses *captures* to assign arbitrary *highlight names* to different nodes in the tree. Each highlight name can then be mapped to a color (as described in the [init-config command][theme]). Commonly used highlight names include `keyword`, `function`, `type`, `property`, and `string`. Names can also be dot-separated like `function.builtin`. #### Example Go Snippet For example, consider the following Go code: ```go func increment(a int) int { return a + 1 } ``` With this syntax tree: ```scheme (source_file (function_declaration name: (identifier) parameters: (parameter_list (parameter_declaration name: (identifier) type: (type_identifier))) result: (type_identifier) body: (block (return_statement (expression_list (binary_expression left: (identifier) right: (int_literal))))))) ``` #### Example Query Suppose we wanted to render this code with the following colors: - keywords `func` and `return` in purple - function `increment` in blue - type `int` in green - number `5` brown We can assign each of these categories a *highlight name* using a query like this: ```scheme ; highlights.scm "func" @keyword "return" @keyword (type_identifier) @type (int_literal) @number (function_declaration name: (identifier) @function) ``` Then, in our config file, we could map each of these highlight names to a color: ```json { "theme": { "keyword": "purple", "function": "blue", "type": "green", "number": "brown" } } ``` #### Highlights Result Running `tree-sitter highlight` on this Go file would produce output like this: 
func increment(a int) int {
    return a + 1
}
### Local Variables Good syntax highlighting helps the reader to quickly distinguish between the different types of *entities* in their code. Ideally, if a given entity appears in *multiple* places, it should be colored the same in each place. The Tree-sitter syntax highlighting system can help you to achieve this by keeping track of local scopes and variables. The *local variables* query is different from the highlights query in that, while the highlights query uses *arbitrary* capture names, which can then be mapped to colors, the locals variable query uses a fixed set of capture names, each of which has a special meaning. The capture names are as follows: - `@local.scope` — indicates that a syntax node introduces a new local scope. - `@local.definition` — indicates that a syntax node contains the *name* of a definition within the current local scope. - `@local.reference` — indicates that a syntax node contains the *name*, which *may* refer to an earlier definition within some enclosing scope. When highlighting a file, Tree-sitter will keep track of the set of scopes that contains any given position, and the set of definitions within each scope. When processing a syntax node that is captured as a `local.reference`, Tree-sitter will try to find a definition for a name that matches the node's text. If it finds a match, Tree-sitter will ensure that the *reference*, and the *definition* are colored the same. The information produced by this query can also be *used* by the highlights query. You can *disable* a pattern for nodes, which have been identified as local variables by adding the predicate `(#is-not? local)` to the pattern. This is used in the example below: #### Example Ruby Snippet Consider this Ruby code: ```ruby def process_list(list) context = current_context list.map do |item| process_item(item, context) end end item = 5 list = [item] ``` With this syntax tree: ```scheme (program (method name: (identifier) parameters: (method_parameters (identifier)) (assignment left: (identifier) right: (identifier)) (method_call method: (call receiver: (identifier) method: (identifier)) block: (do_block (block_parameters (identifier)) (method_call method: (identifier) arguments: (argument_list (identifier) (identifier)))))) (assignment left: (identifier) right: (integer)) (assignment left: (identifier) right: (array (identifier)))) ``` There are several types of names within this method: - `process_list` is a method. - Within this method, `list` is a formal parameter - `context` is a local variable. - `current_context` is *not* a local variable, so it must be a method. - Within the `do` block, `item` is a formal parameter - Later on, `item` and `list` are both local variables (not formal parameters). #### Example Queries Let's write some queries that let us clearly distinguish between these types of names. First, set up the highlighting query, as described in the previous section. We'll assign distinct colors to method calls, method definitions, and formal parameters: ```scheme ; highlights.scm (call method: (identifier) @function.method) (method_call method: (identifier) @function.method) (method name: (identifier) @function.method) (method_parameters (identifier) @variable.parameter) (block_parameters (identifier) @variable.parameter) ((identifier) @function.method (#is-not? local)) ``` Then, we'll set up a local variable query to keep track of the variables and scopes. Here, we're indicating that methods and blocks create local *scopes*, parameters and assignments create *definitions*, and other identifiers should be considered *references*: ```scheme ; locals.scm (method) @local.scope (do_block) @local.scope (method_parameters (identifier) @local.definition) (block_parameters (identifier) @local.definition) (assignment left:(identifier) @local.definition) (identifier) @local.reference ``` #### Locals Result Running `tree-sitter highlight` on this ruby file would produce output like this: 
def process_list(list)
  context = current_context
  list.map do |item|
    process_item(item, context)
  end
end

item = 5
list = [item]
### Language Injection Some source files contain code written in multiple different languages. Examples include: - HTML files, which can contain JavaScript inside `