tree-sitter/docs/section-3-creating-parsers.md

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Creating Parsers	creating-parsers

Creating parsers

Developing Tree-sitter grammars can have a difficult learning curve, but once you get the hang of it, it can be fun and even zen-like. This document will help you to get started and to develop a useful mental model.

Getting Started

Dependencies

In order to develop a Tree-sitter parser, there are two dependencies that you need to install:

• Node.js - Tree-sitter grammars are written in JavaScript, and Tree-sitter uses Node.js to interpret JavaScript files. It requires the node command to be in one of the directories in your PATH. You'll need Node.js version 6.0 or greater.
• A C Compiler - Tree-sitter creates parsers that are written in C. In order to run and test these parsers with the tree-sitter parse or tree-sitter test commands, you must have a C compiler installed. Tree-sitter will try to look for these compilers in the standard places for each platform.

Installation

To create a Tree-sitter parser, you need to use the tree-sitter CLI. You can install the CLI in a few different ways:

• Build the tree-sitter-cli Rust crate from source using cargo, the Rust package manager. This works on any platform. See the contributing docs for more information.
• Install the tree-sitter-cli Node.js module using npm, the Node package manager. This approach is fast, but is only works on certain platforms, because it relies on pre-built binaries.
• Download a binary for your platform from the latest GitHub release, and put it into a directory on your PATH.

Project Setup

The preferred convention is to name the parser repository "tree-sitter-" followed by the name of the language.

mkdir tree-sitter-${YOUR_LANGUAGE_NAME}
cd tree-sitter-${YOUR_LANGUAGE_NAME}

You can use the tree-sitter CLI tool to set up your project, and allows your parser to be used from multiple languages.

# This will prompt you for input
tree-sitter init

Once you have installed the CLI and run through the init command's prompts, a file called grammar.js should exist with the following contents:

/// <reference types="tree-sitter-cli/dsl" />
// @ts-check

module.exports = grammar({
  name: 'YOUR_LANGUAGE_NAME',

  rules: {
    // TODO: add the actual grammar rules
    source_file: $ => 'hello'
  }
});

Now, run the following command:

tree-sitter generate

This will generate the C code required to parse this trivial language, as well as a few files that are needed to compile and load this native parser as a Node.js module.

You can test this parser by creating a source file with the contents "hello" and parsing it:

echo 'hello' > example-file
tree-sitter parse example-file

Alternatively, in Windows PowerShell:

"hello" | Out-File example-file -Encoding utf8
tree-sitter parse example-file

This should print the following:

(source_file [0, 0] - [1, 0])

You now have a working parser.

Finally, look back at the triple-slash and @ts-check comments in grammar.js; these tell your editor to provide documentation and type information as you edit your grammar. For these to work, you must download Tree-sitter's TypeScript API from npm into a node_modules directory in your project:

npm install

Tool Overview

Let's go over all of the functionality of the tree-sitter command line tool.

Command: init

The first command you will likely run is the init command. This command sets up an empty repository with everything you need to get going with a grammar repository. It only has one optional argument, --update, which will update outdated generated files, if needed.

The main file of interest for users to configure is tree-sitter.json, which tells the CLI information about your grammar, such as the queries.

Structure of tree-sitter.json

The grammars field

This field is an array of objects, you typically only need one object in this array, unless your repo has multiple grammars (e.g. like Typescript and TSX)

Basics

These keys specify basic information about the parser:

• scope (required) - A string like "source.js" that identifies the language. Currently, we strive to match the scope names used by popular TextMate grammars and by the Linguist library.

• path - 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 - 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 in order 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 in order 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 in order to determine whether this language should be used for a potential language injection site. Language injection is described in more detail in a later section.

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.
• tags - Path to an tag query. Default: queries/tags.scm.

The behaviors of these three files are described in the next section.

The metadata field

This field contains information that tree-sitter will use to populate relevant bindings' files, especially their versions. A future bump-version and publish subcommand will leverage this version information as well. Typically, this will all be set up when you run tree-sitter init, but you are welcome to update it as you see fit.

• version (required) - The current version of your grammar, which should follow semver
• license - The license of your grammar, which should be a valid SPDX license
• description - The brief description of your grammar
• authors (required) - An array of objects that contain a name field, and optionally an email and url field. Each field is a string
• links - An object that contains a repository field, and optionally a homepage field. Each field is a string
• namespace - The namespace for the Java and Kotlin bindings, defaults to io.github.tree-sitter if not provided

The bindings field

This field controls what bindings are generated when the init command is run. Each key is a language name, and the value is a boolean.

• c (default: true)
• go (default: true)
• java (default: false)
• kotlin (default: false)
• node (default: true)
• python (default: true)
• rust (default: true)
• swift (default: false)

Command: version

The version command prints the version of the tree-sitter CLI tool that you have installed.

tree-sitter version 1.0.0

The only argument is the version itself, which is the first positional argument. This will update the version in several files, if they exist:

• tree-sitter.json
• Cargo.toml
• package.json
• Makefile
• CMakeLists.txt
• pyproject.toml

As a grammar author, you should keep the version of your grammar in sync across different bindings. However, doing so manually is error-prone and tedious, so this command takes care of the burden.

Command: generate

The most important command you'll use is tree-sitter generate. This command reads the grammar.js file in your current working directory and creates a file called src/parser.c, which implements the parser. After making changes to your grammar, just run tree-sitter generate again.

The first time you run tree-sitter generate, it will also generate a few other files for bindings for the following languages:

C/C++

• Makefile - This file tells make how to compile your language.
• bindings/c/tree-sitter-language.h - This file provides the C interface of your language.
• bindings/c/tree-sitter-language.pc - This file provides pkg-config metadata about your language's C library.
• src/tree_sitter/parser.h - This file provides some basic C definitions that are used in your generated parser.c file.
• src/tree_sitter/alloc.h - This file provides some memory allocation macros that are to be used in your external scanner, if you have one.
• src/tree_sitter/array.h - This file provides some array macros that are to be used in your external scanner, if you have one.

Go

• bindings/go/binding.go - This file wraps your language in a Go module.
• bindings/go/binding_test.go - This file contains a test for the Go package.

Node

• binding.gyp - This file tells Node.js how to compile your language.
• bindings/node/index.js - This is the file that Node.js initially loads when using your language.
• bindings/node/binding.cc - This file wraps your language in a JavaScript module for Node.js.

Python

• pyproject.toml - This file is the manifest of the Python package.
• setup.py - This file tells Python how to compile your language.
• bindings/python/binding.c - This file wraps your language in a Python module.
• bindings/python/tree_sitter_language/__init__.py - This file tells Python how to load your language.
• bindings/python/tree_sitter_language/__init__.pyi - This file provides type hints for your parser when used in Python.
• bindings/python/tree_sitter_language/py.typed - This file provides type hints for your parser when used in Python.

Rust

• Cargo.toml - This file is the manifest of the Rust package.
• bindings/rust/lib.rs - This file wraps your language in a Rust crate when used in Rust.
• bindings/rust/build.rs - This file wraps the building process for the Rust crate.

Swift

• Package.swift - This file tells Swift how to compile your language.
• bindings/swift/TreeSitterLanguage/language.h - This file wraps your language in a Swift module when used in Swift.

If there is an ambiguity or local ambiguity in your grammar, Tree-sitter will detect it during parser generation, and it will exit with a Unresolved conflict error message. See below for more information on these errors.

Command: build

The build command compiles your parser into a dynamically-loadable library, either as a shared object (.so, .dylib, or .dll) or as a WASM module.

You can change the compiler executable via the CC environment variable and add extra flags via CFLAGS. For macOS or iOS, you can set MACOSX_DEPLOYMENT_TARGET or IPHONEOS_DEPLOYMENT_TARGET respectively to define the minimum supported version.

You can specify whether to compile it as a wasm module with the --wasm/-w flag, and you can opt to use docker or podman to supply emscripten with the --docker/-d flag. This removes the need to install emscripten on your machine locally.

You can specify where to output the shared object file (native or WASM) with the --output/-o flag, which accepts either an absolute path or relative path. Note that if you don't supply this flag, the CLI will attempt to figure out what the language name is based on the parent directory (so building in tree-sitter-javascript will resolve to javascript) to use for the output file. If it can't figure it out, it will default to parser, thus generating parser.so or parser.wasm in the current working directory.

Lastly, you can also specify a path to the actual grammar directory, in case you are not currently in one. This is done by providing a path as the first positional argument.

Example:

tree-sitter build --wasm --output ./build/parser.wasm tree-sitter-javascript

Notice how the tree-sitter-javascript argument is the first positional argument.

Command: test

The tree-sitter test command allows you to easily test that your parser is working correctly.

For each rule that you add to the grammar, you should first create a test that describes how the syntax trees should look when parsing that rule. These tests are written using specially-formatted text files in the test/corpus/ directory within your parser's root folder.

For example, you might have a file called test/corpus/statements.txt that contains a series of entries like this:

==================
Return statements
==================

func x() int {
  return 1;
}

---

(source_file
  (function_definition
    (identifier)
    (parameter_list)
    (primitive_type)
    (block
      (return_statement (number)))))

• The name of each test is written between two lines containing only = (equal sign) characters.

• Then the input source code is written, followed by a line containing three or more - (dash) characters.

• Then, the expected output syntax tree is written as an S-expression. The exact placement of whitespace in the S-expression doesn't matter, but ideally the syntax tree should be legible. Note that the S-expression does not show syntax nodes like func, ( and ;, which are expressed as strings and regexes in the grammar. It only shows the named nodes, as described in this section of the page on parser usage.

  The expected output section can also optionally show the field names associated with each child node. To include field names in your tests, you write a node's field name followed by a colon, before the node itself in the S-expression:

(source_file
  (function_definition
    name: (identifier)
    parameters: (parameter_list)
    result: (primitive_type)
    body: (block
      (return_statement (number)))))

• If your language's syntax conflicts with the === and --- test separators, you can optionally add an arbitrary identical suffix (in the below example, |||) to disambiguate them:

==================|||
Basic module
==================|||

---- MODULE Test ----
increment(n) == n + 1
====

---|||

(source_file
  (module (identifier)
    (operator (identifier)
      (parameter_list (identifier))
      (plus (identifier_ref) (number)))))

These tests are important. They serve as the parser's API documentation, and they can be run every time you change the grammar to verify that everything still parses correctly.

By default, the tree-sitter test command runs all of the tests in your test/corpus/ folder. To run a particular test, you can use the -f flag:

tree-sitter test -f 'Return statements'

The recommendation is to be comprehensive in adding tests. If it's a visible node, add it to a test file in your test/corpus directory. It's typically a good idea to test all of the permutations of each language construct. This increases test coverage, but doubly acquaints readers with a way to examine expected outputs and understand the "edges" of a language.

Attributes

Tests can be annotated with a few attributes. Attributes must be put in the header, below the test name, and start with a :. A couple of attributes also take in a parameter, which require the use of parenthesis.

Note: If you'd like to supply in multiple parameters, e.g. to run tests on multiple platforms or to test multiple languages, you can repeat the attribute on a new line.

The following attributes are available:

• :skip — This attribute will skip the test when running tree-sitter test. This is useful when you want to temporarily disable running a test without deleting it.
• :error — This attribute will assert that the parse tree contains an error. It's useful to just validate that a certain input is invalid without displaying the whole parse tree, as such you should omit the parse tree below the --- line.
• :fail-fast — This attribute will stop the testing additional tests if the test marked with this attribute fails.
• :language(LANG) — This attribute will run the tests using the parser for the specified language. This is useful for multi-parser repos, such as XML and DTD, or Typescript and TSX. The default parser used will always be the first entry in the grammars field in the tree-sitter.json config file, so having a way to pick a second or even third parser is useful.
• :platform(PLATFORM) — This attribute specifies the platform on which the test should run. It is useful to test platform-specific behavior (e.g. Windows newlines are different from Unix). This attribute must match up with Rust's std::env::consts::OS.

Examples using attributes:

=========================
Test that will be skipped
:skip
=========================

int main() {}

-------------------------

====================================
Test that will run on Linux or macOS

:platform(linux)
:platform(macos)
====================================

int main() {}

------------------------------------

========================================================================
Test that expects an error, and will fail fast if there's no parse error
:fail-fast
:error
========================================================================

int main ( {}

------------------------------------------------------------------------

=================================================
Test that will parse with both Typescript and TSX
:language(typescript)
:language(tsx)
=================================================

console.log('Hello, world!');

-------------------------------------------------

Automatic Compilation

You might notice that the first time you run tree-sitter test after regenerating your parser, it takes some extra time. This is because Tree-sitter automatically compiles your C code into a dynamically-loadable library. It recompiles your parser as-needed whenever you update it by re-running tree-sitter generate.

Syntax Highlighting Tests

The tree-sitter test command will also run any syntax highlighting tests in the test/highlight folder, if it exists. For more information about syntax highlighting tests, see the syntax highlighting page.

Command: parse

You can run your parser on an arbitrary file using tree-sitter parse. This will print the resulting the syntax tree, including nodes' ranges and field names, like this:

(source_file [0, 0] - [3, 0]
  (function_declaration [0, 0] - [2, 1]
    name: (identifier [0, 5] - [0, 9])
    parameters: (parameter_list [0, 9] - [0, 11])
    result: (type_identifier [0, 12] - [0, 15])
    body: (block [0, 16] - [2, 1]
      (return_statement [1, 2] - [1, 10]
        (expression_list [1, 9] - [1, 10]
          (int_literal [1, 9] - [1, 10]))))))

You can pass any number of file paths and glob patterns to tree-sitter parse, and it will parse all of the given files. The command will exit with a non-zero status code if any parse errors occurred. Passing the --cst flag will output a pretty-printed CST instead of the normal S-expression representation. You can also prevent the syntax trees from being printed using the --quiet flag. Additionally, the --stat flag prints out aggregated parse success/failure information for all processed files. This makes tree-sitter parse usable as a secondary testing strategy: you can check that a large number of files parse without error:

tree-sitter parse 'examples/**/*.go' --quiet --stat

Command: highlight

You can run syntax highlighting on an arbitrary file using tree-sitter highlight. This can either output colors directly to your terminal using ansi escape codes, or produce HTML (if the --html flag is passed). For more information, see the syntax highlighting page.

The Grammar DSL

The following is a complete list of built-in functions you can use in your grammar.js to define rules. Use-cases for some of these functions will be explained in more detail in later sections.

• Symbols (the $ object) - Every grammar rule is written as a JavaScript function that takes a parameter conventionally called $. The syntax $.identifier is how you refer to another grammar symbol within a rule. Names starting with $.MISSING or $.UNEXPECTED should be avoided as they have special meaning for the tree-sitter test command.

• String and Regex literals - The terminal symbols in a grammar are described using JavaScript strings and regular expressions. Of course during parsing, Tree-sitter does not actually use JavaScript's regex engine to evaluate these regexes; it generates its own regex-matching logic as part of each parser. Regex literals are just used as a convenient way of writing regular expressions in your grammar.

• Regex Limitations - Currently, only a subset of the Regex engine is actually supported. This is due to certain features like lookahead and lookaround assertions not feasible to use in an LR(1) grammar, as well as certain flags being unnecessary for tree-sitter. However, plenty of features are supported by default:

  • Character classes
  • Character ranges
  • Character sets
  • Quantifiers
  • Alternation
  • Grouping
  • Unicode character escapes
  • Unicode property escapes

• Sequences : seq(rule1, rule2, ...) - This function creates a rule that matches any number of other rules, one after another. It is analogous to simply writing multiple symbols next to each other in EBNF notation.

• Alternatives : choice(rule1, rule2, ...) - This function creates a rule that matches one of a set of possible rules. The order of the arguments does not matter. This is analogous to the | (pipe) operator in EBNF notation.

• Repetitions : repeat(rule) - This function creates a rule that matches zero-or-more occurrences of a given rule. It is analogous to the {x} (curly brace) syntax in EBNF notation.

• Repetitions : repeat1(rule) - This function creates a rule that matches one-or-more occurrences of a given rule. The previous repeat rule is implemented in terms of repeat1 but is included because it is very commonly used.

• Options : optional(rule) - This function creates a rule that matches zero or one occurrence of a given rule. It is analogous to the [x] (square bracket) syntax in EBNF notation.

• Precedence : prec(number, rule) - This function marks the given rule with a numerical precedence which will be used to resolve LR(1) Conflicts at parser-generation time. When two rules overlap in a way that represents either a true ambiguity or a local ambiguity given one token of lookahead, Tree-sitter will try to resolve the conflict by matching the rule with the higher precedence. The default precedence of all rules is zero. This works similarly to the precedence directives in Yacc grammars.

• Left Associativity : prec.left([number], rule) - This function marks the given rule as left-associative (and optionally applies a numerical precedence). When an LR(1) conflict arises in which all of the rules have the same numerical precedence, Tree-sitter will consult the rules' associativity. If there is a left-associative rule, Tree-sitter will prefer matching a rule that ends earlier. This works similarly to associativity directives in Yacc grammars.

• Right Associativity : prec.right([number], rule) - This function is like prec.left, but it instructs Tree-sitter to prefer matching a rule that ends later.

• Dynamic Precedence : prec.dynamic(number, rule) - This function is similar to prec, but the given numerical precedence is applied at runtime instead of at parser generation time. This is only necessary when handling a conflict dynamically using the conflicts field in the grammar, and when there is a genuine ambiguity: multiple rules correctly match a given piece of code. In that event, Tree-sitter compares the total dynamic precedence associated with each rule, and selects the one with the highest total. This is similar to dynamic precedence directives in Bison grammars.

• Tokens : token(rule) - This function marks the given rule as producing only a single token. Tree-sitter's default is to treat each String or RegExp literal in the grammar as a separate token. Each token is matched separately by the lexer and returned as its own leaf node in the tree. The token function allows you to express a complex rule using the functions described above (rather than as a single regular expression) but still have Tree-sitter treat it as a single token. The token function will only accept terminal rules, so token($.foo) will not work. You can think of it as a shortcut for squashing complex rules of strings or regexes down to a single token.

• Immediate Tokens : token.immediate(rule) - Usually, whitespace (and any other extras, such as comments) is optional before each token. This function means that the token will only match if there is no whitespace.

• Aliases : alias(rule, name) - This function causes the given rule to appear with an alternative name in the syntax tree. If name is a symbol, as in alias($.foo, $.bar), then the aliased rule will appear as a named node called bar. And if name is a string literal, as in alias($.foo, 'bar'), then the aliased rule will appear as an anonymous node, as if the rule had been written as the simple string.

• Field Names : field(name, rule) - This function assigns a field name to the child node(s) matched by the given rule. In the resulting syntax tree, you can then use that field name to access specific children.

In addition to the name and rules fields, grammars have a few other optional public fields that influence the behavior of the parser.

• extras - an array of tokens that may appear anywhere in the language. This is often used for whitespace and comments. The default value of extras is to accept whitespace. To control whitespace explicitly, specify extras: $ => [] in your grammar.
• inline - an array of rule names that should be automatically removed from the grammar by replacing all of their usages with a copy of their definition. This is useful for rules that are used in multiple places but for which you don't want to create syntax tree nodes at runtime.
• conflicts - an array of arrays of rule names. Each inner array represents a set of rules that's involved in an LR(1) conflict that is intended to exist in the grammar. When these conflicts occur at runtime, Tree-sitter will use the GLR algorithm to explore all of the possible interpretations. If multiple parses end up succeeding, Tree-sitter will pick the subtree whose corresponding rule has the highest total dynamic precedence.
• externals - an array of token names which can be returned by an external scanner. External scanners allow you to write custom C code which runs during the lexing process in order to handle lexical rules (e.g. Python's indentation tokens) that cannot be described by regular expressions.
• precedences - an array of array of strings, where each array of strings defines named precedence levels in descending order. These names can be used in the prec functions to define precedence relative only to other names in the array, rather than globally. Can only be used with parse precedence, not lexical precedence.
• word - the name of a token that will match keywords for the purpose of the keyword extraction optimization.
• supertypes an array of hidden rule names which should be considered to be 'supertypes' in the generated node types file.

Writing the Grammar

Writing a grammar requires creativity. There are an infinite number of CFGs (context-free grammars) that can be used to describe any given language. In order to produce a good Tree-sitter parser, you need to create a grammar with two important properties:

1. An intuitive structure - Tree-sitter's output is a concrete syntax tree; each node in the tree corresponds directly to a terminal or non-terminal symbol in the grammar. So in order to produce an easy-to-analyze tree, there should be a direct correspondence between the symbols in your grammar and the recognizable constructs in the language. This might seem obvious, but it is very different from the way that context-free grammars are often written in contexts like language specifications or Yacc/Bison parsers.

2. A close adherence to LR(1) - Tree-sitter is based on the GLR parsing algorithm. This means that while it can handle any context-free grammar, it works most efficiently with a class of context-free grammars called LR(1) Grammars. In this respect, Tree-sitter's grammars are similar to (but less restrictive than) Yacc and Bison grammars, but different from ANTLR grammars, Parsing Expression Grammars, or the ambiguous grammars commonly used in language specifications.

It's unlikely that you'll be able to satisfy these two properties just by translating an existing context-free grammar directly into Tree-sitter's grammar format. There are a few kinds of adjustments that are often required. The following sections will explain these adjustments in more depth.

The First Few Rules

It's usually a good idea to find a formal specification for the language you're trying to parse. This specification will most likely contain a context-free grammar. As you read through the rules of this CFG, you will probably discover a complex and cyclic graph of relationships. It might be unclear how you should navigate this graph as you define your grammar.

Although languages have very different constructs, their constructs can often be categorized in to similar groups like Declarations, Definitions, Statements, Expressions, Types, and Patterns. In writing your grammar, a good first step is to create just enough structure to include all of these basic groups of symbols. For a language like Go, you might start with something like this:

{
  // ...

  rules: {
    source_file: $ => repeat($._definition),

    _definition: $ => choice(
      $.function_definition
      // TODO: other kinds of definitions
    ),

    function_definition: $ => seq(
      'func',
      $.identifier,
      $.parameter_list,
      $._type,
      $.block
    ),

    parameter_list: $ => seq(
      '(',
       // TODO: parameters
      ')'
    ),

    _type: $ => choice(
      'bool'
      // TODO: other kinds of types
    ),

    block: $ => seq(
      '{',
      repeat($._statement),
      '}'
    ),

    _statement: $ => choice(
      $.return_statement
      // TODO: other kinds of statements
    ),

    return_statement: $ => seq(
      'return',
      $._expression,
      ';'
    ),

    _expression: $ => choice(
      $.identifier,
      $.number
      // TODO: other kinds of expressions
    ),

    identifier: $ => /[a-z]+/,

    number: $ => /\d+/
  }
}

Some of the details of this grammar will be explained in more depth later on, but if you focus on the TODO comments, you can see that the overall strategy is breadth-first. Notably, this initial skeleton does not need to directly match an exact subset of the context-free grammar in the language specification. It just needs to touch on the major groupings of rules in as simple and obvious a way as possible.

With this structure in place, you can now freely decide what part of the grammar to flesh out next. For example, you might decide to start with types. One-by-one, you could define the rules for writing basic types and composing them into more complex types:

{
  // ...

  _type: $ => choice(
    $.primitive_type,
    $.array_type,
    $.pointer_type
  ),

  primitive_type: $ => choice(
    'bool',
    'int'
  ),

  array_type: $ => seq(
    '[',
    ']',
    $._type
  ),

  pointer_type: $ => seq(
    '*',
    $._type
  )
}

After developing the type sublanguage a bit further, you might decide to switch to working on statements or expressions instead. It's often useful to check your progress by trying to parse some real code using tree-sitter parse.

And remember to add tests for each rule in your test/corpus folder!

Structuring Rules Well

Imagine that you were just starting work on the Tree-sitter JavaScript parser. Naively, you might try to directly mirror the structure of the ECMAScript Language Spec. To illustrate the problem with this approach, consider the following line of code:

return x + y;

According to the specification, this line is a ReturnStatement, the fragment x + y is an AdditiveExpression, and x and y are both IdentifierReferences. The relationship between these constructs is captured by a complex series of production rules:

ReturnStatement          ->  'return' Expression
Expression               ->  AssignmentExpression
AssignmentExpression     ->  ConditionalExpression
ConditionalExpression    ->  LogicalORExpression
LogicalORExpression      ->  LogicalANDExpression
LogicalANDExpression     ->  BitwiseORExpression
BitwiseORExpression      ->  BitwiseXORExpression
BitwiseXORExpression     ->  BitwiseANDExpression
BitwiseANDExpression     ->  EqualityExpression
EqualityExpression       ->  RelationalExpression
RelationalExpression     ->  ShiftExpression
ShiftExpression          ->  AdditiveExpression
AdditiveExpression       ->  MultiplicativeExpression
MultiplicativeExpression ->  ExponentiationExpression
ExponentiationExpression ->  UnaryExpression
UnaryExpression          ->  UpdateExpression
UpdateExpression         ->  LeftHandSideExpression
LeftHandSideExpression   ->  NewExpression
NewExpression            ->  MemberExpression
MemberExpression         ->  PrimaryExpression
PrimaryExpression        ->  IdentifierReference

The language spec encodes the twenty different precedence levels of JavaScript expressions using twenty levels of indirection between IdentifierReference and Expression. If we were to create a concrete syntax tree representing this statement according to the language spec, it would have twenty levels of nesting, and it would contain nodes with names like BitwiseXORExpression, which are unrelated to the actual code.

Using Precedence

To produce a readable syntax tree, we'd like to model JavaScript expressions using a much flatter structure like this:

{
  // ...

  _expression: $ => choice(
    $.identifier,
    $.unary_expression,
    $.binary_expression,
    // ...
  ),

  unary_expression: $ => choice(
    seq('-', $._expression),
    seq('!', $._expression),
    // ...
  ),

  binary_expression: $ => choice(
    seq($._expression, '*', $._expression),
    seq($._expression, '+', $._expression),
    // ...
  ),
}

Of course, this flat structure is highly ambiguous. If we try to generate a parser, Tree-sitter gives us an error message:

Error: Unresolved conflict for symbol sequence:

  '-'  _expression  •  '*'  …

Possible interpretations:

  1:  '-'  (binary_expression  _expression  •  '*'  _expression)
  2:  (unary_expression  '-'  _expression)  •  '*'  …

Possible resolutions:

  1:  Specify a higher precedence in `binary_expression` than in the other rules.
  2:  Specify a higher precedence in `unary_expression` than in the other rules.
  3:  Specify a left or right associativity in `unary_expression`
  4:  Add a conflict for these rules: `binary_expression` `unary_expression`

Note: The • character in the error message indicates where exactly during parsing the conflict occurs, or in other words, where the parser is encountering ambiguity.

For an expression like -a * b, it's not clear whether the - operator applies to the a * b or just to the a. This is where the prec function described above comes into play. By wrapping a rule with prec, we can indicate that certain sequence of symbols should bind to each other more tightly than others. For example, the '-', $._expression sequence in unary_expression should bind more tightly than the $._expression, '+', $._expression sequence in binary_expression:

{
  // ...

  unary_expression: $ => prec(2, choice(
    seq('-', $._expression),
    seq('!', $._expression),
    // ...
  ))
}

Using Associativity

Applying a higher precedence in unary_expression fixes that conflict, but there is still another conflict:

Error: Unresolved conflict for symbol sequence:

  _expression  '*'  _expression  •  '*'  …

Possible interpretations:

  1:  _expression  '*'  (binary_expression  _expression  •  '*'  _expression)
  2:  (binary_expression  _expression  '*'  _expression)  •  '*'  …

Possible resolutions:

  1:  Specify a left or right associativity in `binary_expression`
  2:  Add a conflict for these rules: `binary_expression`

For an expression like a * b * c, it's not clear whether we mean a * (b * c) or (a * b) * c. This is where prec.left and prec.right come into use. We want to select the second interpretation, so we use prec.left.

{
  // ...

  binary_expression: $ => choice(
    prec.left(2, seq($._expression, '*', $._expression)),
    prec.left(1, seq($._expression, '+', $._expression)),
    // ...
  ),
}

Hiding Rules

You may have noticed in the above examples that some of the grammar rule name like _expression and _type began with an underscore. Starting a rule's name with an underscore causes the rule to be hidden in the syntax tree. This is useful for rules like _expression in the grammars above, which always just wrap a single child node. If these nodes were not hidden, they would add substantial depth and noise to the syntax tree without making it any easier to understand.

Using Fields

Often, it's easier to analyze a syntax node if you can refer to its children by name instead of by their position in an ordered list. Tree-sitter grammars support this using the field function. This function allows you to assign unique names to some or all of a node's children:

function_definition: $ => seq(
  'func',
  field('name', $.identifier),
  field('parameters', $.parameter_list),
  field('return_type', $._type),
  field('body', $.block)
)

Adding fields like this allows you to retrieve nodes using the field APIs.

Lexical Analysis

Tree-sitter's parsing process is divided into two phases: parsing (which is described above) and lexing - the process of grouping individual characters into the language's fundamental tokens. There are a few important things to know about how Tree-sitter's lexing works.

Conflicting Tokens

Grammars often contain multiple tokens that can match the same characters. For example, a grammar might contain the tokens ("if" and /[a-z]+/). Tree-sitter differentiates between these conflicting tokens in a few ways.

1. Context-aware Lexing - Tree-sitter performs lexing on-demand, during the parsing process. At any given position in a source document, the lexer only tries to recognize tokens that are valid at that position in the document.

2. Lexical Precedence - When the precedence functions described above are used within the token function, the given explicit precedence values serve as instructions to the lexer. If there are two valid tokens that match the characters at a given position in the document, Tree-sitter will select the one with the higher precedence.

3. Match Length - If multiple valid tokens with the same precedence match the characters at a given position in a document, Tree-sitter will select the token that matches the longest sequence of characters.

4. Match Specificity - If there are two valid tokens with the same precedence and which both match the same number of characters, Tree-sitter will prefer a token that is specified in the grammar as a String over a token specified as a RegExp.

5. Rule Order - If none of the above criteria can be used to select one token over another, Tree-sitter will prefer the token that appears earlier in the grammar.

If there is an external scanner it may have an additional impact over regular tokens defined in the grammar.

Lexical Precedence vs. Parse Precedence

One common mistake involves not distinguishing lexical precedence from parse precedence. Parse precedence determines which rule is chosen to interpret a given sequence of tokens. Lexical precedence determines which token is chosen to interpret at a given position of text and it is a lower-level operation that is done first. The above list fully captures Tree-sitter's lexical precedence rules, and you will probably refer back to this section of the documentation more often than any other. Most of the time when you really get stuck, you're dealing with a lexical precedence problem. Pay particular attention to the difference in meaning between using prec inside of the token function versus outside of it. The lexical precedence syntax is token(prec(N, ...)).

Keywords

Many languages have a set of keyword tokens (e.g. if, for, return), as well as a more general token (e.g. identifier) that matches any word, including many of the keyword strings. For example, JavaScript has a keyword instanceof, which is used as a binary operator, like this:

if (a instanceof Something) b();

The following, however, is not valid JavaScript:

if (a instanceofSomething) b();

A keyword like instanceof cannot be followed immediately by another letter, because then it would be tokenized as an identifier, even though an identifier is not valid at that position. Because Tree-sitter uses context-aware lexing, as described above, it would not normally impose this restriction. By default, Tree-sitter would recognize instanceofSomething as two separate tokens: the instanceof keyword followed by an identifier.

Keyword Extraction

Fortunately, Tree-sitter has a feature that allows you to fix this, so that you can match the behavior of other standard parsers: the word token. If you specify a word token in your grammar, Tree-sitter will find the set of keyword tokens that match strings also matched by the word token. Then, during lexing, instead of matching each of these keywords individually, Tree-sitter will match the keywords via a two-step process where it first matches the word token.

For example, suppose we added identifier as the word token in our JavaScript grammar:

grammar({
  name: 'javascript',

  word: $ => $.identifier,

  rules: {
    _expression: $ => choice(
      $.identifier,
      $.unary_expression,
      $.binary_expression
      // ...
    ),

    binary_expression: $ => choice(
      prec.left(1, seq($._expression, 'instanceof', $._expression))
      // ...
    ),

    unary_expression: $ => choice(
      prec.left(2, seq('typeof', $._expression))
      // ...
    ),

    identifier: $ => /[a-z_]+/
  }
});

Tree-sitter would identify typeof and instanceof as keywords. Then, when parsing the invalid code above, rather than scanning for the instanceof token individually, it would scan for an identifier first, and find instanceofSomething. It would then correctly recognize the code as invalid.

Aside from improving error detection, keyword extraction also has performance benefits. It allows Tree-sitter to generate a smaller, simpler lexing function, which means that the parser will compile much more quickly.

External Scanners

Many languages have some tokens whose structure is impossible or inconvenient to describe with a regular expression. Some examples:

• Indent and dedent tokens in Python
• Heredocs in Bash and Ruby
• Percent strings in Ruby

Tree-sitter allows you to handle these kinds of tokens using external scanners. An external scanner is a set of C functions that you, the grammar author, can write by hand in order to add custom logic for recognizing certain tokens.

To use an external scanner, there are a few steps. First, add an externals section to your grammar. This section should list the names of all of your external tokens. These names can then be used elsewhere in your grammar.

grammar({
  name: 'my_language',

  externals: $ => [
    $.indent,
    $.dedent,
    $.newline
  ],

  // ...
});

Then, add another C source file to your project. Currently, its path must be src/scanner.c for the CLI to recognize it. Be sure to add this file to the sources section of your binding.gyp file so that it will be included when your project is compiled by Node.js and uncomment the appropriate block in your bindings/rust/build.rs file so that it will be included in your Rust crate.

In this new source file, define an enum type containing the names of all of your external tokens. The ordering of this enum must match the order in your grammar's externals array; the actual names do not matter.

#include "tree_sitter/parser.h"
#include "tree_sitter/alloc.h"
#include "tree_sitter/array.h"

enum TokenType {
  INDENT,
  DEDENT,
  NEWLINE
}

Finally, you must define five functions with specific names, based on your language's name and five actions: create, destroy, serialize, deserialize, and scan.

Create

void *tree_sitter_my_language_external_scanner_create(void) {
  // ...
}

This function should create your scanner object. It will only be called once anytime your language is set on a parser. Often, you will want to allocate memory on the heap and return a pointer to it. If your external scanner doesn't need to maintain any state, it's ok to return NULL.

Destroy

void tree_sitter_my_language_external_scanner_destroy(void *payload) {
  // ...
}

This function should free any memory used by your scanner. It is called once when a parser is deleted or assigned a different language. It receives as an argument the same pointer that was returned from the create function. If your create function didn't allocate any memory, this function can be a noop.

Serialize

unsigned tree_sitter_my_language_external_scanner_serialize(
  void *payload,
  char *buffer
) {
  // ...
}

This function should copy the complete state of your scanner into a given byte buffer, and return the number of bytes written. The function is called every time the external scanner successfully recognizes a token. It receives a pointer to your scanner and a pointer to a buffer. The maximum number of bytes that you can write is given by the TREE_SITTER_SERIALIZATION_BUFFER_SIZE constant, defined in the tree_sitter/parser.h header file.

The data that this function writes will ultimately be stored in the syntax tree so that the scanner can be restored to the right state when handling edits or ambiguities. For your parser to work correctly, the serialize function must store its entire state, and deserialize must restore the entire state. For good performance, you should design your scanner so that its state can be serialized as quickly and compactly as possible.

Deserialize

void tree_sitter_my_language_external_scanner_deserialize(
  void *payload,
  const char *buffer,
  unsigned length
) {
  // ...
}

This function should restore the state of your scanner based the bytes that were previously written by the serialize function. It is called with a pointer to your scanner, a pointer to the buffer of bytes, and the number of bytes that should be read. It is good practice to explicitly erase your scanner state variables at the start of this function, before restoring their values from the byte buffer.

Scan

bool tree_sitter_my_language_external_scanner_scan(
  void *payload,
  TSLexer *lexer,
  const bool *valid_symbols
) {
  // ...
}

This function is responsible for recognizing external tokens. It should return true if a token was recognized, and false otherwise. It is called with a "lexer" struct with the following fields:

• int32_t lookahead - The current next character in the input stream, represented as a 32-bit unicode code point.
• TSSymbol result_symbol - The symbol that was recognized. Your scan function should assign to this field one of the values from the TokenType enum, described above.
• void (*advance)(TSLexer *, bool skip) - A function for advancing to the next character. If you pass true for the second argument, the current character will be treated as whitespace; whitespace won't be included in the text range associated with tokens emitted by the external scanner.
• void (*mark_end)(TSLexer *) - A function for marking the end of the recognized token. This allows matching tokens that require multiple characters of lookahead. By default (if you don't call mark_end), any character that you moved past using the advance function will be included in the size of the token. But once you call mark_end, then any later calls to advance will not increase the size of the returned token. You can call mark_end multiple times to increase the size of the token.
• uint32_t (*get_column)(TSLexer *) - A function for querying the current column position of the lexer. It returns the number of codepoints since the start of the current line. The codepoint position is recalculated on every call to this function by reading from the start of the line.
• bool (*is_at_included_range_start)(const TSLexer *) - A function for checking whether the parser has just skipped some characters in the document. When parsing an embedded document using the ts_parser_set_included_ranges function (described in the multi-language document section), the scanner may want to apply some special behavior when moving to a disjoint part of the document. For example, in EJS documents, the JavaScript parser uses this function to enable inserting automatic semicolon tokens in between the code directives, delimited by <% and %>.
• bool (*eof)(const TSLexer *) - A function for determining whether the lexer is at the end of the file. The value of lookahead will be 0 at the end of a file, but this function should be used instead of checking for that value because the 0 or "NUL" value is also a valid character that could be present in the file being parsed.
• void (*log)(const TSLexer *, const char * format, ...) - A printf-like function for logging. The log is viewable through e.g. tree-sitter parse --debug or the browser's console after checking the log option in the Playground.

The third argument to the scan function is an array of booleans that indicates which of external tokens are currently expected by the parser. You should only look for a given token if it is valid according to this array. At the same time, you cannot backtrack, so you may need to combine certain pieces of logic.

if (valid_symbols[INDENT] || valid_symbols[DEDENT]) {

  // ... logic that is common to both `INDENT` and `DEDENT`

  if (valid_symbols[INDENT]) {

    // ... logic that is specific to `INDENT`

    lexer->result_symbol = INDENT;
    return true;
  }
}

External Scanner Helpers

Allocator

Instead of using libc's malloc, calloc, realloc, and free, you should use the versions prefixed with ts_ from tree_sitter/alloc.h. These macros can allow a potential consumer to override the default allocator with their own implementation, but by default will use the libc functions.

As a consumer of the tree-sitter core library as well as any parser libraries that might use allocations, you can enable overriding the default allocator and have it use the same one as the library allocator, of which you can set with ts_set_allocator. To enable this overriding in scanners, you must compile them with the TREE_SITTER_REUSE_ALLOCATOR macro defined, and tree-sitter the library must be linked into your final app dynamically, since it needs to resolve the internal functions at runtime. If you are compiling an executable binary that uses the core library, but want to load parsers dynamically at runtime, then you will have to use a special linker flag on Unix. For non-Darwin systems, that would be --dynamic-list and for Darwin systems, that would be -exported_symbols_list. The CLI does exactly this, so you can use it as a reference (check out cli/build.rs).

For example, assuming you wanted to allocate 100 bytes for your scanner, you'd do so like the following example:

#include "tree_sitter/parser.h"
#include "tree_sitter/alloc.h"

// ...

void *tree_sitter_my_language_external_scanner_create(void) {
  return ts_calloc(100, 1); // or ts_malloc(100)
}

// ...

Arrays

If you need to use array-like types in your scanner, such as tracking a stack of indentations or tags, you should use the array macros from tree_sitter/array.h.

There are quite a few of them provided for you, but here's how you could get started tracking some . Check out the header itself for more detailed documentation.

NOTE: Do not use any of the array functions or macros that are prefixed with an underscore and have comments saying that it is not what you are looking for. These are internal functions used as helpers by other macros that are public. They are not meant to be used directly, nor are they what you want.

#include "tree_sitter/parser.h"
#include "tree_sitter/array.h"

enum TokenType {
  INDENT,
  DEDENT,
  NEWLINE,
  STRING,
}

// Create the array in your create function

void *tree_sitter_my_language_external_scanner_create(void) {
  return ts_calloc(1, sizeof(Array(int)));

  // or if you want to zero out the memory yourself

  Array(int) *stack = ts_malloc(sizeof(Array(int)));
  array_init(&stack);
  return stack;
}

bool tree_sitter_my_language_external_scanner_scan(
  void *payload,
  TSLexer *lexer,
  const bool *valid_symbols
) {
  Array(int) *stack = payload;
  if (valid_symbols[INDENT]) {
    array_push(stack, lexer->get_column(lexer));
    lexer->result_symbol = INDENT;
    return true;
  }
  if (valid_symbols[DEDENT]) {
    array_pop(stack); // this returns the popped element by value, but we don't need it
    lexer->result_symbol = DEDENT;
    return true;
  }

  // we can also use an array on the stack to keep track of a string

  Array(char) next_string = array_new();

  if (valid_symbols[STRING] && lexer->lookahead == '"') {
    lexer->advance(lexer, false);
    while (lexer->lookahead != '"' && lexer->lookahead != '\n' && !lexer->eof(lexer)) {
      array_push(&next_string, lexer->lookahead);
      lexer->advance(lexer, false);
    }

    // assume we have some arbitrary constraint of not having more than 100 characters in a string
    if (lexer->lookahead == '"' && next_string.size <= 100) {
      lexer->advance(lexer, false);
      lexer->result_symbol = STRING;
      return true;
    }
  }

  return false;
}

Other External Scanner Details

If a token in the externals array is valid at a given position in the parse, the external scanner will be called first before anything else is done. This means the external scanner functions as a powerful override of Tree-sitter's lexing behavior, and can be used to solve problems that can't be cracked with ordinary lexical, parse, or dynamic precedence.

If a syntax error is encountered during regular parsing, Tree-sitter's first action during error recovery will be to call the external scanner's scan function with all tokens marked valid. The scanner should detect this case and handle it appropriately. One simple method of detection is to add an unused token to the end of the externals array, for example externals: $ => [$.token1, $.token2, $.error_sentinel], then check whether that token is marked valid to determine whether Tree-sitter is in error correction mode.

If you put terminal keywords in the externals array, for example externals: $ => ['if', 'then', 'else'], then any time those terminals are present in the grammar they will be tokenized by the external scanner. It is similar to writing externals: [$.if_keyword, $.then_keyword, $.else_keyword] then using alias($.if_keyword, 'if') in the grammar.

If in the externals array use literal keywords then lexing works in two steps, the external scanner will be called first and if it sets a resulting token and returns true then the token considered as recognized and Tree-sitter moves to a next token. But the external scanner may return false and in this case Tree-sitter fallbacks to the internal lexing mechanism.

In case of some keywords defined in the externals array in a rule referencing form like $.if_keyword and there is no additional definition of that rule in the grammar rules, e.g., if_keyword: $ => 'if' then fallback to the internal lexer isn't possible because Tree-sitter doesn't know the actual keyword and it's fully the external scanner resposibilty to recognize such tokens.

External scanners are a common cause of infinite loops. Be very careful when emitting zero-width tokens from your external scanner, and if you consume characters in a loop be sure use the eof function to check whether you are at the end of the file.