Tree-sitter Parsing

Parse source code into concrete syntax trees using Tree-sitter. Based on go-tree-sitter bindings.

Tree-sitter produces syntax trees that:

• Represent the full structure of source code
• Update incrementally as code changes
• Are robust to syntax errors (partial parsing)
• Support pattern-based queries using S-expressions

Loading

local treesitter = require("treesitter")

Supported Languages

local langs = treesitter.supported_languages()
-- {go = true, javascript = true, python = true, ...}

Quick Start

Parse Code

local code = [[
func hello() {
    return "Hello!"
}
]]

local tree, err = treesitter.parse("go", code)
if err then
    return nil, err
end

local root = tree:root_node()
print(root:kind())        -- "source_file"
print(root:child_count()) -- number of top-level declarations

Query Syntax Tree

local code = [[
func hello() {}
func world() {}
]]

local tree = treesitter.parse("go", code)
local root = tree:root_node()

-- Find all function names
local query = treesitter.query("go", [[
    (function_declaration name: (identifier) @func_name)
]])

local captures = query:captures(root, code)
for _, capture in ipairs(captures) do
    print(capture.name, capture.text)
end
-- "func_name"  "hello"
-- "func_name"  "world"

Parsing

Simple Parse

Parse source code into a syntax tree. Creates a temporary parser internally.

local tree, err = treesitter.parse("go", code)

Returns: Tree, error

Reusable Parser

Create a parser for repeated parsing or incremental updates.

local parser = treesitter.parser()
parser:set_language("go")

local tree1 = parser:parse("package main")

-- Incremental parse with old tree
local tree2 = parser:parse("package main\nfunc foo() {}", tree1)

parser:close()

Returns: Parser

Parser Methods

Syntax Trees

Get Root Node

local tree = treesitter.parse("go", "package main")
local root = tree:root_node()

print(root:kind())  -- "source_file"
print(root:text())  -- "package main"

Tree Methods

Incremental Editing

Update the tree when source code changes:

local code = "func main() { x := 1 }"
local tree = treesitter.parse("go", code)

-- Mark edit: changed "1" to "100" at byte 19
tree:edit({
    start_byte = 19,
    old_end_byte = 20,
    new_end_byte = 22,
    start_row = 0,
    start_column = 19,
    old_end_row = 0,
    old_end_column = 20,
    new_end_row = 0,
    new_end_column = 22
})

-- Re-parse with edited tree (faster than full parse)
local parser = treesitter.parser()
parser:set_language("go")
local new_tree = parser:parse("func main() { x := 100 }", tree)

Nodes

Nodes represent elements in the syntax tree.

Node Types

local node = root:child(0)

-- Type information
print(node:kind())        -- "package_clause"
print(node:type())        -- same as kind()
print(node:is_named())    -- true for significant nodes
print(node:grammar_name()) -- grammar rule name

Navigation

-- Children
local child = node:child(0)           -- by index (0-based)
local named = node:named_child(0)     -- named children only
local count = node:child_count()
local named_count = node:named_child_count()

-- Siblings
local next = node:next_sibling()
local prev = node:prev_sibling()
local next_named = node:next_named_sibling()
local prev_named = node:prev_named_sibling()

-- Parent
local parent = node:parent()

-- By field name
local name_node = func_decl:child_by_field_name("name")
local field = node:field_name_for_child(0)

Position Information

-- Byte offsets
local start = node:start_byte()
local end_ = node:end_byte()

-- Row/column positions (0-based)
local start_pt = node:start_point()  -- {row = 0, column = 0}
local end_pt = node:end_point()      -- {row = 0, column = 12}

-- Source text
local text = node:text()

Error Detection

if root:has_error() then
    -- Tree contains syntax errors
end

if node:is_error() then
    -- This specific node is an error
end

if node:is_missing() then
    -- Parser inserted this to recover from error
end

S-Expression

local sexp = node:to_sexp()
-- "(source_file (package_clause (package_identifier)))"

Queries

Pattern matching using Tree-sitter's query language (S-expressions).

Create Query

local query, err = treesitter.query("go", [[
    (function_declaration
        name: (identifier) @func_name
        parameters: (parameter_list) @params
    )
]])

Returns: Query, error

Execute Query

-- Get all captures (flattened)
local captures = query:captures(root, source_code)
for _, capture in ipairs(captures) do
    print(capture.name)   -- "@func_name"
    print(capture.text)   -- actual text
    print(capture.index)  -- capture index
    -- capture.node is the Node object
end

-- Get matches (grouped by pattern)
local matches = query:matches(root, source_code)
for _, match in ipairs(matches) do
    print(match.id, match.pattern)
    for _, capture in ipairs(match.captures) do
        print(capture.name, capture.node:text())
    end
end

Query Control

-- Limit query scope
query:set_byte_range(0, 1000)
query:set_point_range({row = 0, column = 0}, {row = 10, column = 0})

-- Limit matches
query:set_match_limit(100)
if query:did_exceed_match_limit() then
    -- More matches exist
end

-- Timeout (string duration or nanoseconds)
query:set_timeout("500ms")
query:set_timeout(1000000000)  -- 1 second in nanoseconds

-- Disable patterns/captures
query:disable_pattern(0)
query:disable_capture("func_name")

Query Inspection

local pattern_count = query:pattern_count()
local capture_count = query:capture_count()
local name = query:capture_name_for_id(0)
local id = query:capture_index_for_name("func_name")

Tree Cursor

Efficient traversal without creating node objects at each step.

Basic Traversal

local cursor = tree:walk()

-- Start at root
print(cursor:current_node():kind())  -- "source_file"
print(cursor:current_depth())        -- 0

-- Navigate
if cursor:goto_first_child() then
    print(cursor:current_node():kind())
    print(cursor:current_depth())  -- 1
end

if cursor:goto_next_sibling() then
    -- moved to next sibling
end

cursor:goto_parent()  -- back to parent

cursor:close()

Cursor Methods

Language Metadata

local lang = treesitter.language("go")

print(lang:version())           -- ABI version
print(lang:node_kind_count())   -- number of node types
print(lang:field_count())       -- number of fields

-- Node kind lookup
local kind = lang:node_kind_for_id(1)
local id = lang:id_for_node_kind("identifier", true)
local is_named = lang:node_kind_is_named(1)

-- Field lookup
local field_name = lang:field_name_for_id(1)
local field_id = lang:field_id_for_name("name")

Errors

See Error Handling for working with errors.

Query Syntax Reference

Tree-sitter queries use S-expression patterns:

; Match a node type
(identifier)

; Match with field names
(function_declaration name: (identifier))

; Capture with @name
(function_declaration name: (identifier) @func_name)

; Multiple patterns
[
  (function_declaration)
  (method_declaration)
] @declaration

; Wildcards
(_)           ; any node
(identifier)+ ; one or more
(identifier)* ; zero or more
(identifier)? ; optional

; Predicates
((identifier) @var
  (#match? @var "^_"))  ; regex match

See Tree-sitter Query Syntax for complete documentation.