Syntax Tree


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Syntax Tree is a suite of tools built on top of the internal CRuby parser. It provides the ability to generate a syntax tree from source, as well as the tools necessary to inspect and manipulate that syntax tree. It can be used to build formatters, linters, language servers, and more.

It is built with only standard library dependencies. It additionally ships with a plugin system so that you can build your own syntax trees from other languages and incorporate these tools.


Syntax Tree is both a command-line interface and a library. If you're only looking to use the command-line interface, then we recommend installing the gem globally, as in:

gem install syntax_tree

To run the CLI with the gem installed globally, you would run:

stree version

If you're planning on using Syntax Tree as a library within your own project, we recommend installing it as part of your gem bundle. First, add this line to your application's Gemfile:

gem "syntax_tree"

And then execute:

bundle install

To run the CLI with the gem installed in your gem bundle, you would run:

bundle exec stree version


Syntax Tree ships with the stree CLI, which can be used to inspect and manipulate Ruby code. Below are listed all of the commands built into the CLI that you can use.

For many commands, file paths are accepted after the configuration options. For all of these commands, you can alternatively pass in content through STDIN or through the -e option to specify an inline script.


This command will print out a textual representation of the syntax tree associated with each of the files it finds. To execute, run:

stree ast path/to/file.rb

For a file that contains 1 + 1, you will receive:

(program (statements (binary (int "1") + (int "1"))))


This command is meant to be used in the context of a continuous integration or git hook. It checks each file given to make sure that it matches the expected format. It can be used to ensure unformatted content never makes it into a codebase.

stree check path/to/file.rb

For a file that matches the expected format, you will receive:

All files matched expected format.

If there are files with unformatted code, you will receive:

[warn] path/to/file.rb
The listed files did not match the expected format.

To change the print width that you are checking against, specify the --print-width option, as in:

stree check --print-width=100 path/to/file.rb


This command will output to stdout a set of tags suitable for usage with ctags.

stree ctags path/to/file.rb

For a file containing the following Ruby code:

class Foo

class Bar < Foo

you will receive:

!_TAG_FILE_FORMAT   2   /extended format; --format=1 will not append ;" to lines/
!_TAG_FILE_SORTED   1   /0=unsorted, 1=sorted, 2=foldcase/
Bar test.rb /^class Bar < Foo$/;"   c   inherits:Foo
Foo test.rb /^class Foo$/;" c


This command will output a Ruby case-match expression that would match correctly against the first expression of the input.

stree expr path/to/file.rb

For a file that contains 1 + 1, you will receive:

  left: SyntaxTree::Int[value: "1"],
  operator: :+,
  right: SyntaxTree::Int[value: "1"]


This command will output the formatted version of each of the listed files. Importantly, it will not write that content back to the source files. It is meant to display the formatted version only.

stree format path/to/file.rb

For a file that contains 1 + 1, you will receive:

1 + 1

To change the print width that you are formatting with, specify the --print-width option, as in:

stree format --print-width=100 path/to/file.rb


This command will output a JSON representation of the syntax tree that is functionally equivalent to the input. This is mostly used in contexts where you need to access the tree from JavaScript or serialize it over a network.

stree json path/to/file.rb

For a file that contains 1 + 1, you will receive:

  "type": "program",
  "location": [1, 0, 1, 6],
  "statements": {
    "type": "statements",
    "location": [1, 0, 1, 6],
    "body": [
        "type": "binary",
        "location": [1, 0, 1, 5],
        "left": {
          "type": "int",
          "location": [1, 0, 1, 1],
          "value": "1",
          "comments": []
        "operator": "+",
        "right": {
          "type": "int",
          "location": [1, 4, 1, 5],
          "value": "1",
          "comments": []
        "comments": []
    "comments": []
  "comments": []


This command will output a Ruby case-match expression that would match correctly against the input.

stree match path/to/file.rb

For a file that contains 1 + 1, you will receive:

  statements: SyntaxTree::Statements[
    body: [
        left: SyntaxTree::Int[value: "1"],
        operator: :+,
        right: SyntaxTree::Int[value: "1"]

This command will search the given filepaths against the specified pattern to find nodes that match. The pattern is a Ruby pattern-matching expression that is matched against each node in the tree. It can optionally be loaded from a file if you specify a filepath as the pattern argument.

stree search VarRef path/to/file.rb

For a file that contains Foo + Bar you will receive:

path/to/file.rb:1:0: Foo + Bar
path/to/file.rb:1:6: Foo + Bar

If you put VarRef into a file instead (for example, query.txt), you would instead run:

stree search query.txt path/to/file.rb

Note that the output of the match CLI command creates a valid pattern that can be used as the input for this command.


This command will format the listed files and write that formatted version back to the source files. Note that this overwrites the original content, so be sure to be using a version control system.

stree write path/to/file.rb

This will list every file that is being formatted. It will output light gray if the file already matches the expected format. It will output in regular color if it does not.

path/to/file.rb 0ms

To change the print width that you are writing with, specify the --print-width option, as in:

stree write --print-width=100 path/to/file.rb

To ignore certain files from a glob (in order to make it easier to specify the filepaths), you can pass the --ignore-files option as an additional glob, as in:

stree write --ignore-files='db/**/*.rb' '**/*.rb'


Any of the above CLI commands can also read configuration options from a .streerc file in the directory where the commands are executed.

This should be a text file with each argument on a separate line.


If this file is present, it will always be used for CLI commands. You can also pass options from the command line as in the examples above. The options in the .streerc file are passed to the CLI first, then the arguments from the command line. In the case of exclusive options (e.g. --print-width), this means that the command line options override what's in the config file. In the case of options that can take multiple inputs (e.g. --plugins), the effect is additive. That is, the plugins passed from the command line will be loaded in addition to the plugins in the config file.


When running commands with stree, it's common to pass in lists of files. For example:

stree write 'lib/*.rb' 'test/*.rb'

The commands in the CLI accept any number of arguments. This means you could pass **/*.rb (note the lack of quotes). This would make your shell expand out the file paths listed according to its own rules. (For example, here are the rules for GNU bash.)

However, it's recommended to instead use quotes, which means that Ruby is responsible for performing the file path expansion instead. This ensures a consistent experience across different environments and shells. The globs must follow the Ruby-specific globbing syntax as specified in the documentation for Dir.

Baked into this syntax is the ability to provide exceptions to file name patterns as well. For example, if you are in a Rails app and want to exclude files named schema.rb but write all other Ruby files, you can use the following syntax:

stree write "**/{[!schema]*,*}.rb"


Syntax Tree can be used as a library to access the syntax tree underlying Ruby source code. The API is described below. For the full library documentation, see the RDoc documentation.

This function takes a filepath and returns a string associated with the content of that file. It is similar in functionality to, except that it takes into account Ruby-level file encoding (through magic comments at the top of the file).


This function takes an input string containing Ruby code and returns the syntax tree associated with it. The top-level node is always a SyntaxTree::Program, which contains a list of top-level expression nodes.


This function takes an input string containing Ruby code, parses it into its underlying syntax tree, and formats it back out to a string. You can optionally pass a second argument to this method as well that is the maximum width to print. It defaults to 80.


This function yields a new mutation visitor to the block, and then returns the initialized visitor. It's effectively a shortcut for creating a SyntaxTree::MutationVisitor without having to remember the class name. For more information on that visitor, see the definition below., query, &block)

This function takes an input string containing Ruby code, an input string containing a valid Ruby in clause expression that can be used to match against nodes in the tree (can be generated using stree expr, stree match, or Node#construct_keys), and a block. Each node that matches the given query will be yielded to the block. The block will receive the node as its only argument.


This function takes an input string containing Ruby code and returns a list of all of the class declarations, module declarations, and method definitions within a file. Each of the entries also has access to its associated comments. This is useful for generating documentation or index information for a file to support something like go-to-definition.


There are many different node types in the syntax tree. They are meant to be treated as immutable structs containing links to child nodes with minimal logic contained within their implementation. However, for the most part they all respond to a certain set of APIs, listed below.


One of the easiest ways to descend the tree is to use the child_nodes function. It is implemented on every node type (leaf nodes return an empty array). If the goal is to simply walk through the tree, this is the easiest way to go.

program = SyntaxTree.parse("1 + 1")
# => (binary (int "1") :+ (int "1"))


This method returns a copy of the node, with the given attributes replaced.

program = SyntaxTree.parse("1 + 1")

binary = program.statements.body.first
# => (binary (int "1") + (int "1"))

binary.copy(operator: :-)
# => (binary (int "1") - (int "1"))

Pattern matching

Pattern matching is another way to descend the tree which is more specific than using child_nodes. Using Ruby's built-in pattern matching, you can extract the same information but be as specific about your constraints as you like. For example, with minimal constraints:

program = SyntaxTree.parse("1 + 1")
program => { statements: { body: [binary] } }
# => (binary (int "1") :+ (int "1"))

Or, with more constraints on the types to ensure we're getting exactly what we expect:

program = SyntaxTree.parse("1 + 1")
program => SyntaxTree::Program[statements: SyntaxTree::Statements[body: [SyntaxTree::Binary => binary]]]
# => (binary (int "1") :+ (int "1"))


Every node responds to the pretty_print Ruby interface, which makes it usable by the pp library. You can use this API manually, but it's mostly there for compatibility and not meant to be directly invoked. For example:

pp SyntaxTree.parse("1 + 1")
# (program (statements (binary (int "1") + (int "1"))))


Every node responds to the to_json Ruby interface, which makes it usable by the json library. Much like pretty_print, you could use this API manually, but it's mostly used by JSON to dump the nodes to a serialized format. For example:

program = SyntaxTree.parse("1 + 1")
program => { statements: { body: [{ left: }] } }
puts JSON.dump(left)
# {"type":"int","value":"1","loc":[1,0,1,1],"cmts":[]}


Every node responds to format, which formats the content nicely. The API mirrors that used by the pretty_print gem in that it accepts a formatter object and calls methods on it to generate its own internal representation of the text that will be outputted. Because of this, it's easier to not use this API directly and instead to call SyntaxTree.format. You can however use this directly if you create the formatter yourself, as in:

source = "1+1"
program = SyntaxTree.parse(source)
program => { statements: { body: [binary] } }

formatter =, [])

# => "1 + 1"


Every node responds to ===, which is used to check if the given other node matches all of the attributes of the current node except for location and comments. For example:

program1 = SyntaxTree.parse("1 + 1")
program2 = SyntaxTree.parse("1 + 1")

program1 === program2
# => true


Every node responds to construct_keys, which will return a string that contains a Ruby pattern-matching expression that could be used to match against the current node. It's meant to be used in tooling and through the CLI mostly.

program = SyntaxTree.parse("1 + 1")
puts program.construct_keys

# SyntaxTree::Program[
#   statements: SyntaxTree::Statements[
#     body: [
#       SyntaxTree::Binary[
#         left: SyntaxTree::Int[value: "1"],
#         operator: :+,
#         right: SyntaxTree::Int[value: "1"]
#       ]
#     ]
#   ]
# ]


If you want to operate over a set of nodes in the tree but don't want to walk the tree manually, the Visitor class makes it easy. SyntaxTree::Visitor is an implementation of the double dispatch visitor pattern. It works by the user defining visit methods that process nodes in the tree, which then call back to other visit methods to continue the descent. This is easier shown in code.

Let's say, for instance, that you wanted to find every place in source where you have an arithmetic problem between two integers (this is pretty contrived, but it's just for illustration). You could define a visitor that only explicitly visits the SyntaxTree::Binary node, as in:

class ArithmeticVisitor < SyntaxTree::Visitor
  def visit_binary(node)
    if node in { left: SyntaxTree::Int, operator: :+ | :- | :* | :/, right: SyntaxTree::Int }
      puts "The result is: #{node.left.value.to_i.public_send(node.operator, node.right.value.to_i)}"

visitor =
visitor.visit(SyntaxTree.parse("1 + 1"))
# The result is: 2

With visitors, you only define handlers for the nodes that you need. You can find the names of the methods that you will need to define within the base visitor, as they're all aliased to the default behavior (visiting the child nodes). Note that when you define a handler for a node, you have to tell Syntax Tree how to walk further. In the example above, we don't need to go any further because we already know the child nodes are SyntaxTree::Int, so they can't possibly contain more SyntaxTree::Binary nodes. In other circumstances you may not know though, so you can either:

  • call super (which will do the default and visit all child nodes)
  • call visit_child_nodes manually
  • call visit(child) with each child that you want to visit
  • call nothing if you're sure you don't want to descend further

There are a couple of visitors that ship with Syntax Tree that can be used as examples. They live in the lib/syntax_tree directory.


When you're creating a visitor, it's very easy to accidentally mistype a visit method. Unfortunately, there's no way to tell Ruby to explicitly override a parent method, so it would then be easy to define a method that never gets called. To mitigate this risk, there's Visitor.visit_method(name). This method accepts a symbol that is checked against the list of known visit methods. If it's not in the list, then an error will be raised. It's meant to be used like:

class ArithmeticVisitor < SyntaxTree::Visitor
  visit_method def visit_binary(node)
    # ...

This will only be checked once when the file is first required. If there is a typo in your method name (or the method no longer exists for whatever reason), you will receive an error like so:

~/syntax_tree/lib/syntax_tree/visitor.rb:46:in `visit_method': Invalid visit method: visit_binar (SyntaxTree::Visitor::VisitMethodError)
Did you mean?  visit_binary
    from (irb):2:in `<class:ArithmeticVisitor>'
    from (irb):1:in `<main>'
    from bin/console:8:in `<main>'


Similar to visit_method, visit_methods also checks that methods defined are valid visit methods. This variation however accepts a block and checks that all methods defined within that block are valid visit methods. It's meant to be used like:

class ArithmeticVisitor < SyntaxTree::Visitor
  visit_methods do
    def visit_binary(node)
      # ...

    def visit_int(node)
      # ...

This is only checked when the methods are defined and does not impose any kind of runtime overhead after that. It is very useful for upgrading versions of Syntax Tree in case these methods names change.


When you're defining your own visitor, by default it will walk down the tree even if you don't define visit_* methods. This is to ensure you can define a subset of the necessary methods in order to only interact with the nodes you're interested in. If you'd like to change this default to instead raise an error if you visit a node you haven't explicitly handled, you can instead inherit from BasicVisitor.

class MyVisitor < SyntaxTree::BasicVisitor
  def visit_int(node)
    # ...

The visitor defined above will error out unless it's only visiting a SyntaxTree::Int node. This is useful in a couple of ways, e.g., if you're trying to define a visitor to handle the whole tree but it's currently a work-in-progress.


The MutationVisitor is a visitor that can be used to mutate the tree. It works by defining a default visit_* method that returns a copy of the given node with all of its attributes visited. This new node will replace the old node in the tree. Typically, you use the #mutate method on it to define mutations using patterns. For example:

# Create a new visitor
visitor =

# Specify that it should mutate If nodes with assignments in their predicates
visitor.mutate("IfNode[predicate: Assign | OpAssign]") do |node|
  # Get the existing If's predicate node
  predicate = node.predicate

  # Create a new predicate node that wraps the existing predicate node
  # in parentheses
  predicate =
      lparen: SyntaxTree::LParen.default,
      contents: predicate,
      location: predicate.location

  # Return a copy of this node with the new predicate
  node.copy(predicate: predicate)

source = "if a = 1; end"
program = SyntaxTree.parse(source)

SyntaxTree::Formatter.format(source, program)
# => "if a = 1\nend\n"

SyntaxTree::Formatter.format(source, program.accept(visitor))
# => "if (a = 1)\nend\n"


The WithScope module can be included in visitors to automatically keep track of local variables and arguments defined inside each scope. A current_scope accessor is made available to the request, allowing it to find all usages and definitions of a local.

class MyVisitor < Visitor
  prepend WithScope

  def visit_ident(node)
    # find_local will return a Local for any local variables or arguments
    # present in the current environment or nil if the identifier is not a local
    local = current_scope.find_local(node)

    puts local.type # the type of the local (:variable or :argument)
    puts local.definitions # the array of locations where this local is defined
    puts local.usages # the array of locations where this local occurs

Language server

Syntax Tree additionally ships with a language server conforming to the language server protocol. It can be invoked through the CLI by running:

stree lsp

By default, the language server is relatively minimal, mostly meant to provide a registered formatter for the Ruby language. However there are a couple of additional niceties baked in. There are related projects that configure and use this language server within IDEs. For example, to use this code with VSCode, see ruby-syntax-tree/vscode-syntax-tree.


As mentioned above, the language server responds to formatting requests with the formatted document. It typically responds on the order of tens of milliseconds, so it should be fast enough for any IDE.


The language server also responds to the relatively new inlay hints request. This request allows the language server to define additional information that should exist in the source code as helpful hints to the developer. In our case we use it to display things like implicit parentheses. For example, if you had the following code:

1 + 2 * 3

Implicitly, the 2 * 3 is going to be executed first because the * operator has higher precedence than the + operator. To ease mental overhead, our language server includes small parentheses to make this explicit, as in:

1 + ₍2 * 3₎


The language server additionally includes this custom request to return a textual representation of the syntax tree underlying the source code of a file. Language server clients can use this to (for example) open an additional tab with this information displayed.


There are multiple ways to customize Syntax Tree's behavior when parsing and formatting code. You can ignore certain sections of the source code, you can register plugins to provide custom formatting behavior, and you can register additional languages to be parsed and formatted.

Ignoring code

To ignore a section of source code, you can use a special # stree-ignore comment. This comment should be placed immediately above the code that you want to ignore. For example:

numbers = [

Normally the snippet above would be formatted as numbers = [10_000, 20_000, 30_000]. However, sometimes you want to keep the original formatting to improve readability or maintainability. In that case, you can put the ignore comment before it, as in:

# stree-ignore
numbers = [

Now when Syntax Tree goes to format that code, it will copy the source code exactly as it is, including the newlines and indentation.


You can register additional customization that can flow through the same CLI with Syntax Tree's plugin system. When invoking the CLI, you pass through the list of plugins with the --plugins options to the commands that accept them. They should be a comma-delimited list. When the CLI first starts, it will require the files corresponding to those names.

To register plugins, define a file somewhere in your load path named syntax_tree/my_plugin. Then when invoking the CLI, you will pass --plugins=my_plugin. To require multiple, separate them by a comma. In this way, you can modify Syntax Tree however you would like. Some plugins ship with Syntax Tree itself. They are:

  • plugin/single_quotes - This will change all of your string literals to use single quotes instead of the default double quotes.
  • plugin/trailing_comma - This will put trailing commas into multiline array literals, hash literals, and method calls that can support trailing commas.
  • plugin/disable_auto_ternary - This will prevent the automatic conversion of if ... else to ternary expressions.

If you're using Syntax Tree as a library, you can require those files directly or manually pass those options to the formatter initializer through the SyntaxTree::Formatter::Options class.


To register a new language, call:

SyntaxTree.register_handler(".mylang", MyLanguage)

In this case, whenever the CLI encounters a filepath that ends with the given extension, it will invoke methods on MyLanguage instead of SyntaxTree itself. To make sure your object conforms to each of the necessary APIs, it should implement:

  • - usually this is just an alias to, but if you need anything else that hook is here.
  • MyLanguage.parse(source) - this should return the syntax tree corresponding to the given source. Those objects should implement the pretty_print interface.
  • MyLanguage.format(source) - this should return the formatted version of the given source.

Below are listed all of the "official" language plugins hosted under the same GitHub organization, which can be used as references for how to implement other plugins.


Syntax Tree's goal is to seamlessly integrate into your workflow. To this end, it provides a couple of additional tools beyond the CLI and the Ruby library.


Syntax Tree ships with the ability to define rake tasks that will trigger runs of the CLI. To define them in your application, add the following configuration to your Rakefile:

require "syntax_tree/rake_tasks"

These calls will define rake stree:check and rake stree:write (equivalent to calling stree check and stree write with the CLI respectively). You can configure them by either passing arguments to the new method or by using a block.


If you'd like to change the default name of the rake task, you can pass that as the first argument, as in:


If you wanted to configure Syntax Tree to check or write different files than the default (lib/**/*.rb), you can set the source_files field, as in: do |t|
  t.source_files = FileList[%w[Gemfile Rakefile lib/**/*.rb test/**/*.rb]]


If you want to ignore certain file patterns when running the command, you can pass the ignore_files option. This will be checked with File.fnmatch? against each filepath that the command would be run against. For example: do |t|
  t.source_files = "**/*.rb"
  t.ignore_files = "db/**/*.rb"

If you want to use a different print width from the default (80), you can pass that to the print_width field, as in: do |t|
  t.print_width = 100


If you're running Syntax Tree with plugins (either your own or the pre-built ones), you can pass that to the plugins field, as in: do |t|
  t.plugins = ["plugin/single_quotes"]


RuboCop and Syntax Tree serve different purposes, but there is overlap with some of RuboCop's functionality. Syntax Tree provides a RuboCop configuration file to disable rules that are redundant with Syntax Tree. To use this configuration file, add the following snippet to the top of your project's .rubocop.yml:

  syntax_tree: config/rubocop.yml



Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.