# RBS

RBS is a language to describe the structure of Ruby programs.
You can write down the definition of a class or module: methods defined in the class, instance variables and their types, and inheritance/mix-in relations.
It also allows declaring constants and global variables.

The following is a small example of RBS for a chat app.

<!-- run-start:a.rbs:bundle exec rbs -I a.rbs validate -->
```rbs
module ChatApp
  VERSION: String

  class User
    attr_reader login: String
    attr_reader email: String

    def initialize: (login: String, email: String) -> void
  end

  class Bot
    attr_reader name: String
    attr_reader email: String
    attr_reader owner: User

    def initialize: (name: String, owner: User) -> void
  end

  class Message
    attr_reader id: String
    attr_reader string: String
    attr_reader from: User | Bot                     # `|` means union types: `#from` can be `User` or `Bot`
    attr_reader reply_to: Message?                   # `?` means optional type: `#reply_to` can be `nil`

    def initialize: (from: User | Bot, string: String) -> void

    def reply: (from: User | Bot, string: String) -> Message
  end

  class Channel
    attr_reader name: String
    attr_reader messages: Array[Message]
    attr_reader users: Array[User]
    attr_reader bots: Array[Bot]

    def initialize: (name: String) -> void

    def each_member: () { (User | Bot) -> void } -> void  # `{` and `}` means block.
                   | () -> Enumerator[User | Bot, void]   # Method can be overloaded.
  end
end
```
<!-- run-end -->

## The Target Version

* The standard library signatures targets the [latest release of Ruby](https://www.ruby-lang.org/en/downloads/branches/). (`4.0` as of 2026.)
* The library code targets [non-EOL versions of Ruby](https://www.ruby-lang.org/en/downloads/branches/). (`>= 3.3` as of 2026.)

## Installation

Install the `rbs` gem. `$ gem install rbs` from the command line, or add a line in your `Gemfile`.

```rb
gem "rbs"
```

## CLI

The gem ships with the `rbs` command line tool to demonstrate what it can do and help develop RBS.

```console
$ rbs version
$ rbs list
$ rbs ancestors ::Object
$ rbs methods ::Object
$ rbs method Object then
```

An end user of `rbs` will probably find `rbs prototype` the most useful. This command generates boilerplate signature declarations for ruby files. For example, say you have written the below ruby script.

```ruby
# person.rb
class Person
  attr_reader :name
  attr_reader :contacts

  def initialize(name:)
    @name = name
    @contacts = []
  end

  def speak
    "I'm #{@name} and I love Ruby!"
  end
end
```

Running prototype on the above will automatically generate

```console
$ rbs prototype rb person.rb
class Person
  @name: untyped

  @contacts: untyped

  attr_reader name: untyped

  attr_reader contacts: untyped

  def initialize: (name: untyped) -> void

  def speak: () -> ::String
end
```

It prints signatures for all methods, classes, instance variables, and constants.
This is only a starting point, and you should edit the output to match your signature more accurately.

`rbs prototype` offers three options.

- `rb` generates from just the available Ruby code
- `rbi` generates from Sorbet RBI
- `runtime` generates from runtime API (**deprecated**; see [#2841](https://github.com/ruby/rbs/issues/2841))

## Library

There are two important concepts, _environment_ and _definition_.

An _environment_ is a dictionary that keeps track of all declarations. What is the declaration associated with `String` class? An _environment_ will give you the answer.

A _definition_ gives you the detail of the class. What is the type of the return value of `gsub` method of the `String` class? The _definition_ for `String` class knows the list of methods it provides and their types.

The following is a small code to retrieve the definition of the `String#gsub` method.

<!-- run-start:a.rb:bundle exec ruby a.rb -->
```rb
require "rbs"

loader = RBS::EnvironmentLoader.new()

# loader.add(path: Pathname("sig"))   # Load .rbs files from `sig` directory
# loader.add(library: "logger")       # Load logger library

environment = RBS::Environment.from_loader(loader).resolve_type_names

# ::String
string = RBS::TypeName.new(name: :String, namespace: RBS::Namespace.root)

# Class declaration for ::String
decl = environment.class_decls[string]

# Builder provides the translation from `declaration` to `definition`
builder = RBS::DefinitionBuilder.new(env: environment)

# Definition of instance of String
instance = builder.build_instance(string)

# Print the types of `gsub` method:
puts instance.methods[:gsub].method_types.join("\n")
# Outputs =>
#  (::Regexp | ::string pattern, ::string replacement) -> ::String
#  (::Regexp | ::string pattern, ::Hash[::String, ::String] hash) -> ::String
#  (::Regexp | ::string pattern) { (::String match) -> ::_ToS } -> ::String
#  (::Regexp | ::string pattern) -> ::Enumerator[::String, self]

# Definition of singleton of String
singleton = builder.build_singleton(string)
# No `gsub` method for String singleton
puts singleton.methods[:gsub]
```
<!-- run-end -->

## Guides

- [Architecture](docs/architecture.md)
- [Core and standard library signature contribution guide](docs/CONTRIBUTING.md)
- [Writing signatures guide](docs/sigs.md)
- [Stdlib signatures guide](docs/stdlib.md)
- [Syntax](docs/syntax.md)
- [RBS by Example](docs/rbs_by_example.md)
- [RBS collection](docs/collection.md)
- [Using `Data` and `Struct`](docs/data_and_struct.md)
- [Releasing a gem with RBS](docs/gem.md)

## Community

Here is a list of some places you can talk with active maintainers.

- [Ruby Discord Server (invite link)](https://discord.gg/ad2acQFtkh) -- We have `rbs` channel in Ruby Discord server.
- [ruby-jp Slack Workspace (in Japanese)](https://ruby-jp.github.io/) -- We have `types` channel in ruby-jp slack workspace.
- [gem_rbs_collection](https://github.com/ruby/gem_rbs_collection) -- We have a repository of third-party RBS type definitions, for the case your dependency doesn't ship with RBS files.

## Development

After checking out the repo, run `bin/setup` to install dependencies. Then, run `bundle exec rake test` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run `bundle exec rake install`. Releases are cut by the `Release gems` workflow rather than from a working copy -- see [docs/release.md](docs/release.md).

### C Code Formatting

This project uses `clang-format` to enforce consistent formatting of C code with a `.clang-format` configuration in the root directory.

#### Setup

First, install clang-format:

```bash
# macOS
brew install clang-format

# Ubuntu/Debian
sudo apt-get install clang-format

# Windows
choco install llvm
```

#### Usage

Format all C source files:

```bash
rake format:c
```

Check formatting without making changes:

```bash
rake format:c_check
```

#### Editor Integration

For VS Code users, install the "clangd" extension which will automatically use the project's `.clang-format` file.

## Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/rbs.
