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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.

module ChatApp
  VERSION: String

  class User
    attr_reader login: String
    attr_reader email: String

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

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

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

  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

  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.

The Target Version

  • The standard library signatures targets the latest release of Ruby. (3.2 as of 2023.)
  • The library code targets non-EOL versions of Ruby. (>= 3.0 as of 2023.)


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

gem "rbs"


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

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


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.

require "rbs"

loader =

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

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

# ::String
string = :String, namespace: RBS::Namespace.root)

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

# Builder provides the translation from `declaration` to `definition`
builder = 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]



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


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. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at