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Ruby Namespaces

A module can wrap classes and constants under one name. The wrapped names stay separate from names outside the module.

A class inside a module

Book sits inside Store. The full name Store::Book builds a book object.

module Store
  class Book
    def initialize(title)
      @title = title
    end
    def show
      puts @title
    end
  end
end
book = Store::Book.new("Dune")
book.show

Same name, two namespaces

Kitchen and Garage each hold a Tool class. The full name in each line picks which Tool gets built.

module Kitchen
  class Tool
    def use
      puts "Cooks the meal"
    end
  end
end
module Garage
  class Tool
    def use
      puts "Fixes the car"
    end
  end
end
kitchen_tool = Kitchen::Tool.new
garage_tool = Garage::Tool.new
kitchen_tool.use
garage_tool.use

A constant inside a namespace

OPEN_HOUR lives inside Store. The nested Sign class reads it directly, and the last line reaches it with the full name.

module Store
  OPEN_HOUR = 9
  class Sign
    def show
      puts "Opens at #{OPEN_HOUR}"
    end
  end
end
sign = Store::Sign.new
sign.show
puts Store::OPEN_HOUR

Example

module Store
  SECTION = "Fiction"
  class Book
    def initialize(title)
      @title = title
    end
    def show
      puts "#{@title} sits in #{SECTION}"
    end
  end
end
module Archive
  class Book
    def show
      puts "Archived book"
    end
  end
end
store_book = Store::Book.new("Dune")
store_book.show
archive_book = Archive::Book.new
archive_book.show
puts Store::SECTION

Key points

  • A class defined inside a module lives under the module name.
  • The :: shape joins the module name to the wrapped name.
  • Two modules can hold classes with the same name.
  • The full name picks the class that a line builds.
  • Constants can live inside a namespace too.
  • A nested class can read the constants of its module directly.

In this example, Store wraps a Book class and a SECTION constant. Store::Book builds the store book, and its show method reads SECTION directly. Archive holds its own Book, and Archive::Book builds that separate class.

Note

Namespaces keep big programs tidy when names would clash.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

How does a program build an object of the Book class inside Store?