Can a Ruby module be described as a singleton class?

Question

I\'m trying to understand the purpose of a Ruby module from a design pattern perspective.

Is a Ruby module essentially just a class that is only initialized once?

Answer 1

A ruby class is a module you can make instances of. Like a class, a module can have methods, but you cannot make an instance of a module. That's the only difference between them.

In practice, modules are commonly used for:

• Name spaces
• Mixins
• To hold functions

Name Space

Here's an example of a module used as a name space:

module MyLib
  class Foo
  end
  class Bar
  end
end

The full name of these classes is MyLib::Foo and MyLib::Bar. Because they are contained in a namespace (which presumably is unique), the names Foo and Bar cannot conflict with a Foo or Bar defined in your program or in another library.

Mixin

Here's a module used as a mix-in:

module Mixin
  def foo
    puts "foo"
  end
end

Since you can't make an instance of the Mixin module, you get access to foo by including (mixing in) the module:

class MyClass
  include Mixin
end

MyClass.new.foo    # => foo

Functions

Like a class, a module can hold functions that do not operate on any instance. To do that, you define class methods in the module:

module SomeFunctions
  def self.foo
    puts "foo"
  end
end

A class method defined in a module is just like a class method defined in a class. To call it:

SomeFunctions.foo    # => foo