What are modules and mixins used for?

A module is a collection of methods and constants. It cannot be instantiated, but it can be mixed in to add behaviour, which is Ruby's alternative to multiple inheritance.

• include adds the module's methods as instance methods of the class.
• extend adds them to the object or class itself as singleton methods.
• prepend inserts the module before the class in the lookup chain, so it can override methods and call super.

module Greetable
  def greet = "Hello, #{name}"
end

class User
  include Greetable
  attr_accessor :name
end

Modules are also used for namespacing, as in Admin::User. In Rails, ActiveSupport::Concern manages mixin dependencies and class methods.

Because lookup follows the ancestor chain, Module#ancestors is the key to predicting which method wins. Keep mixins cohesive and relatively state-light to avoid surprising interactions.