Modules is a way to achieve Polymorphism is Ruby. A module is a collection of behaviors that can be included in another class via mixins.

Modules are usefull because Ruby is a single inheritance language: a class can only inherits from one superclass. This limitation makes it difficult to design accurately a problem. Modules let the programmer mix in behaviors to achieve what would be otherwise impossible with a single inheritance. A class can sub-class from only one parent (superclass) but it can mix in as many modules as it likes.

Module cannot be instantiated (no object can be created from a module). They are only used for mixins and namespacing.


Mixins comes from “mixed in”: a module is mixed in (mixin) to a class using the include method invocation.

As Ruby’s class can only inherits from one superclass, one can use module and mixins to group common behaviors to create a more flexible design.

module Speakable
  def speak(sound)
    puts sound

class Flamingo
  include Speakable

johnny =
johnny.speak('Gwa gwa!') # => Gwa gwa!

By convention, Rubyist use the “-able” suffix on a verb to describes the behavior that the module is modeling. This is however not enforced.

See Method Lookup Path for how Ruby lookup methods of module mixed in to a class.


Namespacing means grouping similar classes under a common module. Using namespacing has two advantages:

  • Better organize and recognize related classes in our code
  • Reduce the likelihood of our custom classes to collide with classes of the same name in the codebase (specially true when using a lot of gems)
module Mammal
  class Dog

  class Cat

olive =
felix =

Classes in a module are called using two colons: ::.

Container (for methods)

Using modules as a container for methods (called module methods) let us define methods that quite don’t match any class and/or module in our codebase.

module Speakable
  # Other methods

  def self.out_of_place_method(num)

result = Speakable.out_of_place_method(4) # Preferred way to call the method
result = Speakable::out_of_place_method(4)
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