Modules

What are you going to learn?

  • Understand what is a Module
  • Understand the usages of a Module and when to apply it
  • Refactor our code by mixin in modules

What is a Module?

In simple words, a Module is:

  • Only stores behavior, not state
  • A group of methods that can be shared across multiple classes
  • Cannot be instantiate it, just as classes
  • Even though you cannot create instances, Modules can live and run by themselves
  • Also use as a way to encapsulate and isolate classes. To give a Namespace

Instance methods appear as methods in a class when the module is included, module methods do not. Conversely, module methods may be called without creating an encapsulating object, while instance methods may not.

This is how a Module looks like:

module ModuleName
end

Keep in mind a couple of things here:

  • All module names in Ruby are written in camel case
  • The definition starts with the module word
  • They are commonly created in separated files, with the name of the class in snake case.

Now that you have defined your module, let's start using it:

module Area
  # This is how you define constants in Ruby
  PI = 3.1416

  def self.square(n)
    n * n
  end

  def self.rectangle(a,b)
    a * b
  end

  def self.circle(r)
    PI * r * r
  end
end

Area.square(10) #=> 100
Area.rectangle(10, 5) #=> 50
Area.circle(5) #=> 78.54

As you can see, this methods are defined and behave just like in class methods. But this is just the tipo of the iceberg, let's see another examples.

Let's say you have the following classes:

class Image
  def share_on_facebook
  end
end

class Post
  def share_on_facebook
  end
end

class Tweet
  def share_on_facebook
  end
end

Maybe if you come from a technical background, one possible alternative that may cross your mind is inheritance, but we are taking another path, Modules.

In shareable.rb

module Shareable
  def share_on_facebook
  end
end

In image.rb

require 'shareable'

class Image
  include Shareable
end

In post.rb

require 'shareable'

class Post
  include Shareable
end

In tweet.rb

require 'shareable'

class Tweet
  include Shareable
end

The code above shows how to mixin all of the methods from the Shareable module into multiple classes. Take this into consideration:

  • You can access properties on objects from host class. In other words, you can access instance variables from the module, and once they are included into the class, it will behave as you expected.
  • The include directive will inject the methods as instance methods.

Extending a Class

What we mean by extending a class, is to add methods, just like above, but instead of instance methods, add them as class methods. 

class Tweet
  extend Searchable
end

module Searchable
  def find_all_from(user)
    # some code implementation
  end
end

As you can see instead of using the keyword include, we are using extend, this will inject the methods to the class level so the invokation would be:

Tweet.find_all_from("kurenn")

Modules Usages

Exercises

Remember we have provided a repository with a bunch of exercises for you to complete. You can find it here

You can finde them under /ruby-exercises/Module1/modules.

Additional Resources