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8.1 Getting Modular

Mixing it up

Ruby modules allow you to create groups of methods that you can then include or mix into any number of classes. Modules only hold behaviour, unlike classes, which hold both behaviour and state.

Since a module cannot be instantiated, there is no way for its methods to be called directly. Instead, it should be included in another class, which makes its methods available for use in instances of that class. There is, of course, more to this story, but let's keep it simple for now.

In order to include a module into a class, we use the method include which takes one parameter - the name of a Module.

Example Code: 
module WarmUp
module WarmUp
  def push_ups
    "Phew, I need a break!"
  end
end
 
class Gym
  include WarmUp
  
  def preacher_curls
    "I'm building my biceps."
  end
end
 
class Dojo
  include WarmUp
  
  def tai_kyo_kyu
    "Look at my stance!"
  end
end
 
puts Gym.new.push_ups
puts Dojo.new.push_ups
 
 
[reset]

Output Window

In the example above, we have a Gym and a Dojo. They each have their own, distinct behaviour - preacher_curls and tai_kyo_kyu respectively. However, both require push_ups, so this behaviour has been separated into a module, which is then included into both classes.

Some hierarchy and a little exercise

Just like all classes are instances of Ruby's Class, all modules in Ruby are instances of Module.

Interestingly, Module is the superclass of Class, so this means that all classes are also modules, and can be used as such. For detailed lessons on inheritance in Ruby, do take a look at our chapter on the subject in the "Ruby Primer: Ascent."

Example Code: 
module WarmUp
module WarmUp
end
 
puts WarmUp.class      # Module
puts Class.superclass   # Module
puts Module.superclass  # Object
 
 
[reset]

Output Window

Time for some practice! As always, make the tests pass. Note that the perimeter of both a square and a rectangle is calculated by summing up all of its sides.

module Perimeter
module Perimeter
  # Your code here
end
 
class Rectangle
  # Your code here
  def initialize(length, breadth)
    @length = length
    @breadth = breadth
  end
 
  def sides
    [@length, @breadth, @length, @breadth]
  end
end
 
class Square
  # Your code here
  def initialize(side)
    @side = side
  end
 
  def sides
    [@side, @side, @side, @side]
  end
end
 
 
[reset] See the Solution

Output Window

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