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 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

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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.

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 end puts WarmUp.class # Module puts Class.superclass # Module puts Module.superclass # Object

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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 # 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

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