Ruby Building Blocks

I feel like a toddler. My brain hurts and nothing makes sense. The internet insists on explaining things to me like I already know things. Dear Internet, I most certainly do not. I have included here the stuff that did make sense. May these humble building blocks aid you in your own pursuit of understanding.

Modules vs. Classes

From my blog post on the map method, a module is a way of grouping together methods, classes, and constants - a collection of reuseable code. A class, on the other hand, is a Ruby 'thing' (object) with a state and a behavior.

Quick define: superclass and subclass describe the relationship between classes. The subclass acquires the behaviors (methods) of the superclass. It's a hierarchy. Super → top, sub → bottom.

ModulesClasses
DefinitionA collection of reuseable code.A Ruby object with a state and a behavior.
InstantiationNo.Yes. Class.new()
InclusionInclude in classes or modules with the 'include' command. A module can be included by multiple classes.No.
InheritanceDoes not do inheritance.Can inherit methods from it's superclass, can serve as a base for a subclass to inherit.
PurposeA code container, good for organization. Provides methods that you can use across multiple classes. Use when you want to group things together that don't naturally form a class.To create instances of itself.
Containsmethods, constants, variablesmethods, constants, variables
Syntaxmodule ModuleName .. endclass ClassName .. end

Namespace & Mixins

Stay with me, these terms require some exposition. This wonderfully fabulous article provides a great explanation. In fact, it's so great that I'm going to include it here:


A good example of when the term 'namespace' becomes relevant is when you want to collaborate with other developers and share code.

(Nothing below here is mine)

Say you write a set of trigonometry functions sin, cos, and so on. You stuff them all into a file, trig.rb, for future generations to enjoy. Meanwhile, Sally is working on a simulation of good and evil, and codes up a set of her own useful routines, including beGood and sin, and sticks them into action.rb. Joe, who wants to write a program to find out how many angels can dance on the head of a pin, needs to load both trig.rb and action.rb into his program. But both define a method called sin. Bad news.

The answer is the module mechanism. Modules define a namespace, a sandbox in which your methods and constants can play without having to worry about being stepped on by other methods and constants. The trig functions can go into one module:

module Trig
  PI = 3.141592654

  def Trig.sin(x)
    # ..
  end

  def Trig.cos(x)
    # ..
  end
end
  

and the good and bad action methods can go into another:

module Action
  VERY_BAD = 0
  BAD      = 1

  def Action.sin(badness)
    # ...
  end
end
  

Pretty awesome right?!

More awesomeness that isn't mine: "A module can't have instances, because a module isn't a class. However, you can include a module within a class definition. When this happens, all the module's instance methods are suddenly available as methods in the class as well. They get mixed in. In fact, mixed-in modules effectively behave as superclasses."

There you go. Now you understand namespaces and mixins. If you don't, email me and tell me I did a poor job of explaining it / stringing together internet examples. I will not be offended and will likely thank you for the feedback.