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copied!<p>Modules add functions to things; <code>attr_accessor</code> adds functions for interacting with a variable.</p> <pre><code>module ToMix @module_var = "module_var" attr_accessor :mixed_var def initialize @mixed_var = "mixed_var" end def mix_function puts "mix_function: #{@mixed_var}" end def self.module_function @module_var end end class Mixed include ToMix end Mixed.new.mixed_var </code></pre> <p>Notably,</p> <pre><code>"".extend(ToMix).mixed_var == nil # no error, but no value, interesting! </code></pre> <p>but</p> <pre><code>(a = "".extend(ToMix)).mixed_var = "interesting" a.mixed_var == "interesting" </code></pre> <p>and</p> <pre><code>ToMix.module_function == "module_var" </code></pre> <p><a href="https://stackoverflow.com/a/151774/171916">https://stackoverflow.com/a/151774/171916</a> <a href="http://www.natontesting.com/2009/09/28/accessing-instance-variables-declared-in-ruby-modules/" rel="nofollow noreferrer">http://www.natontesting.com/2009/09/28/accessing-instance-variables-declared-in-ruby-modules/</a> <a href="https://stackoverflow.com/questions/3127069/how-to-dynamically-alter-inheritance-in-ruby">How to dynamically alter inheritance in Ruby</a></p> <p>Edit: Those wiser than I should correct me if I'm wrong, but module and class definitions are themselves objects. Defining a <code>@var</code> in a module definition thus adds the var to the module object itself</p> <p>Edit: Those wiser than I have corrected me: While class and module definitions sort of behave like a singleton, they are not themselves objects. You can think of <code>def self.bacon</code> and <code>@var</code> outside of methods as c++ static methods and variables, but they can /only/ be access like a static, even if you're in an instance.</p>

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