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    <p>The keywords <strong>class</strong>, <strong>module</strong> and <strong>def</strong> are what is known as "scope gates". They create new scopes. </p> <pre><code>#!/usr/bin/env ruby module ModuleA class ClassA def initialize puts "test passed" end end end module ModuleB include ModuleA # test 1 c = ClassA.new # this works as ModuleA has been included into this module class ClassB # class is a scope gate, creates new scope def initialize # def is a scope gate, creates new scope c = ModuleA::ClassA.new # must fully qualify ClassA end end ClassB2 = Class.new do # no scope gate define_method :initialize do # no scope gate c = ClassA.new # this works, no need to fully qualify end end end b = ModuleB::ClassB.new b2 = ModuleB::ClassB2.new </code></pre> <p>I began to understand scopes in Ruby after reading the <a href="http://rads.stackoverflow.com/amzn/click/1934356476">book "Metaprogramming Ruby"</a>. It is truly enlightening.</p> <p><strong>Edit</strong>: In response to also's comment below.</p> <p>A class is essentially a Ruby constant (notice that it is an object with a capitalized name). Constants have a defined lookup algorithm within scopes. <a href="http://rads.stackoverflow.com/amzn/click/0596516177">The Ruby Programming Language</a> O'Reilly book explains it well in section 7.9. It is also briefly described in this <a href="http://coderrr.wordpress.com/2008/03/11/constant-name-resolution-in-ruby/">blog post</a>.</p> <p>Top-level constants, defined outside of any class or module, are like top-level methods: they are implicitly defined in Object. When a top-level constant is referenced from within a class it is resolved during the search of the inheritance hierarchy. If the constant is referenced within a module definition it does an explicit check of Object after searching the ancestors of the module. </p> <p>That's why include ModuleB at the top level makes the class in ModuleB visible in all modules, classes and methods. </p>
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    1. COI recommend Metaprogramming Ruby to everybody that wants to be a hot-shot ruby dev.
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    2. COWhy, then, does `include ModuleB` at the top level help anything? Shouldn't the scope gates prevent this from having an effect inside of `ModuleB` and `ClassB`?
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    3. COTop-level constants, defined outside of any class or module, are like top-level methods: they are implicitly defined in Object. When a top-level constant is referenced from within a class, therefore, it is resolved during the search of the inheritance hierarchy. If the constant is referenced within a module definition, however, an explicit check of Object is needed after searching the ancestors of the
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