title: Ruby: module, require and include
body: <p>I'm trying to use Ruby modules (mixins).</p>
  
  <p>I have test.rb:</p>
  
  <pre><code>#!/usr/bin/env ruby
  require_relative 'lib/mymodule'
  
  class MyApp
    include MyModule
    self.hallo
  end
  </code></pre>
  
  <p>and lib/mymodule.rb:</p>
  
  <pre><code>module MyModule
    def hallo
      puts "hallo"
    end
  end
  </code></pre>
  
  <p>Quite simple setup. But it does not work :( :</p>
  
  <pre><code>ruby test.rb
  test.rb:8:in `&lt;class:MyApp&gt;': undefined method `hallo' for MyApp:Class (NoMethodError)
          from test.rb:6:in `&lt;main&gt;'
  </code></pre>
  
  <p>Where is my error?</p>