title: Get the value of an instance variable given its name
body: <p>In general, how can I get a reference to an object whose name I have in a string?</p>
  
  <p>More specifically, I have a list of the parameter names (the member variables - built dynamically so I can't refer to them directly).</p>
  
  <p>Each parameter is an object that also has an <code>from_s</code> method.</p>
  
  <p>I want to do something like the following (which of course doesn't work...):</p>
  
  <pre><code>define_method(:from_s) do | arg |
      @ordered_parameter_names.each do | param |
          instance_eval "field_ref = @#{param}"
          field_ref.from_s(param)
      end
  end
  </code></pre>