title: factory pattern in F#
body: <p>With this type:</p>
  
  <pre><code>   let Empty =
          {
              Buy = new Dictionary&lt;int64, int64 * float&gt;()
              Sell = new Dictionary&lt;int64, int64 * float&gt;()
          }
  </code></pre>
  
  <p>if I assign it to several entities through the code:</p>
  
  <pre><code>let a = Empty
  let b = Empty
  </code></pre>
  
  <p>they'll represent the same dictionaries.</p>
  
  <p>How can I generate a new instance every time?</p>