title: Declaring a generic variable in a type in F#
body: <p>How do I declare that I want something of type <code>'a</code> which I have no value for yet.</p>
  
  <pre><code>type MyType&lt;'a&gt; =
      let mutable something:'a = ??
      let setSomething item:'a =
          something &lt;- a
  </code></pre>