title: F# Option equivalent of C#'s ?? operator
body: <p>I am looking for a way to get the value of an F# option or use a default value if it is None.
  This seems so common I can't believe something predefined doesn't exist. Here is how I do it right now:</p>
  
  <pre><code>// val getOptionValue : Lazy&lt;'a&gt; -&gt; Option&lt;'a&gt; -&gt; 'a    
  let getOptionValue (defaultValue : Lazy&lt;_&gt;) = function Some value -&gt; value | None -&gt; defaultValue.Force ()
  </code></pre>
  
  <p>I am (sort of) looking for the F# equivalent of the C# ?? operator:</p>
  
  <pre><code>string test = GetString() ?? "This will be used if the result of GetString() is null.";
  </code></pre>
  
  <p>No function in the Option module does what I think is a pretty basic task. What am I missing?</p>