 <p>I think you're looking for <a href="http://msdn.microsoft.com/en-us/library/7977ey2c.aspx">System.Collections.Generic.Queue</a> instead of <a href="http://msdn.microsoft.com/en-us/library/system.collections.queue.aspx">System.Collections.Queue</a>.</p> <p>Change</p> <pre><code>open System.Collections </code></pre> <p>to </p> <pre><code>open System.Collections.Generic </code></pre>