title: Simply parse command line args
body: <p>I want to use the following code to restrict there is only one argument. However, I got the following error at <code>first :: NIL</code>?</p>
  
  <pre>
  Error   1   This expression was expected to have type
      string []    
  but here has type
      'a list 
  </pre>
  
  <pre><code>[&lt;EntryPoint&gt;]
  let main argv = 
  
      match argv with 
      | first :: NIL -&gt; 
           .... do something with first
      | _ -&gt; failwith "Must have only one argument."
  </code></pre>