title: How to check in F# whether object implements interface
body: <p>Prototypical code in C#:</p>
  
  <pre><code>if(obj1 is ISomeInterface) {
     do_something
  }
  </code></pre>
  
  <p>Code in F# that doesn't compile:</p>
  
  <pre><code>match obj1 with 
  | :? ISomeInterface -&gt; do_something
  | _ -&gt; ()
  </code></pre>