title: Extract value from union case
body: <p>In an Fsharp application, I have defined several union case types as </p>
  
  <pre><code>type A = A of String
  type B = B of String
  type C = C of String
  </code></pre>
  
  <p>I would like to define a function to extract the value from the union case instances. </p>
  
  <pre><code>let getValue( ctor: String-&gt; 'a) = 
     ...implementation here
  </code></pre>
  
  <p>Is there anyway to achieve such task?
  Thanks.</p>