title: Checking for a particular data constructor
body: <p>Let's say that I defined my own data-Type like </p>
  
  <pre><code>data MyData = A arg| B arg2| C arg3
  </code></pre>
  
  <p>How would I write a function (for instance: <code>isMyDataType</code>) that checks wether the given argument is one out of the particular types in <code>MyData</code> and successively returns a boolean (True or False) , e.g. typing in Ghci: 
  <code>isMyDataType B</code> returns True and <code>isMyDataType Int</code> returns False.</p>