title: Haskell if-construct that returns nothing on `else`
body: <p>is there a way where a construct like (pseudo code):</p>
  
  <pre><code>test (a,b) = if a &gt; 10 then a else null
  </code></pre>
  
  <p>in Haskell. If you map over a list I can use <code>filter</code> to do this, but I would like a similar construct using <code>if then else</code>.</p>