title: Haskell Filtering but keeping the filtered
body: <p>In my understanding, the Haskell <code>filter</code> function <code>filter a bs</code> would filter all <code>a</code>'s from a list <code>bs</code>.</p>
  
  <p>Is there a simple method or variation of <code>filter</code> that would do the opposite, only keeping the <code>a</code>'s from the list <code>bs</code>, basically creating a list of <code>a</code>'s. </p>