title: How to use filter in list comprehension?
body: <p>So i'm trying to make a setSubtraction from 2 set lists where it returns the set of all elements that occur in xs but not in ys.  For example xs = [1,2,3] and ys = [3,4,5], then setSubtraction xs ys =  [1,2]</p>
  
  <p>this is what i have so far</p>
  
  <pre><code>setSubtract xs ys = [x | x &lt;- xs, y &lt;- ys, filter (==y) xs]
  </code></pre>
  
  <p>but the filter function seems to be erroring out</p>