title: Filtering Nothing from a List with foldr/foldl
body: <p>I'm trying to implement catMaybes with List folding but have no clue how i should start.</p>
  
  <p>My general idea is to use foldl and use a case to filter for Nothing and Just. After that the same function has to concat the found parts together my code this far looks like this:</p>
  
  <pre><code>{-# LANGUAGE LambdaCase #-}
  catMaybes :: [Maybe a] -&gt; [a]
  catMaybes xs = foldl((++) \case Nothing -&gt; []; Just y -&gt; y) [] xs
  </code></pre>
  
  <p>I'm getting alot of errors when compiling...
  I think i need a push in the right direction to solve this. Thank you for your help!</p>