title: Haskell efficient concatenation of lists
body: <p>When I have a function</p>
  
  <pre><code>func :: Int -&gt; Int -&gt; [[a]]
  func a b = func' a ++ func' b
  </code></pre>
  
  <p>where</p>
  
  <pre><code>func' :: Int -&gt; [[a]],
  </code></pre>
  
  <p>what is a good possibility to avoid the (++)?</p>