title: The performance of (++) with lazy evaluation
body: <p>I have been wondering about this a lot, and I haven't found satisfying answers.</p>
  
  <p>Why is <code>(++)</code> "expensive"? Under lazy evaluation, we won't evaluate an expression like</p>
  
  <pre><code>xs ++ ys
  </code></pre>
  
  <p>before necessary, and even then, we will only evaluate the part we need, <em>when</em> we need them.</p>
  
  <p>Can someone explain what I'm missing?</p>