title: What is call-by-need?
body: <p>I want to know what is call-by-need.</p>
  
  <p>Though I searched in wikipedia and found it here: <a href="http://en.wikipedia.org/wiki/Evaluation_strategy" rel="noreferrer">http://en.wikipedia.org/wiki/Evaluation_strategy</a>,
  but could not understand properly. 
  If anyone can explain with an example and point out the difference with call-by-value, it would be a great help.</p>