title: Why purely functional languages allow passing parameters only by value?
body: <p>I'm new to functional languages and I was wondering why we can't pass a parameter by reference. 
  I found anserws saying that </p>
  
  <blockquote>
    <p>you are not supposed to change the state of objects once they have been created</p>
  </blockquote>
  
  <p>but I didn't quite get the idea.</p>