title: Example of "True Polymorphism"? (Preferably, using Haskell)
body: <p>I've seen lots of partial definitions of "True Polymorphism", for example <a href="http://www.haskell.org/haskellwiki/Haskell_Tutorial_for_C_Programmers" rel="nofollow">here</a> and <a href="http://en.wikipedia.org/wiki/Ad_hoc_polymorphism#Example" rel="nofollow">here</a> but nowhere have I been able to find a clear example of the difference with two concrete examples.</p>
  
  <p>I understand that overloading the <code>+</code> operator is some form of polymorphism and that it is implemented differently in Haskell and C++. Can someone show precisely what the difference is with examples in both languages?</p>