title: Can Haskell display an object of polymorphic type ?
body: <p>In the book of <em>The Haskell Road to Logic, Math, and Programming</em> by Doets, at page 152, it is given that</p>
  
  <p><a href="https://i.stack.imgur.com/hccPD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hccPD.png" alt="enter image description here"></a></p>
  
  <p>However, when I type <code>[]</code> into ghc, I get <code>[]</code> and </p>
  
  <pre><code>$&gt; :t []
  [] :: [a]
  </code></pre>
  
  <p><strong>so Haskell does display a polymorphic data type, so am I missing something in here, or does the book ?</strong> </p>
  
  <p>Maybe it is something that is added later into the Haskell; the book is written in 2004.</p>