title: Invalid data type
body: <p>The following code is invalid:</p>
  
  <pre><code>data A = Int | [Int] | (Int, Int)
  </code></pre>
  
  <p>Why is it ok to use the concrete type <code>Int</code> as part of a data type definition, but not the concrete type <code>[Int]</code> or <code>(Int, Int)</code>?</p>