title: Point2D data type in Haskell
body: <p>Learning Haskell....</p>
  <p>Stuck with this message when I write the following code. Looks like there is some problem with the type declaration/ constructor.</p>
  <p><strong>Error message : Not in scope: type constructor or class `Point2D'</strong></p>
  <p>ps: I am facing a similar problem with Marklist which I thought was an alias for (String, Int). Do suggest. Are there newer names for these type declarations ?</p>
  <pre><code>distance :: Point2D -&gt; Point2D -&gt; Int
  distance (x1, y1) (x2, y2) = 
  sqrt(sqr (x1-x2) + sqr (y1-y2))
  where
      sqr :: Float -&gt; Float
      sqr x = x * x
  </code></pre>