title: "fromJust" vs. "Just =" in Haskell
body: <p>Haskell newbie here. I'm trying to understand <a href="http://projects.haskell.org/diagrams/gallery/FibCalls.html" rel="nofollow">this</a> example from Haskell's <code>diagrams</code> library. Specifically, there's a line like this:</p>
  
  <p><code>Just t = &lt;thing&gt;</code> where <code>&lt;thing&gt;</code> is of type <code>Maybe (Tree a)</code></p>
  
  <p>I don't understand what this is doing. I understand that we need to get the value out from the <code>Maybe</code>. I replaced that line of code with</p>
  
  <p><code>t = fromJust &lt;thing&gt;</code></p>
  
  <p>and it works the same. Is there any difference between the two lines, and can anyone explain what the first line is doing?</p>