title: What does the "Just" syntax mean in Haskell?
body: <p>I have scoured the internet for an actual explanation of what this keyword does. Every Haskell tutorial that I have looked at just starts using it randomly and never explains what it does (and I've looked at many).</p>
  
  <p>Here's a basic piece of code from <em>Real World Haskell</em> that uses <code>Just</code>. I understand what the code does, but I don't understand what the purpose or function of <code>Just</code> is. </p>
  
  <pre><code>lend amount balance = let reserve    = 100
                        newBalance = balance - amount
                    in if balance &lt; reserve
                       then Nothing
                       else Just newBalance
  </code></pre>
  
  <p>From what I have observed, it is related to <code>Maybe</code> typing, but that's pretty much all I have managed to learn.</p>
  
  <p>A good explanation of what <code>Just</code> means would be very much appreciated.</p>