title: Assign return value when index check out of bounds
body: <p>In pseudo-code, I'm trying the following:</p>
  
  <pre class="lang-none prettyprint-override"><code>for i in len(array):
      try:
          a = array[i-1]
      except(out_of_bounds_error):
          a = false
  </code></pre>
  
  <p>where <code>array</code> is just made up of booleans.</p>
  
  <p>In the book (Chapter 9.2) it says you can check whether a function returns a result or not with something like:</p>
  
  <pre><code>let a: u32 = array[i-1]
  </code></pre>
  
  <p>which tells me <code>a</code> is indeed a bool. Without a <code>Result</code> type, how do I handle the inevitable (and expected) <code>attempt to subtract with overflow</code> error at runtime?</p>