title: let declarations require an initializer expression
body: <p>I'm reading The Swift Programming Language, in the Simple Values section</p>
  
  <blockquote>
    <p>“Use let to make a constant and var to make a variable. The value of a
    constant doesn’t need to be known at compile time, but you must assign
    it a value exactly once”</p>
  </blockquote>
  
  <p>So I think I can do this</p>
  
  <pre><code>let aConstant:Int
  aConstant = 5
  </code></pre>
  
  <p>But I get <strong>let declarations require an initializer expression</strong> !!</p>
  
  <p>Why is that ? What does they mean by "The value of a constant doesn’t need to be known at compile time" ?</p>