title: How to override a mutable variable in Trait in scala?
body: <p>I'd like override one mutable variable in Trait in constructor. But it will complain that "overriding variable a in trait A of type Int; variable a cannot override a mutable variable".  Why the scala do not allow me do that ? And any best practise for this ? Thanks</p>
  
  <pre><code>trait A{
    var a:Int = _
  }
  
  class B(override var a:Int) extends A
  </code></pre>