title: val-mutable versus var-immutable in Scala
body: <p>Are there any guidelines in Scala on when to use val with a mutable collection versus using var with an immutable collection? Or should you really aim for val with an immutable collection?</p>
  
  <p>The fact that there are both types of collection gives me a lot of choice, and often I don't
  know how to make that choice.</p>