Declaring abstract "var" members

It's a bad practice to declare abstract vars in abstract classes or traits. Do not do this:

trait Foo {
  var value: String
}

Instead, prefer to declare abstract things as def:

trait Foo {
  def value: String
}

// can then be overridden as anything
class Bar(val value: String) extends Foo

The reason has to do with the imposed restriction - a var can only be overridden with a var. The way to allow freedom to choose on inheritance is to use def for abstract members. And why would you impose the restriction to use a var on those that inherit from your interface. def is generic so use it instead.
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