Scala: Example use for early definition / early initializer / pre-initialized fields

Scala allows you to make early definitions like so:

trait A {
  val v: Int

class B extends { val v = 4 } with A

What is an example use of this feature?


Whenever the value is used for the trait initialization. So for eaxmple for this trait:

trait UsefulTrait {
  val parameter : Int
  private val myHelperObject = new MyExpensiveClass(parameter)

The parameter is used to substitute for a constructor parameter. However the parameter should be rather made an abstract method, because it leaves more free space to the implementer.

Let's see a example from the Programming in Scala book (page 451). If we have a definition like this:

trait RationalTrait {
   val numerArg: Int
   val denomArg: Int

Then numerArg and denomArg are called abstract vals & the trait can be used directly without extends, like this:

val x = new RationalTrait {
   val numerArg = 1
   val denomArg = 2


val y = new {
   val numerArg = 1
   val denomArg = 1
} with RationalTrait

The above two are both valid Pre-initializing of abstract val in trait, except that when you need to put an expression value to abstract vals, you can only use the later form, like this:

val z = new {
  val numerArg = 1 * x
  val denomArg = 2 * x
} with RationalTrait

Another interesting example in book is to Pre-initialized fields in a class definition.

class RationalClass(n: Int, d: Int) extends {
  val numerArg = n
  val denomArg = d
} with RationalTrait {
  def + (that: RationalClass) = new RationalClass(
    numer * that.denom + that.numer * denom,
    denom * that.denom

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