Exercise 4.3.5 Scope: questionable "circular dependency"

[eed3si9n]

https://github.com/underscoreio/creative-scala/blob/develop/src/pages/programs/names.md#exercises--

original

object One {
  val a = b - 1
  val b = a + 1

  val answer = a + b
}

Trick question! This code doesn't work. Here a and b are defined in terms of each other which leads to a circular dependency that can't be resolved.

problem

I think the reason because this code doesn't work is actually two folds.

1. Circular declaration itself is not forbidden with Scala. It just requires a type annotation on b as val b: Int = a + 1.
2. The reason it doesn't "work", as in it will actually return incorrect value is because val a = b - 1 is referencing b before it is initialized, which seems to be treated as 0. Making answer into (0 - 1) + (0 - 1 + 1), which is -1.

scala> object One {
     |   val a = b - 1
     |   val b: Int = a + 1
     |
     |   val answer = a + b
     | }
<console>:12: warning: Reference to uninitialized value b
         val a = b - 1
                 ^
defined object One

scala> One.answer
res1: Int = -1

expectation

If we want to expose the readers to val initialization order issue, it might be worth changing the exercise to:

object One {
  val a = b - 1
  val b = 10

  val answer = a + b
}

and note that fact that without -Xfatal-warnings one could end up with incorrect answer?