Lazy Enforcement of Requirements on Abstract Fields

[Hossein]

Thank you for the comprehensive explanation. I have new questions now:

@hmf

val is a value that must be instantiated at the start of the application (unless lazy).

Start of the application? If I understand you correctly, you’re implying there is no real late-binding for the abstract vals. Otherwise, the initialisation should have been postponed to the time when a concrete value is provided further down the inheritance hierarchy. Or, am I missing anything?

@hmf

Note: I don’t think the requirement in your example above is actually checked. You have to instantiate B2. Try using the vale -10.

Well, it indeed is checked:

scala> trait B3 extends B1 {def v = -10; def f(): Int}
defined trait B3

scala> val b3 = new B3 {def f() = 124000}
java.lang.IllegalArgumentException: requirement failed
  at scala.Predef$.require(Predef.scala:212)
  at B1$class.$init$(<console>:17)
  ... 33 elided

Albeit, I would say it should have been checked earlier — making B3 illegal even before its instantiation.

Cheers,
–Hossein