What does the @elidable annotation do in Scala, and when should I use it?

Question

I\'ve noticed in some of the scala library code, notably Predef, there is code like:

/** Tests an expression, throwing an `AssertionError` if false.         


        

Answer 1

As a complement to Tomasz Nurkiewicz's answer two comments.

(1) C++ style

Because I came from C++ I've defined

/** ''Switch'' between '''Debug''' and '''Release''' version. */
object BuildLevel {
  type only = annotation.elidable
  final val DEBUG = annotation.elidable.INFO
}

and use this in good old C++ preprocessor style like

import BuildLevel._
@only(DEBUG)
private def checkExpensive(...) {
  ...
}

override def compare(that: ): Int = {
  checkExpensive(...)
  ...
}

to mark expensive checks (check of pre-conditions or invariants that must always holds true) that I want to switch off in release builds.

Of course that's just similar to the assert use case except for the difference of refactoring out expensive code in a separate method that should be switched off as a whole. But all this is only worthwhile for really expensive checks. In a 10k lines project I have only 3 marked checks. Cheaper tests I wouldn't switch off and leave in the code, because they increase its robustness.

(2) Unit signature

This approach is suitable only for methods with a (...) => Unit signature. If one use a result of such a switched off method like

@only(DEBUG)
def checkExpensive(that: Any): Int = {
  4
}
val n = checkExpensive(this)

at least my Scala 2.9.1.final compiler crashes. However, there is not much sense in such a signature. Because: Which value should such a switched off method return?