Scala and forward references

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Scala: forward references - why does this code compile?

object Omg {

  class A

  class B(val a: A)

  private val b = new B(a)

  private val a = new A

  def main(args: Array[String]) {
    println(b.a)
  }

}

the following code prints "null". In java. similar construction doesn't compile because of invalid forward reference. The question is - why does it compile well in Scala? Is that by design, described in SLS or simply bug in 2.9.1?

The issue that annoys me about this is that it allows a val to change its value. That makes me sad :-( — thoredge, Aug 29 '12 at 21:31
it's a bit strange - lots of error could be caused by that, and I relied on Java behavior, which requires to have values initialized before they are used. — jdevelop, Aug 29 '12 at 21:36
@jdevelop Even java doesn't catch all possible forward references. — Daniel C. Sobral, Aug 30 '12 at 01:25

score 38 · Accepted Answer · edited Mar 16 '14 at 12:45

38

It's not a bug, but a classic error when learning Scala. When the object Omg is initialized, all values are first set to the default value (null in this case) and then the constructor (i.e. the object body) runs.

To make it work, just add the lazy keyword in front of the declaration you are forward referencing (value a in this case):

object Omg {

  class A

  class B(val a: A)

  private val b = new B(a)

  private lazy val a = new A

  def main(args: Array[String]) {
    println(b.a)
  }
}

Value a will then be initialized on demand.

This construction is fast (the values are only initialized once for all application runtime) and thread-safe.

edited Mar 16 '14 at 12:45

corazza

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answered Aug 29 '12 at 20:18

paradigmatic

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16

Adding `lazy` works if you *know* you are executing the statements in the wrong order. But if you *think* you have them in the correct order, you probably won't think of adding that "unnecessary" `lazy`. :/ – kornfridge Jan 09 '13 at 15:02

score 8 · Answer 2 · edited Aug 29 '12 at 20:33

The way that I understand it, this has to do with how the Scala classes are created. In Java, the class defined above would be initializing the variables inline, and since a had not been defined yet, it could not be compiled. However, in Scala it is more the equivalent of this in Java (which should also produce null in the same scenario):

class Omg {
  private B b = null;
  private A a = null;

  Omg(){ 
    b = new B(a);
    a = new A();
  }
}

Alternately, you could make your declaration of b lazy, which would defer setting the value until it is called (at which time a will have been set).

score 6 · Answer 3 · edited May 23 '17 at 12:09

6

If this is a concern, compile with -Xcheckinit during development and iterate until the exceptions go away.

Spec 5.1 for template body statements executed in order; beginning of 4.0 for forward references in blocks.

Forward References - why does this code compile?

edited May 23 '17 at 12:09

Community

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answered Aug 30 '12 at 05:35

som-snytt

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score 3 · Answer 4 · answered Aug 29 '12 at 20:23

As @paradigmatic states, it's not really a bug. It's the initialization order, which follows the declaration order. It this case, a is null when b is declared/init-ed.

Changing the line private val b = new B(a) to private lazy val b = new B(a) will fix issue, since using lazy will delay the init. of b to it's first usage.

It's very likely that this behavior is described in the SLS.

Scala and forward references

4 Answers4

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