Avoiding NPE in trait initialization without using lazy vals

Question

This is probably covered by the blog entry by Jesse Eichar—still I can't figure out how to correct the following without resorting to lazy vals so that the NPE is fixed:

Given

trait FooLike { def foo: String }
case class Foo(foo: String) extends FooLike

trait Sys {
  type D <: FooLike
  def bar: D
}

trait Confluent extends Sys {
  type D = Foo
}

trait Mixin extends Sys {
  val global = bar.foo
}

First attempt:

class System1 extends Mixin with Confluent {
  val bar = Foo("npe")
}

new System1  // boom!!

Second attempt, changing mixin order

class System2 extends Confluent with Mixin {
  val bar = Foo("npe")
}

new System2  // boom!!

Now I use both bar and global very heavily, and therefore I don't want to pay a lazy-val tax just because Scala (2.9.2) doesn't get the initialisation right. What to do?

Actually, it was possible in my concrete case, to add `bar` as constructor argument: `class System3( val bar: Foo ) extends Confluent with Mixin; new System3( Foo( ":-)" ))`—but I'm still interested how to solve this if I hadn't had the possibility of the constructor arg. — 0__, Sep 28 '12 at 21:12

score 11 · Accepted Answer · edited May 23 '17 at 10:29

11

You can use an early initializer:

class System1 extends {
  val bar = Foo("npe")
} with Mixin with Confluent {
  // ...
}

scala> new System1
res3: System1 = System1@1d0bfedd

edited May 23 '17 at 10:29

Community

1
1

answered Sep 28 '12 at 21:35

kiritsuku

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3

Wow, can't believe after >4 years of Scala I'm still encountering new things! – 0__ Sep 28 '12 at 21:51

Avoiding NPE in trait initialization without using lazy vals

1 Answers1