Accessing members of mixin traits - scala

Question

I have the following code in scala:
trait A {
val foo: String => Int = { in =>
in.toInt
}
}
trait B extends A {
def bar(a: String) = {
foo(a)
}
}
class C(a: String) {
self: B =>
val b = bar(a)
}
val x = new C("34") with B
During instantiation of x I get NPE. Couldn't figure out why.
edit
NOTE: Can not figure out why foo of A trait doesn't get initialized

Answer 1

Please refer to the http://docs.scala-lang.org/tutorials/FAQ/initialization-order.html.
The only addition to this is that self-type makes class C abstract. So actually you do:
abstract class C(a: String) {
def bar(a: String): Int
val b = bar(a)
}
val x = new C("34") with B
You can try to replace it in your code and see same result. Some more info here.
In short: the linearization of new C with B will be (B <- A) <- C, so initalization is C -> (A -> B). Please refer to Class Initialization section section:
After the superclass constructor is executed, the constructors for each mixin trait are executed. Since they are executed in right-to-left order within the linearization, but the linearization is created by reversing the order of the traits, this means the constructors for the mixin traits are executed in the order that they appear in the declaration for the class. Remember, however, that when mixins share hierarchy, the order of execution may not be quite the same as how the mixins appear in the declaration.
In your case new C("34") with B equals to class K extends C("34") with B; new K. Note that self type of class C doesn't affect the initialization order.
Simplified example:
scala> trait C {def aa: String; println(s"C:$aa")}
defined trait C
scala> trait D {val aa = "aa"; println(s"D:$aa")}
defined trait D
scala> new C with D
C:null
D:aa
res19: C with D = $anon$1#2b740b6
Solution: If your foo is placed in third party library (so you can't make it lazy), you may just use mix-in instead of self-type or at least mix A into the C class:
trait A {
val foo: String => Int = { in =>
in.toInt
}
}
trait B extends A {
def bar(a: String) = {
foo(a)
}
}
class C(a: String) extends A {
self: B =>
val b = bar(a)
}
val x = new C("34") with B

Answer 2

The short answer on why you get a NullPointerException is, that initialization of C requires initializing b, which invokes the method stored in val foo, which is not initialized at this point.
Question is, why is foo not initialized at this point? Unfortunately, I cannot fully answer this question, but I'd like to show you some experiments:
If you change the signature of C to extends B, then B, as the superclass of C is instantiated before, leading to no exception being thrown.
In fact
trait A {
val initA = {
println("initializing A")
}
}
trait B extends A {
val initB = {
println("initializing B")
}
}
class C(a: String) {
self: B => // I imagine this as C has-a B
val initC = {
println("initializing C")
}
}
object Main {
def main(args: Array[String]): Unit ={
val x = new C("34") with B
}
}
prints
initializing C
initializing A
initializing B
while
trait A {
val initA = {
println("initializing A")
}
}
trait B extends A {
val initB = {
println("initializing B")
}
}
class C(a: String) extends B { // C is-a B: The constructor of B is invoked before
val initC = {
println("initializing C")
}
}
object Main {
def main(args: Array[String]): Unit ={
val x = new C("34") with B
}
}
prints
initializing A
initializing B
initializing C
As you can see, the initialization order is different. I imagine the dependency-injection self: B => to be something like a dynamic import (i.e., putting the fields of an instance of B into the scope of C) with a composition of B (i.e., C has-a B). I cannot prove that it is solved like this, but when stepping through with IntelliJ's debugger, the fields of B are not listed under this while still being in the scope.
This should answer the question on why you get a NPE, but leaves the question open on why the mixin is not instantiated first. I cannot think of problems that may occur otherwise (since extending the trait does this basically), so this may very well be either a design choice, or noone thought about this use case. Fortunately, this will only yield problems during instantiation, so the best "solution" is probably to not use mixed-in values during instantiation (i.e., constructors and val/var members).
Edit: Using lazy val is also fine, so you can also define lazy val initC = {initB}, because lazy val are not executed until they are needed. However, if you do not care about side effects or performance, I would prefer def to lazy val, because there is less "magic" behind it.

Answer 3

Declare A.foo to be a lazy val.