Why is instantiating a trait with default values in Scala a compilation error?

Question

The error can be reproduced by compiling the following code:

object ReproducingMyError {

  trait MyTrait[X] {
    def someFunc: X
  }

  def f[X] = new MyTrait[X] {
    var x: X = _
    def someFunc: X = x
  }

}

2 error messages were generated. Both point to

  def f[X] = new MyTrait[X] {
                 ^

The messages are similar:

Error: Parameter type in structural refinement may not refer to an abstract type defined outside that refinement

Error: Parameter type in structural refinement may not refer to a type member of that refinement

Why is this a compilation error?

To be clear, it's the setter that is the problem, `def x_=(x: X)`. That's the parameter in the error. — som-snytt, Sep 19 '15 at 02:54

score 2 · Accepted Answer · answered Sep 19 '15 at 10:08

If you don't need to expose the var outside (which I assume you don't), then adding explicit return type fixes the problem:

def f[X]: MyTrait[X] = new MyTrait[X] {
  var x: X = _
  def someFunc: X = x
}

Without explicit type ascription, the compiler will infer the problematic "structural refinement", i.e. a type that looks like this: MyTrait[X] { var x: X }.

Yet another way to fix this is to declare the var as private - this way it won't be visible by anyone outside and, thus - won't be included in the inferred refinement type.

def f[X] = new MyTrait[X] {
  private var x: X = _
  def someFunc: X = x
}

Why is instantiating a trait with default values in Scala a compilation error?

1 Answers1