Scala class singleton with non-default constructor parameters

Question

I have already read about issues with objects inheriting from companion classes.

For example:

• Class constructor parameter with default value causes companion object initializer to fail
• super constructor cannot be passed a self reference unless parameter is declared by-name

But I have a bit different issue (I am not sure that it is a bug)

I have the following code:

class Scala(name: String)

import Scala._

object Scala extends Scala(TEST_NAME){
  val TEST_NAME = "test name"
}

Note, that I am using variable defined in scope of companion object and then passing it to super's constructor.

I am getting the following compilation error:

Scala.scala:5: error: super constructor cannot be passed a self reference 
unless parameter is declared by-name

object Scala extends Scala(TEST_NAME){
                           ^
one error found

Other variants that I have tried:

Call by name:

class Scala(name: => String)

import Scala._

object Scala extends Scala(TEST_NAME){
  val TEST_NAME = "test name"
}

Named argument:

class Scala(name: String)

import Scala._

object Scala extends Scala(name = TEST_NAME){
  val TEST_NAME = "test name"
}

Both of them:

class Scala(name: => String)

import Scala._

object Scala extends Scala(name = TEST_NAME){
  val TEST_NAME = "test name"
}

Some environment details:

• java: java version "1.8.0_144"
• javac: javac 1.8.0_144
• scala: Scala code runner version 2.12.3
• scalac: Scala compiler version 2.12.3
• OS: Darwin ***.local 17.0.0 Darwin Kernel Version 17.0.0: Thu Aug 24 21:48:19 PDT 2017; root:xnu-4570.1.46~2/RELEASE_X86_64 x86_64

Update:

For anyone who is interested in resolution of this:

• scala/bug/10537
• Scala users discussion

Answer 1

If your code above would compile then you could write

class A(val x: String)

object B extends A(B.x_again) {

  val x_again = x
}

which certainly doesn't look good.

Answer 2

You can use an obscure feature that is going to be removed in Scala 3: early initializers. It allows you to specify init code to run before you call superclass constructor.

class Scala(name: String)

object Scala extends {
  val TEST_NAME: String = "test name"
} with Scala(TEST_NAME) { /* rest of object code */ }

Note that import is not required - TEST_NAME is already in the scope.

---

Here's a Scastie snippet to prove that it works.

Answer 3

For your restricted use case:

scala> class C(s: String) ; object X extends C(X.x) { final val x = "hello, world" }
defined class C
defined object X

The constant value definition is inlined (per the spec).