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:

@EduardoParejaTobes you mean there is no need in companionship, yes? — Andrii Abramov, Oct 03 '17 at 17:51
yup, that this is similar to https://stackoverflow.com/q/32203867/614394 but for values. I think it's fine for types, and wrong for values :) — Eduardo Pareja Tobes, Oct 04 '17 at 11:36
There is some sense in such kind companions. For example: `scala.util.Random`. Or when you have DTO that returns static data, e.g. description of service, terms of use, etc. — Andrii Abramov, Oct 04 '17 at 12:40
I meant not about companions extending the class/trait they accompany, but about an object having "access to itself" when extending any other class/trait. I think its type should be accessible (so that you can write `object X extends Y[X.type]`), but its values not. — Eduardo Pareja Tobes, Oct 04 '17 at 16:49

score 2 · Answer 1 · answered Oct 09 '17 at 05:14

2

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).

answered Oct 09 '17 at 05:14

som-snytt

39,429
2
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129

score 1 · Answer 2 · answered Oct 03 '17 at 16:38

1

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.

answered Oct 03 '17 at 16:38

Oleg Pyzhcov

7,323
1
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Thank you for answering. So what is the resolution of my question? Is it a bug? Edge case? – Andrii Abramov Oct 03 '17 at 17:51
@AndriiAbramov I don't know. I'd call a bug that by-name parameter isn't working while the message suggests that it should (esp. if older versions of Scala supported that, which I cannot check now), but it's probably enough of an edge case too, since nobody seemed to care for at least since 2.10. – Oleg Pyzhcov Oct 03 '17 at 18:28

score 0 · Answer 3 · answered Oct 03 '17 at 15:37

0

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.

answered Oct 03 '17 at 15:37

Eduardo Pareja Tobes

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Yeah, I agree that it looks a bit strange. – Andrii Abramov Oct 03 '17 at 17:49

Scala class singleton with non-default constructor parameters

3 Answers3