Object inside class

Question

Scala 2.12
What is wrong with my implementation?

object MyJob extends DatasetReader(x=x) {
  val x = "aaa"
  DatasetReader.read()
}

class DatasetReader(x: String) {
  object DatasetReader {
    def read(): String = {
       // ...
    }
  }
}

error:

super constructor cannot be passed a self reference unless parameter is declared by-name

How to fix it?

Can't reproduce your error "super constructor cannot be passed..." https://scastie.scala-lang.org/qc5I3DOOT02MkHPNxz1sww The error is "not found: value x". — Dmytro Mitin, Nov 06 '20 at 12:18
Ok, maybe you have import `import MyJob.x`. Then the error is reproducible https://scastie.scala-lang.org/iXdrwmJzTtCayZibqeRIog — Dmytro Mitin, Nov 06 '20 at 12:24
@Thilo I guess OP wants, using named parameter of parent constructor, to refer to object field. — Dmytro Mitin, Nov 06 '20 at 12:26
But that object field will not be initialized yet (which is why the compiler rejects it). Do `x="aaa"` or define it as a constant somewhere else (outside of the the object that is currently being constructed). — Thilo, Nov 06 '20 at 12:29

Tomer Shetah · Accepted Answer · 2020-11-06T12:41:58.127

2

Another option you have is:

object MyJob extends {
  val x = "aaa"
} with DatasetReader(x) {
  DatasetReader.read()
}

Code run at Scastie.

There are similar post in StackOverflow, issue in github, and a bug in Scala.

edited Nov 06 '20 at 12:41

answered Nov 06 '20 at 11:54

Tomer Shetah

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2

You beat me with early initializer :) – Dmytro Mitin Nov 06 '20 at 12:44
That is a very obscure feature (and removed in Scala 3), no need to reach for that. @DmytroMitin can you restore your earlier, much more pragmatic answer, please? – Thilo Nov 07 '20 at 00:54
@Thilo Done . . – Dmytro Mitin Nov 07 '20 at 01:17

Dmytro Mitin · Answer 2 · 2020-11-07T03:02:41.543

Try

val x = "aaa"

object MyJob extends DatasetReader(x=x) {
  DatasetReader.read()
}

https://scastie.scala-lang.org/DjB31943QxujtmfrzLa3tg

I guess you can do what you want with early initializer

object MyJob extends {
  val x = "aaa"
} with DatasetReader(x=x) {
  DatasetReader.read()
}

https://scastie.scala-lang.org/r6FYtxXeT1SFcFHX6KFM1A

Please notice that early initializers are deprecated in Scala 3

http://dotty.epfl.ch/docs/reference/dropped-features/early-initializers.html

Object inside class

2 Answers2

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