How to avoid scala's case class default toString function being overridden?

Question

Scala case class has a default toString function. But when this case class extends a trait with an existing toString() function, it will be rendered useless. How can I prevent this situation?

There is a similar question at: http://stackoverflow.com/questions/20998866/scala-prevent-mixing-methods — nitishagar, Dec 14 '14 at 03:25

score 28 · Accepted Answer · edited Feb 25 '15 at 09:59

28

OK here is the easist answer:

override def toString = ScalaRunTime._toString(this)

end of story:)

edited Feb 25 '15 at 09:59

Sebastien Lorber

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answered Dec 14 '14 at 07:53

tribbloid

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That is DRY and easy. – som-snytt Dec 14 '14 at 21:56
2

noob question: That gave me errors for several reasons; I got this to work: `override def toString() = ScalaRunTime._toString(this)` (I had to add parens after `toString`). Moreover, I had to have my particular class explicitly `extend Product`. Any way to avoid this? (Also, I used `import scala.runtime.ScalaRunTime` to avoid fully qualifying the name.) – not-just-yeti Feb 23 '17 at 12:48
3

@not-just-yeti declaring as case class is equivalent to extend Product. Other parts are related to scala shortcuts in different versions and not important. – tribbloid Mar 03 '17 at 22:02

score 9 · Answer 2 · answered Dec 14 '14 at 05:18

Here's a workaround I think may work, it may be too much ceremony, you decide. It involves a trait.

trait StandardToString { this:Product =>
  override def toString = productPrefix + productIterator.mkString("(", ",", ")")
}

Now trying it with some samples:

trait Human {
   override def toString() = "Human"
}

case class European(firstName:String) extends Human

and running it with the trait:

scala> new European("Falco") with StandardToString
res0: European with StandardToString = European(Falco)

of course with the trait you are left with

scala> new European("Adele")
res1: European = Human

score 1 · Answer 3 · answered Dec 14 '14 at 22:05

It's more precise to say that the case class toString is not generated, rather than that it is overridden.

This isn't much of an answer or workaround.

scala> trait X { override def toString = "X" }
defined trait X

scala> case class Y(i: Int) extends X
defined class Y

scala> Y(42)
res0: Y = X

scala> case class Y(i: Int)
defined class Y

scala> class Z(x: Int) extends Y(x) with X { override def toString = super[Y].toString }
defined class Z

scala> new Z(42)
res1: Z = Y(42)

You can't do that with a trait:

scala> trait R extends Y { override def toString = super[Y].toString }
<console>:9: error: Implementation restriction: traits may not select fields or methods from super[C] where C is a class
       trait R extends Y { override def toString = super[Y].toString }
                                                   ^

How to avoid scala's case class default toString function being overridden?

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