Product inheritance in case classes

Question

I have some case classes that extend a common superclass and I'd like to access fields from the superclass using productElement method (I've tryed to declare base class as a case class but I get a frightening warning about the dangers of inheritance of case classes and yet doesn't work).

I can imagine some solution like this:

abstract class A(a: Int) extends Product { def productArity = 1 def productElement(n: Int) = if (n == 0) a else throw new IndexOutOfBoundsException } case class B(b: Int) extends A(1) { def productArity = super.productArity + 1 def productElement(n: Int) = if (n < super.productArity) super.productElement(n) else .... }

but it was getting so ugly that I can't even finish.

Does anybody know a better solution?

Yes, I've done it and I get this: Case-to-case inheritance has potentially dangerous bugs which are unlikely to be fixed. You are strongly encouraged to instead use extractors to pattern match on non-leaf nodes. Besides it doesn't work. — jglatre
– jglatre, Commented Oct 31, 2011 at 16:38

Miles Sabin · Accepted Answer · 2011-11-07 10:07:12Z

In Scala trunk a lot of this is done for you already: case classes now extend the appropriate ProductN trait. Only case class direct (ie. not inherited) members are included in the product however, so if you need to include members from a super type, they would have to be abstract in the super type and given a concrete implementation in the case class.

Here's a REPL session (Scala trunk, 2.10.0.r25951-b20111107020214),

scala> trait A { val a: Int } defined trait A scala> case class B(b: Int, a : Int = 1) extends A defined class B scala> val b = B(23) b: B = B(23,1) scala> b.productArity res0: Int = 2 scala> b.productElement(0) res1: Any = 23 scala> b.productElement(1) res2: Any = 1 scala> b._1 // use Product method ... note result type res6: Int = 23 scala> b._2 // use Product method ... note result type res7: Int = 1

Thanks, your solution looks nicer than the previous one (abstract class), but I still can't figure out a way to put inherited members at first positions. I'd like to avoid some clumsy code like x.productElement( x.productArity - 1 )
@Blaisorblade I forget the details the devil was hiding in, but you can follow progress (or the lack thereof) here.

Duncan McGregor · Accepted Answer · 2011-10-31 18:34:40Z

The closest I can get is to not implement anything in A

scala> abstract class A(val a: Int) extends Product defined class A scala> case class B(override val a: Int, b: String) extends A(a) defined class B scala> val anA: A = B(42, "banana") anA: A = B(42,banana) scala> anA.a res37: Int = 42 scala> anA.productArity res38: Int = 2 scala> anA.productElement(1) res39: Any = banana scala> anA.productElement(0) res40: Any = 42

Thanks for the idea! It led me to this other solution: abstract class A(val a: Int) extends Product case class B(b: String, override val a: Int = 42) extends A(a) the only drawback is that 'a' won't be in the same position for all subclasses.

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