Scala: Case class unapply vs a manual implementation and type erasure

Question

I'm trying to understand what Scala does with Case Classes that makes them somehow immune to type erasure warnings.

Let's say we have the following, simple class structure. It's basically an Either:

abstract class BlackOrWhite[A, B]

case class Black[A,B]( val left: A ) extends BlackOrWhite[A,B]

case class White[A,B]( val right: B ) extends BlackOrWhite[A,B]

And you're trying to use it like this:

object Main extends App {

    def echo[A,B] ( input: BlackOrWhite[A,B] ) = input match {
        case Black(left) => println( "Black: " + left )
        case White(right) => println( "White: " + right )
    }

    echo( Black[String, Int]( "String!" ) )
    echo( White[String, Int]( 1234 ) )
}

Everything compiles and runs without any problems. However, when I try implementing the unapply method myself, the compiler throws a warning. I used the following class structure with the same Main class above:

abstract class BlackOrWhite[A, B]

case class Black[A,B]( val left: A ) extends BlackOrWhite[A,B]

object White {

    def apply[A,B]( right: B ): White[A,B] = new White[A,B](right)

    def unapply[B]( value: White[_,B] ): Option[B] = Some( value.right )

}

class White[A,B]( val right: B ) extends BlackOrWhite[A,B]

Compiling that with the -unchecked flag issues the following warning:

[info] Compiling 1 Scala source to target/scala-2.9.1.final/classes...
[warn] src/main/scala/Test.scala:41: non variable type-argument B in type pattern main.scala.White[_, B] is unchecked since it is eliminated by erasure
[warn]         case White(right) => println( "White: " + right )
[warn]                   ^
[warn] one warning found
[info] Running main.scala.Main

Now, I understand type erasure and I've tried to get around the warning with Manifests (to no avail so far), but what is the difference between the two implementations? Are case classes doing something that I need to add in? Can this be circumvented with Manifests?

I even tried running the case class implementation through the scala compiler with the -Xprint:typer flag turned on, but the unapply method looks pretty much like I expected:

case <synthetic> def unapply[A >: Nothing <: Any, B >: Nothing <: Any](x$0: $iw.$iw.White[A,B]): Option[B] = if (x$0.==(null))
    scala.this.None
else
    scala.Some.apply[B](x$0.right);

Are you using the latest version of Scala? I can't reproduce your issue, and this related question from a few months ago determined a similar issue to yours as being a compiler bug. See stackoverflow.com/questions/7008428/… — Destin

Owen Owen · Accepted Answer · 2012-01-09T03:01:42

I cannot give a complete answer, but I can tell you that even though the compiler generates an unapply method for case classes, when it pattern matches on a case class it does not use that unapply method. If you try -Ybrowse:typer using both builtin case matching and your unapply method, you will see a very different syntax tree is produced (for the match) depending on which is used. You can also browse the later phases and see that the difference remains.

Why Scala does not use the builtin unapply I am not sure, though it may be for the reason you bring up. And how to get around it for your own unapply I have no idea. But this is the reason Scala seems to magically avoid the problem.

After experimenting, apparently this version of unapply works, though I'm a bit confused about why:

def unapply[A,B](value: BlackOrWhite[A,B]): Option[B] = value match {
    case w: White[_,_] => Some(w.right)
    case _ => None
}

The difficulty with your unapply is that somehow the compiler has to be convinced that if a White[A,B] extends a BlackOrWhite[C,D] then B is the same as D, which apparently the compiler is able to figure out in this version but not in yours. Not sure why.

Scala: Case class unapply vs a manual implementation and type erasure

2 Answers