how to disallow pattern matching on certain classes in scala

Question

Scala Pattern matching with hibernate proxies do not work with lists of inherited objects. To work around this, I wrap hibernate objects in case classes, see http://oletraveler.com/2011/04/20/20/

What I would like to accomplish is to throw a compile time error (preferrable) or runtime error if someone tries to match on an inherited hibernate entity.

For example:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
class PaymentSource

@Entity
class CreditCard

@Entity User {
  var paymentSources: java.util.ArrayList
}

user.paymentSources.map(_ match {
  case cc: CreditCard => println("oops") // <- this should error
})

I tried overriding unapply on CreditCard, but that didn't work since unnapply is only called when deconstructing the object, not just matching on the instance.

Any thoughs?

Answer 1

I really do not see any way of accomplishing this purely in code:

case x : SomeType =>

Is a fundamental pattern; the only way the code would not compile is if SomeType is not visible. But then, presumably, this is not much use!

It may be the case that a compiler plugin in tandem with a user annotation could do this, however - that's not my area of expertise but I guess it might look like:

@unmatchable class CreditCard( ... )

But then the issue here is that you can't really enforce that your "customers" will use the plugin (unless, I suppose, you ensure that your code will not compile without it - not that I know how you can achieve that)