Is it good practice to make case classes sealed?

Question

The main reason to seal classes seems to be that this allows the compiler to do exthaustivity searches when pattern matching on those classes. Say I have data types meant for pattern matching. Toy example:

sealed trait Statement
case class Assign(name: String, value: Int) extends Statement
case class Print(name: String) extends Statement
case class IfZero(name: String, thenn: Statement, els: Option[Statement]) extends Statement
case class Block(statements: List[Statement]) extends Statement

The use case for these classes would be to consume them through pattern matching:

def execute(statement: Statement): Unit = statement match {
    case Assign(name, value)      => ???
    case Print(name)              => ???
    case IfZero(name, thenn, els) => ???
    case Block(statements)        => statements foreach { execute(_) }
  }

To this end, the Statement trait is sealed so that the compiler can warn me if I forget one statement kind in the match statement. But what about the case classes? Case classes cannot inherit from each other, but traits and ordinary classes can. So, is it good practice to seal the case classes as well? What could go wrong if I don't?

Answer 1

You don't have to seal the case classes but you should mark them as final and therefore forbid any further inheritance relationship. Making them sealed is only useful when you want exhaustiveness checking on its subclasses, which is not a very likely use case.

Marking all classes as final by default is a good thing because it forbids the users of your API to change the behavior of these classes when they override its methods. If you didn't specifically design your class to be subclassed, it may happen that the subclassing leads to bugs in your application because the subclassed class is no longer doing what it was intended to do.