Sealed types

[Brian Goetz]

You are right, either way would be justified.  I get why you like it this way; that’s a valid opinion.  

But, be careful of the “just add a switch to make it go the other way” argument; while that certainly seems like it might “make everyone happy” in that everyone can express what they want, what it really does is make the language more complicated for everyone, because now there are more things that everyone has to learn.  And it seems pretty likely the “return on incremental complexity” here is negative.  So we should go one way or the other.  

Again, I get why you want to go the way you do.  And it’s a valid preference; I think (at least right now) I would still prefer to set the balance of this feature more in favor of the clients.