Well, I assumed you'd want to spot that error before it happens rather than when it does. The exception being thrown by the assert (at run-time) means that the user sees the failure. That's sub-optimal, right? So you'd ensure that there is testing of all code that calls this code in order to trigger the exception ahead of time if that occurs. Or am I misunderstanding? It just seems that the assert applies the constraint too late in the process. It doesn't ensure you have the right type once you've deployed. Instead it ensures failure on wrong type, which is something different.