But that assumes I have some switch case somewhere right? If I passed an array of "orders" to a downstream function, it knows that what it has is orders, not something else, so it doesn't need to check for anything, and in the case I checked for errors upstream (when I try to allocate a new order) all downstream functions know that no invalid order can be passed, in which case you only have one check at creation time.

That's why I haven't fully understood yet how working like this is simpler.