I think that the problem here is actually in signaling a wrong interface. If you in fact want to receive a iterable, then you should not signal that you expect

I think that the problem here is actually in signaling a wrong interface. If you in fact want to receive a iterable, then you should not signal that you expect a list. The keyword yield is in fact a nice language integrated mechanism to create a iterable interface. In languages that lacking this it resorts to using much more verbose mechanism.