I think you can throw anything, including whatever sort of exception you could want. However, the real idea is that you should not use exceptions for anything

I think you can throw anything, including whatever sort of exception you could want. However, the real idea is that you should not use exceptions for anything expected like IO issues. Exceptions are meant to signify errors that could not be handled--unlike in Java, you shouldn't use exceptions for control flow. The real issue is one of semantics--in Java, one concept (exceptions) is used to represent two do disparate things: signal unexpected errors (runtime exceptions) and manage expected conditions (checked exceptions). A lot of people do not like checked exceptions and think exceptions should only really be used for the former case. Thus the lack of checked exceptions is a feature rather than a bug; moreover, it is a feature other people in this thread have already requested. So, the potentially annoying answer is that people using your library should not expect particular exceptions--that is not what they should be used for in Gosu. Coincidentally, this is very similar to how JavaScript deals with exceptions, and I have had no issues there at all.