[nightski]

Not exactly, I'm saying OO still makes it hard to test because it is an opaque blob of state. It would be really hard to use/test the LUA API for example without knowing it is using a stack underneath the hood. That detail leaks through the API whether you want it to or not. If it was switched to a FIFO queue that would completely change the behavior.

Since using this API makes an implicit constraint that it is using a stack underneath the hood, why not just expose the stack for inspection? What advantage does keeping it an opaque blob have? I agree you don't want code manipulating this stack (although if it is immutable as in FP this isn't an issue), but since the external code already knows it is a stack and relies on that fact then it is part of the public API already.