If you don't think about the underlying implementation of erlang:length(), this seems to me like an easy mistake to make. Lots of data structures keep a size f

If you don't think about the underlying implementation of erlang:length(), this seems to me like an easy mistake to make. Lots of data structures keep a size field and access it in O(1), so it's not completely unreasonable to expect erlang:length to be O(1) without making sure. At least that's how I understood the context of the problem, I may be wrong of course.