I honestly don't know a thing about Erlang. But from my humble understanding of the interpreter source in the article: if is_list(list) is true, then the list is not empty. If you ask me, a respectable dequeue operation shoudn't need more than O(1).
The correct implementation for checking if a list is empty would have been a pattern match, something akin to
case List of
[] -> empty;
[_ | _] -> not_empty
end.
Or, you know. Reading the API docs and using queue:is_empty(Queue), which is O(1) instead of O(n) (which queue:len(Queue) is -explicitly stated to be-).
