This question pertains to the (remarkable!) lecture by Rob Pike (https://www.youtube.com/watch?v=HxaD_trXwRE&t=555s)
How can the errorf method of type lexer be considered of type stateFn? It has a different signature than other stateFns. How is lexer's run() method able to run the line state = state(l) after errorf is returned in the previous iteration? nil is returned only after the errorf is actually run. And to run errorf, it needs to be invoked with (format, args...). How can it accept a (*lexer) when invoked as state(l) in the run() method?