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On 5-Aug-05, at 12:53 PM, Glenn Maynard wrote:

Here, I was referring to self-recursion: it may make sense, when debugging,
to want to know which function kicked off a tight recursion, but in the
state machine case it feels wrong and unintuitive for a debugging function being asked "who called this function" to return the function that started
the state machine originally.

Does it? Would you want the debugging function to show you the 327,432 previous states before you saw the function that originally created the maze? Would you want the state machine to fail because of stack overflow?

gcc can be instructed to not insert tailcalls; under some circumstances this is useful, although as the maze example illustrates it can lead to other bugs. Still, for those occasions, and for those who share your intuitions about state machines, it might be reasonable to have some mechanism in Lua for doing the same thing; the nontailcallable closure flag I suggested has the advantage of allowing some selectivity. (Suppose you use a library which unbeknownst to you is implemented with a tailcalled state machine; globally turning tail calls off might then make it impossible to debug your code because the library consistently crashes.)

Yet another possibility would be preprocessing the compiled Lua byte code, replacing every instance of OP_TAILCALL with OP_CALL. This is safe, because the compiler cannot know whether or not the function being tailcalled is a Lua function or a C function, so it always emits an OP_RETURN after an OP_TAILCALL.