Reviewer info: ============== I have an M.Sc. in computing science. Much of my experience is with state-space search, which has certain similarities to the topic at hand. Also, lately I have been working daily with code that ought to have been written using an FSM library, but wasn't! :-( I have been following the FSM discussion on the list, and have focussed on the library for one day, reading through the docs and rationale, plus Harel's initial statechart paper. The rationale document did not convince me of the correctness of some design choices, so I did not attempt to write code with the library. Reviewer comments: ================== Let us consider that we have multiple fsms and we would like to apply operations to these fsms, e.g. direct concatenation (the terminal state of one automata becomes the start state of another), or choice (depending on an action of, or an accept/reject by fsm a, proceed to execute fsm b or fsm c). This is apparently straightforwared when working at the level of states; however, the library draws a distinction between a state_machine and a state. This acts as a barrier to the direct composition of state machines that we would like to use as components within large state machines. The distinction made between state and state_machine, which I consider to be unfortunate, was not justified in the rationale. For naturally recursive structures, distinguishing between the structure's frontier and its interior by type is best avoided. (Analogously, the head of a (singly-linked) list and the root of a tree would not be distinguished by type from their successors and children, respectively.) Composition is critical! See, for instance, section 12.4 of Keller (http://www.cs.hmc.edu/claremont/keller/webBook/ch12/). An example of multiple-machine composition into a large machine should also exist in the tutorial. The simplest useful such case would likely be connecting two half-adders to make a full-adder machine. There is a brief note in the tutorial re: submachines that fails to do this topic justice. If the template approach recommended there is to be the canonical form for a reusable component, that style should be propagated through all but the simplest examples in the tutorial. Each submachine should clearly be labelled as part of its own .hpp file. I was pleased that the tutorial and rationale collectively contain considerable discussion of exception handling. In the speed vs. scalability trade-off section of the rationale page, it is claimed that using an fsm for a parsing task is an abuse, excusing the relatively low performance of the library. However, finite-state automata are inextricably linked with the acceptance of regular languages: see, for instance, section 12.2 of Keller (again, http://www.cs.hmc.edu/claremont/keller/webBook/ch12/); it is not too much to ask that an fsm library to be able to serve as the underpinning of a reasonably efficient regex library. The lack of support for dynamically-configurable automata is also unfortunate, though not unexpected given the requirements list. Other reviewers have also already commented upon the lack of a table-lookup dispatch method. From the rationale: "To warrant fast table lookup, states and events must be represented with an integer. To keep the table as small as possible, the numbering should be continuous, e.g. if there are ten states, it's best to use the ids 0-9. To ensure continuity of ids, all states are best defined in the same header file. The same applies to events. Again, this does not scale." However, I think that for table lookup there is no requirement that ids be consecutive -- a compile-time write-once read-many hash table could do the trick. But even that may be an aside. I am unconvinced by: "To satisfy requirement 6 [to support the development of arbitrarily large and complex state machines], it should be possible that to spread a single state machine over several translation units. This however means that the dispatch table must be filled at runtime and the different trnaslation units must somehow make themselves 'known' so that their part of the state machine can be added to the table. There simply is no way to do this automatically and portably." While perhaps each state (compilation module) must know of its successor states, no central table of "current state x action x next state" encompassing the entire machine need be explicitly created. Under the "Limitations" section, reference is made to "lost events": "It is currently not possible to specially handle events that have not triggered a recation (and would thus be silently discarded). However, a facility allowing this will probably be added in the not-too-distant future." If the event is irrelevant at that point in the state machine, it is most properly ignored. If it is not irrelevant, there ought to be a transition (perhaps a self-transition). Therefore, I don't understand the comment. I have an uneasy feeling about how differently asynchronous and synchronous machines are operated, but did not consider this issue in detail. Allowing the tracking of history is intruiging; the prohibition against deep history of states with orthogonal regions is suspicious. The explanation given is that it would be more complicated (but still possible) to implement. In my experience, that sort of reasoning just doesn't fly at Boost. The discussion of fork and join bars, combined with the event dispatch to orthogonal regions discussion immediately below that, suggests that the implementation is not truly designed to handle concurrent states. I think, however, if one is to claim that UML statecharts are supported that the semantics of execution ought to be the same! I can well imagine that this "would complicate the implementation quite a bit", but Boost is also about interface, not only implementation, and if the wrong interface is standardized (whether de jure or de facto) it's worse for everybody in the end. There is no explicit discussion of non-deterministic fsms. Of course, every such fsm can be re-expressed deterministically. Similarly, a given fsm may be non-minimal, but such minimization procedures exist. Both of these procedures necessarily require a global view of the fsm. As this is contrary to the scalability requirement listed in the rationale, I conclude that neither are present. :-) However, I would like to ask if (excepting possible compiler limitations) is there anything about the library that precludes extending it such that NDFAs could be specified in code, and converted into the minimized DFA during compilation? (http://www2.toki.or.id/book/AlgDesignManual/BOOK/BOOK5/NODE207.HTM# SECTION03177000000000000000 gives a toy example of FSM minimization). We had tuple and mpl separately before fusion; it seems reasonable for boost to have both compile- and run-time fsm libraries, if ultimately they may be similarly fused. Reviewer conclusion: ==================== I like this library, but I'm not satisfied with it. What level of library is good enough for boost? The library is not ready for standardization, however, this doesn't appear to be the standard that most people use when deciding upon their vote. This library is certainly good enough for people to make successful use of. Ultimately, I think there are issues of design and rationale that must be sorted through, appropriate modifications made, followed by re-review. Less weighty issues: ==================== struct Greeting; struct Machine: fsm::state_machine<Machine, Greeting> {}; (and all similar) should be changed to struct Machine: fsm::state_machine<Machine, struct Greeting> {}; If necessary, the old way can be metioned as a workaround for older compilers in a footnote. The "default" black disc in the diagrams is too large! Maybe make the radius about 2/3s of what it is now. Change "Such transitions are currently flagged" to "Transitions across orthogonal regions are currently flagged". It's the first sentence in the last paragraph of the rationale.