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Hans Fugal authored
Summary: spaghetti-monster Finagle uses a nifty state machine pattern in `Promise.scala`. Each state carries its data with it, then you switch statements with a simple state `cas`, within the functions (which are the transition inputs). See https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Promise.scala#L133-L139 and https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Promise.scala#L604-L635 for example. I was thinking we could do something similar in C++, though we don't quite have the same luxury of type cases like scala. (Although as an aside I found this really cool paper on implementing type cases in C++, it's kind of evil but very cool. http://www.stroustrup.com/OOPSLA-typeswitch-draft.pdf) I was looking at having a union of the different state classes, e.g. union U { State base; Done done; Waiting waiting; ... }; and then you could use the memoized `dynamic_cast` trick in that paper to make a fairly efficient type case (and fairly readable, depending on how evil you want to get with the macros). However, `dynamic_cast<Done*>(&u.base)` blissfully segfaults. I'm not sure if this is something that should work and it's a compiler bug, or whether trying to (ab)use a union this way is against some arcane C++ rules. @hannesr suggested maybe a variant type might work. We can also do memory mangling on our own if it comes to it - however those are all more advanced techniques to play with at a later optimization time. So instead, I went for a this-is-context helper base class. The mutable context is just `this`, and you inherit from `FSM`. The magic macros make it more succint to lay out state transitions. See the tests for examples. Maybe in the future we can get really clever and find a way to generate state machine diagrams from code using this, especially when macros are being used. Test Plan: unit tests were written and pass Reviewed By: davejwatson@fb.com Subscribers: meisner, trunkagent, net-systems@, fugalh, exa, njormrod, hannesr FB internal diff: D1613497
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