Re: Rules engine in Lua?

[Duck <duck@...>]

> Our backup alternative is precompile the rules off-line
> and generate the corresponding state machine (expressed
> as a lua script)

Bear in mind (sorry, this doesn't actually answer your question, but is 
just an observation on a neat feature of Lua which you might not be aware 
of when creating state machine code) that Lua supports proper tail calls,
meaning that if the value a function returns is computed _directly by 
calling another function_, that call (the tail call) is implemented as a 
jump, not as a call.

So tail calls don't consume any call stack, and can therefore be 
recursively nested to any depth without ever running out of memory.

This makes them an ideal and readable way to implement state machines.

Each state is a function, and each state transition is a tail call from 
one state function to another. Even for machine-generated state machines 
this is rather handy -- transitions are expressed directly and naturally 
in the language itself, and the code to perform the transitions is 
generated directly, and super-efficiently, by the compiler.

There's a clear example in the Programming in Lua book, V1 of which is 
on-line and V2 of which is in any good bookshop (and at Amazon).