[rules-users] Drools and Clojure

[André Thieme]

With what have you been wrong Greg?
I understood Marks reply in such a way that the mvel dialect allows to
use Maps without eval. But this is just syntactic sugar. When compiled
or interpreted (I don't know which of those Drools will do with mvel
rules), this will get replaced with an eval under the hood.
So, that means that still the optimizations can't be applied.
Maybe the mvel rule is only interpreted, which would make it even slower.

For me it does not matter much which syntax will be my target.
My Clojure lib will allow to write down rules in s-expression
syntax, and it will compile them (compilation is also available at
runtime) into any syntax Drools accepts.
Currently my target is the default syntax - that means I will
produce strings that access Maps with eval.

IF the mvel syntax will result in more performant code because it
makes Drools handle it better without using eval under the hood, then
sure, I can also compile into mvel syntax.
Although with that I still have the little problem to have globals in
place of literals for Map lookups (see my other mail).



Greg Barton schrieb:
> I was wrong. :). See Mark's later email.
> 
> GreG
> 
> On Aug 13, 2009, at 17:25, André Thieme <address.good.until.2009.dec.14 at justmail.de> wrote:
> 
> Greg Barton schrieb:
> There's no reason why a rete based system couldn't use maps as first
> class objects, but Drools is heavily oriented towards POJOS.  Using
> eval in the way you have is pretty much the way to go.
> 
> Thanks for confirming that.
> For now I know that I am doing it right by using eval and that this
> means that rules for typical Clojure objects will not benefit from some
> of the optimizations Drools usually can apply.
> Maybe it will change in the future.
> 
> 
> As long as type information is accessible (both for first class types
> and their members) you should be able to have the left hand side of a
> rule (the conditions) be as it is now.
> 
> I think that if Maps become 1st class objects there could be a different
> Syntax, without using eval.
> 
> 
> If you lobby the devs hard enough and get others on your side you may
> be able to convince them to go in that direction, but I doubt it
> would be possible before version 6 or so, if that early.  (And I'm
> not even sure it's possible.)
> 
> At this point it is mostly interesting for me to get Drools working with
> Clojure together and concentrate on correctness. The goal is that users
> of my lib can use do all typical things people do with Drools without
> writing Java code. Everything, including the rules, would be written in
> Clojure.
> But of course it would be very interesting if the devs could indeed have
> Clojure in mind. I don't know how reusable the existing code is, for
> using Maps without the need for eval and with having the respective
> performance advantages.
> I understand that the looup of the value for a given key can not be
> optimized away. On my hardware get() is limited to "only" 1000 calls
> per msec and core. Reading a field from a POJO is faster.
> If I understand it correctly then the "problem" with eval is that it
> needs to be executed each time, and no clever caching can be done.
> So, eliminating that by having Maps being 1st class is what sounds
> interesting.
> When I refer to Maps and Clojure, then I talk about Maps having only
> very few key/value pairs, just like POJOs, and being immutable.
> This immutability may even be very helpful for optimization. It is
> guaranteed that a given object will never change.
> 
> I hope the devs will find it okay if I make some concrete suggestions
> in the coming weeks.
> 
> 
> Sunny greetings,
> André


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