[rules-users] Drools and Clojure (maps as facts)

[André Thieme]

Hello group!

I am a Clojure user and would like to look into using Drools with it.
 From Clojure I can use all Java classes and call all methods. So,
instantiating a KnowledgeBase, KnowledgePackages or a KBFactory is no
problem, and calling the respective methods, to get the system started,
or insert facts into a session, dispose it and fireAllRules is also
fine.

It is just that in Clojure one typically does not use POJOs.
The most typical data structures used are: hashmaps, vectors, structs,
lists, structuremaps and sets (in no particular order).
Those are persistent and "concurrency ready" Clojure DSs, and btw, also
usable from Java directly.

Now the basic Drools examples I saw all work with POJOs.
They were mostly comparisons of native typed fields in a class.
I however would be interested to compare different qualities of key/
value pairs in a hashmap, or compare structs with each other.
Comparing structuremaps is maybe what comes closest to the POJO examples.
For example, we may have a struct (defstruct person :name :age :type)
and store it's instances in a vector. And we would also insert it into
our Drools session object:
(doseq [p all-persons] (.insert session p))
which would correspond roughly to
for (Person p : allPersons) { session.insert(p); }
Only that "person" is a map, not a class.
Well, of course it is also an object and under the hood a class, a
java.util.Map even - but I mean that unlike POJOs the key/value pairs
are stored differently (fields in classes are also just key/value pairs).

So, is there a way to insert maps (let it be java.util.HashMaps for
example) and also to reason about them? I won't care if the syntax
for that is complicated. As Clojure is a Lisp I would just write a
little macro and have a very nice and readable syntax (probably a bit
similar to the one of Jess or Lisa).
Instead of looking at all instances of the type Person AND then have
their age compared, I would look at all maps/structs which have a key
:type with the value :person AND then compare the value of their :age
key with a number, such as:
(rule "example person rule, matching non-adults"
   :when (= :type :person)
     (< :age 18)
   :then
     (println "No service for minors."))

In Java terms it would mean that we insert several instances of
java.util.HashMap, all having the k/v pairs for name, age and type.
HashMap<String, Object> hm = new HashMap<String, Object>();
hm.put("name", someName);
hm.put("age", someAge);
hm.put("type", "person");
session.insert(hm);

Is that in principle possible with Drools 5?


Sunny greetings,
André
-- 
Lisp is not dead. It’s just the URL that has changed:
http://clojure.org/