Saturday, July 21, 2007

DieSeL for the Engine

Thanks to ANTLR and ANTLRWorks, I have been able to make significant progress on the addition of Domain Specific Language (DSL) support for the Inference Engine of NxBRE. A usable version is available in the SVN repository on SourceForge.

NxBRE DSL, aka NxDSL, main goal is to allow rules authors to use their own natural language to write executable rules. Technically this feature is based on:
  • a language-specific ANTLR grammar that strictly enforces the structure of a rule file: this file is not supposed to be edited by the implementer,
  • a custom definition file that translates statements into RuleML atoms: this file must be created by the implementer to capture, with regular expressions, the natural language fragments and how they translate in RuleML atoms.

In the following example, the words in red are parsed by ANTLR while the ones in green are matched by the regular expression from the definition file.

rule "Discount for regular products"
The customer is rated premium
The product is a regular product
then deduct
The discount on this product for the customer is 5.0 %

The terms in italic are captured by ANTLR and the regular expressions to get values for labels, implication actions and atom values.

As you can see, the ANTLR grammar defines and enforces the structure of the rulebase, i.e. the skeleton of the rules, logical blocks and statements.

NxDSL comes with a grammar that allows using French terms for defining the rule structure, thus opening the door to consistently write the body of the rules in the same language, as shown hereafter:

règle "Remise pour les produits standards"
Le client est en catégorie premium
Le produit est de type standard
alors déduis
La remise pour ce produit pour ce client est de 5.0 %

While NxDSL adds extra work for the engine implementor, the positive impact for the rule writers will justify its usage for projects in need of letting non-technical users manage rules.

NxDSL also opens the possibility of writing a plain-text rule editor by leveraging the definition file to provide code assistance to the writer. Anyone feeling like contributing such an editor?