A DSL with LUA code generation

Overview

This project demonstrates transformation of a DSL into LUA code.

The DSL is defined using a grammar (*.g4) file. The parser for this grammar is generated automatically by ANTLR from the .g4 grammar, at compile time. At run time, we transform a script the follows the DSL's specification, into executable LUA code, with the help of FreeMarker templates.

Compile time

Run time

The DSL

Valid scripts include a collection of rules that have the form if <predicate> then <action>. Preciates are comparisons such as height < 30, weight >= 40 or age >= 6, and actions are only accept and reject.

See 1.rules for an example of a valid script:

if age < 50 then accept
if height >= 180 then reject
if weight == 40 then accept

The formal grammar for our DSL is in the file IfThen.g4:

grammar IfThen;
prog : conditional * ;
conditional : 'if' predicate 'then' action ;
predicate : measurement comp value ;
measurement: 'age' | 'height' | 'weight' ;
comp : '<' | '>' | '<=' | '>=' | '==' | '~=' ;
value : NON_NEGATIVE_NUMBER ;
action : 'accept' | 'reject' ;
WS : [ \t\r\n]+ -> skip ; // skip spaces, tabs, newlines

NON_NEGATIVE_NUMBER : [0-9]+ ;

The use of LUA for executing the rules

The LUA code runs through the generated rules, and stops when one rule returns an accept or reject decision. As output, we get not only the decision, but also information the reasoning behind the decision, i.e. which rule was fired to trigger that decision.

How to build the project and generate the DSL parser

Assuming you have Maven installed, run

mvn clean package

From the project folder. This includes invoking the antlr4 goal, which generate the code for parsing the DSL, under the folder target/generated-sources/, provided by Antlr4 Maven plugin. Notice the <visitor> and <listener> options in pom.xml, which cause the generation of Visitor base classes, as opposed to the Listener pattern.

The antlr4 goal can also be invoked separately, by running

mvn antlr4:antlr4

in the project folder, or by invoking the goal antlr4:antlr4 under Plugins -> antlr4 in the Maven Projects window in IntelliJ.

How to run the LUA code generator

Assuming you have Maven installed, run the class App by typing

mvn exec:java -Dexec.mainClass="com.github.adilevin.App"

in the project folder.

This will read the DSL script in the resource file /dsl-scripts/1.rules

if age < 50 then accept
if height >= 180 then reject
if weight == 40 then accept 

and will turn it into LUA code, generating the file rules.lua:

-- This code was auto-generated using ANTLR4

return {

  { ["pred"] = function(person)
      return (person.age < 50)
    end,
    ["action"] = "accept",
    ["text"] = "if age < 50 then accept" },

  { ["pred"] = function(person)
      return (person.height >= 180)
    end,
    ["action"] = "reject",
    ["text"] = "if height >= 180 then reject" },

  { ["pred"] = function(person)
      return (person.weight == 40)
    end,
    ["action"] = "accept",
    ["text"] = "if weight == 40 then accept" },

}

How to execute the LUA code

As a prerequisite, install LUA. Windows users - install from here. After generating the LUA code, run the following command in the project folder:

lua main.lua

Notice that main.lua "require"s rules.lua.