1. Don’t put closing semi-column at the end of DSL expression line:

   [when]Message status is {1} = m : Message(status == {1});

2.  Avoid different token names on the left hand side (before the “=”) and right hand side (after the “=”) of the DSL expansion:

   [when]Message status is {3} = m:Message(status == {1})
   [then]Log "{message}" = System.out.println("{bobo}");
   

3.  DSL expansion should be on one line. You can even have several expressions on the same line, but you cannot break the line. By putting expression on a new line will cause an exception during parsing:

   [when]Message id is {id} = m:Message(id == {id})
   [then]LogUpdate "{2}" = System.out.println("{2}");
   update(m);
   [when]Message status is {s} = m:Message(status == {s})
   [then]Log "{2}" = 
   System.out.println("{2}");

4. Avoid having DSLs with the same name. It can create a confusion. Remember that DSL is parsed from top to bottom, so when two DSLs with the same name exist, the top one will apply during expansion since it will be matched first:

   [then]SetStatus {s} = System.out.println({s});
   [then]SetStatus {s} = m.setStatus({s});
   

5. If token has quotes on the left hand side of the DSL, then the token must have quotes on the right hand side of the DSL:

   [then]SetMessage "{msg}" = m.setStatus("{msg}");
   

6.  There is no room for typos:

   rule "set-hello"
      when
   	True
      then
   	SetMessage "Hello"
   end
   

   [then]SetMessag "{msg}" = m.setStatus("{msg}");
   

7. Token names can be literal or numeric characters, or both:

   [then]Print1 "{1}" = System.out.println("{1}");
   [then]Print2 "{msg}" = System.out.println("{msg}");
   [then]Print3 "{23msg}" = System.out.println("{23msg}");
   

8.  If token on the left hand side has no quotes, and expression/function on the right hand side can accept parameter of type String, then the token can be placed between quotes on the right hand side:

   [then]PrintInteger {1} = System.out.println("{1}");
   [then]PrintInteger2 {1} = printingFunction("{1}");
   

9.  If you want to use declared variable in right hand side, make sure it is declared on the left hand side of the rule. Remember, DSL template is ONLY a template, it is a not a place to declare variables, declaration occurs only in DRL, DSL contains the expansion. Keep in mind that declaration and usage of the variable is per-rule basis:

   rule "set-hello"
      when
   	Message
      then
   	SetMessage "Hello"
   end
   

   [when]Message = m : Message() //declare and initialize 'm'
   [then]SetMessage "{msg}" = m.setStatus("{msg}");
   

10. White spaces before the “=” and after are allowed when writing a DSL line:

    [then]SetMessage   "{msg}"  =    m.setStatus("{msg}");
    

11. If you are using a function in the right hand side of the DSL of the expansion, then make sure you provide necessary imports in DRL source for all the objects the function is using. For example if your function gets a proxy to a JMS Queue from JBoss, then you have to include imports for all the objects associated with this operation, like you normally would do it in normal Java class. Also the function itself must be present in source DRL.
12. Left hand side of the rule, can evaluate boolean function instead of expression:

    rule "CheeseType"   
       when	
    	Cheese is of type "silton"   
       then	
    	DoSomething
    end
    
    function boolean 
    	someEvaluatingFunction(Cheese cheese,String type) {
       if (cheese.getType().equals(type))
    	return true;
       else
    	return false;
    }
    

    [when]Cheese is of type "{1}" = 
    chz:Cheese() eval(someEvaluatingFunction(chz, "{1}"))
    [then]DoSomething = System.out.println("Something");
    

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Responses[46]

Posted by Alexander Zagniotov on 24 October 2008 5:34:20 PM  | Permalink