Re: [rules-users] Drools Guvnor: DSL with multiple fact fields conditions using "-" and eval

Friday, 27 August 2010

DSL uses regular expression parsing, but the LHS is recursive.  (Chomsky
type-3 vs. type-2, if you want the comp. sci. background.) This means that
using DSL restricts you to a subset of the LHS language, although you may
map arbitrarily complex LHS expressions to a single DSL sentence. (In your
case, you would have to have three mappings, one for name, one for tag and
one for name and tag.)

Heuristics might be use to alleviate certain cases, but this would merely be
a band-aid on a very deep gash.

There's another trick to avoid eval as you use it: Create (what I call:)
"virtual attributes" by adding methods to your fact classes. You have
   String name;
   String getName(){ return name; }
and if you add
   boolean isNameBlank(){ return StringUtils.isBlank( name ); }
you can use
   Asset( nameBlank == true )
and define a straightforward DSL sentence which can be merged with others
into the fact pattern.

-W

2010/8/27 Patricia Bogoevici <patriciabogoevici(a)yahoo.com&gt;

...
 Thx for your answer, Wolfgang.

 To be honest, when I first ran into this, I was in doubt whether that is a
 parsing error in Drools (or  mvel)? Or is an error that I got into because
 of how I constructed the DSL expression?
 It seems that it is an error in Drools and I wonder if that should be
 reported as an issue in Jira.

 --- On *Fri, 8/27/10, Wolfgang Laun <wolfgang.laun(a)gmail.com&gt;* wrote:

 From: Wolfgang Laun <wolfgang.laun(a)gmail.com&gt;
 Subject: Re: [rules-users] Drools Guvnor: DSL with multiple fact fields
 conditions using "-" and eval
 To: "Rules Users List" <rules-users(a)lists.jboss.org&gt;
 Date: Friday, August 27, 2010, 2:58 AM

 It seems that the initial hyphen merges the DSL definition's right hand
 side into the parenthesis of the last preceding term, even if that one is an
 eval, which itself is meant to go into some preceding pattern.

 You could work around that by making all evals Conditional Elements rather
 than Inline Eval Constraints. The disadvantage is that you'll have to use
 full object qualification, e.g.,

 [when]where name is empty=eval(StringUtils.isBlank( asset.getName() )

 Patricia:
 True. But, that is a test case, and does not affect the result of what I
 was testing

 I think that your variant

    [when]-name empty=name=="''"

 has to be written without the apostrophes:

    [when]-name empty=name==""

 -W

 2010/8/27 Patricia Bogoevici
<patriciabogoevici@yahoo.com<http://mc/compose?to=patriciabogoevici@yahoo.com>
 >

  Thx for your answer.

 I tried again the rule, removing the extra space after hyphen from DSL
 expresion. No luck. So, I tried a few more cases, and the only one that
 worked correctly was #3.
 It seems to me that is somehow related to the parenthesis, although it
 might be something else. Hope this info helps.

 1) only eval:
 DSL:

 [when] There is an Asset=asset:Asset()
 [when]-name is empty=eval(name=="''")
 [when]-tag is empty=eval(tag=="''")

 Rule source:
     rule "test_rule"
     dialect "mvel"
         when
             asset:Asset( eval(name=="''",
eval(tag=="''") ) )
         then
     end

 2) no eval, but with the parenthesis:
 DSL:

 [when] There is an Asset=asset:Asset()
 [when]-name is empty=(name=="''")
 [when]-tag is empty=(tag=="''")

 Rule source
     rule "test_rule"
     dialect "mvel"
         when
             asset:Asset( (name=="''", (tag=="''") )
)
         then
     end

 3) simple case
 DSL

 [when] There is an Asset=asset:Asset()
 [when]-name empty=name=="''"
 [when]-tag empty=tag=="''"

 Rule Source:
     rule "test_rule"
     dialect "mvel"
         when
              asset:Asset( name=="''", tag=="''" )
         then
     end

 Thanks,

 --- On *Thu, 8/26/10, Mauricio Salatino
<salaboy@gmail.com<http://mc/compose?to=salaboy@gmail.com>
 >* wrote:

 From: Mauricio Salatino
<salaboy@gmail.com<http://mc/compose?to=salaboy@gmail.com>
 >
 Subject: Re: [rules-users] Drools Guvnor: DSL with multiple fact fields
 conditions using "-" and eval
 To: "Rules Users List"
<rules-users@lists.jboss.org<http://mc/compose?to=rules-users@lists.jboss.org>
 >
 Date: Thursday, August 26, 2010, 10:43 PM

 can you try with this:
 [when] There is an Asset=asset:Asset()
 [when] -name is empty=eval(StringUtils.isBlank(name))
 [when] -tag is empty=eval(StringUtils.isBlank(tag))

 2010/8/26 Patricia Bogoevici
<patriciabogoevici@yahoo.com<http://mc/compose?to=patriciabogoevici@yahoo.com>
 >

 Hi all,

 I am using the latest Guvnor release (5.1), and I have the following DSL
 expression where I use "-" to have conditions on multiple lines:

 [when] There is an Asset=asset:Asset()
 [when]- name is empty=eval(StringUtils.isBlank(name))
 [when]- tag is empty=eval(StringUtils.isBlank(tag))

 Having these DSL expressions, I wanted to create a rule, but I ran into
 some issues.
 To summarize: There seems to be a parsing error, when conditions are added
 in DSL using "-".  Did anybody else run into this problem? I wonder if I do
 something wrong, but, re-creating the DSL to have the 2 conditions into one
 expression, is all working fine. It may be a combination of "-" and eval?

 Below, is the description of rules created, and error I got.

 First, I created a rule using the above DSL expressions in the BRL editor.
 While the rule looked ok in the editor, and it validated fine, only when I
 checked the source I noticed is wrong. So, the rule source looks like this:
 rule "test_dsl"
         dialect "mvel"
         when
             asset:Asset( eval(StringUtils.isBlank(name,
 eval(StringUtils.isBlank(tag)) )) )
         then
             asset.setStatus("INVALID")
     end

 It looks that the conditions are parsed incorrectly, and the second eval
 condition is added as a parameter to the StringUtils.isBlank from the first
 eval. Also, all the right parenthesis are added at the end of the
 expression. If I add one condition at a time, it all works fine. The error
 is when I have more than 1 condition.

 So, I created a new DSL, that contains both conditions into one line,
 eliminating the hyphen between conditions:

 [when] There is an asset with name empty AND tag empty
 =asset:Asset(eval(StringUtils.isBlank(name)),eval(StringUtils.isBlank(tag)))

 Using  BRL editor, I created a second rule using the DSL above. The rule
 validated ok, and the rule source looks fine:

 rule "test_dsl_1"
         dialect "mvel"
         when

 asset:Asset(eval(StringUtils.isBlank(name)),eval(StringUtils.isBlank(tag)))
         then
             asset.setStatus("INVALID")
     end

 I created a test for this rule, that ran correctly.

 Thanks,
 Patricia

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Re: [rules-users] Drools Guvnor: DSL with multiple fact fields conditions using "-" and eval