[rules-users] Drools Guvnor: DSL with multiple fact fields conditions using "-" and eval
Wolfgang Laun
wolfgang.laun at gmail.com
Fri Aug 27 13:53:19 EDT 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 at yahoo.com>
> 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 at gmail.com>* wrote:
>
>
> From: Wolfgang Laun <wolfgang.laun at gmail.com>
> Subject: Re: [rules-users] Drools Guvnor: DSL with multiple fact fields
> conditions using "-" and eval
> To: "Rules Users List" <rules-users at lists.jboss.org>
> 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 at 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 at gmail.com<http://mc/compose?to=salaboy@gmail.com>
> >* wrote:
>
>
> From: Mauricio Salatino <salaboy at 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 at 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 at 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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>
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> - MyJourney @ http://salaboy.wordpress.com
> - Co-Founder @ http://www.jbug.com.ar
>
> - Salatino "Salaboy" Mauricio -
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