[rules-users] MVEL and Maps

Tue Nov 10 05:31:02 EST 2009

Leonardo Gomes wrote:
> Regarding the problem with dates, even when using java beans, what
> options would I have other than accepting the limitations of that
> syntax sugar or to provide a static method, casting and converting
> arguments, returning a boolean, and call this using an inline eval.
If I'm not mistaken, in drools 5 you can declare (and even define)
functions which can be called without eval(). So you'd provide that
static method, declare it in DRL & at least get rid of eval().

Also, I think there's a smoother syntax to acces maps, the mvel page
explains it. Short version this["foo"] -> foo. Or something.

> 
> Thanks again,
> 
> Leo.
> 
> 2009/11/8 Edson Tirelli <ed.tirelli at gmail.com>:
>>    Wolfgang,
>>
>>    As you know, there are a ton of problems when you go down the "date"
>> route. Probably the reason java never had a decent built-in date framework
>> (fingers crossed for JSR-310).
>>
>>    Anyway, back to the point, the syntax sugar for string-based strings is a
>> really simple way for people to use really simple dates in their rules. But,
>> yes, with that, you can't define which calendar, timezone, daylight savings,
>> etc, rules should use. It will just use the defaults for the JVM instance.
>> You will probably never see *me* using that. But there are people that just
>> want something that simple and in this case, it is there.
>>
>>    MVEL page is:
>>
>> http://mvel.codehaus.org/
>>
>>    I don't think that mvel has anything specific to dates, but I might be
>> wrong.
>>
>>    The only thing "not available" in Drools are the control structures
>> (if/while/etc) that we disable explicitly.
>>
>>     Edson
>>
>> 2009/11/8 Wolfgang Laun <wolfgang.laun at gmail.com>
>>> For the sake of clarification: Are you saying that there is, or should be,
>>> some magic which would make a constraint like
>>>    Map ( this["departureDate"] == "07-Jan-2009" )
>>> behave as a comparison between two java.util.Date values if the LHS Object
>>> is of this type? And also if it is a GregorianCalendar object?
>>>
>>> Since even you are "not sure" regarding MVEL: What is the definite
>>> reference for MVEL in Drools? I have a vague recollection of reading
>>> somewhere that not all of MVEL is available...
>>>
>>> -W
>>>
>>>
>>> 2009/11/8 Edson Tirelli <ed.tirelli at gmail.com>
>>>>      Ok, without discussing the merits of using (or not) maps, the reason
>>>> you are having problems with dates is that the "string-based" date is syntax
>>>> sugar in Drools parser/compiler. Although, when you create expressions using
>>>> nested accessors or [] for collection/map element access, drools wraps the
>>>> whole thing into an "eval" and delegates to MVEL. We do some transformations
>>>> to bridge the gap between drools operators and mvel operators, but I don't
>>>> think drools has anything to "transform" the string-based date into
>>>> something mvel understands.
>>>>
>>>>      Having said that, we should probably add this functionality to the
>>>> "MVELDumper" class for better integration. Would you please open a jira for
>>>> that? Also, patches are welcome in case someone would like to help. This is
>>>> an isolated part of the code and easy to understand.
>>>>
>>>>     Meanwhile, best you can do is avoid the use of this Map syntax +
>>>> string-based dates, or write your expressions directly in a way the mvel
>>>> understands (not sure mvel has a special syntax for dates).
>>>>
>>>>     Same explanation for the NPE.
>>>>
>>>>     Edson
>>>>
>>>> 2009/11/8 Leonardo Gomes <leonardo.f.gomes at gmail.com>
>>>>> Hi Wolfgang,
>>>>>
>>>>> Here's the context:
>>>>>
>>>>> We're replacing a very basic home-made rule engine with drools. Today,
>>>>> fact data can be added dynamically b/c for all rules our fact is
>>>>> nothing more than a Map and we can register new fact data through an
>>>>> UI where you say what's the key and what's the type of the value.
>>>>> Rules can also be created dynamically and operate against those newly
>>>>> created attributes of my fact.
>>>>>
>>>>> Incoming requests are key / value structures and we have a service
>>>>> that will look at the list of registered fact data and try to find
>>>>> that information in the incoming request by looking for the keys and
>>>>> getting the corresponding values, then populating the Map I just
>>>>> talked about which will be injected in the working memory for
>>>>> evaluation against our rules.
>>>>>
>>>>> That's why, in Drools, I need to have my rules accessing data from a
>>>>> Map. By using MVEL, as I said, it works fine with strings, integers
>>>>> and list of strings, but I'm having problems with dates and would like
>>>>> some advice on that case.
>>>>>
>>>>> Here's another example of things that work:
>>>>>
>>>>> rule "test rule"
>>>>>        when
>>>>>                Map ( this["airportCode"] == "GIG"
>>>>>                        , this["numberOfPassengers"] > 1
>>>>>                        , this["class"] in ("BUSINESS", "FIRST CLASS") )
>>>>>        then
>>>>>                m.addMessage("rule matched");
>>>>> end
>>>>>
>>>>> And here's where I have problems,
>>>>>
>>>>> Map ( this["departureDate"] ) == "07-Jan-2009" )
>>>>>
>>>>>
>>>>> Thanks a lot,
>>>>> Leo.
>>>>>
>>>>>
>>>>>
>>>>> On Sun, Nov 8, 2009 at 10:56 AM, Wolfgang Laun <wolfgang.laun at gmail.com>
>>>>> wrote:
>>>>>> On Sat, Nov 7, 2009 at 6:00 PM, Leonardo Gomes
>>>>>> <leonardo.f.gomes at gmail.com>
>>>>>> wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> I'm feeding my working memory with Maps (unfortunately, this is a
>>>>>>> requirement and I can't use beans).
>>>>>>>
>>>>>> Before people start racking their brains to overcome all the resulting
>>>>>> difficulties: Could you please explain what this requirement is meant
>>>>>> to
>>>>>> achieve?
>>>>>> -W
>>>>>>
>>>>>>
>>>>>>> To access the values in this maps I use the following MVEL expression
>>>>>>> :
>>>>>>>
>>>>>>>                Map ( this["airportCode"] == "GIG"
>>>>>>>                        , this["numberOfPassengers"] > "1"
>>>>>>>                        , this["class"] memberOf ("BUSINESS", "FIRST")
>>>>>>> )
>>>>>>>
>>>>>>> And it works fine, except for dates where I get a ClassCastException,
>>>>>>> even tough I'm using the right pattern:
>>>>>>>
>>>>>>> Map ( this["departureDate"] > "07-Nov-2009" )   --- fails
>>>>>>>
>>>>>>> Is there anything I need to do to make the dynamic cast of dates
>>>>>>> work,
>>>>>>> in this particular case?
>>>>>>>
>>>>>>> --
>>>>>>>
>>>>>>> Morever, I get NPEs if there's no value for a given key. For example,
>>>>>>> the expression Map ( this ["name"] == 'Leonardo' ) would fail if
>>>>>>> there's no entry with "name" as the key.
>>>>>>>
>>>>>>> Any help is welcome!
>>>>>>>
>>>>>>> Thanks in advance,
>>>>>>> Leo.
>>>>>>> _______________________________________________
>>>>>>> rules-users mailing list
>>>>>>> rules-users at lists.jboss.org
>>>>>>> https://lists.jboss.org/mailman/listinfo/rules-users
>>>>>>
>>>>> _______________________________________________
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>>>>
>>>>
>>>> --
>>>>  Edson Tirelli
>>>>  JBoss Drools Core Development
>>>>  JBoss by Red Hat @ www.jboss.com
>>>>
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>>
>>
>> --
>>  Edson Tirelli
>>  JBoss Drools Core Development
>>  JBoss by Red Hat @ www.jboss.com
>>
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