[rules-users] "from" keyword does not actually check types?

[Simon Thum]

Edson Tirelli wrote:
>    Hi Jevon,
> 
>    That is a discussion we had for some time (whether "from" should check
> return type or not for matching purposes). The only reason it was not
> implemented so far is that it would possibly cause "silent failures".
> Example:
> 
> Person( name == "bob" ) from $people
> 
>     If $people returns an Alien() and the check is in place, the rule will
> not match and the user might not notice the reason, as the way it is today,
> it will raise a ClassCastException.
> 
>     Although, in my opinion, the advantages of the check surpass the "silent
> failure" problem and we should add this check, specially because sometimes
> you have a collection of objects that share a common superclass, but with
> different actual subclasses instances, and you want to operate only on a
> give subclass.
> 
>     Anyway, I will talk with Mark and possibly add this to Drools 5.1.
FWIW, I'd appreciate both options being possible. Maybe an explicit
qualification makes sense, like

 (subset|all)? from ...

so you'd retain error checks where appropriate.


> 
>     Regarding your keyword usage issue, I added a comment to it:
> 
> https://jira.jboss.org/jira/browse/JBRULES-2218
> 
>     Regards,
>       Edson
> 
> 2010/2/24 Jevon Wright <jevon at jevon.org>
> 
>> Hi,
>>
>> I am using Drools 4.0.7, and I had a DRL rule similar to the following:
>>
>> rule "..."
>>   when
>>     other : Foo( )
>>     x : Bar( ) from other.property
>>
>>   then
>>     ...
>> end
>>
>> An unexpected bug occured. In some situations, other.property would _not_
>> be of type Bar, yet Drools would still think that it was. As a result I was
>> getting some unusual exceptions being thrown: "org.mvel.CompileException:
>> unable to resolve property: ..."
>>
>> If I changed the rule to:
>>     x : Bar( ) from other.property
>>     eval ( x instanceof Bar )
>>
>> It would instead throw a ClassCastException (i.e. "Baz cannot be cast into
>> Bar").
>>
>> If, instead, I changed the rule to:
>>     x : Bar( other.property == x )
>>
>> It would start working correctly, and 'x' would only be of type Bar.
>>
>> Is this known behaviour? Is it expected? If so, I would appreciate it if
>> the 4.0.7 documentation was updated to reflect that "from" actually ignores
>> the return type of properties. It took me a couple of hours to detect and
>> fix this bug.
>>
>> I would try these rules in Drools 5.x, but I'm still waiting for the
>> blocking JBRULES-2218 to be resolved.
>>
>> Cheers
>> Jevon
>>
>> _______________________________________________
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>> rules-users at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/rules-users
>>
>>
> 
> 
> 
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