[rules-users] forall is satisfied when there is nothing?

[Davide Sottara]

Indeed it is true by convention, see also
http://en.wikipedia.org/wiki/Universal_quantification

The only other alternative is to deprecate quantifiers altogether ;)

Davide

On 01/07/2014 11:53 AM, Wolfgang Laun wrote:
> Please comment on http://en.wikipedia.org/wiki/First-order_logic#Empty_domains
> -W
>
> On 07/01/2014, Davide Sottara <dsotty at gmail.com> wrote:
>> First order logic does permit empty domains, and the universal
>> quantifier evaluates to true in that case.
>> Davide
>>
>> On 01/07/2014 11:11 AM, Wolfgang Laun wrote:
>>> It is true that first-order logic usually assumes that the domain of a
>>> formula
>>> be a nonempty set. As so often, things aren't quite so simple when
>>> formulae
>>> are evaluated on a computer. What should be done in this case, forall
>>> with an empty domain? Throw an exception? Not very convenient, since
>>> there's no reasonable way of handling exceptions thrown on the LHS.
>>> Return false? That doesn't make sense, because you can't inspect what
>>> isn't there. Return true? If it isn't false - what else?
>>>
>>> If first-order logic does permit empty domains, it must be treated as
>>> a special case.
>>>
>>> -W
>>>
>>> On 07/01/2014, Sonata <plz.write.to at gmail.com> wrote:
>>>> Hi, I am using the "forall" keyword on the LHS and it seems the
>>>> condition
>>>> is
>>>> satisfied when there is nothing to match. e.g. "forall (MyClass(value ==
>>>> "test"))" fires the rule when there is no MyClass() object in the
>>>> working
>>>> memory. My workaround is add "exists (MyClass())".
>>>>
>>>> Also, same for "not (exists (MyClass(value != "test")))", but I can
>>>> understand this, as there is no MyClass() object, it doesn't exists and
>>>> hence "not" gives true.
>>>>
>>>> But for "forall", it doesnt sound right to me. I wonder if "forall" is
>>>> actually implemented as "not exists" in the engine.
>>>>
>>>> Please clarify if this is by design or a bug. Build is 5.5.0.Final
>>>>
>>>> Thank you
>>>>
>>>>
>>>>
>>>> --
>>>> View this message in context:
>>>> http://drools.46999.n3.nabble.com/forall-is-satisfied-when-there-is-nothing-tp4027553.html
>>>> Sent from the Drools: User forum mailing list archive at Nabble.com.
>>>> _______________________________________________
>>>> rules-users mailing list
>>>> rules-users at lists.jboss.org
>>>> https://lists.jboss.org/mailman/listinfo/rules-users
>>>>
>>> _______________________________________________
>>> rules-users mailing list
>>> rules-users at lists.jboss.org
>>> https://lists.jboss.org/mailman/listinfo/rules-users
>>>
>> _______________________________________________
>> rules-users mailing list
>> rules-users at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/rules-users
>>
> _______________________________________________
> rules-users mailing list
> rules-users at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/rules-users
>