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

[Wolfgang Laun]

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
>>>
>>>
>>>
>>> --
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>
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