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

[Davide Sottara]

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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