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

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(a)gmail.com&gt; 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-nothi... >> Sent from the Drools: User forum mailing list archive at Nabble.com. >> _______________________________________________ >> rules-users mailing list >> rules-users(a)lists.jboss.org >> https://lists.jboss.org/mailman/listinfo/rules-users >> > _______________________________________________ > rules-users mailing list > rules-users(a)lists.jboss.org > https://lists.jboss.org/mailman/listinfo/rules-users > _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users