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

Please comment on http://en.wikipedia.org/wiki/First-order_logic#Empty_domains -W On 07/01/2014, Davide Sottara <dsotty(a)gmail.com&gt; 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(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 > _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users