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

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(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 > _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users