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

[Davide Sottara]

The rule engine is still trying to approximate logic, even if not
completely in a mathematical sense.
One property quantifiers have to obey is DeMorgan's laws, so "forall ...
" should be logically equivalent
to "not exists .. such that not...",  which in Drools becomes for example :

forall Cloth( dry )
<->
not Cloth( ! dry )

if forall returned false on an empty set, the equivalence would no
longer hold.
Notice that there is a second negation inside the pattern.
(btw, the "not" CE has the semantics of "not exists")

This is actually the way it is implemented internally, and no, I don't
think that
it would be appropriate to change it.

Davide


On 01/08/2014 12:32 PM, Sonata wrote:
> laune wrote
>> On 08/01/2014, Davide Sottara &lt;
>> dsotty@
>> &gt; wrote:
>>> Indeed it is true by convention, see also
>>> http://en.wikipedia.org/wiki/Universal_quantification
>>>
>>> The only other alternative is to deprecate quantifiers altogether ;)
>>>
>> Ah, *by convention*, yes. One should exercise some care with "vacuous
>> truths", however. Uttering statements such as "All the little green
>> men in my room are from Mars" may not brand you as a liar, but you
>> could be called "batty". ;-)
>>
>> -W
>>
>>> Davide
>>>
> Guys, I am not sure whats the arguing point here. But from a programmer's
> and a Drools engine user's perspective, I would find forall(void) to return
> false more convenient.
>
> Say for example:
>   forall(Cloth(dried)) then collect()
> Programmingwise, I dont want to execute collect() when the Cloth()
> collection is empty and do extra null checkings.
> Performancewise, I also dont want to call collect() when there is actually
> nothing to perform.
> In terms of semantics, I purposely want to check for isDried is true, it is
> easy to realize from the LHS that I only want to collect clothes when all of
> them are dried, rather than I want to collect also empty air.
>
> On the contrary, if I really want to do the RHS even though the LHS has
> nothing, I would have written:
>   not( exists( Customer() ) ) then takeanap()
> and I will not write
>   forall( Customer(status == "gone") ) then takeanap()
> because the latter needs to do extra property visits and string comparisons
> and it is so indirect to express what I really want.
>
> I really dont care what the mathematical definition is unless you tell me
> Drools is a math. engine rather than a rule engine to express business
> requirements. That, I should update myself and take a step back to look at
> Drools again. Just my own opinion :P
>
>
>
>
> --
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