[rules-dev] Backwards chaining: the difference between input and output variables

Mark Proctor mproctor at codehaus.org
Thu Apr 21 12:23:54 EDT 2011 

Wolfgang,

Jess/clips have never differentiated between binding and constraint. has 
that been a major problem for Jess?
(cheese name ?cn )
(person likes ?cn )

In previous DRL we did not allow unification like above. So you were 
forced to separate binding and constraint:
Cheese( cn : name )
Person( likes  == cn )

In most common cases I think the separation is preferred, users do not 
need to be concerned with the concepts of unification. What I have done 
is to allow unification in the manner than Jess and Clips does, this is 
important for prolog itself because the second pattern is not a filter. 
So == does not make sense, it is a unification.

Mark


On 21/04/2011 07:27, Wolfgang Laun wrote:
> Designing syntax well is not easy. With extensions, one should strive 
> for as much
> conformity with the existing language, while trying to follow general 
> principles.
>
> One might have discussed (for instance) the use of field names for 
> referencing
> the query relations, taken from their parameter definition. And then 
> one could write,
> as usual:
>
>     ?editableThings(food: thing, location == loc )
>
> or
>
>     ?editableThings(food: thing, loc: location )
>
> And the in/out is clear to all who know a little legacy DRL.
>
> And the ugly semicolon evaporates.
>
> And the maintainability/readability disadvantage of "positional" is gone.
>
> Cheers
> -W
>
>
> On 20 April 2011 22:52, Michael Anstis <michael.anstis at gmail.com 
> <mailto:michael.anstis at gmail.com>> wrote:
> >
> > Simple yes, but consistent too should be a factor.
> >
> > Most questions we have to the user mailing list involve people 
> writing DRL not using tooling.
> >
> > So DRL, IMO, has to be seen as the "tool" to author rules. Drop the 
> proposed colon altogether or make it's use consistent.
> >
> > On 20 April 2011 17:42, Mark Proctor <mproctor at codehaus.org 
> <mailto:mproctor at codehaus.org>> wrote:
> >>
> >> My personally opinion is to keep the language simple and instead 
> have the tooling inject what ever is necessary as a visulation. Be it 
> different colouring, hover over or graphic symbol. It keeps the 
> language simple and actually achieve the desired result better.
> >>
> >> Mark
> >> On 20/04/2011 14:00, Leonardo Gomes wrote:
> >>
> >> +1 for Michael's suggestion.
> >>
> >> It's a bit more verbose, but makes things clear.
> >>
> >> The semicolon here:
> >> ?editableThings(food : ?, loc;)
> >>
> >> Is a typo, right? You actually meant:
> >>
> >> ?editableThings(food : ?, loc);
> >>
> >> - Leo.
> >>
> >>
> >>
> >> On Wed, Apr 20, 2011 at 11:59 AM, Michael Anstis 
> <michael.anstis at gmail.com <mailto:michael.anstis at gmail.com>> wrote:
> >>>
> >>> Hmmmmm....
> >>>
> >>> Personally, I don't like the use of ":" i isolation as it's what 
> we currently use to bind variables and I feel "cheese:" as an output 
> definition could just make people question whether they've missed 
> something. Perhaps "cheese : ?" would be a viable alternative. This 
> would be in keeping with (a) current variable declaration, (b) the use 
> of "?" to identify a call to a query. Geoffrey's examples would then 
> become:-
> >>>
> >>> rule outputinput
> >>> when
> >>>     Here( loc : location)
> >>>     ?editableThings(food : ?, loc;)
> >>> then
> >>>     System.out.println("Food " + food + " at location " + loc);
> >>>     // Output:
> >>>     // Food crackers at location kitchen
> >>>     // Food apple at location kitchen
> >>> end
> >>>
> >>> rule outputOutput
> >>> when
> >>>     ?editableThings(food : ?, loc : ?;)
> >>> then
> >>>     System.out.println("Food " + food + " at location " + loc);
> >>>     // Output:
> >>>     // Food crackers at location kitchen
> >>>     // Food apple at location kitchen
> >>>     // Food chocolate at location living room
> >>>     // Food chips at location living room
> >>> end
> >>>
> >>> rule typo
> >>> when
> >>>     Here( looc : location)
> >>>     ?editableThings(food : ?, loc : ?;)
> >>> then
> >>>     System.out.println("Food " + food + " at location " + loc);
> >>>     // Output:
> >>>     // Food crackers at location kitchen
> >>>     // Food apple at location kitchen
> >>>     // Food chocolate at location living room
> >>>     // Food chips at location living room
> >>>     // looc is just an unused bound variable
> >>> end
> >>>
> >>> On 20 April 2011 10:16, Geoffrey De Smet <ge0ffrey.spam at gmail.com 
> <mailto:ge0ffrey.spam at gmail.com>> wrote:
> >>>>
> >>>> Mark and I were discussing backwards chaining
> >>>> 
> http://blog.athico.com/2011/04/backward-chaining-emerges-in-drools.html
> >>>> on IRC and we 'd like your opinion on a design issue.
> >>>>
> >>>> The example
> >>>> ========
> >>>>
> >>>> Let's say you have this data:
> >>>>   Location("crackers", "kitchen")
> >>>>   Location("apple", "kitchen")
> >>>>   Location("chocolate", "living room")
> >>>>   Location("chips", "living room")
> >>>>
> >>>> Let's say you have this code:
> >>>>
> >>>> query editableThings( String thing, String location )
> >>>>     Location(thing, location)
> >>>> end
> >>>> And then these 3 rules:
> >>>>
> >>>> rule outputinput
> >>>> when
> >>>>     Here( loc : location)
> >>>>     ?editableThings(food, loc;)
> >>>> then
> >>>>     System.out.println("Food " + f + " at location " + loc);
> >>>>     // Output:
> >>>>     // Food crackers at location kitchen
> >>>>     // Food apple at location kitchen
> >>>> end
> >>>>
> >>>> rule outputOutput
> >>>> when
> >>>>     ?editableThings(food, loc;)
> >>>> then
> >>>>     System.out.println("Food " + f + " at location " + loc);
> >>>>     // Output:
> >>>>     // Food crackers at location kitchen
> >>>>     // Food apple at location kitchen
> >>>>     // Food chocolate at location living room
> >>>>     // Food chips at location living room
> >>>> end
> >>>>
> >>>> rule typo
> >>>> when
> >>>>     Here( looc : location)
> >>>>     ?editableThings(food, loc;)
> >>>> then
> >>>>     System.out.println("Food " + f + " at location " + loc);
> >>>>     // Output:
> >>>>     // Food crackers at location kitchen
> >>>>     // Food apple at location kitchen
> >>>>     // Food chocolate at location living room
> >>>>     // Food chips at location living room
> >>>> end
> >>>>
> >>>> The discussion
> >>>> =========
> >>>>
> >>>> Both rules have the same statement:
> >>>>   ?editableThings(food, loc;)
> >>>>
> >>>> In the outputInput rule, "loc" is an input variable.
> >>>> In the outputOutput rule, "loc" is an output variable.
> >>>>
> >>>> I am wondering if we don't need a visual demarcation that a 
> variable is an output variable,
> >>>> to make it stand out of an input variable?
> >>>>
> >>>> Proposition 1: Suffix output variables with ":"
> >>>>
> >>>> rule outputinput
> >>>> when
> >>>>     Here( loc : location)
> >>>>     ?editableThings(food:, loc;)
> >>>> then ... end
> >>>>
> >>>> rule outputOutput
> >>>> when
> >>>>     ?editableThings(food:, loc:;)
> >>>> then ... end
> >>>> rule typo
> >>>> when
> >>>>     Here( looc : location)
> >>>>     ?editableThings(food:, loc;) // compiler error because input 
> variable loc is not declared
> >>>> then ... end
> >>>>
> >>>> --
> >>>> With kind regards,
> >>>> Geoffrey De Smet
> >>>> _______________________________________________
> >>>> rules-dev mailing list
> >>>> rules-dev at lists.jboss.org <mailto:rules-dev at lists.jboss.org>
> >>>> https://lists.jboss.org/mailman/listinfo/rules-dev
> >>>>
> >>>
> >>>
> >>> _______________________________________________
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> >>> https://lists.jboss.org/mailman/listinfo/rules-dev
> >>>
> >>
> >>
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> >>
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> >
> >
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