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

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