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

[Geoffrey De Smet]

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
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