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;)

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