Re: [rules-users] Use of the binding variable

The unification operator ":=" is a combination of the binding operator ":" and the equality operator "==". In fact, it behaves as either, depending on the context: In this example, we are looking for two different Persons with the same name: rule "Pairs" when $p := Person( $name := name ) Person( this != $p, $name := name ) then System.out.println( "We have two persons with the same name : " + $name ); end The LHS is equivalent to: $p : Person( $name : name ) Person( this != $p, name == $name ) The first time a variable such as $name is used with ":=", it works as ":". Any time later, since the variable now has a value assigned to it, ":=" will work as "==". Apparently, this operator may seem redundant and useless :) but it is actually meant to be used with **queries**. For example: query personsByName( String $name, Person $p ) $p := Person( $name := name ) end The arguments ($name, $p) can be BOTH IN and OUT parameters, depending on the context of the caller. So, the query can be used in 4 different ways: IN/IN : Is this Person in the WM and does it have this name? ?personsByName( "john", p1 ) IN/OUT : Retrieve all Persons with this name ?personsByName( "john", $pers ) OUT/IN : Retrieve this person, if it is in the WM, and return its name ?personsByName( $n, p1 ) OUT/OUT : Retrieve all persons from the WM with their names ?personsByName( $n, $pers ) Fir this reason, it is impossible for the query to know whether, upon invocation, $name and $p will be IN - and thus already have a value - or OUT. So, the unification operator ":=" is necessary because of its polymorphic ability to behave as ":" or "==" as appropriate. Best Davide On 05/09/2013 06:51 AM, Sonata wrote: