[rules-users] single binding for or CE flagged by compiler

[Hehl, Thomas]

Not getting into whether this is a bad practice, the only language I've
ever seen support this syntax is COBOL. Every other language I've worked
in (and there are many) do not support this technique, so it seems
reasonable that this syntax should be disallowed in drools as well.

 

________________________________

From: rules-users-bounces at lists.jboss.org
[mailto:rules-users-bounces at lists.jboss.org] On Behalf Of Edson Tirelli
Sent: Monday, March 16, 2009 9:58 AM
To: Rules Users List
Subject: Re: [rules-users] single binding for or CE flagged by compiler

 


   This is an old discussion. Single binding on "or" is a non-orthogonal
special case on the language syntax that was introduced in the early
days when we only had "infix" CEs. When we started to support prefix CE,
the special case was not updated, and as a result, the single binding
works only with infix.

   IMO, this syntax promotes bad practices and should be disallowed.
Although, some people like it and it is still in there. Just to clarify
why I think it promotes bad practice:

* When you have the same object type for all the patterns inside an "or"
group, you should be using constraint connector ||, not the "or" CE.
Using your example:

$t : Trigger( fa == 1 || == 2 ) // or any variation of the syntax.

   This keeps the network smaller and prevents unexpected behavior when
the constraints are not mutually exclusive and in some cases people
forget that the rule will fire once for each logical branch when using
"or".

* If the object types are different, then you can't use single binding
anyway:

$o  : ( Cheese() or Wine() ) // this will lead to ClassCastExceptions on
the usage of $o.

   So, I really think that the "single binding or" fulfilled its purpose
on Drools 3, when we didn't had all the flexibility in the language we
have today. But now, the only thing it achieves is promoting bad
practices.

   Just my 0.02c.

   []s
   Edson

2009/3/16 Wolfgang Laun <wolfgang.laun at gmail.com>

As far as I know, this should work:

package orel;
import orel.Main.Trigger;
rule ror1
    when
        $t : (or Trigger(fa == 1)
                 Trigger(fa == 2))
    then
        System.out.println( "fired " + $t.getName() );
end

This is what Drools-5.0.0 kbuilder.getErrors().toString() returns:

[5,11]: [ERR 102] Line 5:11 mismatched input 'Trigger' expecting '(' in
rule ror1 in pattern or[6,14]: [ERR 102] Line 6:14 mismatched input
'Trigger' expecting ')' in rule ror1[6,30]: [ERR 102] Line 6:30
mismatched input ')' expecting 'then' in rule ror1

Shouldn't toString() insert line ends? As it is, the result is pretty
much useless.

This works:
rule ror1
    when
        (or $t : Trigger(fa == 1)
            $t : Trigger(fa == 2))
    then
        System.out.println( "fired " + $t.getName() );
end

-W


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-- 
 Edson Tirelli
 JBoss Drools Core Development
 JBoss, a division of Red Hat @ www.jboss.com

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