[rules-users] Condition syntax to access Map

[Edson Tirelli]

   All,

   I think we need to differentiate paradigms here. When using rules,
contrary to imperative code, what we are doing is pattern matching.

X( a.b.c == <value> )

   In the above case, we are looking for Xs that make that whole constraint
true (i.e. match). If a or b are null, the whole expression will be false,
does not matter the value of c or the value it is being compared against.
Raising a null pointer exception, IMO, brings no advantage at all to the
table... on the contrary, makes writing rules more difficult.

   Another example we had in the past:

class Circle implements Shape
class Square implements Shape

rule X
when
    Circle() from $shapes
...

   In the above example, $shapes is a list and the rule is clearly looking
for Circles. If there are Squares in there, they will just not match.
Raising a ClassCastException like it would happen in an imperative language
brings no advantage to the table, IMO.

   So, IMO, all property navigation should be null pointer safe in the LHS
of the rules.

   This is not what happens today, but I think it should be fixed.

   Edson




2011/7/28 Vincent LEGENDRE <vincent.legendre at eurodecision.com>

> Hi all,
>
> I agree with W. : NPE should be the default, and "null" cases behaviour
> should be planned by programmers.
> But I am not sure about using a new operator in rules (and do the update in
> Guvnor ...).
> Why not using some drools annotations on the getter specifying the
> behaviour of an eval on a null value returned by this getter ?
> And may be these annotation could be added to an existing POJO via the
> declared type syntax (just like event role in fusion) ?
>
> Vincent.
>
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>
>


-- 
  Edson Tirelli
  JBoss Drools Core Development
  JBoss by Red Hat @ www.jboss.com
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