[rules-dev] modify( expr ) {....}

[Mark Proctor]

On 07/04/2011 06:54, Wolfgang Laun wrote:
>
>
> On 7 April 2011 07:52, Mark Proctor <mproctor at codehaus.org 
> <mailto:mproctor at codehaus.org>> wrote:
>
>     On 07/04/2011 06:44, Wolfgang Laun wrote:
>>     Perhaps I'm naive, but I think that it should be able to
>>     "compile" this by
>>     a mere expansion:
>>        modify( getFact( var ) ) { setName("yoda"), setColor("blue") }
>>     becomes
>>        getFact( var ).setName("yoda" );
>>        getFact( var ). setColor("blue" );
>>        update( getFact( var ) );
>>     And just warn agsinst side effects.
>     getFact( var) was just a simplificiation. It can allow for any
>     expression of any complexity, it could pull from DB.
>
>
DB again was just an example, that expressions could be heavy, we have 
no control over that. For instance they could pull from the DB to get 
some information about which fact it is that have to lookup in the WM. 
Either way it would not be desirable to execute the entire expression 
for each setter.

Mark
> An existing object which is an existing fact in the WM of this session?
> -W
>
>     It means for each and every setter we would have to evaluate the
>     whole expression.
>
>     Mark
>>
>>     I've written lots of rules for various mini-apps lately, and I've
>>     never had any
>>     reason to use anything except bound variables in a modify anyway.
>>
>>     As for MVEL: I don't trust a "language" that doesn't provide a
>>     definition
>>     of its syntax. The less it is used in Drools (without the user
>>     asking for it)
>>     the better, Sorry if this hurts any feelings, but this reasoning
>>     is backed
>>     up by CENELEC.
>>
>>     Cheers
>>     Wolfgang
>>
>>     On 7 April 2011 07:14, Mark Proctor <mproctor at codehaus.org
>>     <mailto:mproctor at codehaus.org>> wrote:
>>
>>         I'm at the stage where I cannot make this work in a robust
>>         way when
>>         using the java dialect:
>>         modify( getFact( var ) ) { setName("yoda") }
>>
>>         The problem is we have to infer the type of the expression.
>>         If the
>>         expression is complex using variables in methods, we get method
>>         ambiguity if we don't know all variable types. Once we have the
>>         expression type we rewrite this as:
>>         Person obj = ( Person) getFact(var);
>>         obj.setName( "yoda" );
>>
>>         Edson did this originally by using mvel to analyse the
>>         expression. The
>>         problem is that if the expression uses any local variables,
>>         we don't
>>         know what those are. So we need to analyse the entire
>>         consequence, we
>>         started to do this with MVEL but we have reached one too many
>>         stumbling
>>         blocks - most recent of those is MVEL does not understand
>>         java generics.
>>         I've put in about 3 weeks trying to solve this with mvel, and
>>         now had to
>>         stop. When MVEL adds generics we can hopefully resume this
>>         work again.
>>
>>         For now i'm having to roll back to our current behaviour. The
>>         positive
>>         news is that no one has reported issues with this. I'm
>>         guessing most
>>         people use just the fact instance, not expressions, and if
>>         they use an
>>         expression it's simple. So the brittleness is not showing up.
>>
>>         Anyway the work around for now is to explicitley cast, that
>>         should
>>         resolve the issue, if it comes up for anyone. I'm tempted to
>>         say that
>>         expressions are only officially supported when used with casting.
>>         Atleast until we can do robust type inference:
>>         modify( (Person) getFact( var ) ) { setName("yoda") }
>>
>>         Mark
>>
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>>
>>
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