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

[Wolfgang Laun]

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.

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> 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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