Re: [rules-users] Condition syntax to access Map

Yes, that is exactly what I think. Pattern matching constraints are like query parameters. They need to exist and evaluate to true in order to match. So, for this to match: a.b.c == null a needs to exist and be non-null, b needs to exist and be non-null, c needs to exist and be null. So it is not just NP safe navigation... it is an existence test at the same time. So for maps a[x].b[y].c[z] == null The keys x, y and z need to exist, and c[z] must have a value of null. That is what the expression above is asking for, in my understanding. This presents no loss of completeness to the language, as you can still test non-existence of keys if that is what you want, but the most common case you are looking for the opposite and it becomes much simpler to write rules that way. We can probably do that and have a configuration option to turn this feature on/off. Edson 2011/7/29 Mark Proctor <mproctor(a)codehaus.org&gt; > Lets forget that these are nested accessors and the problems they bring. > Lets look at what they would be if they were real relations: > > > On 29/07/2011 08:55, Wolfgang Laun wrote: > > Whoa! See below... > > 2011/7/28 Edson Tirelli <ed.tirelli(a)gmail.com&gt; > >> >> 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. >> > > (Edson: Only if you define it so. The logical implication of c being null > in an absent a.b (i.e., a.b==null) could very well be that "a.b.c does not > exist", and you can't claim that a.b.c exists if a.b. doesn't! > > Is there no house at some address? > (city.street[name].house[number] == null) # true => no such house > > $c : City() > $s : Street( city == $c, street = "name" ) > House( number == null) > > The above is identical logic to the more convenient form of nested > accessors, it's the proper relational form. In this case if there was no > Street, it wouldn't match. > > > > > This test data with false when null: Vienna/TirelliStrasse/42 returns > "false", hence there *is* such a house. But we don't have a Tirelli Street > in Vienna (yet)! > > Confer this to Perl's > ! exists $city{-streets}{"Tirelli"}[42] > ) > > >> Raising a null pointer exception, IMO, brings no advantage at all to the >> table... on the contrary, makes writing rules more difficult. >> > > Edson, Mark,... please do recall the times where you have had an NPE in the > code in a boolean expression? How painful would it have been if Java would > have returned "false", continuing to cover a coding error made elsewhere? > > Why don't other languages tolerate "null" silently? (Perl, the most > pragmatic of all, doesn't - it has introduced an operator I can use or not.) > > I have no problem when folks want to take shortcuts and live la dolce vita, > but > <em>I don't want to be led into the bog without my consent.</em> > > > >> 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. >> > > This is an entirely different matter than the previous one. I see no reason > whatsoever, not to define this "from" as working with an implicit filter. > > -W > > > >> >> 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(a)eurodecision.com&gt; >> >>> 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. >>> >>> _______________________________________________ >>> rules-users mailing list >>> rules-users(a)lists.jboss.org >>> https://lists.jboss.org/mailman/listinfo/rules-users >>> >>> >> >> >> -- >> Edson Tirelli >> JBoss Drools Core Development >> JBoss by Red Hat @ www.jboss.com >> >> _______________________________________________ >> rules-users mailing list >> rules-users(a)lists.jboss.org >> https://lists.jboss.org/mailman/listinfo/rules-users >> >> > > > _______________________________________________ > rules-users mailing listrules-users@lists.jboss.orghttps://lists.jboss.org/mailman/listinfo/rules-users > > > > _______________________________________________ > rules-users mailing list > rules-users(a)lists.jboss.org > https://lists.jboss.org/mailman/listinfo/rules-users > > -- Edson Tirelli JBoss Drools Core Development JBoss by Red Hat @ www.jboss.com