On 29/07/2011 14:28, Edson Tirelli wrote:
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.
> So, a builder option to turn this on is allright with me.
We can probably do that and have a configuration option to turn
this feature on/off.
I'm strongly against configuration options in this case, we decide on
one way and stick with it. We already have too many configurations and a
casual person looking at the code could introduce a bug as they weren't
aware of what configuratino was on for null safety.
I think part of the problem here is we are mixing domains, between
script evaluation and relational constraints. There is a reason why
other rule engines don't do nested accessors :) (ignoring the technical
issues too).
Mark
Mark
Edson
2011/7/29 Mark Proctor <mproc...@codehaus.org
<mailto:mproc...@codehaus.org>>
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.tire...@gmail.com
<mailto:ed.tire...@gmail.com>>
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>
So, a builder option to turn this on is allright with me.
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.legen...@eurodecision.com
<mailto:vincent.legen...@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 <http://www.jboss.com>
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JBoss Drools Core Development
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