[DFDL-WG] assert and discriminator - no more before/after

Mon Sep 17 09:15:35 EDT 2012

Some thoughts so far...added to agenda for next WG call.

I think the IBM implementation was done that way to handle descendent 
references and their implications, and to compensate for the dropping of 
the 'timing' attribute. It uses a notify mechanism to speed up the point 
where a discriminator can be evaluated, and hence backtrack earlier. I 
believe it could have been implemented by evaluating at fixed points - 
evaluate straight away if no reference to self or descendent; evaluate 
when finished component if reference to self or descendent; evaluate if 
processing error occurs within component. 

I have been back through spec revisions, draft documents on speculative 
parsing, emails and WG minutes, to see what we have discussed previously:

- The 'timing' attribute on asserts and discriminators existed in spec 
038. This was dropped in spec 039 when the following was added: "The 
expression is evaluated when the referenced elements are known to exist or 
known not to exist.". This was Tim's suggestion.

- Later on in spec 042 the 'timing' attribute was dropped from asserts 
too, and the wording for both changed to "Any element referred to by the 
expression must have already been processed or is a descendent of this 
element. The expression must have been evaluated by the time this element 
and it descendents have been processed." plus additionally for 
discriminators "or when a processing error occurs when processing this 
element or its descendents".

- The motivating use case for supporting forward references to descendents 
is as follows:  "For the most common use case (choice resolution) the most 
natural place for a DFDL author to place the discriminators is on the root 
of the branch. This may well involve a forward reference into the branch 
content.  If we disallow this, you are forcing the author to place the 
discriminator in the branch content, which might be in a separate global 
element, perhaps in another schema".  And the problem with doing the 
latter is the rule that says a discriminator resolves the nearest active 
point of uncertainty. If you place a discriminator inside a global element 
then it will get evaluated at all points of use - with potentially 
undesirable results. So you place your discriminator as close to the point 
of uncertainty as possible.

- Your dead-simple suggestion leaves the above choice scenario open to the 
problem stated.

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848

From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     Steve Hanson/UK/IBM at IBMGB
Cc:     dfdl-wg at ogf.org
Date:   15/09/2012 20:09
Subject:        Re: [DFDL-WG] assert and discriminator - no more 
before/after

That is a remarkably complex implementation.

I am a bit concerned.... is it correct? Here's the scenario I'm worried 
about: When an expression is evaluated, some node set results for internal 
sub-expression results might be empty node sets. But there's no positive 
way to tell this happened because some part of the infoset had 'not yet' 
been filled in, because we cannot predict the future. So an expression 
could successfully evaluate, just produced a different value than it would 
if evaluated later.  

It seems to me some of this is inherent in XPath and the 
node-set-as-result model. An example of this might be a discriminator with 
an expression that evaluates to true if some subtree does NOT exist. At 
the start of the associated element, that tree doesn't exist, so you get 
empty node set back when you examine it, discriminator expression 
successfully evaluates to true. Later, when recursing into children of the 
element in question you add that sub-tree. After the element is complete 
the answer to the discriminator expression would have been false.

Anyway, in the daffodil project we're trying to reuse an XPath 
implementation (Saxon-B), and create as our infoset trees in the JDOM 
object model. 

So the above strategy, even if it works, isn't available to us anyway 
because we're trying to use an expression evaluator that does call us back 
for variable access, but expects us to hand it a JDOM tree as the infoset, 
and it accesses that tree at will while evaluating the expression. We 
don't have intercept capability to the inquiries on the infoset/jdom. 

Other implementations might also prefer simpler implementation techniques, 
even at the expense of efficiency. 

So the question becomes are there simpler strategies for implementing the 
DFDL expressions on asserts/discriminators? 

I have a dead-simple suggestion that might work.
1.      an assert/discrim which annotates an element behaves as if 
evaluated after the element's value is computed, true whether simple or 
complex type.
2.      an assert/discrim which annotates a sequence or choice behaves as 
if evaluated before any child of the sequence/choice.
Seems very simple. (Almost too simple?) I'm not sure it is correct, but it 
seems workable so far to me, in that you can control before/after by 
syntactic placement. This might have some impact on the model structure, 
but there are other things in DFDL that do as well. 

Is there some obvious flaw I'm missing?

...mike

On Fri, Sep 14, 2012 at 5:14 AM, Steve Hanson <smh at uk.ibm.com> wrote:
Hi Mike 

I believe the rule is that an assert/discriminator expression is evaluated 
"as soon as it can be". In IBM DFDL, we try to evaluate an expression when 
it is encountered, and if it can't be evaluated at that time because it 
references element(s) that do not appear in the infoset, the parser 
continues but the expression manager registers an interest in the 
element(s). The expression manager gets notified when the element(s) 
appear in the infoset and the expression is re-evaluated at that time. If 
the parser gets to the end of the scope for the assert/discriminator and 
it is still not evaluated, it is an error. (Tim please correct any 
mis-information). 

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848 

From:        Mike Beckerle <mbeckerle.dfdl at gmail.com> 
To:        dfdl-wg at ogf.org 
Date:        13/09/2012 16:32 
Subject:        [DFDL-WG] assert and discriminator - no more before/after 
Sent by:        dfdl-wg-bounces at ogf.org 

I am looking in the spec for guidance about the evaluation order of assert 
statements.

We used to have before/after control properties, but eliminated them.

If I annotate a simpleType'd element with an assert that says { . eq 'x' 
}, that of necessity references the current value, so must execute after 
the value has been computed.

If on the other hand I annotate a complexType element with a discriminator 
that says { ../flag eq 'C1' } then this of necessity must execute before I 
go after the contents because the whole point is to evalutate the 
discriminator first.

Did we ever articulate exactly what the rules are here about order of 
evaluation?

Thanks for reminders

..mikeb

-- 
Mike Beckerle | OGF DFDL WG Co-Chair 
Tel:  781-330-0412
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-- 
Mike Beckerle | OGF DFDL WG Co-Chair 
Tel:  781-330-0412

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 
741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
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