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@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* <http://www.ogf.org/dfdl/> IBM SWG, Hursley, UK* **smh@uk.ibm.com* <smh@uk.ibm.com> tel:+44-1962-815848
From: Mike Beckerle <mbeckerle.dfdl@gmail.com> To: dfdl-wg@ogf.org Date: 13/09/2012 16:32 Subject: [DFDL-WG] assert and discriminator - no more before/after Sent by: dfdl-wg-bounces@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
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