For today's call. Regards Steve Hanson Architect, Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh@uk.ibm.com tel:+44-1962-815848 ----- Forwarded by Steve Hanson/UK/IBM on 16/10/2012 13:12 ----- From: Steve Hanson/UK/IBM To: Tim Kimber/UK/IBM@IBMGB Cc: Mike Beckerle <mbeckerle.dfdl@gmail.com> Date: 01/10/2012 15:31 Subject: Re: [DFDL-WG] assert and discriminator - no more before/after My comments: i) Implementations are free to optimize evaluation of discriminators so as to improve performance and diagnostic capability by analyzing the actual discriminator test expressions and evaluating them earlier. Should state that this is allowed only if the outcome of the early evaluation will always give the same result as evaluation after parsing. ii) Need to expand to include discriminators on sequences, choices and group refs. iii) In the case of a processing error, is the discriminator evaluated at the point the processing error occurs, or after rollback has occurred? I think it must be before rollback, else the discriminator may fail when it would have otherwise passed. iv) (TBD: ??? is this right ??). The issue here is which processing error 'wins'. I think the discriminator wins here. v) Need to expand to include asserts? Regards Steve Hanson Architect, Data Format Description Language (DFDL) Co-Chair, OGF DFDL Working Group IBM SWG, Hursley, UK smh@uk.ibm.com tel:+44-1962-815848 From: Tim Kimber/UK/IBM To: Mike Beckerle <mbeckerle.dfdl@gmail.com> Cc: Steve Hanson/UK/IBM@IBMGB Date: 28/09/2012 15:44 Subject: Re: [DFDL-WG] assert and discriminator - no more before/after One comment : the term 'this component' could be understood to refer to either a) the component on which the discriminator is positioned or b) the component that is the nearest enclosing point of uncertainty I think b) is the intended meaning. regards, Tim Kimber, DFDL Team, Hursley, UK Internet: kimbert@uk.ibm.com Tel. 01962-816742 Internal tel. 37246742 From: Mike Beckerle <mbeckerle.dfdl@gmail.com> To: Steve Hanson/UK/IBM@IBMGB, Tim Kimber/UK/IBM@IBMGB, Date: 28/09/2012 15:26 Subject: Re: [DFDL-WG] assert and discriminator - no more before/after Revised to use standard spec terminology about speculative parse behavior. That is, known to exist or known not to exist. -------------------------------------------------------------- Proposal: Evaluation Time for Discriminators A discriminator annotation on an element declaration or element reference is evaluated after the element is parsed. This evaluation occurs regardless of whether the parsing of the corresponding element instance ends with or without a processing error. In the case where parsing ends without error, evaluation of the discriminator then occurs and there are 2 possible outcomes 1. discriminator evaluates to true - The nearest enclosing point of uncertainty is resolved, and this component is known to exist. (NOTE: keep in mind in the above that this element might be one of several in a sequence making up the alternative (it is not necessarily just a single root to the alternative), so just because this discriminator is being evaluated after its element, there may still be more parsing to do within that alternative. So this point of uncertainty can be resolved, but the parse could still encounter a processing error later. ) 2. discriminator evaluates to false, or a processing error occurs during evaluation of the discriminator - the nearest enclosing point of uncertainty is resolved and this component is known not to exist. Any discriminator-caused processing error is the cause of error in this case. Diagnostics would refer to this as the cause of the error if there are no remaining alternatives in the enclosing points of uncertainty. In the case of a processing error, evaluation of the discriminator controls the way that processing error is handled. There are 3 possible outcomes 1. discriminator evaluates to true - the nearest enclosing point of uncertainty is resolved, and this component is determined to be known to exist. As a result, the processing error applies to the next outward enclosing point of uncertainty (if any). 2. discriminator evaluates to false - the nearest enclosing point of uncertainty is resolved, and this component is determined to be known NOT to exist. 3. discriminator evaluation causes a processing error - the processing error (the one from the discriminator evaluation) causes the nearest enclosing point of uncertainty to be resolved. The component is known NOT to exist. The discriminator-caused processing error is the cause of error in this case. Diagnostics would refer to this as the cause of the error if there are no remaining alternatives in the enclosing points of uncertainty. (TBD: ??? is this right ??) Implementations are free to optimize evaluation of discriminators so as to improve performance and diagnostic capability by analyzing the actual discriminator test expressions and evaluating them earlier. On Mon, Sep 17, 2012 at 9:58 AM, Mike Beckerle <mbeckerle.dfdl@gmail.com> wrote: Thanks for this. So, an expression on a discriminator wants to be able to be placed somewhere syntactically such that it is clear that it will apply to a specific point of uncertainty. Yet it then must reference forward into the structure that would normally not have yet been created/processed at that point, for example to look at a tag field. I see the issue now. The problem is that all these forward references must have their points of uncertainty resolved, and then we can evaluate the expression without being fooled about whether something temporarily doesn't exist or not. ...mikeb On Mon, Sep 17, 2012 at 9:15 AM, Steve Hanson <smh@uk.ibm.com> wrote: 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@uk.ibm.com tel:+44-1962-815848 From: Mike Beckerle <mbeckerle.dfdl@gmail.com> To: Steve Hanson/UK/IBM@IBMGB Cc: dfdl-wg@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@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@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 -- Mike Beckerle | OGF DFDL WG Co-Chair Tel: 781-330-0412 -- dfdl-wg mailing list dfdl-wg@ogf.org https://www.ogf.org/mailman/listinfo/dfdl-wg 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 -- 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 -- Mike Beckerle | OGF DFDL WG Co-Chair Tel: 781-330-0412 -- Mike Beckerle | OGF DFDL WG Co-Chair Tel: 781-330-0412 -- 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