[DFDL-WG] public comment "Non-deterministic outcome of setVariable evaluations"

[Mike Beckerle]

Per call on 2013-10-01 revise wording to specify exact order of evaluation
when combining newVariableInstance, and setVariable across multiple
annotation points that are resolved together.



Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are
subject to the OGF Intellectual Property
Policy<http://www.ogf.org/About/abt_policies.php>



On Tue, Oct 1, 2013 at 8:16 AM, Steve Hanson <smh at uk.ibm.com> wrote:

> I think this leaves us in a strange halfway house where the spec says
> which order annotations on the same annotation point are executed, but does
> not say in which order annotations on different annotation points are
> executed. To my mind, we either have a predictable order or we don't.
>
> Regards
>
> Steve Hanson
> Architect, IBM Data Format Description Language (DFDL)
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> IBM SWG, Hursley, UK*
> **smh at uk.ibm.com* <smh at uk.ibm.com>
> tel:+44-1962-815848
>
>
>
> From:        Mike Beckerle <mbeckerle.dfdl at gmail.com>
> To:        "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>,
> Date:        30/09/2013 23:12
> Subject:        [DFDL-WG] public comment "Non-deterministic outcome of
> setVariable        evaluations"
> Sent by:        dfdl-wg-bounces at ogf.org
> ------------------------------
>
>
>
> Some email discussion before posting to the public comment forum may be
> helpful for this issue.
>
> Right now the rules say that all dfdl:newVariableInstance annotations for
> a schema component are always executed before any setVariable annotations.
> This is ok. One can introduce an extra sequence in order to allow a
> setVariable to occur before a newVariableInstance.
>
> (Personally: I would like to get a warning from my DFDL implementation if
> I write statement annotations at one annotation point that are not in the
> canonical order described in Errata 3.25)
>
> The problem is that the order of execution of multiple setVariable
> statements at one annotation point, or similarly, of multiple
> newVariableInstance at one annotation point, is not specified. This is what
> will get users into trouble.
>
> If there are multiple newVariableInstance annotations, then the order is
> important to understanding when exactly a new instance comes into scope,
> and whether the prior-scope's value for that variable can be read as part
> of the default value expressions. Consider this:
>
> <dfdl:newVariableInstance ref="save">{ $delim }</dfdl:newVariableInstance>
> <dfdl:newVariableInstance ref="delim">{  ../hdr/d
> }</dfdl:newVariableInstance>
>
> We need to specify whether $delim in the first annotation is referring to
> that variable before the new instance is created, or after. I suggest that
> before is more intuitive, as that is lexical order. Note however, that this
> means:
>
> <dfdl:newVariableInstance ref="prefix">{  fn:concat($prefix, "_",
> fn:name()) }</dfdl:newVariableInstance>
>
> is allowed and is not recursion. The expression is referring to $prefix
> from the enclosing scope, and then initializing a new instance of $prefix
> for the scope contained by this schema component.
>
> The same logic applies to dfdl:setVariable annotations. The order needs to
> be controlled by the user.
>
> I suggest the fix for this issue is as follows:
>
> Errata 3.20, 3.21, and Errata 3.25 must be modified (and the corresponding
> sections of the spec)
>
> For errata 3.20, and errata 3.21 the sentence:
>
>  "However, the order of execution among them is not specified. Schema
> authors can insert sequences to control the timing of evaluation of
> statements more precisely."
>
> is replaced by:
>
> "The order of execution is specified in errata 3.25."
>
> Errata 3.25 is modified.
>
> Numbered item 3 of both the first list and second list is replaced by:
>
> 3. dfdl:setVariable(s) - in lexical order
>
> Numbered item 2 of the second list is replaced by:
>
> 2. dfdl:newVariableInstance(s) - in lexical order
>
> These two paragraphs are added below the numbered lists to clarify what
> 'lexical order' means.
>
> The dfdl:setVariable annotations at any one annotation point of the schema
> are always executed in lexical order. However, dfdl:setVariable annotations
> can also be found in different annotation points that are combined into the
> resolved set of annotations for one schema component. In this case, the
> order of execution of the dfdl:setVariable statements from any one
> annotation point remains lexical, but it is unspecified what the order of
> execution is across dfdl:setVariable statements coming from different
> annotation points.
>
> The dfdl:newVariableInstance annotations at any one annotation point of
> the schema are always executed in lexical order. However,
> dfdl:newVariableInstance annotations can also be found in different
> annotation points that are combined into the resolved set of annotations
> for one schema component. In this case, the order of execution of the
> dfdl:newVariableInstance statements from any one annotation point remains
> lexical, but it is unspecified what the order of execution is across
> dfdl:newVariableInstance statements coming from different annotation points.
>
>
> ----------------------
>
> Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | *
> www.tresys.com* <http://www.tresys.com/>
> Please note: Contributions to the DFDL Workgroup's email discussions are
> subject to the *OGF Intellectual Property Policy*<http://www.ogf.org/About/abt_policies.php>
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