[DFDL-WG] DFDL Statement Evaluation Timing (Assert, Discriminator, SetVariable, NewVariableInstance)

[Tim Kimber]

I agree with most of that. I certainly would want newVariableInstance to 
be evaluated before a complex element is parsed.
The complication with the setVariable rule is that an absolute path may be 
equivalent to a path which uses the self-axis but it is not possible, in 
general, to determine this by static analysis of the xsd. Furthermore, one 
XPath expression can contain zero to many path refs, any or all of which 
might ( or might not ) start with the self-axis.

I can only see two possible solutions to this. 
- Asserts/discriminators/setVariable are always executed after the 
component on which they are positioned has been fully parsed. With the 
exception of asserts/discriminators with test=pattern.
- Asserts/discriminators/setVariable have a 'timing' flag that defaults to 
'after'. If it is set to 'before' and the expression does not evaluate 
successfully then it is a schema definition error.
This would allow earlier rejection of the wrong branch in the model ( and 
therefore more efficient parsing ) in cases where the expression only 
refers to items that have already been parsed.

regards,

Tim Kimber, DFDL Team,
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 37246742




From:   Steve Hanson/UK/IBM at IBMGB
To:     Mike Beckerle <mbeckerle.dfdl at gmail.com>, 
Cc:     dfdl-wg at ogf.org
Date:   30/10/2012 13:12
Subject:        Re: [DFDL-WG] DFDL Statement Evaluation Timing (Assert, 
Discriminator, SetVariable, NewVariableInstance)
Sent by:        dfdl-wg-bounces at ogf.org



Thanks for writing this up, I have a couple of comments from thinking 
about this in parallel, and reading the write-up. 

- In 2), replace element with object as applies to sequence/choice as well 

- newVariableInstance should be evaluated before the object is parsed 
(note: self-axis is not allowed) 
- setVariable should be evaluated before the object is parsed unless it 
uses self-axis 
- setVariable should be evaluated after the object is parsed if it uses 
self-axis 
- consider only allowing self-axis in setVariable for simple 
elements/types 
- add statement about early evaluation of non-pattern 
asserts/discriminators if that can be done (as per IBM implementation) 

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:        29/10/2012 22:54 
Subject:        [DFDL-WG] DFDL Statement Evaluation Timing (Assert, 
Discriminator,        SetVariable, NewVariableInstance) 
Sent by:        dfdl-wg-bounces at ogf.org 



I'll write this up like an errata, but this is for discussion of whether 
we believe this is clear and complete.

-------------------------------------

Glossary: DFDL Statements are the annotation elements dfdl:assert, 
dfdl:discriminator, dfdl:setVariable, and dfdl:newVariableInstance.

Errata: Locations where DFDL Statements are allowed to appear are extended 
to also include Global Element Declarations, and on Simple Type 
Definitions.

Errata: Clarification about discriminators: Discriminators exclude 
Assertions even when combining across references.

Beyond the stipulation that there can be only one dfdl:discriminator at 
any annotation point of the DFDL schema, there are further constraints.

A single dfdl:discriminator annotation may appear on an element reference, 
or on the global element declaration it refers to, or on the simple type 
appearing immediately within or referenced from the global element 
declaration. But only one of those places. In addition, if a discriminator 
occupies one of those places, then no dfdl:assert annotations may appear 
in any of those locations. 

A dfdl:discriminator annotation may appear on a group reference or on the 
model group within the global group definition it refers to. But only one 
of those places, and similarly, if a discriminator appears in any of those 
places, then no dfdl:assert annotations may appear in any of those 
locations.

Errata: Clarification about the execution order of DFDL Statements when 
they appear on an element reference or element declaration.

DFDL Statement annotations for a given schema component are executed as 
follows:

1) all relevant DFDL statement annotations are gathered to form a single 
list which preserves schema-definition order. 
For a simpleType definition, the DFDL statement annotations found 
immediately on it are kept in schema-definition order, and are appended to 
the end of  a list of those from any base simpleType definition. 
For an element declaration having simple type, the DFDL statement 
annotations found immediately on the declaration are appended to the end 
of the list of those from its simple type. 
For an element reference, DFDL statement annotations found immediately on 
the element reference are appended to the end of the list of those from 
the global element declaration it references.
2) given the combined list, the annotations are executed as follows: 
1.        before any parsing of the element, a dfdl:discriminator with 
testKind="pattern" is executed. 
2.        if there is no discriminator, then before any parsing of the 
element, all dfdl:asserts (there could be several) with testKind="pattern" 
are executed in the order they appear in the list of DFDL statements. 
3.        The element itself is parsed, or its inputValueCalc property is 
evaluated to create its value. 
4.        all newVariableInstance annotations are executed and new 
variables are placed into scope for the duration of these remaining steps. 
The statements are executed in the order they appear in the list of DFDL 
statements. 
5.        all setVariable annotations are executed. The statements are 
executed in the order they appear in the list of DFDL statements. 
6.        if a discriminator is present it is executed 
7.        if no discriminator is present, then assert annotations can be 
present, and they are executed. If there are multiple assert annotations 
the statements are executed in the order they appear in the list of DFDL 
statements. 
If the element reference or local element declaration is an array, then 
this evaluation is repeated for each occurrence of the array.

Discussion: 

The above allows the default expressions associated with any statement to 
refer to the value of the element itself as "." 

However, there's this anomaly of syntax where things don't seem right:

<dfdl:newVariableInstance ref="foo:bar" default="{....some expression 
...}"/>  creates a new variable for the scope of the entity it annotates.

It's clear what this means if this annotation is placed on a sequence or 
choice. For the children of that sequence/choice the new variable instance 
is in effect.

On a simpleType or element having simple type, similarly it is clear (in 
that case it's a very local variable, just for the expressions in other 
newVariableInstance statemsnts, setVariableStatements, and discriminators 
and assertions).

The rub: on an element declaration (or reference) when there is a complex 
type, its not clear.

<element name="foo">
<annotation><appinfo ....>
   <dfdl:newVariableInstance......> <!-- what can refer to this? -->
</appinfo></annotation>
<complexType>
   <sequence>
      <annotation><appinfo ....>
           <dfdl:assert>... according to rules above this cannot refer to 
the newVariableInstance...</dfdl:assert>
     </appinfo></annotation>
     ....
   </sequence>
</complexType>
</element>

The above rules say the newVariableInstance isn't evaluated until AFTER 
the element is parsed, so the assert down inside on the sequence will NOT 
see this new variable even tho textually, the newVariableInstance 
annotation looks like it would be in scope over the sequence.

Possible ways to avoid this oddity without messing up evaluation order: 
allow newVariableInstance only on simpleType, element declarations and 
element references having simpleType, group references, and 
sequence/choice. Disallow them on element declarations of complex type or 
on element references to those. The schema author can always introduce an 
extra tier of sequence to provide the exact behavior they need, and this 
otherwise error-prone issue can be avoided.


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