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@uk.ibm.com
tel:+44-1962-815848

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: dfdl-wg@ogf.org,
Date: 29/10/2012 22:54
Subject: [DFDL-WG] DFDL Statement Evaluation Timing (Assert, Discriminator, SetVariable, NewVariableInstance)
Sent by: dfdl-wg-bounces@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......> 
</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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