[DFDL-WG] Action 186: Re: DFDL Statement Evaluation Timing (Assert, Discriminator, SetVariable, NewVariableInstance)

[Steve Hanson]

Mike, I think we agreed on 'resolved set of annotations' instead of 
'combined annotations' ? 

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:   19/11/2012 17:16
Subject:        [DFDL-WG] Action 186: Re: DFDL Statement Evaluation Timing 
(Assert, Discriminator, SetVariable, NewVariableInstance)
Sent by:        dfdl-wg-bounces at ogf.org



I had the action to create the final errata wording for this item (Action 
186). 

Last action issue was to change wording to insure it was all dealing with 
combined annotations uniformly, and change "annotated construct" to 
"schema component".

I've done that, but also reorganized this as errata on specific sections 
of the spec consistent with the rest of our errata, and turned the 
'ordering' section into numbered lists for clarity.

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

Errata section 6.2. Clarification: At any single annotation point of the 
schema, there can be only one format annotation (dfdl:format, 
dfdl:element, dfdl:sequence, dfdl:choice, dfdl:group, dfdl:simpleType).  

Errata section 3. Glossary entries

Glossary: Annotation point: A location within a DFDL schema where DFDL 
annotation elements are allowed to appear. 

Note that annotation point is defined in section 6.2, but it belongs also 
in the glossary.

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

Glossary: DFDL Defining Annotations are the annotation elements 
dfdl:defineFormat, dfdl:defineVariable, and dfdl:defineEscapeScheme

Glossary: DFDL Format Annotations are the annotation elements dfdl:format, 
dfdl:element, dfdl:simpleType, dfdl:group, dfdl:sequence, and dfdl:choice.

Glossary: Remove existing definition for "Format Annotations"

Glossary: Physical layer - revised to: "Physical Layer - A DFDL Schema 
adds DFDL annotations onto an XSDL language schema. The annotations 
describe the physical representation or physical layer of the data."

Glossary: Combined annotations: When DFDL annotations appear on a group 
reference and the sequence or choice of the referenced global group, or 
appear among an element reference, an element declaration, and its type 
definition, then they are combined together and the resulting set of 
annotations is referred to as the combined annotations for the schema 
component.

Errata Section 7.3.1:  

Remove "DFDL asserts can be placed on components within a DFDL model." as 
this is made more specifically subsequently.

Replace "More than one dfdl:assert may be used at an annotation point. The 
dfdl:asserts will be evaluated in the order defined in the schema." with 
"If the combined annotations for a schema component contain multiple 
dfdl:assert statements, then those with testKind='pattern' are executed 
before those with testKind='expression' (the default). However, within 
each group the order of execution among them is not specified. Schema 
authors can insert sequences to control the timing of evaluation of 
statements more precisely. See Section REF: Evaluation Order for Statement 
Annotations."

Property testPattern: Add this to the description:

Note that the pattern is used to match against the entire representation 
of the component; hence, the framing (including initiators) is visible to 
the pattern. 

It is a schema definition error if dfdl:alignment is not 1 . 
It is a schema definition error if dfdl:leadingSkip is not 0.
It is a schema definition error if dfdl:encoding is not defined. 

Errata Section 7.3.1, and 7.4.1: Assert and Discriminator Placement

Correct description to be consistent with: dfdl:assert and 
dfdl:discriminator can be placed as annotations on sequence, choice, group 
references, local and global element declarations, element references, and 
simple type definitions. 

Errata Section 7.4.1:

Replace: "Any one annotation point can contain only a single 
dfd:discriminator or one or more dfdl:asserts,
but not both. It is a schema definition error otherwise." with:

"The combined annotations for a schema component can contain only a single 
dfd:discriminator or one or more dfdl:asserts,
but not both. It is a schema definition error otherwise."

Property testPattern: Add this to the description:

Note that the pattern is used to match against the entire representation 
of the component; hence, the framing (including initiators) is visible to 
the pattern. 

It is a schema definition error if dfdl:alignment is not 1 . 
It is a schema definition error if dfdl:leadingSkip is not 0.
It is a schema definition error if dfdl:encoding is not defined.

Errata Section 7.8 newVariableInstance

dfdl:newVariableInstance can be placed as an annotation on sequence, 
choice, and group references.

Replace: "It is a schema definition error to have more than one 
newVariableInstances for the same variable at any given point in the 
document." with "The combined annotations for any schema component can 
contain multiple dfdl:newVariableInstance annotations, but they must each 
refer to a different variable. It is a schema definition error otherwise."

Errata Section 7.9: setVariable 

Correct language to be consistent with: "dfdl:setVariable may be placed as 
an annotation on sequence, choice, group references, local and global 
element declarations for elements of simple type, element references to 
elements of simple type, and simple type definitions. "

Replace "It is a schema definition error to have more than one 
dfdl:setVariable for the same variable at any given point in the 
document." with "The combined annotations for any schema component can 
contain multiple dfdl:setVariable annotations, but they must each refer to 
a different variable. It is a schema definition error otherwise."

Errata: New Section 7.10: Evaluation Order for Statement Annotations

Of the combined annotations for a schema component, some are statement 
annotations and the order of their evaluation relative to the actual 
processing of the schema component itself (parsing or unparsing per its 
format annotations) is as given in the ordered lists below.

Implementations are free to optimize by recognizing and executing 
discriminators or assertions with testKind='expression' earlier so long as 
the resulting behavior is consistent with what results from the this 
description:  

For elements: 
1.      dfdl:discriminator with testKind='pattern' (parsing only)
2.      dfdl:assert(s) with testKind='pattern' (parsing only)
3.      Processing of the element schema component itself (as per format 
annotations)
4.      dfdl:setVariable(s) - See note below.
5.      dfdl:discriminator with testKind='expression' (parsing only) - 
Evaluated even on failure of steps 3, or 4. See note below.
6.      dfdl:assert(s) with testKind='expression' (parsing only)
For sequences and choices:
1.      dfdl:discriminator with testKind='pattern' (parsing only)
2.      dfdl:assert(s) with testKind='pattern' (parsing only)
3.      dfdl:newVariableInstance(s)
4.      dfdl:setVariable(s)
5.      dfdl:discriminator with testKind='expression' (parsing only) - 
Evaluated even on failure of steps 3 or 4. See note below.
6.      dfdl:assert(s) with testKind='expression' (parsing only)
7.      Processing of the sequence/choice component itself (as per format 
annotations)
Note on Discriminators with testKind='expression'

When parsing, an attempt to evaluate a discriminator must be made even if 
preceding statements or the parse of the schema component ended in a parse 
error. 

This is because a discriminator's expression could evaluate to true 
thereby resolving a point of uncertainty even if the complete parsing of 
the construct ultimately caused a parse error. 

Such discriminator evaluation has access to the DFDL Infoset of the 
attempted parse as it existed immediately before detecting the parse 
failure. Attempts to reference parts of the DFDL Infoset that do not exist 
are parse errors. 

It is also a parse error to read any variables defined by any 
dfdl:newVariableInstance statement evaluations that failed, or to read any 
variables set by any dfdl:setVariable statements that failed. 

Note on the dfdl:setVariable Statement for Elements

The combined dfdl:setVariable statements for an element schema com ponent 
(combined from an element reference, element declaration, and base and 
derived simpleType definitions) are executed after the parsing of the 
element. 

This implies that these variables are set after the evaluation of 
expressions corresponding to any computed DFDL format properties for the 
same schema component, and so the variables may not be referenced from 
expressions that compute DFDL properties.

That is, if an expression is used to provide the value of a format 
property (such as dfdl:terminator, or dfdl:byteOrder), the evaluation of 
that format property expression occurs before any dfdl:setVariable 
annotation from the combined annotations for that schema component are 
executed; hence, the expression providing the value of the format property 
may not reference the variable. Schema authors can insert sequences to 
provide more precise control over when variables are set.




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