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

[Mike Beckerle]

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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